Genetics

Chapter 14

Gregor Mendel

• Father of Genetics

• Used peas to study heredity

Vocabulary

• Heredity: passing of traits from parent to offspring

• Genetics: study of heredity

• True-breeding: self-pollinating plant always produces the same offspring

• Allele: alternative versions of a gene

More Vocabulary

• Dominant: A

• Recessive: a

• Homozygous: AA or aa

• Heterozygous: Aa

Law of Segregation

• Separation of alleles into separate gametes

• 50% chance that one allele will go into a gamete

Test Cross

Law of Independent Assortment

Incomplete Dominance

Multiple Alleles

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)

Polygenic Traits

• Additive effect of multiple genes on a single phenotype

• Results in normal distribution

• Examples: skin color, height

Effect of Environment on Phenotype

More acidic soil Less acidic soil

Pleiotropic Effects

Pedigree Analysis

Pedigree

Male

Female

Heterozygous

Shows trait

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

Recessive characteristicsInheritance pattern

•Trait tends to skip generations

Dominant characteristics

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

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

Problem:

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%