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Aslam o alikum • I am Hassam ur Rahman and I am teaching Fsc biology since 2014 .I am always trying to improve myself and provide best lecture to students.i am taking content for the lecture from authentic and relevant sources but human errors are possible . you are requested to please highlight the mistakes.My lectures are equally reliable for Fsc and mdcat students • #visiblescience #mdcatbiology #alevelbiology #neetbiology #fscbiology • My personal channel, I am sure it will change your life and very beneficial for you • https://youtube.com/@hassamurRahman?f... • My Facebook page link • https://www.facebook.com/hassam.urahm... • This lecture is about • DOMINANCE RELATIONS • Dominance is a physiological efect of an allele over its partner allele on the same gene • locus. There are four types of dominance relations among alleles, each indicating a • diferent style of their functional efect upon each other. • 1.Complete dominance 2.Incomplete dominance • 3.Cociominance 4. Over dominance • Complete Dominance • When one allele (R) is completely dominant over the other (r), presence of the recessive • allele is functionally hidden, so the heterozygote (Rr) has the same round phenotype • as (RR) homozygote. • The contrasting pairs of alleles for all the seven characters chosen by Mendel showed • complete dominance. After Mendel, further breeding experiments were carried out • on diferent plants and animals. Many novel phenotypes and phenotypic rat tios were • observed that could not be explained on the basis of complete dominance. • Incomplete Dominance • In 1899 Carl Correns was working on a lowering plant named 4 O’clock. When he • crossed a true breeding fed lowered plant with a true breeding white lowered 4 O’clock, • all the F] hybrids had pink lowers. This new phenotype had a shade intermediate • between those of the parents due to an intermediate amount of pigment in petals. • When Correns self-fertilized Fi pink, the F2 showed all three phenotypes of lowers’in • the ratio of 1 red : 2 pink : 1 white. Red was homozygous for red alleles, and white was • homozygous for white alleles. But when allele for red and allele for white were present • together in the same plant, neither of them masked the efect of other; rather these • alleles showed incomplete dominance in the form of pink colour. • When the phenotype of the heterozygote is intermediate between phenotypes of the • two homozygotes, it is called incomplete or partial dominance. • As there is no truly dominant allele, the usual capital and small letter distinction for • dominant and recessive trait is not necessary. Both the alleles are represented by the • same letter ‘R’ but are numbered diferently to distinguish white from red. Allele for • red is designated as R|, and the allele for white as R2 (Fig. 22.7). • Punnett square indicates that the phenotypic ratio is the same as the genotypic ratio. • There is absolutely no need of a test cross. Do these results make Mendel’s principles • invalid? The lower colour does show blending at phenotypic level in Fi, which is quite • contrary to what Mendel observed. But the re-appearance of red and white lowers in • F2 conirms that blending does not occur at genetic level. • Codominance • • The phenotype of heterozygote is distinct in quality from those of the two homozygotes. • It is not an intermediate quantitative expression like incomplete dominance. Each allele • of the gene pair is associated with a diferent substance, e.g., • • Codominance occurs when both the alleless express independently in heterozygote; • (A|A2) and form their respective products X and Y. The codominant heterozygote • would have both substances at the same time. • Diferent alleles of a gene that are both expressed in a heterozygous condition are • called codominant. • Over Dominance • This dominance relation is fascinating because the over dominant heterozygote exceeds • in quantity the phenotypic expression of both the homozygotes. In fruit ly Drosophila • the heterozygote (w+ / w) has more quantity of luorescent pigments in eyes than wild • (w+ / w+) or white eye (w / w) homozygotes.
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