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WEB TUTORIAL 5.1
Linkage and
Recombination
OVERVIEW
Genes located on the same chromosome are said to be linked. These genes do not segregate according to the law of independent assortment but can be transmitted together during gamete formation. This tutorial demonstrates how gene linkage can affect the gametes
formed during meiosis.
TEXTBOOK REFERENCES
Genes Linked on the Same Chromosome Segregate Together (pp. 101-102)
Independent Assortment
From meiosis, what combinations of chromosomes and alleles are possible in the gametes
if we consider the effect of independent assortment? We will use the example of two pairs
of nonhomologous chromosomes. The first chromosome is colored orange and the second
blue for tracking. Sister and nonsister chromatids are marked, as are the alleles for the first
gene (a or A) and the second gene (b or B).
Also indicated are the two random possibilities for relative orientation of the chromosomes at first metaphase (top and bottom). Because of random segregation, these occur
with equal probability.
During anaphase I, the chromosomes from each homologous pair move to opposite poles.
During anaphase II, the chromatids from each chromosome move to opposite poles.
In this example of independent assortment with two pairs of chromosomes, gametes with
four equally possible combinations of chromosomes can result.
Gene Linkage with No Crossover
IFrom meiosis, what combinations of chromosomes and alleles are possible in the gametes
if we consider the effect of gene linkage (without crossing over)?
Here we have alleles of two linked genes. That is, the two genes are located on the same
chromosome. In our analysis, we will omit any crossing over between nonsister chromatids.
First, in anaphase I, the nonsister chromatids separate from each other.
In anaphase II, the sister chromatids separate from each other.
In this example in which no crossing over occurs between the genes (called complete linkage) we see that only two kinds of gametes result.
These are called parental gametes or noncrossover gametes, because they correspond to
the same combination of linked genes as found in the parent cells. The two parental
gametes are formed in equal proportions in this case of complete linkage.
Gene Linkage with Crossover
From meiosis, what combinations of chromosomes and alleles are possible in the gametes
if we consider the effect of gene linkage with crossing over?
The alleles of two genes are linked on the same chromosome, and crossing over occurs
between nonsister chromatids.
In anaphase I, the homologous chromosomes separate from each other. Notice, however,
that there are new pairings of alleles because a crossover occurred earlier in prophase I. In
anaphase II, the sister chromatids separate.
Four kinds of gametes result. The AB and ab gametes are called parental gametes because
they represent the same gene combinations as in the parent cell. The other two gametes are
called crossover gametes or recombinant gametes because they correspond to new combinations of genes not found in the parent cell.
Linkage Ratios
Complete linkage is unusual; usually some amount of crossing over between genes on
nonsister chromatids occurs. However, it is useful to consider the theoretical limit of complete linkage as a benchmark against which crossing over can be compared. In the case of
complete linkage, a characteristic F2 phenotypic ratio results.
We illustrate complete linkage using two traits in Drosophila that are linked on a single
chromosome: recessive mutations for brown eyes, bw, and heavy wing veins, hv, (compared with dominant wild-type alleles for red eyes, bw+, and thin wing veins, hv+).
The P1, F1, and F2 generations are illustrated here. The F2 phenotypic ratio of 1:2:1 for
brown-thin: red-thin: red-heavy is characteristic of complete linkage.
CONCLUSION
Genes on different chromosomes follow the law of independent assortment during meiosis. However, when genes are located on the same chromosome, a reciprocal exchange of
chromosome segments between homologous chromosomes may occur. This process,
called crossing over, results in the formation of recombinant gametes containing new combinations of alleles not found in the parent cell, thereby contributing to genetic variability
in the gametes. Genes that are completely linked never undergo crossing over and can
exhibit a unique phenotypic ratio in the F2 generation.
YOU
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SHOULD NOW BE ABLE TO
Explain the difference between a parental and recombinant gamete.
Compare the effects of gene linkage with or without crossing over on gamete formation.
Explain the basis for the phenotypic F2 ratio exhibited by genes that are completely linked.
KEY TERMS
complete linkage
crossing over
linkage
parental gametes
recombinant gametes