Dihybrid Crosses: Punnett squares for two traits

Genes on Different Chromosomes If two genes are on different chromosomes, all four

possible alleles combinations for two different genes in a heterozygous cross are _____________ due to independent assortment.

If parents are RrYy (heterozygous for both traits) Ry rY RY ry Gametes: ___________________________________

R

y

r

Y

R r

Y y

OR

Equally likely to line up either way when dividing

Each side of a Punnett Square represents all the possible allele combinations in a gamete from a parent.

Parent gametes always contain one allele for___ _______ .(____________-R or r & Y or y in this case).

Four possible combinations of the alleles for the two genes are possible if heterozygous for both traits. (For example: ___________________)

Due to independent assortment, each possible combination is equally likely if genes are on separate chromosomes. Therefore Punnett squares indicate probabilities for each outcome.

Setting up a Dihybrid Cross: RrYy x RrYy

Which is correct for a dihybrid cross of two heterozygous parents RrYy x RrYy?

A. R r Y y R r

Y y

B. RR Rr rr rR YY Yy yy yY

C. RY Ry rY ry RY Ry rY ry

Explain how the correct answer relates to the genes passed down in each gamete (egg or sperm)

Discuss with your table partner:

Dihybrid Cross of 2 Heterozygotes

9

3

3

1

Heterozygous Dihybrid Cross Dominant for both

traits

Dominant 1st trait

Recessive 2nd trait

Recessive 1st trait

Dominant 2nd trait

Recessive for both

traits

Round Yellow

Round Green

Wrinkled Yellow

Wrinkled Green

__ __ __ __ ______ =

___ ______ =

____

______ = ____

_______ = _____

Heterozygous cross __________ratio if independent assortment

Mendel developed the Law of Independent Assortment because he realized that the results for his dihybrid crosses matched the probability of the two genes being inherited independently.

Exceptions to the Law of Independent Assortment: Linked Genes

Studying fruit flies, Thomas Hunt Morgan identified dihybrid crosses that did not result in the phenotype ratios expected through independent assortment.

Review of Recombinants Recombinants have new combinations of the genes. If the parental generation of cats are white furred with short tails (SS bb) and brown fur with long tails (ss BB), the F1 will be SsBb. In the gametes for the next generation, ________________ are the parental combinations for the gametes (egg or sperm) and ________________ are the recombinant gametes.

Parental Genotypes

___________ Combinations

____________ gametes

Linked Genes When two genes are linked

(relatively close together on the same chromosome), it ________ _____________________________________________________.

Recombinants do occur due to crossing over.

The further apart the genes are the more likely recombinants are because crossing over is more likely to occur.

sb

SB SsBb Short, Brown

Sb Ssbb Short, White

sB ssBb Long, Brown

sb ssbb Long, White

SsBb x ssbb One possible gamete from homozygous recessive parent.

Four

pos

sible

gam

etes

from

het

erzy

gote

. Eq

ually

like

ly if

inde

pend

ently

ass

ortin

g.

Test cross (heterozygote crossed with a recessive) results if independently assorting: ________ (_____ of each of 4 possible combinations.)

Parental combinations: Gray with normal wings

and Black with vestigal wings

Cross of the heterozygous offspring with a double recessive.

Expect 1:1:1:1 if independently assorting.

Results show more of ____________________because genes are on the same chromosome.

Discuss with your table partner: Mythbuster Many people assume that dominant traits are always more common. It is true that if both parents are heterozygous, it is more likely that the offspring will have the dominant trait. However, a recessive trait can be more common in the population. For example, Achrondoplasia dwarfism is caused by a dominant allele, and yet the trait is very rare. Explain how and why this is possible. ______________________________________________________________________________________________________

Genetics and ______________ _________________can change the allelic frequency.

(Allelic frequency = how common an allele is in the gene pool.)

Initial allelic frequency of b = 50% (12/24 alleles)

If the white rabbits are more likely to _________ , the frequency of b allele will increase in the gene pool.

Allelic frequency of b after natural selection= 96% (23/24)

Bb BB bb bb Bb Bb bb BB Bb Bb BB Bb

bb bb bb bb bb Bb bb bb bb bb bb bb bb bb

Discuss with your table partner:

________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Review the meaning of the words genotype and phenotype.

Do you think natural selection acts upon the genotype or phenotype of an organism? Explain your reasoning.