Black''''s veterinary dictionary 21st edition - G docx

Genetics, Heredity and Breeding PAGE Homozygous and heterozygous 299Inheritance through multiple genes 300Inheritance and high milk yields 300 of the individual are represented in the fe

(see FLIES;WARBLES) In Britain, warble flies are

on the wing from late May onwards

Gadding

Excitement, restlessness, uncontrolled rushing

around in horses or cattle due to the presence

of biting flies; also, in the case of cattle, warble

flies

Gag

A device to facilitate oral examination or

treatment by holding the mouth open

Gait, Abnormal

(see ATAXIA;‘GOOSE-STEPPING’;LAMENESS)

Gall-Bladder

The little pouch-like sac in which bile produced

by the liver is stored until it is required during

the process of digestion It is a hollow,

pear-shaped organ lying in a depression on the

posterior surface of the liver The gall-bladder

is not present in the horse and in animals of

the horse tribe, but is found in the other

domesticated animals

Blockages of the bile-duct by liver flukes or

by gallstones may result in jaundice as well as

severe local inflammation Acute inflammation

of the gall-bladder is painful, and there is

danger of rupture or gangrene

‘Gall Sickness’

(see ANAPLASMOSIS)

Gallstones

Gallstones, which are also known as biliary

calculi (see under CALCULI), are concretions

which are formed in the gall-bladder or in the

bile-ducts of the liver As a rule they are hard,

brownish in colour, coated with mucus, and of

a more or less rounded shape They may be

composed of cholesterol; of cholesterol and bile

pigments; or of pigment and lime salts One

or several may be present, causing pain and

jaundice

Gallstones are more prevalent in sheep than

in cows, dogs, cats, and horses

In human medicine, ursodeoxycholic acid

has been used to dissolve gallstones

Galvanised bins, used to store swill, have led to

ZINC POISONINGin pigs

Galvayne’s Groove

A vertical groove in the front surface of thehorse’s upper corner incisor teeth It firstappears at the gum margin at about 10 years oldand gradually moves down the surface of thetooth as the horse ages until it grows out atabout 30 years old (see DENTITION– Horses)

Game Birds, Mortality

This may be considered under 2 headings:

From farm chemicalsMany farm chemicalscan cause poisoning in game and other birds.Deaths have resulted from the use, as seed dress-ings, of compounds such as dieldrin, aldrin andheptachlor, now banned in the UK Some of theorganophosphorus insecticides; dimethoate; andthe ‘nitro-type’ of weedkillers such as DNC,which stains the carcase yellow; are among otherchemicals hazardous to birds

Pheasant poults have died as a result of beingtreated for lice with a 5 per cent gamma benzenehexachloride (BHC) dusting powder

An organophosphorus insecticide does notnecessarily act quickly Death may occur 8 weeksafter eating the poisoned food The symptomsshown by poisoned birds include ruffled feathers,saliva around the beak, high-stepping gait orunsteadiness on the legs, distressed breathing,and paralysis However, as their use is nowreduced, problems caused by organophosphoruscompounds are less frequent

Spraying an orchard with either DDD orDDT (now banned) has caused heavy game-bird losses A partridge was found dead in afield where blackcurrants had been sprayedwith the insecticide endrin It was reportedfrom the farm that 8 or 9 partridges died with-

in a few hours of eating earthworms whichcame to the surface of the soil soon after spray-ing Rat poisons may perhaps be included inthe term ‘farm chemicals’ Owls die after eatingpoisoned rodents

From natural causes Impacted gizzard,tuberculosis, aspergillosis, swine erysipelas,fowl-pox, fowl cholera, fowl typhoid, infectioussinusitis Gapes is another cause of death; also inthe USA, encephalomyelitis Deaths from fowlpest (Newcastle disease) have been reported inthe UK; blackhead in pheasants and partridges

‘Grouse disease’ is the colloquial name for

infestation with Trichostrongylus tenuis Mortality

G

occurs when food is in short supply as a result

of poor growth or overpopulation of birds on

a moor In some circumstances, it has been

concluded that grouse have died because not

enough were shot the previous year Grouse tend

to remain in a locality and not move to other

moors

Louping ill, transmitted by sheep ticks, is

generally fatal to red grouse (Lagopus lagopus

scoticus), the commonest game bird on British

heather moorland, and can reduce stocks to

very low densities

Inclusion-body hepatitisA 9-day outbreak

resulted in an 18 per cent mortality among

1000 intensively reared pheasant poults (19 days

old when the outbreak began)

SalmonellosisAn outbreak killed 50 per cent

of 2800 pheasant poults, deaths beginning

in 3-day-olds The infection was one of

S typhimurium An antibiotic achieved control

later

Coccidiosisis an important disease of

pheas-ants and other game birds, in chicks 2 to 4

weeks old Milky-white droppings are the most

obvious sign (but these are also seen with excess

urate excretion due to kidney disease)

Yersiniosis is another important disease of

pheasants

Moniliasis causes lethargy, stunted growth

and a heavy mortality in partridges Treatment

with formic acid, sprayed on food, has proved

successful (See also BOTULISM.)

Gametes

These are the ova and spermatozoa, and contain

half the number (haploid) of chromosomes

present in all other body cells (diploid)

Gametocide

Gametocide for bird control (See TEM.)

Gametocyte

An oocyte or spermatocyte, the cells which

produce an ovum or spermatozoon

Gamma Globulin

Gamma globulin is a protein fraction of the

blood serum which contains the antibodies

against certain bacteria or viruses (See

COLOSTRUM;IMMUNOGLOBULINS.) It can be

prepared in a concentrated form and can be

used to give protection against infection

Gamma Glutamyl Transferase (GGT)

An enzyme that tends to increase in liverdisease Higher than normal concentrationsare found in liver-fluke infection of sheep andcattle

Gammexane

Gammexane products contain the gamma mer of benzene hexachloride, a highly effective,persistent insecticide Not now used in treatingfarm animals (See BENZENE HEXACHLORIDE;

in CNS tissue and may be seen in cattle, pigs,dogs and cats

Gangrene

The presence of dead tissue in a live animal Inprimary gangrene, bacteria which cause thenecrosis also bring about the putrefactivechanges In secondary gangrene the putrefac-tion is caused by organisms which have invadeddead tissue (e.g following a burn) There are

2 varieties of gangrene, dry and moist; dry grene is a condition of mummification in whichthe circulation stops and the part withers up,while in moist gangrene there is inflammationaccompanied by putrefactive changes

gan-Infection following necrosis may lead togangrene after burns, scalds, frostbite, crushwounds, puncture wounds, etc

Poisoning by ergot results in the same tion in the most distant parts of the body, e.g.the feet, tip of tail, ears, and the combs andwattles of poultry

condi-SignsThere is at first a degree of pain when theaffected part is handled, and in a short time itbecomes reddened and swollen Later it turnsblue or black, the hair falls from it, and there is

a distinct line of demarcation between the grenous and the healthy surface Around thedividing line there is usually some degree ofinflammation, and pus production

gan-Moist gangrene is considerably more serious,since it is accompanied by putrefaction and the

Gangrene 293

G

absorption of toxins The whole area turns

black or greenish, the hair falls out, an offensive

smell is evident, and much fluid exudes from

the decomposing tissues A high temperature,

disturbed heart’s action, and rapid breathing,

are shown (See also GAS GANGRENE.)

Treatmentis mainly surgical, backed up by

the use of appropriate antibiotics or

sulfon-amides In advanced cases, euthanasia becomes

necessary (See also FROST-BITE.)

Gangrenous Dermatitis

A disease of poultry caused by Clostridium

sep-ticum; often associated with infectious bursal

disease and inclusion-body hepatitis, it usually

affects birds between 25 and 100 days of age

Mortality can be very high

Ganjam Ulrus

The Indian name for a bunyavirus infection

transmitted by ticks

Gapes

Gapes is a disease of young chickens, turkeys,

pheasants and other game birds particularly,

although all the domesticated and many

wild birds may also be affected It is caused by

infection with the gapeworm, Syngamus trachea.

The presence of worms in the bronchial

tubes and trachea of the bird causes it to gasp

for breath or ‘gape’, from which the name of

the disease originated Part of the life-history of

the worm is passed in the body of the

earth-worm, and young chickens eating earthworms

may become affected Earthworms can live for

16 years (See also under CAPILLARIASIS.)

Nitroxynil, given in the drinking water, is an

effective treatment

Garden Chemicals

Birds, dogs, and cats may be poisoned as a

result of the use of pesticides For the poisoning

of birds, see preparations listed under GAME

BIRDS,MORTALITY Dieldrin is highly toxic for

cats, and like DDT, should not be used on

them or in their vicinity In fact, all the

CHLO-RINATED HYDROCARBONS are best avoided in

places where small domestic animals or their

food may become contaminated

For the dangers of slug-baits, see

METALDE-HYDE POISONING (See ORCHARDSfor the dangers

of fruit-tree sprays For seed dressings, see under

SEED CORN See also PARAQUAT;HERBICIDES.)

Garden Nightshade Poisoning

Garden nightshade poisoning results from

animals eating Solanum nigrum, which is found

in many parts of the world Its toxicity appears

to vary in different localities The berriescontain an active alkaloidal glycoside calledsolanine, which is readily converted into sugarand the poisonous solanidine by the action ofthe gastric juices in the stomach

Signs Staggering, loss of sensation and sciousness, and sometimes convulsions First-aid:strong black tea or coffee

con-Garron

A useful type of horse for hill-farm work andcarrying deer Garrons do not constitute a sep-arate breed, but were a cross between WesternIsland ponies and the Percheron Nowadays,the Garron is regarded as a larger version of theHighland pony

Gas

(see AIR; BLOAT; CARBON MONOXIDE;OZONE;

ANAESTHETICS;SLURRY;NITROGEN DIOXIDE.)

Gas Bubble Disease

A condition in which fish swim ‘belly up’ cally; the cause is supersaturation of gases inthe water in very intensive farming It can beprevented by proper maintenance of pumpsand normal (not pressurised) aeration of thewater

verti-G

Garden nightshade (Solanum nigrum), also known

as black nightshade, has small purple flowers, and large black shiny berries, several of which are attached to a single stalk Height: 1.3 to 2 m (4 to 6 ft).

Gas Gangrene

Gas gangrene is an acute bacterial disease due to

the inoculation of wounds with organisms

belonging to the ‘gas gangrene’ group

Gas gangrene may attack any of the

domes-tic animals and man The horse is least resistant

and the cow least susceptible

Causes Gas gangrene is produced by

Clostridium oedematiens, Cl welchii, Cl septicum

and Cl chauvei gaining access to the tissues of an

animal through a small wound; after castration

or docking, or parturition, etc

Signs A few hours after the organisms gain

entrance, the area of invasion is found swollen,

hot, painful on pressure, and may crackle when

handled This latter effect is due to gas

forma-tion below the skin The skin and underlying

tissues rapidly become discoloured

In a series of 9 cases in horses, the signs were

fever, depression, painful muscular swellings,

and toxaemia All were dehydrated Colic had

been evident in 6 of the horses; laminitis in

2 Infection had followed intra-muscular

injec-tions in 8 of the horses, and a puncture wound

in 1 The Clostridia isolated were: chauvei (1);

septicum (6); and perfringens (6).

Prevention Vaccination is effective (See also

BRAXY;BLACKQUARTER;GANGRENE.)

Gastrectomy is an operation for the removal of

the whole or part of the stomach

Gastric

Gastric means anything connected with the

stomach, e.g gastric ulcer, gastric juice

Gastric Ulcers

These are seen in pigs in some cases (but not

all) of SWINE FEVER They have also been found

in piglets under a fortnight old, due to Rhizopus

microsporus, isolated from both stomachs and

bedding (See MUCORMYCOSIS.) Associated

with this infection may be another fungal one –

MONILIASIS– caused by the yeast-like organism

Candida albicans.

Gastric ulcers may also be produced by the

toxin of Aspergillus flavus (see AFLATOXIN), and

Gastrodiscus

Amphistome flukes, e.g G aegyptiacus, are

common parasites of horses and pigs in thetropics and subtropics A heavy infestation hascaused collapse in the horse

Gastroenteritis

Inflammation of the stomach and intestines,causing vomiting and diarrhoea It is an acutecondition commonest in young animals It may

be specific or due to irritant organic or

inorgan-ic poisons (See also HAEMORRHAGIC,PARASITIC,and TRANSMISSIBLE GASTROENTERITIS; also DIARRHOEA.)

car-in its normal position agacar-inst the diaphragm by

7 to 10 rows of silk sutures (7 or 10 to a row).The incision into the abdominal wall isclosed by absorbable synthetic sutures

This operation is also known as fundupexy

taking place in 4 to 6 weeks The sponge may be

sterilised in dry heat, and applied either dry or

moistened with normal saline, an antibiotic

solution, or a solution of thrombin Absorbable

gelatin sponge complies with the requirements

of the British Pharmacopoeia.

Gelatin, Succinylated

A modified, fluid, gelatin used as a plasma

sub-stitute to restore body fluid volume in cases

of hypovolemic shock

Gelbviehs

This German yellow breed of cattle, as it is also

known, was evolved by crossing Swiss breeds

with German breeds, and is dual-purpose,

aver-aging nearly 3640 litres (800 gallons) of milk at

4 per cent butterfat Fattening stock give a daily

liveweight gain of 1.1 kg (2.5 lb) and are ready

for the butcher at 405 days in Germany

Gelding

A castrated horse Occasionally a horse which

has had both testicles completely removed

shows stallion-like behaviour, when it is known

as a ‘false rig’ Such an animal may mountmares and achieve both erection and intromis-sion The chasing, or rounding up, of mares,and nipping them, may also occur This behav-iour is not hormonally induced or hormonedependent; it has been suggested that it is part

of the normal social interaction between horses

‘False rigs’ and cryptorchids may show similarbehaviour

Blood samples from 104 horses with eithersexual and/or aggressive male behaviour, butwhich had no palpable or visible testes, wereassayed for testosterone levels 30 to 100 minutesafter an intravenous injection of human chorion-

ic gonadotrophin All but 8 horses were classified

as either geldings (<40 pg/ml) or cryptorchids(τ100 pg/ml) Surgical investigation confirmedthe diagnosis in 23 geldings and 47 cryptorchids;the remaining horses were not operated on.(See also CASTRATION – Immuno-castration, fortreatment of an aggressive cryptorchid stallion.)

Generic Products

Those sold under their Pharmacopoeia names

rather than brand names

A bovine/hamster heterokaryon formed by the fusion of Theileria parva-infected bovine lymphoid

cells and baby hamster kidney cells The cell contains 3 hamster nuclei and 1 bovine nucleus

(arrowed) with a prominent nucleolus The intracytoplasmic masses are macroschizonts of T parva.

( × 1600) (With acknowledgements to the Institute for Research on Animal Diseases.) Monoclonal antibodies from mouse hybridomas have been produced for use in blood typing in cattle.

The biological units of heredity, arranged along

the length of the CHROMOSOMES (See also CELLS.)

Genetic Engineering

This may be defined as the recombination of

genes from different organisms into 1 organism

in a way that would never occur naturally;

e.g from a plant into an animal However, in

practice, the terminology also includes genetic

manipulation, although some changes brought

about by manipulation may also occur quitenaturally

Advances in knowledge of nucleic acids led tothe creation in the laboratory of new combina-tions of genetic material This was achieved bysplicing together DNA from entirely differentsources to form hybrid molecules that are lesslikely to occur during evolutionary processes.Micro-organisms virulent for cattle (forexample) will, if they can be adapted to grow

in laboratory animals, become less virulent for

Genetic Engineering 297

G

HYBRIDOMA TECHNIQUE FOR PRODUCING ANTIBODIES TO PARASITE ANTIGENS (DIAGRAMMATIC)

Spleen cells from mice previously injected with parasite, and secreting antibody to the parasite, can be fused with other cells to form hybrid cells that live and multiply From these a cell line is selected by

‘cloning’ (i.e a colony that represents the progeny of a single cell is isolated) which secretes appropriate

the original host, and may have usefulness for

later vaccine purposes One method of adapting

the micro-organisms to grow in laboratory

animals is to fuse, artificially, cells to form a

HETEROKARYON (Seeillustration.)

UsesGenetic engineering has provided

infor-mation on the molecular basis of gene action,

on bacterial virulence and bacterial resistance

ln agriculture it offers the hope of being able

to transfer from bacteria to plants the genes

which confer the ability to fix nitrogen – and

so reduce farmers’ dependence on scarce and

costly nitrogen fertilisers

In the UK, at the AFRC’s Unit of Nitrogen

Fixation, genes for nitrogen fixation were

transferred with the aid of a PLASMIDfrom a

naturally occurring nitrogen-fixer Klebsiella

pneumoniae to E coli, which had never fixed

nitrogen before The plasmid, of the

exchange-able class, was exchange-able, when transferring, to take

along fragments of its host’s chromosomes,

including pieces bearing the nitrogen-fixation

genes

Some of the new nitrogen-fixing E coli

strains converted these fragments of Klebsiella

chromosome into new, separate plasmids

Geneticists in the unit therefore constructed,

by ordinary genetic manipulations, plasmids

carrying nitrogen-fixation genes which would

transfer themselves alone, without the aid of

another plasmid

In veterinary medicine the greatest potential

lies in the preparation of completely safe viral

vaccines One of the first successes was with

foot-and-mouth virus The specific viral protein, free

of infectivity, was produced by a genetically

manipulated E coli This protein from a

bacteri-al culture is capable of stimulating antibody

pro-duction in animals The technique is used also in

the production of a number of other vaccines

Recombinant DNA techniques

Develop-ment of such techniques involved 3 lines of

research: (1) recognition and isolation of

extra-chromosomal DNA, or plasmids; (2) the

manip-ulation of DNA with ‘restriction’ enzymes which

selectively split DNA into fragments which

could then be rejoined; (3) reinserting the

frag-mented DNA into living cells so that it became

part of the genetic material of the cells

In this way, genetic instructions for

produc-ing mammalian enzymes could be transferred

into E coli, the cell most used for propagating

such plasmid vectors to produce insulin, for

example

The next step was to develop synthetic

nucleotides, actually to construct genes This

has been done successfully and nucleotides of

up to 500 characters have been constructed.Another technique in genetic engineeringinvolves monoclonal antibodies These are pro-duced by fusing antibody-producing cells from

an immunised donor with another type ofwhite blood cell, thereby producing hybrid cellswhich in tissue culture could provide thedesired antibodies Among potential uses are,again, vaccines, but also diagnostic reagents; forblood-typing, in race-horses, for example

Genetics, Heredity and Breeding

PAGE

Homozygous and heterozygous 299Inheritance through multiple genes 300Inheritance and high milk yields 300

of the individual are represented in the fertilisedegg in which the individual has its beginning.Stock-breeding is a craft concerned with themaintenance of the desirable qualities of astock, the improvement of these qualities gen-eration by generation, and the eliminationthrough breeding of qualities which are held to

be undesirable The problems of the geneticistand of the stock-breeder are identical, thoughtheir interests are dissimilar

The geneticist has made much progress bystudying, quickly maturing, very highly fertileanimals such as the mouse, rat, guinea-pig,

rabbit, and above all the fruit fly Drosophila.

A better understanding of heredity was dered possible by the concept that the individ-ual as a whole was not the unit in inheritance,

ren-G

but could be regarded as a definite orderly

combination of independently heritable units

Breed was now interpreted as signifying

different combinations of independently

herita-ble characters, all drawn from the common

source of the stock in which modern

domesti-cated cattle had their origin – just as different

arrangements and combinations of letters make

different words, though all words are made up

of letters derived from a common source: the

alphabet

The breeder has employed the methods of

hybridisation and inbreeding associated with

selection in the creation of the modern breeds

He has practised inbreeding with selection in

order that the desired type of his stock may

be fixed, and he has sought hybrid vigour in

outcrossing The geneticist has employed these

very same methods in his studies The method

of genetics is character-analysis The object of

the breeder is character-synthesis

Instead of the hereditary mechanism being a

simple affair as was first thought, it is one of the

most complex

Phenotype and genotype Allan Fraser

MD, DSc, when senior lecturer in animal

hus-bandry at Aberdeen University, once drew a

helpful analogy between heredity and a game

of cards, with each card representing a gene,

and Honours cards representing genes most

desirable to a breeder

A game of cards is preceded by the shuffling

of packs, and so also is the conception of an

ani-mal preceded by a shuffling of genes Each pack

is then halved – just as before sperm and ovum

meet, and the number of genes in both is

halved by what is called the reduction division

Fertilisation then reunites the 2 half-packs to

form 1 new pack The cards in this represent

the genes in the new individual animal

‘The cards in any one hand or the gene

sample in any individual animal are the result of

pure chance – no one can predict how the run

of the cards or of the genes will go.’

Of 2 animals, sharing the same sire and dam,

1 may have a much better genotype, be a more

valuable breeding animal than its full brother or

sister

If the cards dealt at conception be called the

unalterable genotype of the animal, the playing

of that hand may be called the environment,

which includes climate, nutrition, exposure to

infections, stocking rates, and every aspect of

husbandry and animal management

Some stockmen can make a surprisingly

good job with poor genetic material; others a

sorry job with the best stock; but of course the

most skilful stockman cannot improve uponthe hand of genes once dealt

Genotype can be defined as the entire array

of genes carried by an individual (or, in

anoth-er sense, the genetic constitution of an ual with respect to any limited number of genesunder examination)

individ-Phenotype is the appearance and/or the formance of an individual animal Phenotypicvariation of a population results from the com-bined effects of inheritance and environment.Genetic variation is that part of the phenotypicvariation which is due to genes

per-Homozygous and heterozygous Inorder to illustrate one of the simpler aspects ofheredity in relation to stock-breeding, theappearance of red calf in a herd in which the fash-ionable coat colour is black, and in which all redanimals are eliminated, will serve as an example.Black-and-red coat colours in cattle consti-tute a typical pair of Mendelian characters,black being the dominant and red being therecessive member of the pair A red calf can only

be produced by black parents when both ofthese are heterozygous in respect of their coat-colour character For the character black-coatcolour, there is a determiner or factor: thisfactor may be present in the zygote in theduplex state, having been conveyed thereinto

by both egg and sperm When the factor forblack is present in the duplex state, the individ-ual that arises for that fertilised egg or zygote isspoken of as being ‘homozygous’ for the char-acter black-coat colour On the other hand,into the zygote there may have been brought afactor for black from 1 parent and a factor forred, the alternative character, from the otherparent Under these circumstances, of these

2 factors (that for black and that for red) it isthe former alone that determines what the coatcolour shall be Black is said to be dominant inits relation to red Homozygous and heterozy-gous blacks will be indistinguishable on inspec-tion If 2 heterozygous blacks are mated therewill occur on the average in every 4, 3 blackcalves to 1 red To explain this 3:1 ratio it isassumed that half of both male and femalegametes of such heterozygous individuals (i.e.sperm and ovum) carry the factor for the dom-inant character black, and the other half thefactor for the alternative recessive red, and the

2 sorts of egg and of sperm occur in equal bers If it is assumed that for every pair offactors that correspond to a pair of characters,only 1 can pass into the ripe gamete, it followsthat a 3:1 ratio in the next generation will beobtained, and of the individuals exhibiting the

num-Genetics, Heredity and Breeding 299

G

dominant character, 1 will be homozygous for

that character and 2 heterozygous, whilst the

individual exhibiting the recessive character

must of necessity carry the factor for that

character in the duplex state, since if in its

hereditary constitution it carries a factor for the

dominant, it will exhibit the dominant

charac-ter It is possible by examining the records of

the coat colours of the offspring to define the

hereditary constitution of the parent in respect

of the coat colours black and red The following

matings are possible:

Homozygous black to homozygous black

will give none but blacks, all homozygous

Homozygous black to heterozygous black

will give all blacks, of which 50 per cent will be

homozygous and 50 per cent heterozygous

Homozygous black to homozygous red will

give all heterozygous blacks

Heterozygous black to heterozygous black

will give 25 per cent homozygous blacks, 50 per

cent heterozygous blacks, and 25 per cent reds

Heterozygous black to red will give 50 per

cent heterozygous blacks and 50 per cent red

Red to red will give all reds, of necessity

homozygous The only mating of blacks that

can yield a red calf is that of two individuals

heterozygous in respect of this coat-colour

character

The coat-colour character has to be

consid-ered quite apart from all the rest of the

charac-ters that in their association make the animal

what it is An individual is a pure black when it

is in respect of this character homozygous,

when in its hereditary constitution the

deter-miner or factor for this character has been

received from both its parents

Inheritance through multiple genes

The above example shows how a character –

coat colour – may be inherited through single

genes This is the mechanism of heredity at its

simplest Most characters, however, including

many of economic importance to the farmer,are inherited in a far more complex mannerthrough multiple genes

Multiple genes may have an additive effect asregards the expression of some character; orthey may interact, one with another, in theproduction of a character, inheritance of which

is even more complex

Inheritance and high milk yields Itseems that it is easier to increase the butter-fatcontent and solids-not-fat content than it is toincrease the milk yield through breeding Theheritability of milk yield is not as high as that ofsome other characters

There is a correlation between high yieldsand body size, but conformation is by no meansalways associated with high yields A few ofthe highest-yielding cows have had, to put itmildly, an unfashionable conformation.The diagram below shows how 2 bulls, fullbrothers, may influence milk production intheir daughters in opposite ways It also showshow the ‘gene lottery’ can make nonsense out ofthe expectations of a breeder

For this reason, progeny testing has proved

of the greatest importance in the selection ofbulls, each of which – through AI – may havenot 100 offspring but tens of thousands

It is possible for a farmer to use (by means ofAI) a bull with the proved ability to producedaughters with a high milk yield, as comparedwith the yield resulting from use of animproved or average bull Proven bulls are list-

ed in terms of ‘a bonus of 50 or of anything up

to 100 gallons’ and also in terms of a butterfatbonus (See also PROGENY TESTING.)

Researchers have reported an associationbetween blood groups and production charac-ters in cattle and other animals The work ontransferrin and milk yield is an example; that ofblood groups and milk yield another However,the results were not sufficiently significant to be

of practical value in selecting for productivity

Selectionis the systematic choice of animals

in a population (defined in the genetic sense as

a group of interbreeding animals sharing a mon gene pool, e.g a closed herd, an AI district,

com-a breed) com-as pcom-arents for the next genercom-ation.Family selection means the selection of indi-viduals on the performance of their relatives(sibs, half-sibs, or progeny), i.e selection betweenfamilies instead of between individuals.Genotypic selection is that based on progenytesting with a very large number of progeny, sothat the breeding value of the parent is exactlyknown

G

Black × Red (Red is necessarily white in the

diagram.)

Inbreeding may be defined as the mating

of individuals more closely related than the

average relationship of the population

Both crossbreeding and inbreeding are

methods of bringing about genetic change

Inbreeding was practised by the early

devel-opers and improvers of livestock breeds in order

to fix the type of their animals Inbreeding can

be expected to increase the proportion of

ani-mals homozygous for a given desired character

As the process proceeds, however, individuals

with undesirable characters are likely to appear

– animals which are abnormal in some respect,

sterile, or weak Inbreeding could prove

disas-trously expensive if the proportion of such

animals were high (Test mating of a bull to

related or carrier females may be carried out

in order to detect specific genes such as lethal

factors; the carriers then being culled from the

herd.)

Prolonged inbreeding will lead to

disappoint-ing regression, diminution of vigour, decreased

fertility, and a reduction in body size

The fusion of 2 inbred linesBy

inbreed-ing for a specific character, and by practisinbreed-ing

rigid selection for 2, 3, or more generations,

a strain of relatively homozygous individuals

for the selected character can be created If 2

such strains are developed separately but

simul-taneously for 2 different but highly desirable

characters, and if these 2 strains are thencrossed, the resulting progeny can be expected

to possess both desirable characters to a usefuldegree The mechanism has been notablysuccessful in producing strains of poultry withlarge egg size and high annual yields, and hasbeen exploited commercially

When the generation in which the 2 desirablecharacters are expressed is bred from again, animmediate reassortment of characters occurs,and only in a small percentage of the individu-als will the desirable characters be expressed.The others are most likely to be useless

Hybrid vigour(Heterosis), usually strated by increase in size, better liveweightgains and greater resistance to disease and in theearlier attainment of sexual maturity, occurs inthe first cross-bred generation out of the mating

demon-of 2 widely dissimilar pure-bred parental stocks.Hybrid vigour is an indication of heterozygosis.The 2 parental breeds must be within reason asdissimilar in their characterisation as possible;then in the pooling of those hereditary consti-tutions there will be a very considerable degree

of heterozygosis in the 1st cross-bred offspring;the desirable characters are pooled, and inrespect of those characters exhibited by the 2parents the offspring will be heterozygous.Generally, a characteristic with poor heritability

is enhanced more than one of high heritability

The effect of inbreeding and crossing These 2 sheep were sired by the same ram, but the smaller one was highly inbred (59 per cent) and when 10 months old had attained little more than 40 per cent of the weight of its half-sib (left), which was a 3-way cross of inbred lines, its dam being a 2- way line cross These hoggs are of Chevoit × Welsh extraction (Animal Breeding Research Organisation photograph.)

Genetics, Heredity and Breeding 301

G

If the 1st crosses which exhibit this hybrid

vigour so markedly are interbred, their

off-spring will not exhibit this vigour to the same

extent

‘Nicking’Two individuals not remarkable in

themselves may produce superior offspring

This fact can be explained on the assumption

that the mating brings together the chance

asso-ciation of factors which are complementary or

supplementary, and that these in conjunction

determine characters that are greatly esteemed

‘Pedigree and purity’Pedigrees and

regis-tration in the appropriate herd book have been

an essential part of breeders’ work in all but

‘commercial’ herds The system has been of

service, though not free from abuse Pedigrees

have from time to time been falsified or

genuine mistakes made (including use of the

wrong semen in AI work), and until the advent

of blood typing there was no means of checking

such errors

It is easy to exaggerate the importance of

‘family name’ or of remote ancestors; and in

terms of production traits or characters it has

been stated that ‘the concept of “purity” of pure

breeds is not only something of a myth, but

might actually be detrimental to the possibilities

for improvement’

ChromosomesA male can be distinguished

from a female not only by external appearances

and by differences in the architecture of the

reproductive system, but also by differences

in the organisation of the cells of which their

bodies are composed

The nucleus of the resting cell appears in

stained microscopic preparations as a vesicle

containing a network of delicate threads upon

which are borne, like beads upon a tangled

skein, minute masses of a deeply staining

mate-rial known as chromatin As the cell proceeds

to divide into 2, this tangled mass of fine

threads resolves itself into a constant number

of filaments of definite shape and these become

progressively shorter to assume the form of

stout rods known as chromosomes The

num-ber of chromosomes is usually some multiple

of 2, and is constant and characteristic of

the species to which the individual belongs,

e.g dog 78; horse 64; cow 60; sheep 54; pig 38;

cat 38

(The study of chromosomes is known as

CYTOGENETICS.)

The gametes, egg and sperm respectively,

differ remarkably in size and form, but they are

alike in that each contains half the number of

chromosomes that is characteristic of thesomatic cells of that species

At fertilisation, the characteristic number isrestored, and in the case of each pair 1 member

is derived from one parent, the other from theother In this distribution of the chromosomesone finds a mechanism by which offspringmay inherit from both parents by means of theconstituent genes

It is possible to distinguish male from female

by differences in the chromosome content.While all other pairs of chromosomes consist of

2 chromosomes exactly alike in size and shape,

1 pair differs in the 2 sexes As they influencesex determination, these chromosomes arereferred to as the sex chromosomes

The ovum (or egg) contains the X some

The sperm may contain either the Y some, which will produce male offspring (XY), orthe X chromosome, which will produce femaleoffspring (XX)

chromo-In most female mammals the paired chromosomes are identical, and called the Xchromosomes on account of their shape

sex-In the male, the sex-chromosomes aredissimilar, one being the X chromosome andthe other the Y

The autosomes are all the other chromosomesexcept the sex-chromosomes

Genetic aspects of infertilityThese arebriefly discussed in the following paragraphs.Chromosome screening (see CYTOGENETICS)and, to a lesser extent, blood-typing are likely to

be of increasing value in eliminating a tion of bovine infertility due to hereditary causes (See also INFERTILITY.)

propor-Inheritance of twinningIt is mainly cowsover 3 years old which have twins, and although

G

A bull’s lymphocyte showing chromosomes in the 2nd stage (metaphase) of cell division (mitosis) (With acknowledge- ments to Dr C R E Halnan

and the Veterinary Record.)

twinning may be a desirable character for the

farmer it is not one which is easy to obtain

through breeding It seems that there is a rather

low level of genetic variation in frequency of

twinning, and that prospects for rapidly

increasing litter size in dairy cattle do not at

present seem good In some breeds of sheep,

and perhaps also in cattle, it may not be the

ovulation rate which is the limiting factor but,

as indicated above, the survival rate of fertilised

eggs

Heritability of certain traitsThe

follow-ing table, compiled at the Animal Breedfollow-ing

Research Organisation, shows the degree of

heritability of certain traits sought by the pig

breeder

%

Daily gain, after weaning 0

Percentage of lean meat 45

It will be seen that litter performance has a low

heritability, and must be achieved by suitable

crossbreeding The crossbred sow has a marked

superiority in this respect

Lethal and semi-lethal factors Lethal

factors have been defined as genes which, when

present in the homozygous condition, cause

the death of the embryo and when present in

the heterozygous conditions cause a serious

impairment in the individual often leading to

non-survival

Lethals may be dominant or incompletely

dominant, but many are certainly recessive

They are not always recognised since they may

cause death of the embryo early in development,

and the mating may be regarded as having been

infertile – another illustration of the difficulty of

defining the genetics of infertility

Several lethals are met with in cattle, such as

parrot-mouthed, in which calves die at a few

hours of age, or amputated, in which calves are

born dead with legs and lower jaws absent

Semi-lethals include over-shot jaws in calves

Heredity and disease Diseases exhibit a

spectrum according to the genetic influence

Canine haemophilia is entirely genetic Swedish

gonad hypoplasia is mainly genetic, mastitis is

mainly environmental and injuries are entirely

environmental Simply dividing diseases into

genetic and non-genetic is, therefore, inaccurate

Recessive inheritance: many diseases areinherited as autosomal recessives Neither parent

is usually affected, but the disease comes fromboth; the sexes are affected equally, inbreeding isoften being practised, the incidence is generallylow and exact genetic ratios are obtainable Fewdiseases, however, fulfil all the criteria of simplerecessives

Diseases due to sex-linked recessives, e.g.canine haemophilia, are uncommon in domes-tic mammals and, not being transmitted byunaffected males – the carrier female transmitsthe disease to half her male offspring – areunlikely to be a major problem In poultry,however, sex-linked abnormalities such asfamiliar cerebellar degeneration are transmitted

by the male to half his female progeny and arerelatively common

Diseases such as cryptorchidism and sexes are sex-limited and sometimes regarded asdue to recessives, the homozygote only express-ing itself in 1 sex While both parents are proba-bly involved, their exact inheritance is unknownand cryptorchidism is certainly subject toenvironmental modification

inter-Irregular inheritance: many defects have acomplex inheritance Sporadic abnormalities,which increase on inbreeding, such as chicken

‘crooked toes’ or pig ‘kinky tails’, are calledpheno-deviants, and are probably caused byrecessives exhibiting a threshold of manifestation.Dominant inheritance: the disease usuallycomes from 1 affected parent, and half its off-spring are affected Few dominant diseases areknown in livestock, except in poultry Irregulardominants, exemplified by ‘curved limbs’,where an unaffected male transmits a geneproducing defective offspring out of unrelatedfemales, and less than half are affected, do,however, occur Environment or modifyinggenes may cause the defect inherent in the gene

to develop

Semi-dominant inheritance: several diseasesare due to semi-dominants, the heterozygotebeing distinguishable from both homozygotes

A single dose of a semi-dominant gene duces a Dexter, a double dose a bulldogcalf, and the homozygous normal allele pro-duces a long-legged Dexter Some Americandwarf cattle result from semi-dominants (withacknowledgements to Dr G B Young, AnimalBreeding Research Organisation)

pro-Genetic defectsAll breeds of livestock mayharbour some genetic defects, but their inci-dence is usually low However, specific defectssometimes become more frequent in certainbreeds, and give cause for concern

Genetics, Heredity and Breeding 303

G

CattleExamples of genetic defects include the

following:

Arthrogryposis in Charolais cattle This defect

is characterised by twisted limbs, cleft palate, and

twisted spine In France about 1 per cent of

Charolais cattle are affected AI records show that

while many bulls transmit an occasional defect of

this nature, a few sire about 5 per cent of

off-spring having this abnormality Another defect

encountered in some Charolais cattle involves

the eyes (See EYE,DISEASES OF –Coloboma.)

Decapitated sperms This defect causes the

rejection for AI in Britain of many Hereford

bulls ‘with superior test performance’ because

their semen contains a high percentage of

sperms with the head separated from the tail

Tibial hemimelia A dog-sitting position in the

newborn Galloway calf is suggestive of this defect,

which involves bones missing from the hind legs

(but has to be differentiated from another defect

involving the pelvis, which also prevents the calf

from standing on its hind legs) It is estimated

that about 16 per cent of Galloways now carry

the gene which transmits this defect (The

Galloway Breed Society has an excellent scheme,

requiring compulsory insurance and compulsory

slaughter of any bulls leaving offspring with this

defect.) (See also MANNOSIDOSIS.)

SheepGenetic defects include achondroplasia

in some South Country Cheviot flocks A

‘squashed-in’ face, shortened forelimbs and

defective hooves are characteristic of these

‘dwarf lambs’

Cerebellar ataxia is seen in some Border

Leicester flocks, and these ‘daft lambs’ have a

staggering gait and incoordination of the head

This is due to a recessive gene

Genital Organs

(see diagrams for PENIS and UTERUS)

Genome

A complete set of chromosomes derived from 1

parent; or the total gene complement of a set of

chromosomes

Genotype

This can mean the entire array of genes carried

by an individual; or the genetic constitution of

an individual with respect to any limited

num-ber of genes under examination; or (more

loose-ly) the individual within a given genotype (See

also under GENUS.)

Genotypic Selection

Genotypic selection is that based on

progeny-testing with a very large number of progeny

so that the breeding value of the parent is

exact-ly known The expression is also looseexact-ly used

as a synonym for progeny testing without thisproviso

Gentian

The dried and powdered root of the yellow

gentian plant (Gentiana lutea) It is a bitter

tonic used as an appetiser

Gentian Violet

A stain used in microscopical work and a able antiseptic, of use against fungal and bacter-ial skin infections (See also ANTISEPTICS – Crystalviolet.)

valu-Genus

Genus is a group of species One of the species

is chosen as being typical, and referred to as thegenotype

Gerbil

A small burrowing rodent, originating in deserts,popular as pets.They live for 3 to 5 years; adultsweigh 50 to 90 g, the females being larger thanmales Sexual maturity occurs at 10 weeks Thegestation period is 24 to 26 days and the youngare weaned at 21 to 24 days Rectal temperatue

is 37.4 to 39°C They are naturally healthy mals, remarkably free from infectious diseases:Tyzzer’s disease is usually the only finding Inthe wild they ‘play dead’ when attacked; this is aself-induced epileptic seizure and may occurwhen they are handled (See also PETS.)

ani-Geriatrics

The study of the problems and diseases of theolder animal

German Shepherd Dog

Previously known as the Alsatian, this is a

medi-um to large-sized dog with a tan and black coat.The breed is extremely popular and, to fulfildemand, many unsuitable animals were used forbreeding In consequence a number of inheriteddefects are associated with the breed Theseinclude achalasia, calcinosis circumscripta, cleftpalate and epilepsy A campaign to eliminate hipdysplasia, which is very common, requires allpotential breeding dogs to be X-rayed for evi-dence of the defect Dogs in which it is foundare not used for breeding