LESSON 6: Chromosomal Basis of Inheritance

• heterogametic sex - sex with 2 different sex chromosomes
• homogametic sex - sex with two of the same sex chromosomes
• X chromosome - contains more than 1,500 genes
• Y chromosome - has 231 protein-encoding genes
• in meiosis in a male, the X and Y chromosomes act as if they are a pair of homologs
• Y chromosome - has distinctive overall structure with a short arm and a long arm

  • has pseudoautosomal regions, called PAR1 and PAR2 at both tips, comprising only 5% of the chromosome
  • 63 pseudoautosomal genes are so-called because they have counterparts on X chromosome and can cross over with them
• pseudoautosomal genes - encode a variety of proteins that function in both sexes, participating in or controlling such activities

  • bone growth
  • cell growth
  • immunity
  • signal transduction
  • synthesis of hormones and receptors
  • fertility
  • energy metabolism
• male-specific region (MSY) - large central area comprising 955 of the Y chromosome

  • lies between the two pseudoautosomal regions and consists of three classes of DNA sequence
• sex-determining region Y/testis-determining factor - stimulates male development by sending signals to undifferentiated gonads to develop into a testis
• sustentacular cell - secrete anti-Mullerian hormone, which stopes the development of female structures
• interstitial cells - secrete testosterone that stimulates development of male internal structure
• gonadal sex - undifferentiated structure begins to develop as testis or ovary

  • timing: 6 weeks after fertilization
• phenotypic sex - development of external and internal reproductive structures continues as male or female in response to hormones

  • timing: 8 weeks after fertilization, puberty
• gender identity - strong feelings of being male or female develop

  • timing: from childhood, possibly earlier
• sexual orientation - attraction to same or opposite sex

  • from childhood
• Y-linked - genes carried on Y chromosome
• X-linked - genes carried on X chromosome
• In females, two copies are required for expression of a recessive allele and one copy for a dominant allele
• In males, a single copy of X-linked allele causes expression of the trait or illness because there is no copy of the gene on a second X chromosome to mask the other's effect
• X-linked recessive trait is always expressed in the male
• a male who inherits the allele is usually more severely affected because he has no other allele to offset it
• X-linked dominant trait is expressed in female in one copy