Chapter 13 Mendel and the Gene

I a i individuals have type a i b i individuals have

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I A i individuals have type A, I B i individuals have type B. Ii individuals have blood type O. A and B produce a codominant phenotype when paired with each other in heterozygotes, but both produce a dominant phenotype when paired with i. Dominance relationships vary among alleles. How Many Alleles and Phenotypes Exist? The existence of more than two alleles of the same gene is know as multiple allelism. The ABO blood group in humans is multiallelic because most populations have the I A, I B , and i alleles. When more than two distinct phenotypes are present in a population due to multiple allelism, the trait is polymorphic. Does Each Gene Affect Just One Trait?
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A gene that influences many traits, rather than just one trait, is said to be pleiotropic. The gene responsible for Marfan syndrome in humans, called FBN1 is a good example of pleiotropicity. Pleiotropy is common. In many cases a change in a single allele affects more than one trait. Bare Phenotypes Determined by Genes? Two aspects of the environment affect phenotypes 1. The individual's physical surroundings 2. The alleles present at other gene loci. The phenotypes produced by most genes and alleles are strongly affected by the individual's physical environment. Consequently, an individual's phenotype is often as much a product of the physical environment as it is a product of the genotype. Gene-by-environment interactions have a profound effect on how physicians treat people with the genetic disease phenylketonuria (PKU). Individuals with PKU lack an enzyme that helps convert the amino acid phenylalanine to the amino acid tyrosine. As a result, phenylalanine and a related molecule, phenylpyruvic acid, accumulate in their bodies. Through a simple change in their environment (their diet), individuals with a PKU genotype can have a normal phenotype. When these types of gene-by-gene interactions occur, the phenotype produced by an allele depends on the action of alleles of other genes. What About Traits Like Human Height and Intelligence? Mendel worked with discrete traits- characteristics that are qualitatively different from each other. Height and many other characteristics exhibit quantitative variation- meaning that individuals differ by degree- and are called quantitative traits. Quantitative traits are greatly influenced by the physical environment. Quantitative traits share a common characteristic: when the frequencies of different values observed in a population are plotted on a histogram or frequency distribution, they usually form a bell-shaped curve. If many genes each contribute a small amount to the value of a quantitative trait, then a continuous, bell- shaped distribution results for the population as a whole. Nilsson-Ehle's proposed hypothesis: 1.
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