Chapter 13 Mendel and the Gene

Genes on non sex chromosomes are said to show

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Genes on non-sex chromosomes are said to show autosomal inheritance. The discovery of X-linked inheritance convinced most biologists what the chromosome theory of inheritance was correct. Extending Mendel's Rules Mendel analyzed the simplest possible genetic system. The traits he was studying were not sex-linked. Linkage: What Happens When Genes Are Located on the Same Chromosome? The physical association among genes on the same chromosome is called linkage. Linkage and sex- linkage are different in meaning. If two or more genes are linked, it means that they are located on the same chromosome. If a single gene is sex-linked, it is located on a sex chromosome. Linked genes are physically part of the same chromosome, it is logical to predict that they should always be transmitted together during gamete formation. Linked genes should violate the principle of independent assortment.
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Independent assortment is observed when genes are on different chromosomes, because the alleles of unlinked genes segregate to gametes independently of one another during meiosis I. But when genes are on the same chromosome, their alleles are carried to gametes together. Most of the male Drosophila carried an X chromosome with one of the two combos of alleles found in their mothers: X wY or X wy . The white and yellow alleles did not segregate independently. A small percentage of males had novel phenotypes and genotypes: X wy and X WY . Referred to as recombinant because combo of alleles on their X chromosome was different from the combinations of alleles present in the parental generation. Morgan proposed that gametes with new, recombinant genotypes were generated when crossing over occurred during prophase of meiosis I in the females. A genetic map is a diagram showing the relative positions of genes along a particular chromosome. Linked genes are inherited together unless crossing over occurs between them. When crossing over takes place, genetic recombination occurs. Do Heterozygotes Always Have a Dominant or Recessive Phenotype? The terms dominant and recessive describe which phenotype is observed when two different alleles of a gene occur in the same individual. Not all combos of alleles produce a completely dominant or recessive phenotype, however. The actual phenotypes observed in heterozygous individuals conflict with the genotype and phenotype ratios that Mendel observed. In certain flowers, for example, a pure line of purple flowers mated with a pure line of white flowers produces lavender flower offspring. This process is known as incomplete dominance. When incomplete dominance occurs, heterozygotes have an intermediate phenotype. Dominance is not necessarily an all-or-none phenomenon. Codominance occurs when heterozygotes have the phenotype associated with each individual allele. I A I A is blood type A, and I B I B is blood type B. I A I B is blood type AB. The alleles are codominant.
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