Recombination Among Linked Genes Help
When two or more genes reside on the same chromosome, they are said to be linked. They may be linked together on one of the autosomes or connected together on the sex chromosome. Genes on different chromosomes are distributed into gametes independently of one another (Mendel's Law of Independent Assortment). Genes on the same chromosome, however, tend to stay together during the formation of gametes. Thus, the results of testcrossing dihybrid individuals will yield different results, depending upon whether the genes are linked or on different chromosomes.
EXAMPLE 6.1 Genes on different chromosomes assort independently during meiosis, giving a 1 : 1 : 1 : 1 testcross ratio.
EXAMPLE 6.2 Linked genes do not assort independently, but tend to stay together in the same combinations as they were in the parents. Genes to the left of the slash line (/) are on one chromosome and those to the right are on the homologous chromosome. Very closely linked genes may not recombine in the formation of gametes.
Large deviations from a 1 : 1 : 1 : 1 ratio on the testcross progeny of a dihybrid could be used as evidence for linkage. Linked genes do not always stay together, however, because homologous nonsister chromatids may exchange segments of varying length with one another during meiotic prophase. Recall from Chapter 1 that homologous chromosomes pair with one another in a process called "synapsis" and that the points of genetic exchange, called "chiasmata," produce recombinant gametes through crossing over.
In preparation for meiosis, the DNA of each chromosome replicates, producing two genetically identical (barring mutation) sister chromatids. During prophase I, homologous chromosomes form pairs called synapases [Fig. 6-1 (a)] with the aid of proteins in the synaptonemal complex. Very large protein complexes, called recombination modules [about 90 nanometers (nm) in diameter], occur at intervals along the synaptonemal complex; each of these recombination modules is thought to function as a multienzyme "recombination machine" that affects synapsis and recombination. A nick is the removal of a phosphodiester bond between adjacent nucleotides in a DNA strand. Endonucleases in the recombination modules nick a single strand of each chromatid, allowing nonsister strands to be exchanged [Fig. 6-1(b)], and thus affecting the recombination of linked genes. A DNA polymerase may extend the exchanged strands, and an enzyme called DNA ligase repairs the nicks [Fig. 6-1 c)]. If the top chromatid strand is rotated by 180°, a cross-shaped structure called a chi (Χ) form can be seen under the microscope. This structure is also referred to as the Holliday model [Fig. 6-1(d)]. An endonuclease nicks the two previously uncut strands at tetranucleotide sequences 5'-(A/T)TT(G/C)-3'. Gaps and nicks are then repaired, creating four recombinant chromatids [Fig. 6-1(e)] which will segregate during the second meiotic division to be incorporated into different gametes. Note that if only the A and B loci are being studied in the progeny of dihybrid parents (AB/ab), two of the four possible gametes will retain the linkage relationships of the dihybrid parents (AB and ab) and are thus referred to as parental or noncrossover types; the two other gametes will be recombinant or crossover types (Ab and aB). Thus, each crossover or chiasma event is expected to produce four gametes (AB, Ab, aB, ab) with equal frequencies. However, if a crossover between the two genes under study does not occur in every meiosis, then among all of thegametes (both those with and without crossovers in this region) produced by a dihybrid individual, the frequency of noncrossover-type gametes will exceed that of crossover-type gametes.
The alleles of double heterozygotes (dihybrids) at two linked loci may appear in either of two positions relative to one another. If the two dominant (or wild-type) alleles are on one chromosome and the two recessives (or mutants) on the other (AB/ab), the linkage relationship is called coupling phase, or cis configuration. When the dominant allele of one locus and the recessive allele of the other occupy the same chromosome (Ab/aB), the relationship is termed repulsion phase, or trans configuration. Parental and recombinant gametes will be of different types, depending upon how these genes are linked in the parent.
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