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Genetic linkage and distances

Mendel’s work suggested that traits are inherited independently of each other. Morgan identified a 1:1 correspondence between a segregating trait and the X chromosome, suggesting that the random segregation of chromosomes was the physical basis of Mendel’s model. This also demonstrated that linked genes disrupt Mendel’s predicted outcomes. The fact that each chromosome can carry many linked genes explains how individuals can have many more traits than they have chromosomes. However, observations by researchers in Morgan’s laboratory suggested that alleles positioned on the same chromosome were not always inherited together. During meiosis, linked genes somehow became unlinked.

Homologous recombination

In 1909, Frans Janssen observed chiasmata—the point at which chromatids are in contact with each other and may exchange segments—prior to the first division of meiosis. He suggested that alleles become unlinked and chromosomes physically exchange segments. As chromosomes condensed and paired with their homologs, they appeared to interact at distinct points. Janssen suggested that these points corresponded to regions in which chromosome segments were exchanged. It is now known that the pairing and interaction between homologous chromosomes, known as synapsis, does more than simply organize the homologs for migration to separate daughter cells. When synapsed, homologous chromosomes undergo reciprocal physical exchanges at their arms in a process called homologous recombination    , or more simply, “crossing over.”

To better understand the type of experimental results that researchers were obtaining at this time, consider a heterozygous individual that inherited dominant maternal alleles for two genes on the same chromosome (such as AB ) and two recessive paternal alleles for those same genes (such as ab ). If the genes are linked, one would expect this individual to produce gametes that are either AB or ab with a 1:1 ratio. If the genes are unlinked, the individual should produce AB , Ab , aB , and ab gametes with equal frequencies, according to the Mendelian concept of independent assortment. Because they correspond to new allele combinations, the genotypes Ab and aB are nonparental types that result from homologous recombination during meiosis. Parental types are progeny that exhibit the same allelic combination as their parents. Morgan and his colleagues, however, found that when such heterozygous individuals were test crossed to a homozygous recessive parent ( AaBb × aabb ), both parental and nonparental cases occurred. For example, 950 offspring might be recovered that were either AaBb or aabb , but 50 offspring would also be obtained that were either Aabb or aaBb . These results suggested that linkage occurred most often, but a significant minority of offspring were the products of recombination.

Art connection

The illustration shows the possible inheritance patterns of linked and unlinked genes. The example used includes fruit fly body color and wing length. Fruit flies may have a dominant gray color (G) or a recessive black color (g). They may have dominant long wings (L) or recessive short wings (l). Three hypothetical inheritance patterns for a test cross between a heterozygous and a recessive fruit fly are shown, based on gene placement. The actual experimental results published by Thomas Hunt Morgan in 1912 are also shown. In the first hypothetical inheritance pattern in part a, the genes for the two characteristics are on different chromosomes. Independent assortment occurs so that the ratio of genotypes in the offspring  is 1 GgLl:1 ggll:1 Ggll:1 ggLl, and 50% of the offspring are nonparental types. In the second hypothetical inheritance pattern in part b, the genes are close together on the same chromosome so that no crossover occurs between them.  The ratio of genotypes is 1 GgLl:1 ggll, and none of the offspring are recombinant. In the third hypothetical inheritance pattern in part c, the genes are far apart on the same chromosome so that crossing over occurs 100% of the time. The ratio of genotypes is the same as for genes on two different chromosomes, and 50% of the offspring are recombinant, nonparental types. Part d shows that the number of offspring that Thomas Hunt Morgan actually observed was 965: 944: 206:185 (GgLl:ggll:Ggll:ggLl). Seventeen percent of the offspring were recombinant, indicating that the genes are on the same chromosome and crossing over occurs between them some of the time.
Inheritance patterns of unlinked and linked genes are shown. In (a), two genes are located on different chromosomes so independent assortment occurs during meiosis. The offspring have an equal chance of being the parental type (inheriting the same combination of traits as the parents) or a nonparental type (inheriting a different combination of traits than the parents). In (b), two genes are very close together on the same chromosome so that no crossing over occurs between them. The genes are therefore always inherited together and all of the offspring are the parental type. In (c), two genes are far apart on the chromosome such that crossing over occurs during every meiotic event. The recombination frequency will be the same as if the genes were on separate chromosomes. (d) The actual recombination frequency of fruit fly wing length and body color that Thomas Morgan observed in 1912 was 17 percent. A crossover frequency between 0 percent and 50 percent indicates that the genes are on the same chromosome and crossover occurs some of the time.

In a test cross for two characteristics such as the one shown here, can the predicted frequency of recombinant offspring be 60 percent? Why or why not?

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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