Although we know quite a bit about how genes are inherited we often can’t predict what traits acouple’s children will have no matter what we know about the couple. Take perfect pitch forexample.

Although we know quite a bit about how genes are inherited we often can’t predict what traits acouple’s children will have no matter what we know about the couple. Take perfect pitch forexample. This trait is known to have a strong genetic component. Two people with perfect pitchtend to have children with the trait, but usually there’s no way to predict whetherany given childoftheirs will have it. In fact, it often happens that parents with perfect pitch pass the trait on to someof their children and not others. You can’t even be sure that two people without perfect pitch won’thave a child with perfect pitch.Despite all this uncertainty we do know a lot about inheritance. Our modern understanding beganwith experiments performed by Gregor Mendel. Mendel’s story and his discoveries are reiterated inevery general biology textbook, so I won’t restate them here. However, as a direct result of hisdiscoveries we now know how traits like perfect pitch are inherited.It turns out that perfect pitch probably acts like the seven traits of pea plants that Mendel studied inthe middle of the nineteenth century. Evidence suggests that this trait isautosomal, meaning it isdetermined by a gene on an autosomal chromosome as opposed to a sex chromosome. It is also adominant trait, meaning that perfect pitch is expressed in the heterozygote. Therefore, there arethree possible genotypes;AAandAa, which confer tendency towards perfect pitch; andaa, thegenotype of people without perfect pitch. However, there is a twist — if a child with the propergenotype to develop perfect pitch, eitherAAorAa, does not receive musical training before theyare about nine years old they tend not to develop the trait.EXAMPLE 1.Nancy’s parents are both musicians and studied music at an early age. Nancy andher mother both have perfect pitch, but her father does not. What is Nancy’s genotype?Solution:Since Nancy’s father does not have perfect pitch he’s probablyaa. SinceNancy inherits one allele from each of her parents, she inherited theaallele from her father.But, since Nancy has perfect pitch, she must also have theAallele, so her genotype isAa.EXAMPLE 2.Suppose Nancy’s mother is also heterozygous. What’s the probability that Nancy’sbrother, Tom, has perfect pitch?Solution:Geneticists would (without remorse) represent the mating of Nancy’s parentsasAaxaa. In a mating like this, with only one simple trait involved, Mendel suggested thatparents pass only one of their two alleles to their offspring. So, Nancy’s mother can give herchildren either the A or theaallele, and the child is equally likely to inherit either allele fromher mother. On the other hand, Nancy’s father can only give his children theaallele.Knowing this, we can solve the puzzle using a simple technique, called a Punnett square, tokeep track of every possible combination.Start by drawing an array of four boxes. At the head of each column, list the possible alleles thatone parent can pass on to the offspring, and likewise label each row with alleles the other parentcan pass on, like so

 
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