Question

In humans, the alleles for unattached earlobes is dominant over the allele for attached earlobes. The allele for dark hair dominates the allele for light hair. If both parents are heterozygous for earlobe attachment and hair color, what types of offspring can they produce, and what is the probability for each type?

Answer 1

First, let's define the alleles:

(Dominant) allele for unattached earlobes is E

Allele for attached earlobs is e

(Dominant) allele for dark hair is H

Allele for light hair is h

Heterozygous would mean that for both sets of alleles, they will be EeHh.

Now for a Punnett Square:

       EH        Eh      eH       eh

EH EEHH  EEHh  EeHH  EeHh

Eh  EEHh  EEhh   EeHh  Eehh

eH EeHH  EeHh  eeHH  eeHh

eh  EeHh  Eehh   eeHh  eehh

These are all the possibilities for the parents' offspring. Since the square is 4x4, then the number of genotypes is 16.

Now for the probabilities of each genotype (the amount of times a certain genotype appears divided by the total number of genotypes):

EEHH: $\frac{1}{16}$

EEHh: $\frac{2}{16}$

EEhh: $\frac{1}{16}$

EeHH: $\frac{2}{16}$

EeHh: $\frac{4}{16}$

Eehh: $\frac{2}{16}$

eeHH: $\frac{1}{16}$

eeHh: $\frac{2}{16}$

eehh: $\frac{1}{16}$

(If you ever want to check if your probabilities are off, just add them together and they should add up to the total!)

Hope this helps! :)