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Dihybrid Crosses. Dihybrid Cross: a cross that shows the possible offspring for two traits. Fur Color: B: Black b: White. Coat Texture: R: Rough r: Smooth. In this example, we will cross a heterozygous cow with heterozygous bull. Their genotypes will be:. BbRr x BbRr.

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slide2

Dihybrid Cross:a cross that shows the possible offspring for two traits

Fur Color: B: Black b: White

Coat Texture: R: Rough r: Smooth

In this example, we will cross a heterozygous cow with heterozygous bull. Their genotypes will be:

BbRr x BbRr

dihybrid cross
Dihybrid Cross

BbRr x BbRr

First, you must find ALL possible gametes that can be made from each parent.

Remember, each gamete must have one B and one R (haploid).

BbRr

Possible Gametes

BR

bR

Br

br

dihybrid crosses a cross that shows the possible offspring for two traits

BR

Br

bR

br

BR

BBRR

BBRr

BbRR

BbRr

Br

BBRr

BBrr

BbRr

Bbrr

bR

BbRR

BbRr

bbRR

bbRr

br

BbRr

Bbrr

bbRr

bbrr

Dihybrid Crosses:a cross that shows the possible offspring for two traits

BbRr x BbRr

Fur Color: B: Black b: White

Coat Texture: R: Rough r: Smooth

slide5

BR

Br

bR

br

BR

BBRR

BBRr

BbRR

BbRr

Br

BBRr

BBrr

BbRr

Bbrr

bR

BbRR

BbRr

bbRR

bbRr

br

BbRr

Bbrr

bbRr

bbrr

9

How many of the offspring would have a black, rough coat?

How many of the offspring would have a black, smooth coat?

How many of the offspring would have a white, rough coat?

How many of the offspring would have a white, smooth coat?

3

3

1

Phenotype Ratio: 9:3:3:1

Fur Color: B: Black b: White

Coat Texture: R: Rough r: Smooth

law of independent assortment
Law of Independent Assortment

During gamete formation, the two alleles for one gene segregate or assort independently of the alleles for other genes

Although, other scientists did find exceptions to this law… as you will discover later in this course.

practice
Practice

In pea plants, yellow seeds (E) are dominant over green seeds (e), and rounded peas (R) are dominant over wrinkled peas (r).

Cross a plant that is heterozygous for both traits with a plant that is homozygous recessive for both traits. Draw a Punnett square to show all possible offspring, and determine the genotypic and phenotypic ratios.

more practice
More practice
  • About 70% of Americans perceive a bitter taste from the chemical phenylthiocarbamide (PTC). The ability to taste this chemical results from a dominant allele (T) and not being able to taste PTC is the result of having two recessive alleles (t). Albinism is a trait with normal pigment being dominant (A) and the lack of pigment being recessive (a). A normally pigmented woman who cannot taste PTC has a father who is an albino taster. She marries a homozygous, normally pigmented man who is a taster but who has a mother that does not taste PTC. What are the genotypes of the possible children (choose all that apply)?
practice this one is a little bit harder
Practice (this one is a little bit harder)
  • Suppose in a strain of soybeans, high oil (H) content in the seeds is dominant to low oil content and four seeds (E) in a pod is dominant to two seeds in a pod. A farmer crosses two soybean plants, both with high oil content and four seeds per pod. The resulting crop has:
    • High oil/four seeds: 456
    • High oil / two seeds: 162
    • Low oil / four seeds : 148
    • Low oil / two seeds): 45
  • What genotype were the parent plants?