1. Which is the correct set up for a dihybrid cross of two heterozygous parents RrYy x RrYy?

See following slides:

RR rR Rr rrYY

Yy

yY

yy

A

R r Y y

R

r

Y

y

B.

RY Ry rY ry

RY

Ry

rY

ry

C.

The correct set up for a heterozygous dihybrid cross is

A. AB. BC. C

A B C

33% 33%33%

• Each side of a Punnett Square represents all the possible alleles in a gamete from each parent.

• Parent gametes always contain one allele for each gene. (one R or r and one Y or y in this case).

• Four possible combinations of the alleles for the two genes are possible- RY, Ry, rY and ry.

2. For the Punnett square of RrYy x RrYy, what phenotype ratio would be expected in the offspring?

A. 1:1:1:1B. 9:3:3:1C. 3:1D. 1:2:1

1:1:1:1

9:3:3:13:01

1:02:01

25% 25%25%25%

Dominant for both : Dom 1st/Rec. 2nd : Rec for 1st/Dom. for 2nd : Recessive for both

9 : 3 : 3 : 1

3. The 9:3:3:1 ratio for a heterozygous dihybrid cross will apply if

A. the traits are on separate chromosomes

B. the traits are closely linked on the same chromosome

C. Either A or BD. Neither A nor B

the tr

aits are

on separa

..

the tr

aits are

close

ly lin...

Either A

or B

Neither A

nor B

25% 25%25%25%

A dihybrid Punnett square assumes that all 4 gamete combinations are equally likely.

This will occur if the two genes are on different chromosomes. Independent assortment will make all 4 combinations equally likely.

When two genes are on the same chromosome, genetic recombination can occur due to crossing over.

However, if the two genes are close together on the chromosome, the recombinants will not be equally like as the original combination.

4. Determine the phenotype ratio for a cross between GgHh and gghh.

A. 9:3:3:1B. 1:1:1:1C. 3:1:3:1D. 1:2:1E. 3:1

9:3:3:1

1:1:1:1

3:1:3:1

1:02:013:01

20% 20% 20%20%20%

GH Gh gH gh

gh GgHh GGhh ggHh gghh

Don’t need to fill out additional rows.All gametes of gghh will be gh.

Ratio of 1:1: 1:125% chance dominant phenotype for both traits25% chance dominant for G, recessive for h25% chance recessive for g, dominant for H25% chance recessive for both

1. C2. B3. A4. B