Linkage and pleiotropy

 A close association between two or more characters may result either due to linkage or pleiotropy or both. Pleiotropy refers to the control of two or more characters by a single gene.

A tight linkage between two loci can be confused with pleiotropy. The only way to distinguish between linkage and pleiotropy is to find out a crossover product between linked characters.

Inter-mating in segregating populations may break a tight linkage, but a huge population has to be raised to find out the crossover product. If a crossover product is not found in spite of repeated inter-mating, there seems to be the case of pleiotropy rather than linkage.

Significance of linkage in plant breeding

In plant breeding, the linkage has an impact in three principal ways.

(i) Effect on selection- Linkage between two or more loci controlling different desirable characters is advantageous for a plant breeder. Because desirable alleles will occur together more frequently in segregating populations than would be expected with independent assortment. Thus, the breeder can easily obtain individuals with desirable alleles for two characters. Linkage is undesirable when desirable and undesirable genes are linked together.

To obtain segregation with desirable alleles, the breeder has to grow larger populations than with unlinked loci.

(ii) Effect on genetic variance- The estimates of genetic variances for quantitative characters are greatly influenced by the presence of linkage. Linkage disequilibrium influences the gene action by way of causing bias in the estimates of genetic variances. Linkage in the repulsion phase in segregating population causes an upward bias in the estimates of dominance variance and downward bias in the estimates of additive genetic variance.

(iii) Effect on genetic correlation- Linkage has a great impact on the genetic correlation. The linked characters generally show high values of genetic correlation and also of co-heritability.

A linkage between genes controlling two different desirable characters will help in the simultaneous improvement of both characters. If there is no linkage between two desirable characters, the breeder has to combine such characters from two different sources.

Crossing over

Crossing over may be defined as an exchange of genetic material between non-sister chromatids of homologous chromosomes resulting in a new combination of genes. In other words, crossing over is the interchange of parts between non-sister chromatids of homologous chromosomes during meiotic prophase (pachytene). The term crossing over was first used by

Morgan and Cattell in 1912. A crossing over between linked genes allows their recombination during meiosis.

Features of crossing over

The main features of crossing over are as follows

• Crossing over takes place during meiotic prophase, i.e., during pachytene. Each pair of chromosomes has four chromatids at that time.

• Crossing over occurs between non-sister chromatids of homologous chromosomes (bivalent).

• It is universally accepted that crossing over takes place at four strand stage (tetrad).

• Each crossing over involves only two of the four chromatids of two homologous chromosomes. However, double or multiple crossing over may involve all four, three, or two of the four chromatids, which is very rare.

• Crossing over leads to re-combinations or new combinations between linked genes. Crossing over generally yields two recombinant types of crossover types and two parental types or non-crossover types.

• Crossing over generally leads to an exchange of equal segments or genes and recombination is always reciprocal. However, unequal crossing over has also been reported.

• The value of crossover or recombinants may vary from 0-50%.

• The frequency of recombinants can be worked out from the testcross progeny. It is expressed as the percentage ratio of recombinants to the total population (recombinants + parental types).

Cases of more than two-strand crossing over, somatic crossing over, sister strand crossing over, and unequal crossing over are also known. However, the frequency of such cases is extremely low, i.e. in fractions.

Difference between linkage and crossing over