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BIOLOGY CH-14 (Tay-Sachs disease is fatarl; a dysfunctional enzyme causes…

- - - - The enzyme encooded by the lA aallele the A carbohydrate, whereas the enzyme encoded by the lB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither
      - Most genes exist in populations in more than two allelic forms
      - For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme that attaches A or B carbohydrates to red blood cells: lA,lB, and i.
      - Pleitropy
        
        Most genes have multiple phenotypic effects, a property called pleiotropy
        
        For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease.
        
        Some traits may be determined by two or more genes
        
        In epistasis, one gene affects the phenotype of the another due to interaction of their gene products
        
        In polygenic inheritance, multiple genes independently affect a single trait
        
        Epistasis
        
        One gene determines the pigment color (with alleles B for black and b for brown)
        
        Th other gene (with alleles E for color e for no color) determines whether the pigment will be deposited in the hair
        
        For example, in Labrador retrievers and many other mammals, coat color depends on two genes
        
        If heterozygous black labs (genotype BbEe) are mated, we might expect the dihybrid F2 ratio of 9:3:3:1
        
        In epistasis, expression of a gene at one locus alters the phenotypic expression of a gene at a second locus
        
        Epistatic interactions produce a variety of ratios, all of which are modified versions of 9:3:3:1
        
        However, a Punnett square shows that the phenotypic ratio will be 9 black to 3 chocolate to 4 yellow labs
        
        Polygenic Inheritance
        
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- - - - The true-breeding parents are referred to as the P generation (parental generation), and their hybrid offspring are the F1 generation.
        
        Advantages of using peas
        
        Large numbers of offspring
        
        Mating could be controlled; plants could be allowed to self-pollinate or could be crross-pollinated
        
        Short generation time
        
        The law of segregation
        
        Mendel discovered a ratio of about three purple flowers to one white flower in the F2 generation
        
        When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white
        
        When Mendel crossed contrasting, true-breeding white-and purple-flowered pea plants, all of the F1 hybrids were purple
        
        Mendel reasoned that the only purple flower factor was affecting flower color in the F1 hybrids
        
        Mendel called the purple flower color dominant trait and the white flower color a recessive trait
        
        The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation
        
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        When F1 individuals self-pollinate or cross-pollinate with other F1 hybrids, the F2 generation is produced