The gene linkage. The Morgan’s rule

We may conclude from the principles of genetic analysis that independent trait combinations may occur only if genes responsible for such traits are on different chromosome pairs. Consequently, every organism has limited trait group numbers for independent assortment. This number is limited by chromosome number. On the other side, it is evident that organism’s traits are very numerous, but the chromosome number is limited and small in compare to them.

So we may conclude that each chromosome has many genes. If it is correct, we may state that third Mendel’s rule (of independent assortment) related only to chromosome assortment . Studying traits combinations according to the third rule, Morgan discovered that in some cases there were no new combinations between genes. That means it was a full gene linkage. He obtained the ratio 1:1. In some other cases, he obtained ratios different from classical Mendel’s ratio. So he suggested calling this gene heredity, limiting independent assortment, as gene linkage.

The studies of Morgan’s researching group showed that there is regular gene exchange between homologous chromosomes. The process of gene exchange is called crossing over. It occurs in meiosis during reproductive cell formation. It provides combinations of genes, which are localized in one chromosome. The cells of animals, plants and bacteria have crossing over. The exceptions are drosophila males and silkworms females.

Crossing over provides gene recombination and by this it increases evolutionary role of combinative diversity. We may find crossing over analyzing postbreeding traits assortment. When genes are in different chromosomes, we may write diheterozygote genotype as

When genes are in a same pair of homologues chromosomes, we may write genotype as

The gametes, which were subject to crossing over, are called crossed over gametes. So those which weren’t in crossing over are called non crossed over gametes (pic 7.5). AB/ab = AB and ab (non crossed over gametes) + Ab and aB (crossed over gametes)

Ріс. 7.5. The scheme of two pair allelic genes splitting, which are localized in one chromosome pair: 1 - without crossing over, 2 - with crossing over in meiosis (by K. Shrem. 1965)

Accordingly, organisms, from crossed over gametes, are called crossed over organisms. And organisms, from non crossed over gametes, are called non crossed over or non recombinant organisms. We can confirm previous statements in Morgan’s classic experiment demonstrating the chromosomal basis of gene linkage in Drosophila. He examined such traits as body color and wing length, which are localized on one chromosome (pic 7.6). You may see results of such breeding on a picture 7.6.

Pic. 7.6. The herediting of linked traits in Drosophila (Th. Morgan’s experiment)

Assumed all of this T. Morgan formulated the thesis: genes, which are located in one chromosome, are linked, and this linkage as strong as close they are to each other. This rule was named Morgan’s rule, after Tomas Hunt Morgan. It is hard to study gene linkage in a human. But anyway, we may list some examples of human gene linkage.

Gene linkage of А, В, C, D/DR locuses of HLA system responsible for histocompatibility antigen synthesis in 6th chromosome. Gene linkage of A BO blood group genes and genes of nail defects in one chromosome. Gene linkage of Rh- factor gene and gene responsible for oval erythrocyte shape. Gene linkage in 3rd chromosome locus of Lutheran blood group and locus which has a genes responsible for A and В antigens excretion with saliva. Gene linkage of polydactilia genes and cataract genes. X-chromosome gene linkage of hemophilia genes and colour-blindness genes; and also colour-blindness genes and muscular Duchene dystrophy.

It was suggested that distance between genes are related with crossing over percentage between them. One unit of gene distance was defined as 1% of crossing over between genes and was called one centimorgan, after Thomas Hunt Morgan. To measure gene distance in testcross we may use the following formula.

Here X is a distance between genes, a is a number of individuals in first crossed over group, b is a number of individuals of second crossed over group, n is total hybrids number, 100 is a coefficient for percentage measurement.