Sex influenced inheritance indicates that an allele

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Sex-influenced inheritance indicates that an allele is dominant in one sex, but recessive in the other. An example in humans in male-pattern baldness. The trait is dominant in males but recessive in females (Figure 4.15, 4.16). Sex-influenced traits do not need to be on the sex chromosomes. A sex-limited trait is expressed in only one of the sexes. Examples are breast development in human females, beard growth in human males, and hen-feathering in chickens (Figure 4.17). The expression of these traits is determined by sex hormones. DHT, a molecular relative of testosterone, is produced in males in much higher amounts than seen in females, and causes baldness. Mutations that cause a loss of function in an essential gene result in a lethal phenotype. Essential genes are those that if lost to the individual produce a lethal phenotype. Lethal alleles are those that have potential to cause death to the organism. Nonessential genes are not absolutely needed for survival, but probably provide a benefit to the organism. Loss of function mutations in essential genes cause death, while loss of function mutations in nonessential genes will not cause death. Lethal alleles may kill an organism at a very early age (during development) or later in life. The age of onset refers to when the symptoms appear. Lethal alleles that only have an effect on the organism under certain environmental conditions are called conditional lethal alleles. If the environmental condition is temperature, these are called temperature- sensitive (ts) lethals. A ts lethal frequently encodes a protein that does not function correctly at a given temperature. Lethal alleles that do not affect all members of the population equally are called semilethal alleles.
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8 The analysis of lethal alleles may appear to deviate from Mendelian patterns because the allele causes death at an early age and the homozygous individual is not apparent in the population. The Manx cat carries a dominant mutation that is homozygous lethal, skewing the results of crosses (Figure 4.18). Single genes have pleiotropic effects. Multiple effects of a single gene on the phenotype of an organism is called pleiotropy. Pleiotropy occurs for several reasons including the following: The expression of a single gene can affect cell function in more than one way. For example, a defect in a microtubule protein may affect cell division and cell movement. A gene may be expressed in different cell types in a multicellular organism. A gene may be expressed at different stages of development. As an example of a pleiotropic mutation, consider cystic fibrosis, which is a recessive human disorder. The gene for cystic fibrosis encodes a protein called the cystic fibrosis transmembrane conductance regulator (CFTR), which regulates ionic balance by allowing the transport of chloride ions (Cl –) across epithelial-cell membranes.
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