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

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Presentation transcript:

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: