Gene Linkage

 GENE LINKAGE

According to the chromosome theory of inheritance, units of heredity called genes reside on the chromosomes. The number of genes in a cell is greater than the number of chromosomes. Human beings for example have 46 chromosomes and thousands of genes. Therefore, each chromosome has hundreds and thousands of genes. Such genes tend to inherit together exhibiting gene linkage.  Gene linkage is the phenomenon in which the genes on the same chromosome stay together and tend to inherit together. This phenomenon was first explained in 1910 by Thomas H. Morgan and his student, Alfred H. Sturtevant while working on Drosophila.

 

Linked genes

Genes that are located on the same chromosome and tend to be inherited together in genetic crosses are said to be linked genes. The linked genes are not free to undergo independent assortment. These genes can only segregate if crossing over occurs during the gametogenesis. The crossing-over event results in the reshuffling, or recombination, of the alleles between homologs. This results in the formation of recombinants. The frequency with which crossing over occurs between any two linked genes is generally proportional to the distance separating the respective loci along the chromosome.

 

Linkage group

A linkage group refers to all the genes located on a single chromosome that are inherited together as a unit. The number of linkage groups should correspond to the haploid number of chromosomes. In humans, the number of linkage groups corresponds to the number of chromosome pairs, which is 23 (or 24 in males, due to the X and Y chromosomes).

 

 

Autosomal linkage and Sex linkage

The gene linkage phenomenon occurs on both autosomes and sex chromosomes. When a group of genes are linked together on an autosome it is called autosomal linkage. When genes are linked on a sex chromosome, their linkage is called sex linkage. In human beings, the genes for sickle cell anemia, leukemia, and albinism are found on chromosome 11. Thus these genes are linked genes and the type of linkage is autosomal linkage. These genes tend to be inherited together in the offspring.

 

22.6.2 Detection of gene linkage

A dihybrid test cross (between two gene pairs) can detect gene linkage. In a dihybrid test cross, a heterozygous individual for two traits is crossed with a recessive parent for two traits. If only parental variety is produced then a tight linkage exists between the genes for the two traits. When both parental and recombinants (four phenotypic combinations) are produced in equal 1:1:1:1 ratio, then there would be no linkage between the genes. When this ratio is deviated i.e. more parental types and less recombinant types, this indicates incomplete or partial linkage.

 

T. H. Morgan performed a dihybrid test cross to see how the linkage between genes affects the inheritance of two different characters in Drosophila. In Drosophila, the normal shape of wings is dominant over the vestigial wing. Similarly, grey body color is dominant over black body color.

 

Character	Dominant trait	Recessive trait
Wing shape	Normal wing	Vestigial wing
Body color	Grey color	Black color

 

Morgan made a cross between the individual having a grey body and normal wings with another individual having a black body and vestigial wings, all the F1 progeny inherited grey bodies and normal wing phenotypes, When F1 flies were test crossed with their P1 recessive, following results were observed:

a.      Grey body and normal wings (parental type) = 965

b.     Black body and vestigial wings (parental types) =944

c.      Grey body and vestigial wings (recombinant types) =206

d.     Black body and normal wings (recombinant types) =185

From the above parental and recombinant ratio, Morgan concluded that the genes for body color and wing size are located on the same chromosome (linkage exists). However, a small number of recombinants indicated that occasionally this linkage breaks (due to crossing over).

 

Crossing over breaks gene linkage

Crossing over is an exchange of maternal and paternal chromatid parts between homologous chromosomes. This exchange of chromosomal segments occurs during the prophase of meiosis I. This recombination brings alleles together in new combinations, resulting in a variety of gametes. Crossing over results in the breaking of gene linkage and the formation of recombinants. The farther apart two genes are, the higher the probability that a crossover will occur and therefore the higher the recombination frequency.

Linkage and Mendel’s laws

Mendel's laws do not follow gene linkage. Gene linkage is an exception to Mendel’s second law (the Law of Independent Assortment). Mendel’s Law of Independent Assortment assumes that genes are located on different chromosomes or are far apart on the same chromosome, so they assort independently during meiosis. Gene linkage happens when genes are close together on the same chromosome, so they are often inherited together. This means they do not assort independently.