Patterns of Inheritance

I: Mendel's laws

Two factors called genes control each trait

For each gene, organisms receive one allele (form) from each parent randomly.

If an organism receives different alleles for the same trait, one allele is dominant over the other

II: Test crosses

Monohybrid cross Breeding individuals to

observe the inheritance of one trait

Ex: Mendel’s round vs wrinkled peas

Expected phenotypic ratio

3 dominant: 1 recessive

Test Crosses Continued…

Dihybrid cross Breeding individuals to

observe the inheritance of two separate traits

Ex: Mendel’s cross for shape and color

Expected phenotypic ratio: 9 dom dom: 3 dom rec: 3

rec dom: 1 rec rec

Practice time

Assume the following: A = round head a = square head B = belly button b = no button

If two parents are heterozygous for belly buttons what are the odds/percentages that their child will not have a belly button?

¼ or 25%

If parent #1 is homozygous dominant for both traits and parent #2 is homozygous recessive for both traits what are the odds/percentages their child will have a round head but no belly button?

0/16 or 0%

III: Complex inheritance

Incomplete Dominance: Blending of traits Heterozygotes, and homozygotes have

different phenotypes

Continued…

Co-dominance: Heterozygotes express both dominant

phenotypes

Continued…

Sex determination 23rd chromosome=X or Y XX=female XY=male

Sex linked traits: Genes for the traits

located on the sex chromosome

Ex: fruit fly eye color XR= Red Xr= white

Human color blindness

Continued…

Polygenic: Poly=many gen=genes Multiple genes are

responsible for the expression of the trait