Dihybrid Crosses: Understanding Genetic Traits and Independent Assortment, Study notes of Law

Download Dihybrid Crosses: Understanding Genetic Traits and Independent Assortment and more Study notes Law in PDF only on Docsity!

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

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

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.

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 possible gametes from heterzygote. Equally likely if independently assorting.

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

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