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 = 5%) Example from Table 3.1:

Human Pedigrees Pedigree = Female = Male =Unknown *Proband (p)

Chapter 4: Modification of Mendelian Ratios Allele *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 Dominance Cross 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 *A Y = 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-linkedon X or Y sex-chromosome Sex-limitedall or none expression by sex Sex-influencedgenotype + 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