Autosomal Recessive Disorders The unusual phenotype of a recessive disorder is determined by homozygosity for a recessive allele , and the unaffected phenotype is determined by the corresponding dominant allele. In Chapter 3 we saw that phenylketonuria PKU is a recessive phenotype. Figure The human achondroplasia pheno-type, illustrated by a family of five sisters and two brothers. Most members of the human population more
Figure The human achondroplasia pheno-type, illustrated by a family of five sisters and two brothers. A typical pedigree for a dominant disorder is shown in Figure The three panels at the bottom of more Figure Pedigree of a dominant phenotype determined by a dominant allele A. The genetic disorders of human beings can be dominant or recessive phenotypes and can be either autosomal or X-linked. The reason is a matter of probability: The inheritance and molecular genetics of albinism are integrated in Figure The pheno-type is determined by a dominant allele, which we can call D, that interferes with bone growth during development. Once again, notice that Mendelian ratios are not necessarily observed in families. When we know that both male and female phenotypic proportions are equal, we can assume that we are dealing with autosomal inheritance, not X-linked inheritance. Many more males than females show the phenotype under study. However, it is a late-onset disease, the symptoms generally not appearing until after the person has begun to have children. Testes may be present either in the labia or in the abdomen. Therefore, when the progeny of such matings are totaled, a 1: Albinism Figure is another rare condition that is inherited in a Mendelian manner as an autosomal recessive phenotype in many animals, including humans. Alexis, the son of the last czar of Russia, inherited the allele ultimately from Queen Victoria, who was the grandmother of his mother Alexandra. The four categories are discussed in the following sections. Figure Genetics and the molecular biology of albinism. Abnormal alleles can arise de novo by mutation. An example of a cousin marriage is shown in Figure Note that individuals II-1 andII-5 marry into the family; they are assumed more Such insight holds out hope for a better understanding of the physiology of this condition and, ultimately, a therapy. If the recessive allele is very rare, almost all individuals showing the phenotype are males. Autosomal Recessive Disorders The unusual phenotype of a recessive disorder is determined by homozygosity for a recessive allele , and the unaffected phenotype is determined by the corresponding dominant allele. Figure Pedigree of a rare recessive phenotype determined by a recessive allelea.
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