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Since dominant traits mask recessive traits (assuming no epistasis), 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 the expected outcome when crossing two double-heterozygous parents with unlinked genes.
This cross results in the expected phenotypic ratio of 9:3:3:1. Another example is listed in the table below and illustrates the process of a dihybrid cross between pea plants with multiple traits and their phenotypic ratio patterns. Dihybrid crosses are easily visualized using a 4 x 4 Punnett square.
A Punnett square visualizing the genotype frequencies of a Hardy–Weinberg equilibrium as areas of a square. p (A) and q (a) are the allele frequencies . Genetic variation in populations can be analyzed and quantified by the frequency of alleles .
Punnett square: If the other parent does not have the recessive genetic disposition, it does not appear in the phenotype of the children, but on the average 50% of them become carriers. A hereditary carrier ( genetic carrier or just carrier ), is a person or other organism that has inherited a recessive allele for a genetic trait or mutation ...
Figure 1: Inheritance pattern of dominant (red) and recessive (white) phenotypes when each parent (1) is homozygous for either the dominant or recessive trait. All members of the F 1 generation are heterozygous and share the same dominant phenotype (2), while the F 2 generation exhibits a 6:2 ratio of dominant to recessive phenotypes (3).
Punnett square for three-allele case (left) and four-allele case (right). White areas are homozygotes. Colored areas are heterozygotes. Consider an extra allele frequency, r. The two-allele case is the binomial expansion of (p + q) 2, and thus the three-allele case is the trinomial expansion of (p + q + r) 2.
Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants. The letters B and b represent genes for color, and the pictures show the resultant phenotypes. This shows how multiple genotypes (BB and Bb) may yield the same phenotype (purple petals).
We can notice that, because of the dominance of T-[frequency (0.25 + 0.5)] over tt [frequency 0.25], the 3:1 ratio is still obtained. A cross such as Mendel's, where true-breeding (largely homozygous) opposite parents are crossed in a controlled way to produce an F1, is a special case of hybrid structure.