Mendel’s hypotheses (to explain his results) 1. Alternative versions of genes (alleles) account for variation in inherited characters 2. For each character, an organism inherits two alleles, one from each parent 3. If two alleles differ, one is dominant, the other recessive 4. The two alleles for each character segregate (separate) during gamete production.

Modifications of Mendelian Ratios *Incomplete dominance*Codominance Example: MN Blood group *We can predict genotypic and phenotypic ratios

*Multiple Alleles Examples: *100 alleles at a given locus in Drosophila *ABO Blood group in humans Genotype I A I A I A I O I B I B I B I O I A I B I O I O Antigen A A B B A,B Neither Phenotype A A B B AB O Modifications of Mendelian Ratios

*Pleiotropy Modifications of Mendelian Ratios

*Epistasis Example: In Drosophila, gene:eyeless Modifications of Mendelian Ratios *Black (B) is dominant to brown (b) *Second gene responsible for allowing pigment to be deposited in hair C = presence, c = absence (colorless)

*Quantitative characters *often due to polygenic inheritance Example- *skin color – controlled by 3 genes *each gene with two alleles light and dark incomplete dominance Modifications of Mendelian Ratios

*Gene interactions

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)

*Phenotype depends on environment and genes Modifications of Mendelian Ratios

How do we account for genetic variation? *Independent assortment *Crossing over *Random fertilization Independent Assortment: Cross over: