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 recessive4. 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 IAIA IAIO IBIB IBIO IAIB

IOIO

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

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: