Dihybrid Cross
More complicated crosses can be made by looking at two or more genes. The Punnett square works, however, only if the genes are independent of each other, which means that having a particular allele of gene A does not alter the probability of possessing an allele of gene B. This is equivalent to stating that the genes are not linked, so that the two genes do not tend to sort together during meiosis.
The following example illustrates a dihybrid cross between two heterozygous pea plants. R represents the dominant allele for shape (round), while r represents the recessive allele (wrinkled). A represents the dominant allele for color (yellow), while a represents the recessive allele (green). If each plant has the genotype RrAa, and since the alleles for shape and color genes are independent, then they can produce four types of gametes with all possible combinations: RA, Ra, rA, and ra.
| RA | Ra | rA | ra | |
|---|---|---|---|---|
| RA | RRAA | RRAa | RrAA | RrAa |
| Ra | RRAa | RRaa | RrAa | Rraa |
| rA | RrAA | RrAa | rrAA | rrAa |
| ra | RrAa | Rraa | rrAa | rraa |
Since dominant traits mask recessive traits, there are nine combinations that have the phenotype round yellow, three that are round green, three that are wrinkled yellow, and one that is wrinkled green. The ratio 9:3:3:1 is typical for a dihybrid cross.
Read more about this topic: Punnett Square
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