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.
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.
A testcross is used to determine the genotype of an individual exhibiting a dominant phenotype because this individual could have either a homozygous or heterozygous genotype. The testcross parent is always homozygous recessive for all of the genes under consideration. The purpose of a testcross is to discover how many different kinds of gametes are being produced by the individual whose genotype is in question. A homozygous dominant individual will produce only one kind of gamete; a heterozygous individual will produce two kinds of gametes with equal frequency.
EXAMPLE 2.10 Consider the case in which a testcross is performed with a white-coat male guinea pig and a black-coat female of unknown genotype.
Conclusion: The black female parent must be producing only two kinds of gametes and therefore she is heterozygous dominant Bb.
In the backcross, the F1 progeny are mated back to one of their parents (or to individuals with a genotype identical to that of their parents). Sometimes "back-cross" is used synonymously with "testcross" in genetic literature. The testcross is different in that a recessive homozygote is always used as the testcross parent; this is not necessarily true in a backcross.
EXAMPLE 2.11 A homozygous black female guinea pig is crossed to a white male. An F1 son is backcrossed to his mother. This cross and backcross is diagrammed as follows, using the symbols ♀ for female and ♂ for male.
P: BB♀ x bb♂ black-coat female white-coat male F1: Bb black males and females F1 backcross: Bb♂ x BB♀ black-coat son black-coat mother Backcross progeny: 1/2 BB : 1/2 Bb All-black offspring
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