Last week you were wondering what Jessica and Tom's babies might look like with respect to their eyebrows and earlobes (both Tom
and Jessica have thick eyebrows, controlled by gene E, with thick eyebrows dominant to thin eyebrows, and both of them have free earlobes, controlled by gene F, with free earlobes dominant to attached earlobes). You know that Tom's mom has attached earlobes. Additionally, Tom's mom has thin eyebrows, and so does Jessica's dad. No one in Jessica's pedigree going back to her great grandparents are known to have had attached earlobes, so you assume that she must be homozygous for this trait. If your inferences based on their known pedigrees are correct, what is the probability that Jessica's and Tom's first baby will have free earlobes and thin eyebrows?
Tom and Jessica have thick eyebrows and free earlobes. Tom's mother has attached earlobes, so Tom must have inherited an <em>f</em> allele from her, and because he has free earlobes he must be heterozygous<em> Ff. </em>We assume Jessica is homozygous for free earlobes <em>FF</em>.
Tom's mother and Jessica's father have thin eyebrows, so they are both <em>ee</em> and therefore both Tom and Jessica must be heterozygous <em>Ee</em>.
From the above information, Tom has the genotype EeFf and Jessica is EeFF. If you do a Punnett Square of this cross, you'll get that the probability of having a child with free earlobes and thin eyebrows (eeF_) is 25%.
