Genetics Practice

Gentoypes and Phenotypes Monohybrid Crosses

Dihybrid Crosses

In certain breeds of dogs, deafness is due to a recessive allele (d), and normal hearing is due to its dominant allele (D).

What percentage of the offspring of a normal heterozygous (Dd) dog and a deaf dog (dd) would be expected to have normal hearing?

D d

d Dd dd

d Dd dd

DD = normalDd = normal dd = deaf

2/4 = 50%

What about a cross between a homozygous deaf dog and a homozygous normal? What % of the offspring will have normal hearing?

d d

D Dd Dd

D Dd Dd

DD = normalDd = normal dd = deaf

100%

In humans, normal pigmentation is due to a dominant allele (C), and albinism is due to it’s recessive allele (c). A normal man marries an albino woman. Their first child is albino. What arethe genotypes of these three people?

Dad = Cc, Mom = cc, kid= ccDad had to carry the recessive allele to be able to pass it on

to the child.

If Dad had the genotype (CC), then every offspring would be (Cc), which would be normal pigmentation.

In minks, brown is dominant over silver-blue color. What are the possible genotypes for the offspring if you cross a homozygous brown mink with a silver-blue mink?

100% of offspring = Bb

b b

B Bb Bb

B Bb Bb

In snapdragons, red is not completely dominant over white flowers. What color flowers would you expect when you cross a red flower with a white flower? What would be the genotypic and phenotypic ratios of the offspring be?

Red (RR) x White (rr) = Pink (Rr)

Genotypic ratio = 4:0 Rr Phenotypic ratio = 4:0 pink

This is an example of Incomplete Dominance

What type of inheritance pattern is shown?

Incomplete

Dominance

• RR x Rr = parent genotypes

In cattle, a cross between a white (rr) and a red (RR) cow produces a roan colored cow. Cross a red-haired and roan colored cow.What are the genotypes of the parents?What are the phenotypic ratios of theoffspring?

R R

R RR RR

r Rr Rr

RR to Rr = 2:2

50% Red50% Roan

In humans, acondroplasia “dwarfism” (D) is dominant over normal (d).

A homozygous dominant (DD) person dies before the age of one.

A heterozygous (Dd) person is dwarfed.

A homozygous recessive individual is normal.

D d

D DD Dd

d Dd dd

A heterozygous dwarf man marries a dwarf heterozygous woman. Give the genotypic ratio and the phenotypes of the possible offspring.

1:2:1 genotypic ratio

25% chance of a DD, homozygous dwarf which dies before the age of one

50% chance of having a dwarf child

25% chance of having a child with normal height

• If Sponge Bob is heterozygous squarepants, homozygous yellow body:

• What is his genotype?• SsYY• Using the FOIL method, what would the 4

possible gamete combinations be?• SY• SY• sY• sY

Sponge Bob Dihybrid Genetics

(S = squarepants, s = roundpants; Y = yellow body, y = blue body)

• If Sponge Susie is roundpants, heterozygous yellow body:

• What is her genotype?• ssYy• Use the FOIL method to find the 4 possible

gamete combinations• sY• sy• sY• sy

Sponge Susie Dihybrid Genetics

• If Patrick is heterozygous tall head, heterozygous pink body:

• What is his genotype?• TtPp• Use the FOIL method to find his 4 possible

gamete combinations• TP• Tp• tP• tp

Patrick Dihybrid Genetics

(T = tall head, t = short head; P = pink body, p= orange body)