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Extensions of Mendelian Inheritance

Extensions of Mendelian Inheritance - Extensions of...

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1 Extensions of Mendelian Inheritance Introduction Inheritance patterns that obey the law of segregation and the law of independent assortment are classified as examples of Mendelian inheritance. Simple Mendelian inheritance indicates that two alleles are involved, and one is dominant over the other. Inheritance Patterns of Single Genes General information Table 4.1 summarizes the inheritance patterns and molecular basis of the known types of Mendelian inheritance patterns. Recessive alleles often cause a reduction in the amount or function of the encoded protein. The prevalent alleles in a population are called wild-type alleles, which typically encode for normally expressed and functioning proteins. Different alleles of the same gene are called genetic polymorphisms (Figure 4.1). Mutant alleles are those that have been changed by a mutation and are less common. Mutant alleles often have altered expression (typically decreased) or abnormal protein function. Examples from human diseases are presented in Table 4.2 For an autosomal trait with a simple dominant/recessive relationship, the presence of a single recessive allele does not affect the phenotype of the heterozygote (Figure 4.2). The dominant allele produces enough protein to produce the phenotype. The dominant allele normally produces more protein than is needed. Therefore, the loss of protein production by the recessive allele is not enough to influence the phenotype. Homozygous dominant individuals have an excess of protein, heterozygous individuals have adequate protein, and homozygous recessive individuals have a deficiency of protein. Incomplete dominance occurs when two alleles produce an intermediate phenotype. Incomplete dominance is characterized by heterozygous individuals with a phenotype that is intermediate between the dominant and recessive phenotypes.
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2 Carl Correns was the first to describe incomplete dominance in his studies of the four o’clock plant, Mirabilis jalapa (Figure 4.3). Homozygous red-flowered plants that are crossed with homozygous white- flowered plants yield pink-flowered plants. When pink-flowered heterozygous plants are crossed, their offspring yield ! white- flowered offspring, " pink-flowered offspring, ! red-flowered offspring. This is a 1:2:1 phenotypic ratio, versus the predicted 3:1 ratio expected from a single-factor cross of heterozygous individuals. At the molecular level, 50% of the protein is not able to produce the dominant phenotype, and thus heterozygotes have a lighter color (pink). Whether a trait is dominant or incompletely dominant may depend on how closely we examine the trait in the individual; often, the heterozygote is not quite the same as the wild-type homozygote. With seed shape in peas, for example, RR and Rr genotypes produce round seeds
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