14.2 - Human Chromosomes

• • There are 44 autosomes (pairs 1-22) and 2 sex chromosomes (23rd pair)
  • Chromosomes vary in size and the number of gene they contain
  • Chromosomes 21 and 22 are the smallest
• Human Chromosomes and Linked Genes

  • The types of genes contained on each chromosome are the same from human to human
  • two chromosomes with the same types of genes are homologous chromosomes
  • Genes contained on the same chromosome may be linked
  • They are usually inherited together
• Human Chromsomes and Linked Genes

  • Sex-Linked Genes
  • sex-linked traits tend to be passed from mom to son
  • sex-linked traits are visible mostly in males
  • ex. color-blindness, baldness, duchenne's muscular dystrophy, hemophilia
  • males have only one copy of the gene that is only on the X chromosome, so the trait shows up regardless of whether or not it's dominant
  • there's no homologous allele to mask the recessive allele
• Human Chromosome Disorders

  • Sometimes problems are caused by a problem with entire chromosomes, not just the genes contained on them
  • Nondisjunction
  • Homologous chromosomes don't separate properly during meiosis, so offspring has an extra or missing chromosome
  • Can also occur in sex chromosomes
  • Extra 23rd (XXY) = Klinefelter syndrome
  • Missing 23rd (X) = Turners syndrome
  • Effects can be severe or deadly
  • Trisomy
  • Extra (3rd) copy of an autosomal chromosome
  • Extra 21st chromosome = Down Syndrome