Inheritance of the colour of body, and the length of wings in Drosophila insect:

When crossing occurred between a male homozygous fly with grey-coloured, and long-winged (GLGL) and a female fly homozygous with black colour, and vestigial wings (glgl), where:

(G): Represents the gene for the grey colour of the body that dominates (g) the gene for the black colour of the body.

(L): Represents the gene for the long wings that dominates (l) the gene for the vestigial wings.

F1 generation resulted will be heterozygous grey-coloured, and with long-winged (dihybrid) (GLgl).

Breeding the F1 generation produce grey-coloured and long-winged insects, and black-coloured and vestigial-winged insects in the ratio 3:1; thus the genes didn’t assort in the gametes according to the law of independent assortment (Mendel's second law) but were inherited as one pair of genes as if it is one genetic trait. This can be illustrated as follows:

Test cross for the dihybrid (the heterozygous Drosophila insect in two traits grey-coloured and long-winged (GLgl) supposing that there is a case of complete linkage). (the two pairs of genes are carried on the same pair of homologous chromosomes):

Produce grey-coloured and long-winged insects, and black-coloured and vestigial-winged insects in the ratio 1:1. This ratio looks like we are dealing with one pair of genes (one trait), because linked genes act as one gene. This can be illustrated as follows:

Grey-coloured and Black-coloured and

P1 long-winged insect vestigial-winged

(GLgl) X (glgl)

G 1

(GLgl) (glgl)

F1 Grey-coloured and Black-coloured and

long-winged insect vestigial-winged

1 : 1

Test cross for the dihybrid (the heterozygous Drosophila insect in two traits grey-coloured and long-winged (GLgl) supposing that there is a case of independent assortment (the two pairs of genes controlling the two characteristics are carried on two different pairs of homologous chromosomes):

Grey-coloured and Black-coloured and

P1 long-winged insect X vestigial-winged

GgLl ggll

G 1

GgLl Ggll ggLl ggll

F1 Grey-coloured Grey-coloured Black-coloured Black-coloured

And long-winged and vestigial-winged and long-winged and vestigial-winged

1 : 1 : 1 : 1

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

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:

1. 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.

2. 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:

1. Pairing of the homologous chromosomes in the prophase of the first meiotic division.

2. Crossing over and exchange of pieces between two non-homologous chromatids.

3. Separation of the chromosomes in the first anaphase of the meiotic division.

4. Separation of the chromatids into chromosomes in the second anaphase and its distribution in the gametes.

An example that illustrates the genetic ratios in the case of incomplete linkage (where crossing over occurs):

In Drosophila, when mating occurred between dihybrid grey long-winged female (heterozygous in the two characteristics) and black vestigial-winged male (homozygous in the two characteristics). This mating represents a test cross for the dihybrid individual. Small percent of the offspring carried recombinant characteristics are resulted, whereas, most of the offspring looks like parents as follows:

The offspring resulted:

1. Grey long-winged 41.5% parental individuals.

2. Black vestigial-winged 41.5% parental individuals.

3. Grey vestigial-winged 8.5% recombinant individuals.

4. Black long-winged 8.5% recombinant individuals.

Analysis of these results:

If independent assortment occurred, these individuals would have appeared in equal ratios: 1 : 1 : 1 : 1 , but the parental traits appeared in 83% of the offspring, and 17% had new traits due to crossing over, and thus the linkage is not completed. We can make use of this result in deducing a relative distance between the two linked genes, which is 17 units, and the rate of occurrence of crossing over equals double this number, that is 34%

The importance of crossing over:

1. Crossing over increases the chances of diversity in the traits among members of the same species, which helps in their survival, and evolution.

2. Crossing over is used in drawing maps that illustrate the distribution of the genes in what is known as chromosomal maps.

The chromosomal maps:

The American geneticist Morgan introduced a method to explain the results of linkage and crossing over and to draw maps that determine the location of genes on the chromosomes of some plants, and animals, and for some chromosomes in Man.

Ex.:

In the previous example of Drosophila, the percentage of crossing over between the black colour gene, and the vestigial wing gene was 17%. If another ratio of crossing over of 5% appeared between the black colour gene and the gene for purple colour of the eye, and both are located on the chromosome no. 2. Then the percentage of crossing over between the gene for the purple colour of eye, and the gene for the vestigial wing is 12%, and then it is possible to draw a map for its chromosome as follows:

Nowadays, genetic maps are drawn for the sequence of the nitrogenous nucleotides in DNA molecules.

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