How do we account for genetic variation?

*Independent assortment *Crossing over *Random fertilization

Independent Assortment:

Cross over:

Chi-Square Analysis:

Mendel’s monohybrid and dihybrid ratios are predictions based on the following assumptions:

1. Each allele is dominant or recessive

2. Random segregation of alleles

3. Independent assortment

4. Fertilization is random

Chi-Square Analysis:p value (probability): consider as a percentage (i.e. 0.05 = 5%)

Example from Table 3.1:

Human PedigreesPedigree

= Female

= Male

=Unknown

*Proband (p)

Chapter 4: Modification of Mendelian RatiosAllele

*Wild-type allele

*Mutant allele

Conventional symbols for alleles:

recessive allele- initial letter of the name of the recessive trait, lowercased and italicized

dominant allele- same letter in uppercase

Genetic nomenclature is extremely diverse!

Incomplete or Partial DominanceCross between parents

with contrasting traits: Red flowers or white

flowers

Offspring with an intermediate

phenotype: pink flowers

Codominance:

Example:

MN Blood group- red blood cells contain a transmembrane glycoprotein (glycophorin); two different forms of this protein exist, M and N

Examples:

*Table 4.1: over 100 alleles at a given locus in Drosophila

*ABO Blood group in humans

Multiple Alleles:

*Characterized by the presence of glycoprotein antigens on the surface of red blood cells

*Distinct from the M and N antigens

*Also exhibits codomiance

Lethal Alleles:

Example: Coat color in mice

*A = agouti = wild-type allele

*AY = yellow = mutant allele

Dominant Lethal: Huntington’s disease (H); heterozygous individuals (Hh) have late onset

Combining modified modes of inheritance:

Gene interaction:

*Epistasis

Example:

In Drosophila, the recessive gene eyeless (when homozygous) prevents the expression of eye color genes present in genome

*Novel phenotypes due to gene interaction

Example:

disc-shaped fruit (AABB) X long fruit (aabb)

Genes on the X Chromosome:

*Sex chromosomes*Autosomes

Example: In Drosophila and all mammals

sex chromosomes designated as X and Y

XX=female

XY=male

*1909 Thomas Hunt Morgan

II III IV

XX

XY

or

*X-linkage

X-linkage in Drosophila: white mutation (eyes)

Genes on the X Chromosome con’t:

½ red

½ white

Genes on the X Chromosome con’t:

½ red

Sex-limited Inheritance:*Sex-limited trait

*holandric genes: genes on the Y chromsome

Example: hypertrichosis (ear hair)

*autosomal genes

Example: milk production in mammals; L=lots, l=little

*Sex-influenced trait

Sex-influenced Inheritance:

Examples:

*cleft palate in humans*horns in sheep *pattern baldness in

humans

Summary:

Sex-linked on X or Y sex-chromosome

Sex-limited all or none expression by sex

Sex-influenced genotype + sex determines phenotype

Phenotypic Expression:

*Penetrance

*Expressivity

Gene expression often governed by genotype and environment

Example:

if 9/10 of individuals carrying an allele express the trait, the trait is said to be 90% penetrant

*Onset of genetic expression

*Temperature