More Genetics & Pedigrees

Incomplete Dominance

Multiple Alleles & Codominance

Polygenic Traits

• Additive effect of multiple genes on a single phenotype

• Results in normal distribution

• Examples: skin color, height

Epistasis

• A gene at one locus alters the expression of a gene at another locus.

• Example: – Black coat (B) is

dominant to brown (b)– Pigment color (C) is

dominant to no pigment color (c)

Effect of Environment on Phenotype

More acidic soil Less acidic soil

Pleiotropic Effects

Pedigree

MaleFemale

HeterozygousShows trait

Recessive characteristics

Inheritance pattern•Trait tends to skip generations

Probabilities

Aa aaChild will have recessive trait?

aa

Chance of “a” allele from dad?

1/2

Chance of “a” allele from mom?

2/2 (1)

Total

1/2 x 1 = 1/2

Dominant characteristics•Trait does not skip generations•Occurs frequently within a family

Pedigree Analysis

Sex linked characteristics•Tend to occur more in males than in females

•Color-blindness •Hemophilia

X-linked Recessives: More commonly seen in Males

• Why?• Males only need one recessive

allele to see trait.• E.g. Color-blindness

• XC – 92 % of alleles• Xc – 8 % of alleles

Females Males

92%

8%

(0.92)(0.92) = 85%

2x (0.92)(0.08) = 15%

(0.08)(0.08) = 0.64%

XCXC

XCXc

XcXc

XCyXcy

A woman with normal vision whose father was color-blind marries a color-blind male. What percent of the couple’s sons will be color-blind?

1. All

2. 75%

3. 50%

4. 25%