4.7.4Additional lessons
Sex Determination — IB Biology SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 126 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Sex Chromosomes
Karyotype
- Karyotype Human body cells have 46 chromosomes: 44 autosomes and two sex chromosomes.
Female
- Female Females are XX, so every egg cell carries one X chromosome.
Male
- Male Males are XY, so half of the sperm carry an X and half carry a Y.
Chance of each sex
- Chance of each sex At fertilisation the sperm decides the sex of the child, giving a 50% chance of male or female.
| Parent | Gametes produced |
|---|---|
| Mother (XX) | All eggs X |
| Father (XY) | Half X, half Y |
Part 2 of 6How the Y Chromosome Acts
1
The Y chromosome carries the SRY gene, which codes for the testis-determining factor.
2
If SRY is present in the embryo, the gonads develop into testes from about the seventh week of development.
3
The testes secrete testosterone, which directs development of male reproductive organs and characteristics.
4
If SRY is absent, as in XX embryos, the gonads develop into ovaries and female structures form.
5
Hormones therefore link the genotype of the sex chromosomes to the sex phenotype.
Part 3 of 6X-linked Genes
X-linked Genes
Location
- Location An X-linked gene lies on the X chromosome, which is much larger than the Y and carries many genes the Y lacks.
Males
- Males A male has one X, so a single recessive allele on it is expressed;
- males are described as hemizygous for these genes.
Females
- Females A female needs two recessive alleles to be affected, so heterozygous females are unaffected carriers.
Examples
- Examples Haemophilia, in which blood clotting is impaired, and red–green colour blindness are both caused by recessive alleles on the X chromosome.
Pattern
- Pattern More males than females show the condition, and an affected father cannot pass an X-linked allele to his sons.
Part 4 of 6Notation
Always show the Y chromosome in male genotypes so it is clear that there is no second allele.
Alleles
- Alleles Write the allele as a superscript on X, such as Xᴴ for normal clotting and Xʰ for haemophilia.
Male genotypes
- Male genotypes XᴴY is unaffected and XʰY is affected, since the Y carries no allele for this gene.
Female genotypes
- Female genotypes XᴴXᴴ is unaffected, XᴴXʰ is a carrier, and XʰXʰ is affected.
Part 5 of 6Worked Example
Question
- A woman who is a carrier of haemophilia (XᴴXʰ) has children with an unaffected man (XᴴY).
- Predict the chance that a child is a son with haemophilia.
- Mother's gametesXᴴ or Xʰ; father's gametes: Xᴴ or Y.
- OffspringXᴴXᴴ, XᴴY, XᴴXʰ, XʰY, each with probability 1/4.
- Only XʰY is an affected son.
- Of the sons, one in two is affected; of all children, one in four.
Answer There is a 25% chance of a haemophiliac son, and no daughters are affected, though half are carriers.
Part 6 of 6Exam Checklist & Tips
4.7.4 – Key Points for the Exam
- Autosome Any chromosome that is not a sex chromosome.
- SRY gene Gene on the Y chromosome that triggers testis development.
- Sex linkage Inheritance of a gene that is located on a sex chromosome.
- Hemizygous Having only one copy of a gene, as in X-linked genes in males.
- Carrier A heterozygous female with one recessive X-linked allele.
Common Mistakes
- Writing that the mother decides the sex of the child; the egg always carries an X, so the sperm decides.
- Forgetting the Y chromosome in genotypes for sex-linked traits.
- Stating that X-linked recessive conditions pass from father to son; sons receive the Y from their father.
- Assuming that male children have to be affected when the mother is a carrier; each son has a 50% chance.
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