X-linked inheritance

• Locus - A particular position, point or location
• Loci - Plural of locus. More than one location.
• Gamete - Reproductive cell (e.g. sperm or egg).
• Synapsis: Homologous chromosomes pair, form a bivalent and cross over
• Crossing over - The exchange of genetic information between homolog (non-sister) chromatids during meiosis.
• Genes on the same chromosome can undergo independent assortment due to crossing over
• During crossing over in prophase I, alleles of two genes on a chromosome can get “recombined” if crossing over occurs between where they are located (their loci) along the chromosome
• When crossing over occurs outside the interval between genes, there is no recombination between the alleles of these genes.

  • The resulting chromatids are recombinant but nonrecombinant for A and B genes. Independent assortment did not occur
• Frequency of recombination between genes on a single chromosome depends on their physical distance from each other on the chromosome
• The closer the two genes, the less likely crossing over will occur
• Genes really close to each other rarely get recombined, and are said to be linked to one another
• Scientists can use the frequency of recombination across generations to map the relative locations of genes on a chromosome
• Mendel identified one (relatively uncommon) pattern of inheritance with his pea plants but there are many others
• Different inheritance patterns for a trait can be due to...

  • different degrees of dominance
  • genes carried in the sex chromosomes are transmitted differently in males and in females (x-linked genes)
• Different inheritance patterns for a trait can be due to...

  • genes that are close together on the same chromosome do not undergo independent assortment (linkage).
  • mitochondria and chloroplast DNA follow their own inheritance pattern
• • Sex chromosomes are paired, just like autosomes
  • Sex chromosomes go through the same process of meiosis that all the other chromosomes do
• In humans (and other sexually reproducing species) one sex typically has non-homologous sex chromosomes

  • Females have the homologous sex chromosomes
• Very little crossing over between X and Y because almost none of the genes in the X chromosome have counterparts in the Y chromosome
• The tips of the arms of the X and Y chromosomes share a small region of homology (red) so they can pair up during synapsis
• Regardless, when it comes to inheritance, sex chromosomes segregate and get independently assorted like any other inherited chromosome
• Red-green color blindness is a sex-linked, recessive trait, carried on the X-chromosome
• X-linked traits differ from Mendelian inheritance patterns because inheritance of recessive phenotypes is sex-specific
• X-linked recessive phenotypes are expressed more often in males because males have only one X chromosome