Genetic Linkage

Genetic linkage describes the way in which two genes that are located close to each other on a chromosome are often inherited together. In 1905, William Bateson, Edith Rebecca Saunders, and Reginald C. Punnett noted that the traits for flower color and pollen shape in sweet pea plants appeared to be linked together. A few years later, in 1911, Thomas Hunt Morgan, who was studying heredity in fruit flies, noticed that the eye color of a fly was associated with the fly’s sex and hypothesized that the two traits were linked together.

These observations led to the concept of genetic linkage, which describes how two genes that are closely associated on the same chromosome are frequently inherited together. In fact, the closer two genes are to one another on a chromosome, the greater their chances are of being inherited together or linked. In contrast, genes located farther away from each other on the same chromosome are more likely to be separated during recombination, the process that recombines DNA during meiosis. The strength of linkage between two genes, therefore, depends upon the distance between the genes on the chromosome.

Definition of Genetic Linkage

Genetic linkage is the coupling of two genes’ patterns of inheritance because they are located on the same chromosome.

Genetic Linkage Example

Remember, genes often come in more than one allele. This means that it matters which of the two copies gets packaged in a gamete, since the two copies might be different alleles.

Imagine that chromosome-1 has a gene for hair color and a gene for eye color. Perhaps one of your copies of chromosome-1 contains alleles for red hair and red eyes, and your other copy has alleles for blue hair and blue eyes.

Since these genes are on the same chromosome, if a gamete gets the allele for red hair, it also gets the allele for red eyes. A gamete can’t get red hair and blue eyes because the genes are not distributed individually. Meiosis distributes one copy of each chromosome to a gamete rather than distributing one copy of each gene.

Complete Linkage
The genes closely located in the chromosome show complete linkage as they have no chance of separating by crossing over and are always transmitted together to the same gamete and the same offspring. Thus, the parental combination of traits is inherited as such by the young one.

Incomplete Linkage
The genes distantly located in the chromosome show incomplete linkage because they have a chance of separation by crossing over and of going into different gametes and offspring.

Types of Linkage:

Linkage is of two types, complete and incomplete.

1. Complete Linkage (Morgan, 1919):

The genes located on the same chromosome do not separate and are inherited together over the generations due to the absence of crossing over. Complete linkage allows the combination of parental traits to be inherited as such. It is rare but has been reported in male Drosophila and some other heterogametic organisms.

Example 1:

A red eyed normal winged (wild type) pure breeding female Drosophila is crossed to homozygous recessive purple eyed and vestigial winged male. The progeny or F₁ generation individuals are heterozygous red eyed and normal winged. When F₁ males are test crossed to homozygous recessive female (purple eyed and vestigial winged), only two types of individuals are produced— red eyed normal winged and purple eye vestigial winged in the ratio of 1 : 1 (parental phenotypes only). Similarly during inbreeding of F₁ individuals, recombinant types are absent. In practice, this 1: 1 test ratio is never achieved because total linkage is rare.

Example 2:

In Drosophila, genes of grey body and long wings are dominant over black body and vestigial (short) wings. If pure breeding grey bodied long winged Drosophila (GL/ GL) flies are crossed with black bodied vestigial winged flies (gl/gl), the F₂ shows a 3 : 1 ratio of parental phenotypes (3 grey body long winged and one black body vestigial winged).

This is explained by assuming that genes of body colour and wing length are found on the same chromosome and are completely linked.

2. Incomplete Linkage:

Genes present in the same chromosome have a tendency to separate due to crossing over and hence produce recombinant progeny besides the parental type. The number of recombinant individuals is usually less than the number expected in independent assortment. In independent assortment all the four types (two parental types and two recombinant types) are each 25%. In case of linkage, each of the two parental types is more than 25% while each of the recombinant types is less than 25%.

Example 1:

A red eyed normal winged or wild type dominant homozygous female Drosophila is crossed to homozygous recessive purple eyed and vestigial winged male. The progeny or F₁individuals are heterozygous red eyed and normal winged. F₁ female flies are test crossed with homozygous recessive males. It does not yield the ratio of 1: 1: 1: 1. Instead the ratio comes out to be 9: 1: 1: 8. This shows that the two genes did not segregate independently of each other. The data obtained by Bridges (1916) is as follows:

Only 9.3% recombinant types were observed which is quite different from 50% recom­binants in case of independent assortment. This shows that in the oocytes of the F₁, genera­tion only some of the chromatids undergo cross-over while the majority is preserved intact. This produces 90.7% parental types in the progeny.

Example 2:

In Sweet Pea (Lathyrus odoratus) blue flower colour (B) is dominant over red flower colour (b) while the trait of long pollen (L) is dominant over round pollen (1). A Sweet Pea plant heterozygous for both blue flower colour and long pollen (BbLl) was crossed with double recessive red flowered plant with round pollen (bbll). It is similar to test cross. In case the genes of the two traits are unlinked, the progeny should have four phenotypes (Blue Long, Blue Round, Red Long, and Red Round) in the ratio of 1: 1: 1: 1 (25% each). In case the two genes are completely linked the progeny should have both the parental types (Blue Long and Red Round) in the ratio of 1: 1(50% each).

Recombinants should not appear. However, in the above cross Bateson and Punnett (1906) found both parental and recombi­nant types but with different frequencies in the ratio of 7: 1: 1: 7. (7 + 7 Parental and 1 + 1 recombinant types).

Only 12.6% recombinant types were observed against the expected percentage of 50% in case of independent assortment. Therefore, the genes are linked but undergo recombination due to crossing over in some of the cases.

Example 3:

Morgan and his students have found that linked genes show varied recom­binations, some being more tightly linked than others, (i) In Drosophila, crossing of yellow bodied (Y) and white eyed (W) female with brown bodied (Y⁺) red eyed (W⁺) male produced F₁ to be brown bodied red eyed. On intercrossing of F₁ progeny, Morgan observed that the two genes did not segregate independently of each other and, therefore, the F₂ ratio deviated significantly from expected 9: 3: 3: 1 ratio. He found 98.7% to be parental and only 1.3% recombinants (Fig. 5.18). (ii) In a second cross in Drosophila between white eyed and miniature winged (wwmm) female with wild type or red eyed normal winged (w⁺w⁺m⁺m⁺) males, all the F₁ were found to be of wild type, i.e., red eyed and normal winged. An F₁female fly was then test crossed with white eyed and miniature winged male. 62.8% of the progeny was of parental types while 37.2% were recombinants .