Crossing over

The exchange of segments between non-sister chromatids of homologous chromosomes during meiosis is called crossing over. Linked genes can be separated by crossing over. Two genes with closer gene loci arc strongly linked. There is greater chance of their separation through crossing over if the gene lie father apart.

I. Suppose, there is a single pair of homologous chromosome. The homologous chromosomes pair up lengthwise. This is point to point and locus to locus pairing. One homologue carries genes A and 13. Other homologue has a and b.

  1. Chiasmata are formed at many places between non-sister

chromatids. Crossing over occurs at 4 strand stage between non-sister chromatids. Crossing over can take place at more than one place. Exchange of segments of chromosome means exchange of DNA, i.e. genes or alleles. There are different alleles on the non-sister chromatids. Thus an exchange of segments causes recombination of genes. Allele b crosses over to homologue containing allele A. similarly allele B comes on the homologue of a.

  1. Then chiasmata open up during meiosis I and the homologous chromosomes are separated. The sister chromatids also separate from each other during meiosis II. Each chromatid becomes an independent chromosome.

4 Each chromatid (chromosomes) moves singly in each four haploid gametes. Four types of gametes are formed:

(a)Two gametes have parental combinations of linked genes, i.e. AB and ab,

(b) Two with recombination of genes, i.e. Ab and aB.

Only two parental types of gametes are formed without crossing over. Parental types of gametes produce parental types of offspring. But recombinant gametes produce recombinant offspring.

Examples

Linked genes are located on the same chromosomes. So they do not assort independently. They move together during meiosis and fertilization. Therefore, linked genes should not recombine by the assortments of alleles. But, in fact, recombination between link,: genes occurs.

(a)      The offspring of the testcross did not give 1:1:1:1 plienot). ratio. This ratio conies if the genes for these two characters m : present on different chromosomes and assorted independently

(b)      But if the two genes were completely linked then we shuck observe a 1:1 ratio. The parental phenotypes should be present among the offspring.


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The actual results are different from both these expectations. Most of the offspring had parental phenotypes. It suggests linkage between the two genes. But about 17% of the flies were recombinant. Thus the linkage appeared incomplete. Morgan proposed the mechanism of crossing over for these different results. Crossing over break“ linkage between the two genes. Crossing over occurs when homologous chromosomes are paired in synapsis during prophase of MelOSiS 1. The non- sister chromatids break at some points and exchange fragments. A crossover between chromatids of homologous chromosomes breaks linkages in the parental chromosomes. It forms recombinant. The recombinant bring together alleles in new combination. The later stages of meiosis distribute the recombinant chromosomes to gametes.

Crossing over

Mitotic crossing over

Mitotic crossing over is a rare type of genetic recombination. It may occur in some types of somatic cells during mitosis., Mitotic crossover may occur in organisms that do not have a cycle of sexual reproduction. It can only occur in diploid cells and a pair of chromosomes is required for the crossover to occur.

The mitotic crossover produce homozygous allele combinations in all heterozygous genes located on that chromosome arm. Thus when

a mitotic crossover occurs, genes that were previously recessive are expressed creating a new phenotype. Mitotic crossover occurs in some asexually reproducing fungi, and in human cancer cells.

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