Crosses Involving Single Genes Help (page 2)

Six Different Single Gene Crosses

The monofactorial cross is a mating in which only a single gene is analyzed or monitored in the cross. This cross demonstrates Mendel's principle of segregation. There are six types of crosses that can be performed to uncover various genetic relationships; these are described in Section 1 below. These crosses are also sometimes referred to as monohybrid crosses.

This begins with a cross between a pair of homozygous, pure-breeding parents in the first round (P in cross #1 below) to yield the first filial generation (F₁). The resulting offspring are all monohybrids (heterozygous for a single pair of alleles). A cross is then carried out between these F₁ individuals (cross #2 below) to yield a second filial generation (F₂). Unless otherwise specified in the problem, the F₂ generation is produced by crossing the F₁ individuals among themselves randomly. If plants are normally self-fertilized, they can be artificially cross-pollinated in the parental generation and the resulting F₁ progeny may then be allowed to pollinate themselves to produce the F₂ progeny. In this generation, the alleles are observed to segregate from one another and four possible gametic combinations result in three classes of genotypes. Dominance and recessiveness relationships between the alleles for each specific gene set will determine the phenotypic outcomes.

Crosses 1 and 2 are solved using a branch diagram or a forked-line method to help solve the offspring of the cross. This procedure was introduced in Chapter 1 as a means for determining all possible ways in which any number of chromosome pairs could orient themselves on the first meiotic metaphase plate. It can also be used to find all possible genotypic or phenotypic combinations. The next example shows how to solve cross #2 and Example 2.8 using another method called the Punnett square.

There are four other types of monofactorial crosses:

3. homozygous black (BB) × homozygous black (BB)
4. homozygous white (bb) × homozygous white (bb)
5. heterozygous black (Bb) × homozygous black (BB)
6. heterozygous black (Bb) × homozygous white (bb)

The results from each of the six monofactorial crosses are summarized in Table 2-1. Crosses 3 and 4 show pure breeding lines.