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This ratio indicates that the genes were distributed on the gametes independently.




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There are two types of genes, according to their distribution on chromosomes:

1. Free or independent genes:

These genes are carried on different chromosomes and are distributed independently on the gametes during meiosis. Thus the traits of the progeny follow Mendel's second law of the independent assortment of the genetic factors.

2. Linked genes:

These are different genes carried on the same chromosome and carried together during meiosis and gamete formation and don’t follow the rule of independent assortment but produces other genetic ratios. The transfer of these genes carried on the same chromosome from parents to offspring together as one unit and is known as LINKAGE and there are two kinds of linkage:

a) Complete linkage:

As what happened in the previous example, where the genes are carried on the same chromosome as one genetic unit from one generation to the other through the gametes. But this kind of linkage is not the normal case in all the genes. The linked genes do not stay always like this unless they are very close to each other on the chromosome.

b) Incomplete linkage:

The linked genes on the chromosome may separate from each other and move from one chromosome to its homologous. This incomplete linkage occurs when crossing over occurs.

Crossing Over:

It occurs during the first prophase of the meiotic division when pairing of the homologous chromosomes, a stage called tetrads, where each pair of homologous chromosomes appears as four chromatids. Each point of turning between the internal chromatids is known as chiasma, where exchange between the chromatids occurs along with its genes. When the pair of homologous chromosomes separate in the first anaphase, exchange or crossing of the genes occurs between the non homologous chromatids. When the centromere divides in the second meiotic anaphase, the chromatids of each chromosome separate.

 

The internal chromatids, where exchange of genes occurs are called the new chromosomes, or the new combinant chromosomes (as the have new combinations of genes). Whereas the chromatids where crossing did occur are called the parental chromosomes as they carry the same sequence of genes on the parental chromosomes. Thus, by the end of meiosis there will be gametes carrying the parental chromosomes, and others containing new chromosomes. Therefore, crossing over leads to incomplete linkage that results in a change in the genetic characteristics but in a limited ratio that depends on the distance between the genes on the chromosome. Sometimes, more than one crossing over occurs in the same chromosome. If crossing over occurs between two chromatids having the same alleles as in the case of homozygous dominant or homozygous recessive genes, no change will occur in the resulting ratio.



The steps of crossing over:


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