Punnett square?

Question

What genotype and phenotype ratios are possible when two heterozygous brown eyed individuals with dimpled chins were to have children?

Answer 1

To draw a Punnett square for two sets of alleles, remember that, above each column or next to each row, you're labeling the genes carried by a gamete.

Let's call denote the eyes trait by E and e, where E is the brown-eyed allele and e is the other allele. Additionally, let's denote the chin trait by C and c, where C is the dimpled chin allele and c is the other allele.

We know that both individuals are heterozygous in both traits; therefore, they are capable of producing zygotes with any combination of these alleles. So, we set up our square:

------EC eC Ec ec

EC

eC

Ec

ec

Now it's a simple matter of filling the right four letters into each square. Write the two letters corresponding to the square's row and column (doesn't matter which order) in the square, then count up how many of each combination you have. You'll notice the following pattern:

1 EECC

2 EECc

1 EEcc

2 EeCC

4 EeCc

2 Eecc

1 eeCC

2 eeCc

1 eecc

This is the set of genotypes possible when these two individuals mate. The ratios, thus, are 1:2:1:2:4:2:1:2:1, in the order I listed them above.

Now, reading the problem statement more closely, we notice that the individuals, despite being heterozygous, are both brown-eyed and dimple-chinned. Thus, these alleles are dominant. If we want to find all the phenotypes, we ignore the fact that each individual has two copies of each gene (one from each parent), and instead, focus only on the dominant ones (if any). Thus, for a given trait (eyes or chin), we pick only the most dominant allele, which will be displayed in the individual's phenotype:

EC - Brown-eyed, dimple-chinned

Ec - Brown-eyed, not dimple-chinned

eC - Not brown-eyed, dimple-chinned

ec - Not brown-eyed, not dimple-chinned

These are the possible phenotypes for the individuals' children. To determine the ratios, go back and look at the genotypes, again crossing out recessive alleles when a dominant allele for the same trait is present:

9 EC

3 Ec

3 eC

1 ec

Thus, the statistically expected phenotypic ratio of the two dindividuals' offspring is 9:3:3:1 in the order I listed them.

Answer 2

The cross might be represented as BbDd X BbDd. A Punnett square is not going to be the easiest approach to answering this, but if you need to do the square, you'll have a 4X4 square. The gametes from each parent will be:

BD

Bd

bD

bd

When you fill in the square you'll have 16 squares. A square like this should be shown in any basic biology or genetics book.

I'll let you make the square and count out the genotypic ratios.

The phenotypic ratios, though are easy and very standard. From a dihybrid cross, where both traits exhibit straight dominant/recessive phenotypes, you'll have a ratio of 9:3:3:1. That is, 9/16 of the offspring from this cross will be brown eyed and dimpled, 3/16 will be brown eyed without dimples, 3/16 will be blue eyed with dimples, and 1/16 will have blue eyes and no dimples.