Syllabus Edition

First teaching 2023

First exams 2025

|

Sex Determination (SL IB Biology)

Revision Note

Emma

Author

Emma

Last updated

Sex Determination in Humans

  • 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

Sex chromosomes_1, downloadable IB Biology revision notes

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 squareInheritance of sex, IGCSE & GCSE Biology revision notes

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

Sex Linked Disorders: Haemophilia

  • 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

Examiner Tip

The expected notation when writing about sex linked alleles is to use upper case 'X' and 'Y' for the chromosome, next to superscript letters to represent the allele. For example

  • XfXf Homozygous female who has haemophilia or XFXf Heterozygous female who is a carrier
  • XfY Male who has haemophilia 

Worked example

The genetic diagram below shows how two parents with normal factor VIII can have offspring with haemophilia

Parental phenotypes: carrier female x normal male

Parental genotypes:      XFXf                              XFY

Parental gametes:      XF or Xf                        XF or Y

Monohybrid Punnett Square with Sex-linkage Table

Monohybrid Crosses_1, downloadable AS & A Level Biology revision notes

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 XFXF : 1 XFXf : 1 XFY : 1 XfY

Examiner Tip

Make sure to include all of your working out when constructing genetic diagrams. It is not enough just to complete a Punnett grid, you need to show that you have thought about the possible gametes that can be produced by each parent. Also, remember to state the phenotype as well as the genotype of the offspring that result from the cross. Read the questions carefully when answering sex-linked inheritance questions – is the question asking for a probability for all children or is it asking about a specific sex (males or females).

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Emma

Author: Emma

Expertise: Biology

Prior to working at SME, Emma was a Biology teacher for 5 years. During those years she taught three different GCSE exam boards and two A-Level exam boards, gaining a wide range of teaching expertise in the subject. Emma particularly enjoys learning about ecology and conservation. Emma is passionate about making her students achieve the highest possible grades in their exams by creating amazing revision resources!