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Chapter 13 Mendel and the Gene

Parental strains differ with respect to three genes

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Parental strains differ with respect to three genes that control kernel color- AABBCC produces dark-red kernels, aabbcc produces white kernels. 1. The three genes assort independently. When AaBbCc F 1 individuals self-fertilize, white F 2 individuals would occur at a frequency of 1/64 aabbcc. 1. The a,b, and c alleles don’t contribute to pigment production, but the A, B, and C alleles contribute to pigment production in an equal and additive way. Quantitative traits are produced by the independent actions of many genes. The transmission of quantitative traits is said to result from polygenic inheritance. In polygenic inheritance, each gene adds a small amount to the value of the phenotype. Applying Mendel's Rules to Humans
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To understand the transmission of human traits, investigators have to analyze human genotypes and phenotypes that already exist. They do so by constructing a pedigree, or family tree, of affected individuals. A pedigree records the genetic relationships among the individuals in a family along with each person's sex and phenotype with respect to the trait in question. Identifying Human Alleles as Recessive or Dominant If a phenotype is due to an autosomal recessive allele, then: o Individuals with the trait must be homozygous. o If the parents of an affected individual do not have the trait, then the parents are likely to be heterozygous for the trait. Heterozygous individuals who carry a recessive allele for an inherited disease are referred to as carriers of the disease. These individuals carry the allele and transmit it even though they do not exhibit signs of the disease. In general, a recessive phenotype should show up in offspring only when both parents have that recessive allele and pass it on to their offspring. When a trait is autosomal dominant, individuals who are homozygous or heterozygous for it must have the dominant phenotype. Huntington's disease is a degenerative brain disorder. Identifying Human Traits as Autosomal or Sex-Linked If a trait appears about equally often in males and females, it is likely that the trait is autosomal. But if males are much more likely to have the trait than females are, then the allele responsible is likely to be recessive and found on the X chromosome. Y-linked inheritance is rare because so few genes occur on the Y chromosome. Hemophilia is caused by a defect in an important blood-clotting factor Hemophiliacs are at a high risk of bleeding to death, because even minor injuries result in prolonged bleeding. The appearance of a n X- linked recessive trait skips a generation in a pedigree. A good indicator of an X-linked dominant trait is a pedigree in which an affected male has all affected daughters but no affected sons.
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