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Download Mendel`s Law of Segregation “The two members of a gene pair
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Mendel’s Law of Segregation “The two members of a gene pair segregate from each other into the gametes; so half the gametes carry one member of the pair and the other half of the gametes carry the other member of the pair”. - Allele pairs separate during the formation of gametes and randomly unite at fertilisation. He bred; pure tall and pure short pea plants to generate the tall F1 progeny; he then crossed tall progeny with themselves and found that the offspring were 3 tall and 1 short. This led Mendel to conclude that for short plants to arise from two tall plants – the tall and short factors must separate sometime between gamete production and fertilisation. The molecular mechanism behind this is meiosis – special type of cell division/replication involved in sexual reproduction. In human reproduction; meiosis produces four daughter cells – all with different genetic information. A chromatid is one half of the two identical copies of a duplicated chromosome which are joined at the centromere. The centromere is a part of the chromosome that links sister chromatids and attaches to the spindle during division. Mendel’s Law of Independent Assortment “The assortment of the alleles of different traits during gamete formation is totally independent of their original combinations in the parents”. - The presence of one allele does not depend on what other alleles are present in the oocyte r spermatozoa. Mendel noticed that the height of the plant, colours of the flowers or shape of the seeds had no impact on one another which led him to conclude that different traits are inherited independently from one another. This is found to be true for genes that are on different chromosomes, however, genes on the same chromosome generally do not assort independently from one another. The molecular principle behind this is also meiosis. Mendel’s Law of Dominance; “When two unlike factors responsible for a single character are present in the one individual, one factor can mask the expression of the other”. - The phenotype of one allele dominates over the other. Mendel crossed pure tall plants with pure short plants – all off the F1 progeny were tall. When factors are mixed, the tall factor dominates over the short factor. The molecular mechanism behind this is the fact that genes code for proteins. Each gene/allele encodes a protein which is transcribed into mRNA and translate into protein. Two different alleles at the same locus are the heterozygous genotype and produce two different protein products. This different protein expression leads to the development of different phenotypes and the simplest explanation is that one product is nonfunctional.