Final answer:
In genetics, a double heterozygote has two alleles for each of two genes, resulting in combinations like AB and BA. Use of Punnett squares illustrates how these allele combinations arise and affect offspring phenotypes. Examples include the flower color of pea plants and AB blood typing, which exhibit classic Mendelian ratios and codominance, respectively.
Step-by-step explanation:
For a double heterozygote, two genes are under consideration, each with two alleles. The two possible arrangements of alleles for such an individual are AB and BA. These arrangements become apparent when we examine the inheritance of traits using tools like the Punnett square. For instance, if we cross two heterozygous pea plants for flower color (both Bb), we will see the following allele combinations in the offspring: BB, Bb, Bb, and bb. The Bb genotype appears twice because you can inherit a dominant allele from one parent and the recessive allele from the other, or vice versa, leading to two possible heterozygote combinations. However, for the phenotype, due to the dominance of B over b, approximately 75% of the offspring exhibit purple flowers—a 3:1 phenotypic ratio.
In the example of ABO blood typing, which is an instance of codominance, offspring can have A, AB, or B blood types when both parents are type AB. Here, the alleles IA and IB are codominant, allowing both to be expressed equally if inherited together, resulting in the AB blood type. With two heterozygous parents, the phenotype ratio of their offspring would be 1A:2AB:1B. Similarly, when self-crossing a YyRr heterozygote, which follows Mendel's law of independent assortment, we can predict a 9:3:3:1 phenotypic ratio from a set of 16 possible genotypic combinations.