Variation

• How many pairs of chromosomes do humans have?

  23 pairs
• What do 22 of the chromosome pairs contain?

  Genes determining inherited characteristics
• What is the role of the sex chromosomes?

  They determine sex
• What are the sex chromosomes for males?

  X Y
• What are the sex chromosomes for females?

  X X
• What gametes does a woman produce?

  All X chromosomes
• What gametes does a man produce?

  Half X and half Y chromosomes
• What are the possible sex chromosomes in the offspring?

  XX or XY
• What is the average ratio of female to male offspring?

  1 to 1
• What does the ratio of offspring represent?

  Probabilities of male and female births
• Why might a couple have several offspring of the same sex?

  Due to probabilities in sex inheritance
• Where can you find questions on the inheritance of sex?

  In the revision workbook
• What is the process of sex inheritance in humans?

  • Males have X Y chromosomes
  • Females have X X chromosomes
  • Males produce gametes with X or Y
  • Females produce gametes with X only
  • Offspring can be XX (female) or XY (male)
  • Average ratio of female to male is 1 to 1