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Genetics – Part 2

Animal Breeding

Pensacola Jr College

Review of Genetics in Simple Dominant / Recessive Traits

Genetic Complexities

w Codominance

w Multiple alleles

w Linked genes

Codominance

w Both alleles are expressed

w Heterozygous genotype has some intermediate phenotype (trait) expressed

w Example 1:

• Ficticious banded snake, B=dark bands, b= no bands, Bb= light bands

Codominance (cont’d)

w Example 2:

• Hb^A = normal hemoglobin

• Hb^S = sickle-cell hemoglobin

• Hb^A Hb^S = mostly normal hemoglobin, malaria resistant

Multiple Alleles

w Traits controlled by genes with more than 2 possible alleles in the population

w NOTE there are still only 2 alleles in any *individual*!

w Example: human blood types

• i = recessive

• I^A and I^B are codominant

Linked Genes

w Genes for more than one trait carried on the same chromosome (thus always go into sperm / egg together)

w If you find dihybrid cross not working right, this may be the reason!

w If sexes have different proportions of trait, may be sex-linked (carried on X)

Sex-linked Genes

w Genes present on the X chromosome but absent on the Y chromosome

w Female is diploid and male haploid for these, so recessive shows in males

Sex-linked Genes (cont’d)

w Example: red-green colorblindness

• X^C = normal

• X^c = red-green colorblind

• Remember male always has XY and female XX

^• Passed on to males through MATERNAL parent only