17 Dihybrid & Sex-linkage

  • Created by: lee8444
  • Created on: 01-03-20 09:49

How dihybrid inheritance works

  • Mendel's experiment
  • Pea seed shape - round is dominant to wrinkled
  • Pea colour - yellow is dominant to green
  • Cross between one that always has round, yellow seeds (RRGG) and one that always has green, wrinkled seeds (rrgg)
  • F1 - offspring - all RrGg so all yellow and round
  • F1 offspring were then crossed with each other to give F2
  • 9:3:3:1 - round+yellow, round+green, wrinkled+yellow, wrinkled+green
  • Will always be some statistical error
  • Can use chi-squared to compare with expected results >0.05
  • Led to Mendel's law of random assortment
  • "Each member of a pair of alleles may combine randomly with either of another pair"
1 of 2

Sex-linkage - haemophilia

  • Any gene on the X or Y gene is sex-linked
  • The x chromosome is much longer than the Y chromosome
  • For a lot of the X chromosome, there is no equivalent on the Y chromosome
  • The characteristics controlled by recessive alleles will appear more frequently in males as there is no homologous portion on the Y chromosome that may have the dominant allele which would stop the expression of the recessive allele
  • Haemophilia is on the X chromosome which causes blood clots to form slowly which can lead to persistent internal bleeding - especially in joints - can be lethal
  • Only 1 in 20,000 in Europe
  • Nearly always males, as females need both copies of the recessive allele and females, would usually die during menstruation at puberty
  • One cause is a recessive allele that makes a faulty protein required for clotting
  • This protein can be produced by genetically modified organisms to give haemophiliacs near-normal lives
  • Males only obtain the Y chromosome from their father the X comes from their mother so males only inherit the disease through their mother
  • Affected fathers' daughters will always be carriers of the disease as they take the defective X chromsome
2 of 2


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Genetics, Populations, Evolution & Ecosystems resources »