April 2008

Mendelian GeneticsMendelian GeneticsDihybrid CrossesDihybrid Crosses

March 12, 2009March 12, 2009Mr. BromwellMr. Bromwell

Mendel’s Law of Independent AssortmentMendel’s Law of Independent Assortment

• Genes on separate chromosomes segregate randomly and independently of each other during meiosis.– used when examining the inheritance of two

traits simultaneously

Making Predictions: Blue-Eyed Freckle Juice Making Predictions: Blue-Eyed Freckle Juice

Problem:In at least one of the genes for eye color, brown eyes are dominant to blue eyes.* Having freckles is also a dominant trait.

A brown-eyed, freckle-faced couple are heterozygous for these two traits. What is the probability that they will have a blue-eyed child with freckles?

* Disclaimer: There are at least three genes that control eye color. The brown/blue gene is only one of them. Therefore it is not the only determiner of a person’s eye color.

Solution A:From the information in the problem you know that both parents have the heterozygous genotype ‘BbFf’.

B = dominant allele (brown eyes)b = recessive allele (blue eyes)F = dominant allele (freckles)f = recessive allele (no freckles)

First, determine the types of gametes that each parent can produce.

In other words, according to the Law of Independent Assortment, how many combinations of one eye color allele and one freckle allele can be made?

Alleles in Parent Cell

BbFf

gam

ete

type

s

Making Predictions: Blue-Eyed Freckle Juice Making Predictions: Blue-Eyed Freckle Juice

BF Bf bF bf

Solution A:Second, complete the Punnett square with the allele combinations for both parents.

– You will need a Punnett square that is 4 x 4 boxes.

– Be careful to list the alleles for each parent correctly.

– Place alleles for the same gene together. This makes for easier interpretation.

Making Predictions: Blue-Eyed Freckle Juice Making Predictions: Blue-Eyed Freckle Juice

BF Bf bF bf

BF BBFF BBFf BbFF BbFf

Bf BBFf BBff BbFf Bbff

bF BbFF BbFf bbFF bbFf

bf BbFf Bbff bbFf bbff

♂ male gamete types

♀ fe

mal

e ga

met

e t

ypes

Finally, create a genotype pattern.– The pattern bbF_ will result in blue

eyes and freckles.– The probability of this occurring is

3/16.

Once again… a Word About RatiosOnce again… a Word About RatiosA cross that involves two organisms that are heterozygous for two traits is called a dihybrid crossdihybrid cross.

There are nine possible genotypes in a ratio of 1:2:1:2:4:2:1:2:11:2:1:2:4:2:1:2:1.

There are four possible phenotypes in a ratio of 9:3:3:19:3:3:1.

BF Bf bF bf

BF Brownfreckles

Brownfreckle

s

Brownfreckle

s

Brownfreckle

s

Bf Brownfreckles

Brown no

freckles

Brownfreckle

s

Brown no

freckles

bF Brownfreckles

Brownfreckle

s

Blue Freckle

s

Blue Freckle

s

bf Brownfreckles

Brown no

freckles

Blue Freckle

s

Blue no

Freckles

♂ male gamete types

♀ fe

mal

e ga

met

e t

ypes

Solution B:Instead of dealing with both traits at the same time, first perform a monohybrid cross for each individual trait.

– You will need a Punnett square for each trait.

Making Predictions: Blue-Eyed Freckle Juice Making Predictions: Blue-Eyed Freckle Juice

B b

B BB Bb

b Bb bb

F f

F FF Ff

f Ff ff

Inheritance of eye color Inheritance of freckles

Solution B:Next, find the probability of each individual outcome separately, then multiply their probabilities together to determine the chance of them happening together.

Making Predictions: Blue-Eyed Freckle Juice Making Predictions: Blue-Eyed Freckle Juice

Therefore…– The probability of getting blue eyes is ¼.– The probability of getting freckles is ¾.– ¼ x ¾ = 3/16 (the probability of getting blue eyes and freckles)

B b

B BB Bb

b Bb bb

F f

F FF Ff

f Ff ff

Inheritance of eye color Inheritance of freckles

ReferencesReferencesBiggs, Alton, et. al. Biology. New York: The McGraw Hill Companies, Inc., 2007.“Freckle Juice” Kathinthehat’s Weblog. Oct 3, 2007. Retrieved Apr 7, 2008 from

<http://katinthehat.wordpress.com/2007/10/03/freckle-juice/>.