Final answer:
A Punnett square is used to determine the possible offspring genotypes and phenotypes by combining alleles from two parents. By listing all possible gametes along the top and side of a grid, the resulting genotypes can be deduced and phenotypic ratios inferred.
Step-by-step explanation:
To predict the likelihood of a particular offspring genotype and phenotype from a cross of two individuals with known genotypes, we can use a Punnett square. A Punnett square is a simple graphical way to show all possible combinations of parental alleles and helps to foresee the genotypes of their potential offspring. For example, let's consider a monohybrid cross between two pea plants, where yellow seeds (Y) are dominant over green seeds (y). We have one parent with genotype YY (homozygous dominant) and another with genotype yy (homozygous recessive).
The possible gametes for the YY parent are both Y, and for the yy parent, they are both y. When we set up a Punnett square, we place each of the gametes of one parent on the top row and the gametes of the other parent along the left column. We then fill in the squares by combining the alleles from the top and side to find the possible genotypes of the offspring. In this case, all offspring will be Yy, illustrating that they will exhibit the dominant yellow seed phenotype.
For a cross between two heterozygous parents (Bb) with a dominant (B) purple flower allele and a recessive (b) white flower allele, the Punnett square would show a 3:1 ratio of purple to white flowers in their offspring. This is determined by listing the potential gametes (B and b) from each parent along the top and side, and then pairing the alleles within the squares to find the genotypes: BB, Bb, Bb, and bb. Thus, the phenotypic ratio can be deduced from the genotypes produced in the Punnett square.