Chromosomes and Human Inheritance

  1. The Chromosomal Basis of Inheritance–An Overview
    1. Genes and Their Chromosome Locations
      1. Genes are units of information about heritable traits.
      2. Alleles are slightly different molecular forms of the same gene, which are shuffled during meiosis.
    2. Autosomes and Sex Chromosomes
      1. Sex chromosomes (23) determine gender.
        1. Human females XX
        2. Males XY.
    3. Karyotype Analysis
      1. Chromosomes are visualized in a lab preparation called a karyotype.
      2. Each chromosome has distinct size, length, centromere location, and banding (staining) patterns.

  2. Sex Determination in Humans
    1. The Y chromosome carries a male-determining gene that leads to formation of male organs and tissues
      1. Primary male sexual characteristics (form in the embryo from 4-10 weeks): testes, vas deferens, glans penis, prostate gland, cowpers gland…)
      2. Secondary male sexual characteristics (develop at puberty in response to androgenic hormonal signals): Facial hair, deepened voice, pubic hair, production of semen…)
    2. Absence of the male chromosome in females results in formation of female organs and tissues.
      1. Primary sexual characteristics (form in the embryo from 4-10 weeks) (Ovaries, uterine tubes, uterus, vaginal canal, labia, mammary glands…)
      2. Secondary female sexual characteristics (develop at puberty in response to estrogenic hormonal signals): Adipose deposition in the breast tissue and hips, pubic hair, menarche, widening of hips…)
    3. The X chromosome obviously codes for sexual traits, but it also carries many genes for non-sexual traits.
    4. "Sex" and "gender" are concepts are merely semantic delineations. Some variations in genotype or phenotype can result in sexual ambiguity. These conditions are surprisingly common. Sometimes, only genetic screening can identify the condition.
  3. Autosomal Inheritance
    1. Autosomal Recessive Inheritance
      1. Galactosemia (the inability to metabolize lactose) is an example of autosomal recessive inheritance in which a single gene mutation prevents manufacture of an enzyme needed in the conversion pathway. Tay-Sachs is another example.
    2. Autosomal Dominant Inheritance
      1. Huntington disorder is serious degeneration of the nervous system with an onset from age 40 onward, by which time the gene has (usually) been passed to offspring unknowingly.
      2. Achondroplasia (dwarfism) is a benign abnormality that does not affect persons to the point that reproduction is impossible so the gene is passed on.
  4. X-Linked Inheritance
    1. X-Linked Recessive Inheritance
      1. A serious X-linked recessive condition is hemophilia A, (affects 1/7,000 males), which is the inability of the blood to clot because the genes do not code for the necessary clotting agent(s).
      2. Duchenne muscular dystrophy is a serious failure of the muscles with an early onset leading to death by the early twenties.
    2. X-Linked Dominant Inheritance
      1. In faulty enamel trait the enamel coating of the teeth fails to develop properly.
  5. Changes in Chromosome Number
    1. Categories and Mechanisms of Change
      1. Aneuploidy - Changes by the addition or loss of a single chromosome. (-somy)
      2. Euploidy - changes by the addition or loss of entire genomes. (-ploidy) (common in plants but fatal in humans)
      3. Nondisjunction at anaphase I or anaphase II frequently results in a change in chromosome number.

    2. Changes in the Number of Autosomes

      1. Down syndrome results from trisomy 21; 1 in 1,100 liveborns in North America are affected.
      2. Most children with Down syndrome show mental retardation, and 40 percent have heart defects.
      3. Down syndrome occurs more frequently in children born to women over age 35.
    3. Changes in the Number of Sex Chromosomes
      1. XO condition (Turner Syndrome)
        1. Turner syndrome involves females whose cells have only one X chromosome (designated XO).
        2. Affected individuals (1/2,500 to 10,000 girls) are infertile and have other phenotypic problems such as premature aging and shorter life expectancy.
        3. About 75 percent of the cases are due to nondisjunction in the father; furthermore, about 98 percent of all XO zygotes spontaneously abort.
      2. XXY Condition (Klinefelter Syndrome)
        1. Nondisjunction results in an extra X chromosome in the cells (XXY) of these affected males (1/500 to 2,000 liveborn males).
        2. About 67 percent of these result from nondisjunction in the mother, 33 percent in the father.
        3. Slight mental retardation, sterility, and body feminization are symptoms.
      3. XYY Condition (Supermale syndrome)
        1. The extra Y chromosome in these males (1/1,000) does not affect fertility, but they are taller than average and are slightly mentally retarded.
        2. Erroneous correlations have linked these persons with predisposition to crime.
    4. Other genetic changes that result in sexual ambiguity
      1. Point Mutations (next unit)
        1. SRY mutations
        2. TFS (androgen receptor deficiency)
        3. Alpha-5 reductase deficiency