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EX—allele for blue eyes and allele for brown eyes • Phenotype —the physical expression of a gene or allele.. Blue eyes • Genotype —the genetic composition of an individual... EX Brown

Patterns of Gene Inheritance

Chapter 23

Genetics is the study of genes

What is a gene?

A A factor that controls a heritable characteristic

B Something on a chromosome

C Information stored in a segment of DNA

D Something that encodes a protein

Gregor Mendel

LOTS O’ DEFINITIONS

• Gene -A piece of DNA that encodes a particular trait EX a gene for eye-color

• Allele —an alternate form of a gene EX—allele for blue eyes and allele for brown eyes

• Phenotype —the physical expression of a gene

or allele Ex Blue eyes

• Genotype —the genetic composition of an

individual

MORE DEFINITIONS

• Locus—the location of a gene on a

chromosome Plural=loci

• Dominant allele—expressed whether

alone or in pairs Symbolized by a capital letter EX Brown eye allele=B

• Recessive allele—expressed only in the absence of a dominant allele Symbolized

by lowercase EX blue eye allele=b

• What is the maximum # of alleles a diploid individual can have at any given locus?

STILL MORE DEFINITIONS!!!

• Genotype—the genetic composition of an individual

– Homozygous containing a pair of the same alleles Can be

• Homozygous recessive—two recessive alleles EX bb, or

• Homozygous dominant– two dominant alleles EX BB

– Heterozygous—containing two different

alleles EX Bb

Law of Segregation

• Each individual has two factors (called

• Factors segregate during gametogenesis Ploidy level of gametes?

• Fertilization gives each new individual two factors again

• Ploidy level after fertilization?

Gene locus

Defs

Example Widow’s Peak

∴ If the primary spermatocyte is genotype

Ww, the sperm cell would contain either a W

or a w, but not both.

• Ww represents the genotype of an

individual

• Gametes are represented by W or w

One-Trait Crosses

GENOTYPE?

GENOTYPE?

GENOTYPE?

Punnett Square EXAMPLE 1—A Monohybrid Cross

Monohybrid cross

The One-Trait Testcross

QUESTION: How can we determine the

genotype of a dominant phenotype?

Testcross: cross the dominant phenotype (unknown genotype) with the recessive phenotype (known genotype).

The Law of Independent

Segregation and Independent

Assortment

MEIOSIS & GENETIC VARIATION

Independent

Assortment

• Diploid organisms can

produce 2 n diff gametes

• Ea homologous pair can orient in two different ways

• Humans: 223 = 8,388,608 (est 8.4 million)

Each homologous pair can

orient in either of two ways

HYPOTHETICAL EXAMPLE OF INDEPENDENT ASSORTMENT

Gene for black hair

r

Gene for red hair Hair color

INDEPENDENT ASSORTMENT

OR During meiosis I, tetrads can line up 2 n different ways.

Brown eyes

Black hair

Blue eyes Red hair Brown eyes

Red hair

Blue eyes Black hair

Meiosis I & II

l L L

What are the possible genotypes of this

individuals gametes?

Probabilities

Two-trait Testcross-How?

Genetic Disorders

• Pedigree charts- show patterns of inheritance

Autosomal Dominant Disorders

• Autosomes -Non-sex chromosomes

• Dominant allele

∀ ∴when under what genotype(s)will an

individual show the disorder?

Examples: Autosomal Dominant

Huntington disease

Autosomal recessive pedigree

Autosomal Recessive Disorders

• Tay-Sachs Disease

– Common among United States Jews of central and eastern European descent

– Death by the age of three or four

– Defective enzyme in lysosomes.

• Cystic Fibrosis

– Most common lethal genetic disorder among

Caucasians

– Defective chloride ion transport protein

– Osmotic imbalance results in thick mucous in lungs and pancreatic ducts

Cystic fibrosis therapy

Phenylketonuria (PKU)

• allele on chromosome 12

• lack an enzyme needed for metabolism

of phenylalanine (an amino acid)

• Urine test diagnostic

• Brain damage unless controlled by diet.

• Polygenic (Quantitative) Traits

• Governed by more than one gene pair

• Several genes determine the phenotype

• Produce bell-shaped curve

• EX: Skin color

Polygenic Inheritance

EX 2 Polygenic Inheritance :

Polygenic Disorders

• Examples: cancer, schizophrenia, hypertension, diabetes, etc

– several genes involved

– also environmental influences

Multiple Allelic Traits

• Often more than two alleles exist for a

particular gene locus

• Maximum # of alleles at any given locus?

• Each individual inherits only two

alleles for these genes!!!

• EX: Human Blood type

Inheritance of blood type

Details

Incompletely Dominant Traits

• Codominance both alleles equally

expressed in a heterozygote EX ABO

Blood type

• Incomplete dominance heterozygote

shows intermediate phenotype,

representing a blending of traits EX Wavy hair in Caucasians

• Phenotypic ratio ?

• 1 : 2 : 1

Incomplete dominance

Example Incomplete Dominance

2-Sickle-Cell Disease

• Caused by incompletely dominant alleles

• Single nucleotide mutation causes abnormal hemoglobin

• HbA = normal hemoglobin

• HbS = sickled condition

• Genotype of normal?

• Genotype of sickled homozygote?

• Genotype of intermediate phenotype?

Sickle-Cell Disease

Molecular Explanation-Dominance

• Simple Dominance

– One-half the amount of gene product

sufficient for phenotype

Everything’s Relative

WHAT’S HAPPENING??

• Physical Basis of Genes

• Lots o’ Definitions

• Relate genetics to meiosis

• For genetics problems—first try to figure the genotype(s) of the

gametes