Day 26 -- Y's and y's

Crosses consist of a single trait that, when combined with another subject of the same trait, output four possible offspring from the parents. The trait is represented by two letters: both capital, one capital and one lowercase, or both lowercase. Because the capital letters represent the dominant trait, the recessive trait cannot show unless both letters are lowercase. (If there is an uppercase present, the trait shown is the dominant. This only applies for now....)

Take, for example, the cross of a yellow heterozygous (one dominant and one recessive) and a green homozygous (both recessive) pea. When crossed, a method called the Punnett Square will show all possible offspring from the two parents. In this case, the offspring could be either yellow or green. However, if any offspring do turn out to be yellow, they will always carry the ability to give the recessive trait. (From this example, a child cannot be pure dominant YY because both parents must have the dominant allele.) 

This is a simple cross, meaning there are two letters in each. For dihybrid crosses, we have, basically, the same concept. However, this time, there are four letters (two of two different letters) instead of two! 

These types of crosses are still relatively easy to accomplish. Since we have two different letters in each one, we need to take each possibility and place it up against all possibilities from the second. The hypothetical combination SsYy has four possibilities:

SsYy -- SY // Sy // sY // sy

Now we do the same to the second dihybrid, and we make a slightly larger Punnett square (as seen above). This one, because it is 4 x 4, may have 16 different possible outcomes. However, on the chance that the crosses are exactly the same (e.g. AACC x AACC), there may only be 1 possible outcome (AACC).