Crosses Involving Single Genes Help
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.
EXAMPLE 2.8 A pair of alleles governs coat color in the guinea pig; a dominant allele B produces black and its recessive allele b produces white. The results of an F1 cross (Bb x Bb) are summarized in the table below. The numbers shown in the table above represent the expected F2 ratio of offspring from this cross: 1 : 2 : 1. Note the phenotypic ratio is 3 : 1 black to white. These expected results can also be shown as fractions (in parentheses).
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.
EXAMPLE 2.9 A Punnett square uses a checkerboard or table format to show the genotypes of possible gametes from each parent, one on the top row (bold) and the other on the left-most column (bold). The possible offspring genotypic combinations of each gamete combination are shown in the ''remaining'' squares. For production of the F2 in Example 2.8, the same genotypic combinations (BB, Bb, bb) and ratios result (1 BB : 2 Bb : 1 bb) from this method, as do the same phenotypic ratios (3 black : 1 white).
There are four other types of monofactorial crosses:
- homozygous black (BB) × homozygous black (BB)
- homozygous white (bb) × homozygous white (bb)
- heterozygous black (Bb) × homozygous black (BB)
- 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.
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