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  • autosomal dominant inheritance

    human genetic disease: Autosomal dominant inheritance
    A disease trait that is inherited in an autosomal dominant manner can occur in either sex and can be transmitted by either parent. It manifests itself in the heterozygote (designated Aa), who receives a mutant gene (designated a) from one parent and a normal (“wild-type”) gene (designated A) from the other. In such a case the...
  • combination of alleles

    ...alleles affecting the expression (phenotype) of a particular trait. If the paired alleles are the same, the organism is said to be homozygous for that trait; if they are different, the organism is heterozygous. A dominant allele will override the traits of a recessive allele in a heterozygous pairing. In some traits, however, alleles may be codominant—i.e., neither acts as...
  • contrasted with homozygote

    ...the two gametes (sex cells) that fuse during fertilization carry the same form of the gene for a specific trait, the organism is said to be homozygous for that trait. In a heterozygous organism, or heterozygote, the genes for a specific trait are different.
  • gene frequencies

    heredity (genetics): Population genetics
    ...AA = 0, Aa = 0, aa = 1 Intuitively it seems that, in the intermediate stages, there must be more-complex proportions, including some heterozygotes. One possible intermediate stage is as follows:AA = 0.30, Aa = 0.20, aa = 0.50 The allele frequencies at such an...
    heredity (genetics): Selection
    ...dominant alleles is relatively efficient, because these are by definition expressed in the phenotype. Selection against recessive alleles is less efficient, because these alleles are sheltered in heterozygotes. Even though populations under selection technically are not in Hardy-Weinberg equilibrium, the proportions of the formula can be used as an approximation to show the relative...
  • relationship to consanguinity

    consanguinity: Homozygosity and heterozygosity
    In genetics an allele that is carried at the same position in both of a pair of chromosomes is called homozygous. An allele may be rare in the general population, but, if the parent possesses it, it is transmitted from parent to child with the same probability as any common allele. Therefore, the chance of receiving a rare allele in the chromosomes derived from both mother and father—that...
  • structure of genes

    evolution: Measuring gene variability
    ...and finding out how many are variable and how variable each one is. One simple way of measuring the variability of a gene locus is to ascertain what proportion of the individuals in a population are heterozygotes at that locus. In a heterozygous individual the two genes for a trait, one received from the mother and the other from the father, are different. The proportion of heterozygotes in the...
    heredity (genetics): Discovery and rediscovery of Mendel’s laws
    ...instance, both having come from white-flowered parents (rr)—the plant is termed a homozygote. The union of gametes with different genes gives a hybrid plant, termed a heterozygote (Rr). Since the gene R, for purple, is dominant over r, for white, the F1 generation hybrids will show purple flowers. They are phenotypically...
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