Answer:
The value of studying genetics is in understanding how we can predict the likelihood of inheriting particular traits. This can help plant and animal breeders in developing varieties that have more desirable qualities. It can also help people explain and predict patterns of inheritance in family lines.
One of the easiest ways to calculate the mathematical probability of inheriting a specific trait was invented by an early 20th century English geneticist named Reginald Punnett click this icon to hear the preceding name pronounced. His technique employs what we now call a Punnett square. This is a simple graphical way of discovering all of the potential combinations of genotypes that can occur in children, given the genotypes of their parents. It also shows us the odds of each of the offspring genotypes occurring.
Setting up and using a Punnett square is quite simple once you understand how it works. You begin by drawing a grid of perpendicular lines:
basic Punnett square grid framework--essentially the beginning of tick-tack-toe game box
Next, you put the genotype of one parent across the top and that of the other parent down the left side. For example, if parent pea plant genotypes were YY and GG respectively, the setup would be:
Punnett square with the genotype of one parent on the top with one letter in each square and the genotype of the other parent on the left side with one letter in each square
Note that only one letter goes in each box for the parents. It does not matter which parent is on the side or the top of the Punnett square.
Next, all you have to do is fill in the boxes by copying the row and column-head letters across or down into the empty squares. This gives us the predicted frequency of all of the potential genotypes among the offspring each time reproduction occurs.
same as the previous Punnett square but with the expected genotype frequencies of offspring are indicated in the 4 empty squares on the lower right
In this example, 100% of the offspring will likely be heterozygous (YG). Since the Y (yellow) allele is dominant over the G (green) allele for pea plants, 100% of the YG offspring will have a yellow phenotype, as Mendel observed in his breeding experiments.
In another example (shown below), if the parent plants both have heterozygous (YG) genotypes, there will be 25% YY, 50% YG, and 25% GG offspring on average. These percentages are determined based on the fact that each of the 4 offspring boxes in a Punnett square is 25% (1 out of 4). As to phenotypes, 75% will be Y and only 25% will be G. These will be the odds every time a new offspring is conceived by parents with YG genotypes.
Punnett square with both parents heterozygous (YG) showing that the offspring probablities are 25% YY, 50% YG, and 25% GG
An offspring's genotype is the result of the combination of genes in the sex cells or gametes (sperm and ova) that came together in its conception. One sex cell
