Sex Determination (HL IB Biology)

• Sex is determined by an entire chromosome pair (as opposed to most other characteristics that are just determined by one or a number of genes)
• Females have the sex chromosomes (pair 23 in humans) XX
• Males have the sex chromosomes (pair 23 in humans) XY
  • Note that the rule XX for females and XY for males applies to mammals, but not to all species
• All other chromosomes (pairs 1 - 22 in humans) are autosomes and have no influence on determining the sex of offspring
• Because only a father can pass on a Y chromosome, he is responsible for determining the sex of the child
  • Due to meiosis, half of his sperm cells will carry his X chromosome, half his Y chromosome
  • The chromosome carried by the sperm that fertilises the egg will determine the sex of the child
  • His daughters receive a copy of his X chromosome
  • His sons receive a copy of his Y chromosome

Sex determination in humans diagram

Sperm cells determine the sex of offspring

• The inheritance of sex can be shown using a genetic diagram (known as a Punnett square), with the X and Y chromosomes taking the place of the alleles usually written in the boxes

Sex determination Punnett square

Punnett square showing the inheritance of sex due to the combination of the X and Y chromosomes from each of the gametes

Genes carried by X and Y chromosomes

• The X chromosome is larger than the Y, and has its centromere more central than on the Y chromosome
  • Fewer genes are coded for on the Y chromosome as a result
    • The X carries around 16 × more genes than the Y chromosome
  • Non-sex phenotypic traits, including certain blood clotting factors, are coded for on the X chromosome but not on the Y
• The Y chromosome carries genes that code for male characteristics
• One of these genes is the SRY gene which is involved in
  • Development of testes in male embryos
  • Production of testosterone
• Females don't receive these genes, so instead, ovaries develop and female sex hormones are expressed

• Some genetic diseases in humans are sex-linked
• Inheritance of these diseases is different in males and females
  • Sex-linked genes are only present on one sex chromosome and not the other
  • This means the sex of an individual affects what alleles they pass on to their offspring through their gametes
• If the gene is on the X chromosome, males (XY) will only have one copy of the gene, whereas females (XX) will have two
• There are three phenotypes for females:
  • normal
  • carrier
  • has the disease,
• Males have only two phenotypes
  • norma
  • has the disease

• Haemophilia is a well known sex-linked disease
• There is a gene found on the X chromosome that codes for a protein called factor VIII. Factor VIII is needed to make blood clot
• There are two alleles for factor VIII
  • The dominant F allele which codes for normal factor VIII
  • The recessive f allele which results in a lack of factor VIII, meaning a person has haemophilia
• When a person possesses only the recessive allele f, they don’t produce factor VIII and their blood can't clot normally
• If males have an abnormal allele, f,  they will have the condition as they have only one copy of the gene
• Females can be heterozygous for the faulty gene and not suffer from the condition but act as a carrier
• This means that haemophilia is a potentially fatal genetic disease which affects males more than females

Monohybrid Punnett Square with Sex-linkage Table

Predicted ratio of phenotypes in offspring

1 female with normal blood clotting : 1 carrier female : 1 male with haemophilia : 1 male with normal blood clotting

Predicted ratio of genotypes in offspring: 1 X^FX^F : 1 X^FX^f : 1 X^FY : 1 X^fY