After formulating his Law of segregation, Mendel studied inheritance patterns using the dihybrid cross, which is a cross between individuals that differ in two traits. For example, Mendel studied inheritance in plants that differed in both seed color and seed shape. Yellow, or big-Y, is dominant to green, or small-y, and round, or big-R, is dominant to wrinkled, or small-r. If a true-breeding plant with yellow, round seeds is crossed with one with green, wrinkled seeds, Mendel reasoned that the big-Y-big-R gametes and the small-y-small-r gametes would make doubly-heterozygous yellow, round offspring. This is exactly what he found.

Next, Mendel wondered if the big-Y and big-R alleles and the small-y and small-r alleles from the original parent plants would somehow stay together when the F1 plant formed its gametes, or if they’d mix and match to make gametes with new allele combinations. If the parental alleles somehow remained together, Mendel would have observed a 3:1 ratio of yellow-round to green-wrinkled offspring. Mendel didn’t observe this ratio. Instead, all possible phenotype combinations occurred in a 9 to 3 to 3 to 1 ratio. Mendel reasoned that the seed-color and seed-shape alleles combine in many different ways to make gametes.

Let’s use a Punnett square to see how these four gamete types combine to give this offspring ratio. Yes, in fact, a 9 yellow-round to 3 green-round to 3 yellow-wrinkled to 1 green-wrinkled ratio is predicted by the genotypes in our Punnett square. Mendel observed similar 9-to-3-to-3-to-1 ratios with all of his dihybrid crosses. These results led to Mendel’s second Law, the Law of independent assortment, which states that each allele pair segregates independently during gamete formation. This makes sense to us, because we discussed independent assortment in the previous chapter about meiosis. Independent assortment is the random distribution of the maternal and paternal members of each homologous chromosome pair to the daughter cells.If we look at a cell that’s heterozygous for three genes, A, B, and C, we see that according to the law of independent assortment, 8 different genotypes of gametes can be formed.

Mendel’s laws apply to all diploid organisms, including humans. For example, the PTC bitterness gene has alleles big-T for taster, and small-T for nontaster. The taster allele is dominant, and the nontaster allele is recessive. A person can taste PTC if they are homozygous big-T or heterozygous big-T-small-t, while a homozygous small-t person can’t taste it.

The earlobe attachment gene is also inherited in a Mendelian way. The big-F allele is for free earlobes, and small-f is for attached earlobes. Free earlobes are dominant, and attached earlobes are recessive. A person with free earlobes is either homozygous big-F or heterozygous big-F-small-f, while a homozygous small-f person has attached earlobes. A homozygous PTC taster man with attached earlobes has big-T-little-f gametes. A PTC nontaster woman homozygous for free earlobes has small-t-big-f gametes. If they have a son, he’ll be heterozygous for both PTC bitterness and earlobe attachment. Because the alleles of these two traits assort independently, the son can make four types of gametes, two of which are different from the parental gametes. If the son has children with a woman who’s heterozygous for PTC bitterness with attached earlobes, their gametes can combine in eight different ways, to yield four possible offspring phenotypes.

To learn about a statistical method for determining whether alleles always assort independently, see the Reference Sheet.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education