Recombination Frequency | MDCAT | Genetic Map Of Chromosome | NMDCAT 2021 скачать в хорошем качестве

Recombination Frequency | MDCAT | Genetic Map Of Chromosome | NMDCAT 2021

About This Video! CROSSING OVER Linked genes can be separated by crossing over. Closer the two gene loci, more strongly are their genes linked. The farther apart two genes lie, greater are chances of their separation through crossing over. Crossing over is an exchange of segments between non-sister chromatids of homologous chromosomes during meiosis.’ Let us visualize crossing over by considering only one pair of homologous chromosome . The homologous chromosomes pair up lengthwise, point to point and locus to locus. One homologue carries genes ‘A’ and ‘B \ the other homologue has ‘a’ with ‘b’. Chiasmata are formed at many places between non-sister chromatids of homologous chromosomes. Crossing over occurs at 4 strand stage between non-sister chromatids. It may take place at more than one place along a chromosome. Exchange of chromosome segments logically means exchange of DNA, i.e. genes or alleles. As alleles of non-sister chromatids are diferent, an exchange between their segments results in recombination of genes. Allele ‘b’ crosses over to homologue containing allele ‘A’; and allele ‘B’ comes on the homologue of ‘a’. Then homologous chromosomes separate by opening up chiasmata. The sister chromatids also separate from each other and each becomes an independent chromosome to move singly in each of the four haploid gametes. Four types of gametes are formed; two with parental combinations of linked genes, i.e. AB and ab, and two with recombination of genes, i.e. Ab and aB. If crossing over does not occur, only the two parental types of gametes are formed. Parental types of gametes produce parental types of ofspring, while recombination gametes produce recombinant types of ofspring. Cross Over or Recombination Frequency It is the proportion of recombinant types between two gene pairs as compared to the sum of all combinations. The recombination frequencies between two linked genes can be calculated by backcrossing the heterozygote to a homozygous double recessive The recombination frequency is directly proportional to the distance between the linked gene loci. Genes can be mapped on a chromosome on the basis of their recombination frequencies. If 1% of recombination frequency is equal to 1 unit map distance, the two linked genes A and B with a 20% recombination frequency must be 20 units apart. Crossing over produces genetic variations among ofspring. Genetic variations lead to tremendous variations in their traits. Variations provide raw material for evolution by letting them adapt successfully to the changing environment