UNIT 5: Mendelian Genetics Notes (5)
Dihybrid Crosses
LT BI3.b*:
Dihybrid Crosses
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A simple dihybrid cross involves 2 genes for 2 distinct traits located on 2 different chromosomes
Each gene has 2 alleles: a dominant allele and a recessive allele.
Here are the steps to follow:
Step 1:
Figure out the genotypes of the parents.
Step 2:
Figure out what kinds of gametes the parents can produce.
Step 3:
Set up a Punnett Square for your mating.
Step 4:
Fill in the babies inside the table by matching the egg allele at the
top of the column with the sperm allele at the head of the row.
Step 5:
Figure out the genotypic ratio for your predicted babies.
Step 6:
Figure out the phenotypic ratio for your predicted babies.
Step 7:
Answer the question you've been asked.
Here's our problem: John and Elizabeth are have just had the first of what they hope will be
several children. Both John and Elizabeth are brown eyed, but baby Elvis is blue eyed. John
and Elizabeth also have free earlobes, though Elvis's tiny earlobes are completely attached.
We know the following about these genes. Brown eyes* (B) is completely dominant to blue
eyes (b), and free earlobes (A) is completely dominant to attached earlobes (a).
[* Actually, the genetics of eye color is more complicated than this, but the old fashioned
Brown/Blue system works almost all of the time.]
Here's our preliminary information:
So baby Elvis is homozygous recessive for both of these genes (bb,aa). We can thus figure out
that both John and Elizabeth are heterozygous for both of the genes. Here is our mating:
Now comes the tricky part. This is where most students go astray on this kind of problem —
figuring out the gametes. Every gamete contains exactly one allele for every gene. So every
one of Elizabeth's gametes must have one B and one A, but never two of either. And her eggs
must include all possible combinations of the alleles that she carries. The same goes for John.
So our gametes end up like this:
Now we plug these into a Punnett Square, and get on with the baby making part. Your Punnett
Square must have a column for each kind of egg and a row for each kind of sperm, so you
must build a four by four table. Here it is:
Note that, in the inside squares representing the kinds of babies made, the B's are grouped
together and the A's are grouped together. Be consistent about this, and about the order in
which you write the symbols for the two genes. It's easy to get confused if you get sloppy.
When you analyze the contents of this table, this is what you get for the genotypes:
1BBAA : 1BBaa : 1bbAA : 1bbaa : 2BbAA : 2Bbaa : 2BBAa : 2bbAa : 4BbAa
When you ask yourself the phenotype question, "Do any of these genotypes look alike," you
answer, "Yes; a lot of them do." After you figure out which ones look alike, you will get this
phenotypic ratio:
9 Brown Eyes with Free Earlobes : 3 Blue Eyes with Free Earlobes : 3 Brown Eyes with
Attached Earlobes : 1 Blue Eyes with Attached Earlobes.
And there you have it. All done exactly the same way our monohybrid cross was done. This
kind of cross — in which we are following two genes, and both parents are heterozygous for
both genes — is called a dihybrid cross (di = 2).
SAMPLE PROBLEM
Set up a punnett square using the following information:
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Dominate allele for yellow peas = Y
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Recessive allele for green peas = y
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Dominate allele round peas = R
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Recessive allele for wrinkled peas = r
Cross 2 heterozygous parents :
YyRr
YyRr
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1. What is the probability of producing round, yellow peas? Possible genotype(s)?
2. What is the probability of producing round, green peas? Possible genotype(s)?
3. What is the probability of producing wrinkled, yellow peas? Possible genotype(s)?
4. What is the probability of producing wrinkled, green? Possible genotype(s)?