BIOLOGY Volume 3 - A Guide to General Biology - 2004

24. VARIATION AND GENETICS

24.3. Linkage

All the scenarios and Examples discussed in this chapter so far have dealt with the inheritance of genes located on different Chromosomes. According to cytological studies, all human somatic Cells contain 46 chromosomes. Since humans possess thousands of different traits—such as Blood Groups, eye color, and The ability to secrete Insulin—each chromosome must carry A large number of genes.

Genes located on the same chromosome are called linked genes. All the genes on a single chromosome form a linkage group; they typically segregate into the same gamete and are inherited together. Consequently, genes belonging to the same linkage group usually do not conform to Mendel's law of independent assortment. Therefore, in a dihybrid cross, these genes do not yield the expected 9:3:3:1 phenotypic ratio. Instead, they produce A wide variety of ratios that can now be readily explained using the principles discovered by Mendel. (It is worth emphasizing once again that Mendel was fortunate in choosing to study traits whose genes are localized on different chromosomes.) In Drosophila, the genes controlling body color and wing length have the following allelomorphs (phenotypic traits determined by different alleles): gray body vs. black body, and long wings vs. vestigial (short) wings. Gray body and long wings are dominant. When a homozygous gray-bodied, long-winged fly is crossed with a homozygous black-bodied, vestigial-winged fly, the expected phenotypic ratio in F₂ is 9:3:3:1. This would indicate typical Mendelian inheritance in a dihybrid cross, resulting from the random assortment of the body color and wing length genes located on different, non-homologous chromosomes. However, instead of this, the F₂ generation predominantly showed parental phenotypes in a ratio of approximately 3:1. This can be explained by assuming that the genes for body color and wing length are localized on the same chromosome—that is, they are linked (Fig. 24.8).

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Fig. 24.8. Genetic explanation of the 3:1 F₂ phenotypic ratio As a result of linkage.

In practice, however, a 3:1 ratio is never truly achieved, and all four phenotypes inevitably appear in F₂. This is because complete linkage is extremely rare. In most breeding experiments involving linkage, In addition to flies with parental phenotypes, individuals with new combinations of traits are also observed. These novel phenotypes are termed recombinants. All of this leads to the following definition of linkage: two or more genes are said to be linked if offspring with novel Gene combinations (recombinants) occur less frequently than parental phenotypes.

The events that led the American researcher Thomas Morgan to discover linkage can be illustrated by one of his experiments with Drosophila, in which he predicted the outcome of a testcross between a gray, long-winged heterozygote (from the F₁ Generation of the cross shown in Fig. 24.8) and a recessive homozygote with a black body and vestigial wings. Two possible outcomes were proposed:

1. If the two pairs of alleles determining gray or black body color and long or vestigial wings lie on different chromosomes (i.e., are unlinked), they should assort independently, yielding the following phenotypic ratio:

1 gray body, long wings : 1 gray body, vestigial wings;

1 black body, long wings : 1 black body, vestigial wings.

2. If the alleles determining body color and wing length belong to the same chromosome pair (i.e., are linked), the phenotypic ratio will be different:

1 gray body, long wings : 1 black body, vestigial wings.

An explanation of these predictions is presented in Fig. 24.9.

Fig. 24.9. Genetic explanation of Morgan's predictions.

Morgan performed this testcross several times and never obtained either of the predicted outcomes. Each time, he observed the following results:

41.5% — gray body, long wings

41.5% — black body, vestigial wings

8.5% — gray body, vestigial wings

8.5% — black body, long wings

Based on these results, Morgan postulated that:

1) the genes under study are localized on chromosomes;

2) both genes reside on the same chromosome, i.e., they are linked;

3) the alleles of each gene are located in homologous chromosomes;

4) during Meiosis, an exchange of alleles occurs between homologous chromosomes.

The appearance of recombinant allele combinations in 17% of the offspring was explained based on point 4. This phenomenon was named Crossing-over.

24.7. A homozygous plant with purple flowers and a short stem was crossed with a homozygous plant with red flowers and a long stem; the F₁ hybrids had purple flowers and a short stem. When the F₁ plants were test-crossed with a double homozygote for the recessive genes, the following offspring were obtained:

52 with purple flowers and a short stem

47 with purple flowers and a long stem

49 with red flowers and a short stem

45 with red flowers and a long stem

Provide a full explanation of these results.

24.3.1. Crossing-over and Recombination Frequency

In 1909, the Belgian cytologist Janssens observed The formation of chiasmata during prophase I of meiosis (section 23.4). The GENETIC BASIS OF this process was elucidated by Morgan, who suggested that crossing-over (the exchange of alleles) results from the breakage and recombination of homologous chromosomes during chiasma formation. Subsequently, comparing microscopic data with ratios of recombinant phenotypes confirmed that the EXCHANGE OF GENETIC material occurs between virtually all homologous chromosomes. Alleles comprising a linkage group in parental individuals separate and form new combinations that end up in Gametes—a process termed genetic recombination. Offspring derived from such gametes with "novel" allele combinations are called recombinants or crossovers. Thus, crossing-over represents a major source of the genetic variation observed in populations.

To illustrate THE PRINCIPLE OF crossing-over, we can examine The behavior of a pair of homologous chromosomes in Drosophila carrying alleles for grey body color and long wings (both alleles dominant) and black body color and vestigial wings (both alleles recessive) during chiasma formation. Crossing a heterozygous grey, long-winged male with a homozygous black, vestigial-winged female yielded heterozygous offspring with grey bodies and long wings in F₁ (Fig. 24.10).

Fig. 24.10. Genetic explanation of crossing-over and the appearance of recombinant genotypes. By counting the number of individuals exhibiting recombination (x) and the total number of individuals (y), the recombination frequency can be calculated using the formula:

Recombination frequency (%) = x/y x 100

Test-crossing the flies from the F₁ generation with homozygotes for two recessive genes yielded the following results:

As these results show, the genes determining body color and wing length are linked. (Recall that if these genes were located on different chromosomes and therefore assorted independently, crossing an F₁ heterozygote with a double-recessive homozygote would produce a phenotypic ratio of 1:1:1:1.) From the values given, the recombination frequency for the body color and wing length genes can be calculated.

Recombination frequency is calculated using the formula

In our example, the recombination frequency is

This value corresponds to the number of recombination events occurring during gamete formation. One of Morgan's students, A. H. Sturtevant, proposed that recombination frequencies indicate a linear arrangement of genes along the chromosome. An even more important hypothesis put forward by Sturtevant was that recombination frequency reflects the relative positions of genes on a chromosome: the farther apart linked genes are, the greater the probability that crossing-over will occur between them—that is, the higher the crossover frequency (Fig. 24.11).

Fig. 24.11. Three gene loci—A, B, and C—are located on the same chromosome. The probability of crossing-over and Separation is higher for genes B and C than for genes A and B, because crossover frequency depends on the distance between the genes.

24.8. The diagram below illustrates the loci of 12 alleles located on a pair of chromosomes, showing their relative distances from the centromere.

a) What are the chromosomes shown here called?

b) Between which two loci is Crossing Over most likely to occur?