Edexcel iGCSE Biology-4.26 &4.27 Sex Chromosomes- Study Notes- New Syllabus
Edexcel iGCSE Biology-4.26 &4.27 Sex Chromosomes- Study Notes- New syllabus
Edexcel iGCSE Biology-4.26 &4.27 Sex Chromosomes- Study Notes -Edexcel iGCSE Biology – per latest Syllabus.
Key Concepts:
4.26 understand how the sex of a person is controlled by one pair of chromosomes, XX in a female and XY in a male
4.27 describe the determination of the sex of offspring at fertilisation, using a genetic diagram
Sex Determination in Humans
📝 Introduction
- In humans, the sex of a baby is determined by one pair of chromosomes, called the sex chromosomes.
- There are 23 pairs of chromosomes in every body cell, and one pair is responsible for determining whether a person is male or female.
- Females have the sex chromosomes XX, while males have XY.
- During fertilisation, the combination of sex chromosomes inherited from the parents determines the sex of the offspring.
- There is an equal chance (50%) of having a male or a female child.
🧬 What are Sex Chromosomes?
- Sex chromosomes are the pair of chromosomes that determine the biological sex of a person.
- They are different from the other 22 pairs of chromosomes (autosomes), which carry genes for other body characteristics.
- Humans have two types of sex chromosomes:
- X chromosome
- Y chromosome
- Only one pair of these chromosomes determines whether an individual is male or female.
Female Sex Chromosomes (XX)
- A female has two X chromosomes (XX).
- Since both chromosomes are X, every egg cell produced by a female carries only an X chromosome.
- Therefore, the mother always passes an X chromosome to her child.
Male Sex Chromosomes (XY)
- A male has one X chromosome and one Y chromosome (XY).
- During sperm formation, the two sex chromosomes separate.
- As a result:
- Half of the sperm carry an X chromosome.
- The other half carry a Y chromosome.
- This means the father can pass either an X chromosome or a Y chromosome to the child.
How is the Sex of a Baby Determined?
- During fertilisation:
- The egg always contributes an X chromosome.
- The sperm contributes either an X chromosome or a Y chromosome.
- The combination of these chromosomes determines the baby’s sex.
| Egg Chromosome | Sperm Chromosome | Offspring | Sex |
|---|---|---|---|
| X | X | XX | Female |
| X | Y | XY | Male |
Probability of Having a Male or Female Child
- Each fertilisation is a separate event.
- There are two equally likely outcomes:
- XX = Female
- XY = Male
- Therefore:
- Probability of a female child = 1/2 (50%)
- Probability of a male child = 1/2 (50%)
- The sex of one child does not affect the sex of future children.
Why Does the Father Determine the Sex of the Child?
- The mother’s egg always contains an X chromosome.
- The father’s sperm may contain either an X chromosome or a Y chromosome.
- Therefore, the chromosome contributed by the father decides whether the child will be XX (female) or XY (male).
Summary of Sex Determination
| Feature | Female | Male |
|---|---|---|
| Sex chromosomes | XX | XY |
| Gamete produced | X only | X or Y |
| Determines child’s sex? | No (always contributes X) | Yes (contributes X or Y) |
📌 Quick Recap
– 23rd chromosome pair = sex chromosomes.
– XX = female, XY = male.
– Male sperm determines the sex → X = girl, Y = boy.
– Probability of male/female = 1:1.
💡 Tip: “Father decides the child’s sex.”
Sex Determination at Fertilisation
📝 Introduction
- The sex of a baby is determined at the time of fertilisation.
- Fertilisation is the process in which the nucleus of a sperm cell fuses with the nucleus of an egg cell to form a zygote.
- The sex of the offspring depends on which sex chromosome is carried by the fertilising sperm.
- A genetic diagram (Punnett square) is used to predict the possible sex of the offspring.
- There is an equal probability (50%) of producing a male or a female child.
Sex Chromosomes in Humans
- Humans have 23 pairs of chromosomes.
- 22 pairs are called autosomes.
- 1 pair consists of the sex chromosomes, which determine whether a person is male or female.
| Person | Sex Chromosomes |
|---|---|
| Female | XX |
| Male | XY |
Sex Chromosomes in Gametes
- During meiosis, the chromosome pairs separate, so each gamete receives only one sex chromosome.
Female (XX)
- Every egg cell receives only one X chromosome.
- Therefore, all eggs are genetically the same for sex chromosomes.
Egg cells:
- X
- X
Male (XY)
- Half of the sperm receive an X chromosome.
- The other half receive a Y chromosome.
Sperm cells:
- X
- Y
🧬 Fertilisation and Sex Determination
- During fertilisation:
- The egg always contributes an X chromosome.
- The sperm contributes either an X chromosome or a Y chromosome.
- The combination of these chromosomes determines the sex of the offspring.
- X egg + X sperm = XX (Female)
- X egg + Y sperm = XY (Male)
Genetic Diagram (Punnett Square)
Parents
Female (XX) × Male (XY)
Gametes
- Female → X, X
- Male → X, Y
Punnett Square
| X (Sperm) | Y (Sperm) | |
| X (Egg) | XX | XY |
| X (Egg) | XX | XY |
📊 Results of the Genetic Diagram
Genotypes
- 2 XX
- 2 XY
Genotype Ratio
1 XX : 1 XY
Phenotypes
- 2 Females
- 2 Males
Phenotype Ratio
1 Female : 1 Male
Probability of Each Sex
| Offspring | Probability | Percentage |
|---|---|---|
| Female (XX) | 1/2 | 50% |
| Male (XY) | 1/2 | 50% |
This means that every pregnancy has:
- 50% chance of producing a female child.
- 50% chance of producing a male child.
Why Does the Father Determine the Sex of the Child?
- The mother always contributes an X chromosome because all her eggs contain only X.
- The father contributes either an X chromosome or a Y chromosome through his sperm.
- Therefore, the chromosome carried by the sperm determines whether the child is male or female.
🌟 Important Points About Sex Determination
- Sex is determined at fertilisation, when the sperm joins with the egg.
- The egg always carries an X chromosome.
- The sperm may carry either an X chromosome or a Y chromosome.
- If an X-bearing sperm fertilises the egg, the offspring will be XX (female).
- If a Y-bearing sperm fertilises the egg, the offspring will be XY (male).
- The chance of having a male or female child is equal (1 : 1).
- The sex of one child does not affect the sex of future children because each fertilisation is an independent event.
📊 Summary Table
| Feature | Female | Male |
|---|---|---|
| Sex chromosomes | XX | XY |
| Sex chromosome in gametes | X only | X or Y |
| Chromosome contributed during fertilisation | X | X or Y |
| Possible offspring | XX | XY |
| Probability | 50% | 50% |
⚡ Quick Recap
✔ Sex is determined at fertilisation.
✔ Females have XX chromosomes and males have XY chromosomes.
✔ Every egg carries an X chromosome.
✔ Sperm carry either an X or a Y chromosome.
✔ XX = Female and XY = Male.
✔ The father determines the sex of the child.
✔ The probability of having a male or female child is 50% (1 : 1).
✔ Each pregnancy is independent, so previous children do not affect future outcomes.