lecture 20 - sex determination & genetic linkage

• What is a testcross and its purpose in genetics?

  • A testcross is a cross between an individual with a dominant phenotype and a homozygous recessive individual.
  • It is used to determine the unknown genotype of the dominant phenotype.
• Who started the field of Drosophila genetics?

  Thomas Hunt Morgan
• What did Thomas Hunt Morgan discover in 1910?

  He found a white-eyed male fly and confirmed the chromosome theory of inheritance.
• What is the significance of the SRY gene on the Y chromosome?

  It is responsible for the development of testes in an embryo.
• What are the anatomical signs of sex in humans and when do they begin to emerge?

  • Anatomical signs of sex begin to emerge when the embryo is 2 months old.
  • Gonads can develop into testes or ovaries depending on:

  1. Presence of a Y chromosome
  2. Active genes
• What is required for a biological female to express a recessive X-linked trait?

  She needs two copies of the allele (homozygous).
• Why are X-linked recessive disorders more common in XY individuals than in XX individuals?

  Because males need only one copy of the allele to express the trait.
• How can XX individuals be affected by X-linked recessive diseases?

  They must inherit a mutant maternal allele and a mutant paternal allele.
• What happens to one of the two X chromosomes in mammalian females during embryonic development?

  • One X chromosome is randomly inactivated.
  • The inactive X condenses into a Barr body.
  • Heterozygous females become mosaics for that character.

  
• What does it mean for a gene to be linked?

  • Linked genes are located close together on the same chromosome.
  • They tend to be inherited together during meiosis.
• How do linked genes affect the outcome of genetic crosses?

  • Linked genes can reduce the variety of combinations in offspring.
  • They can lead to non-Mendelian inheritance patterns.
• Sex-Linked Genes


  Genes located on either of the sex chromosomes
• WNT4


  Required gene for the development of female gonads, on chromosome 1 (an autosome)
• Sex is determined by the interaction of a network of gene products.