Transcript Mendelian Genetics

Mendelian Genetics
Dihybrid Crosses
April 2008
March 12, 2009
Mr. Bromwell
Mendel’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
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.
Making Predictions: Blue-Eyed Freckle Juice
Solution A:
Alleles in Parent Cell
From the information in the problem you
know that both parents have the
heterozygous genotype ‘BbFf’.
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?
gamete types
B = dominant allele (brown eyes)
b = recessive allele (blue eyes)
F = dominant allele (freckles)
f = recessive allele (no freckles)
BbFf
BF
Bf
bF
bf
Making Predictions: Blue-Eyed Freckle Juice
♂ male gamete types
Solution A:
– 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.
Finally, create a genotype pattern.
– The pattern bbF_ will result in blue
eyes and freckles.
– The probability of this occurring is
3/16.
♀ female gamete types
Second, complete the Punnett
square with the allele
combinations for both parents.
BF
Bf
bF
bf
BF
BBFF
BBFf
BbFF
BbFf
Bf
BBFf
BBff
BbFf
Bbff
bF
BbFF
BbFf
bbFF
bbFf
bf
BbFf
Bbff
bbFf
bbff
Once again… a Word About Ratios
♂ male gamete types
There are nine possible
genotypes in a ratio of
1:2:1:2:4:2:1:2:1.
There are four possible
phenotypes in a ratio of
9:3:3:1.
♀ female gamete types
A cross that involves two
organisms that are
heterozygous for two traits
is called a dihybrid cross.
BF
Bf
bF
bf
BF
Brown
freckles
Brown
freckles
Brown
freckles
Brown
freckles
Bf
Brown
freckles
Brown
no
freckles
Brown
freckles
Brown
no
freckles
bF
Brown
freckles
Brown
freckles
Blue
Freckles
Blue
Freckles
bf
Brown
freckles
Brown
no
freckles
Blue
Freckles
Blue
no
Freckles
Making Predictions: Blue-Eyed Freckle Juice
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.
Inheritance of eye color
B
b
B
BB
Bb
b
Bb
bb
Inheritance of freckles
F
f
F
FF
Ff
f
Ff
ff
Making Predictions: Blue-Eyed Freckle Juice
Solution B:
Next, find the probability of each individual outcome separately, then
multiply their probabilities together to determine the chance of them
happening together.
Inheritance of eye color
B
b
B
BB
Bb
b
Bb
bb
Inheritance of freckles
F
f
F
FF
Ff
f
Ff
ff
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)
References
Biggs, 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/>.