1. \"What is crossing over? How is meiosis related to this phenomenon? 2. Two pl
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1. "What is crossing over? How is meiosis related to this phenomenon? 2. Two plants in a cross were each heterozygous for two gene pairs (AB/ab) whose loci are linked and 30 map units (mu) apart. (Recall that 1 mu is equal to 1 % recombination between two genes.) Assuming that crossing over occurs during the formation of both male and female gametes and that the A and B alleles are dominant, determine the phenotypic ratio of their offspring a. What gametes and in what proportions will the individual plants produce? b. What will be the phenotypic ratio of the offspring from these two plants (both AB/ab)? 3. * A female has the following genotype: + This female produces 100 meiotic tetrads. Of these, 68 show no crossover events. Of the remaining 32, 20 show a crossover between a and b, 10 show a crossover between b and c, and 2 show a double crossover between a and b and between b and c a. Assuming the order a-b-c and the allele arrangement shown above, what is the map distance between a and b loci? b. Assuming the order a-b-c and the allele arrangement shown above, what is the map distance between b and c loci?Explanation / Answer
Q1) CROSSING OVER:- Crosing over or Genetic Recombination is the mutual exchange of corresponding segments of the adjectent paternal and maternal chromatids of vthe synapsed homologous chromosomes in the pachyteneof meiosis I, producing new combinations of genes. The chromatids resulting from the interchange of segments are called as cross overs, or recombinants. Crossing over may be single, double, or multiple according to the number of points at which it occurs. A single crossing occurs only at one or single point between two sister chromatids. A double crossing over occurs at two points. It may be reciprocal or complementary. A multiple crossing over occurs at many points , but it is rare. The mechanism or the proces s of crossing over occurs by Breakage and Reunion theory og Darlington. It comprises of four steps: Synapsis, Tetrad formation, Exchange of chromatids and Disjunction.
Synapsis:- the homologous chromosome come to lie close together in pairs in the zygotene substage of the prophase of meiosis i. Their pairing is called synapsis.
Tetrad formation:- The chromatids of each synapsed chromosome slightly seperate and become visible in the pachytene substage of the prophase of meiosis. Crossing over occurs in the four- stranded or tetrad stage.
Exchange of chromatids:- Chromatids break at homologous sites mutually exchange small corresponding segments and rejoin.
Disjunction;- After the completion of crossing over the synaptic forces end and the homologous move apart
During meiosis, an outcome known as chromosomal crossing over or genetic recombination occurs as a part of recombination. During this process, a part of one chromosome is for a part of another chromosome, thereby producing unique chromosomal combinations that further divide into haploid daughter cells. This is how the crossing over is linked with the meiosis phenomenon
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