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
🌱 Key Idea
A person’s sex is determined by one specific pair of chromosomes called sex chromosomes.
Humans have 23 pairs of chromosomes, but the 23rd pair decides male or female.
🔹 Sex Chromosome Combinations
| Sex | Chromosome Pair | Key Feature |
|---|---|---|
| Female | XX | Both chromosomes are X → female characteristics develop |
| Male | XY | One X and one Y → male characteristics develop |
How Sex is Determined
- All eggs from the female carry an X chromosome.
- Sperm from the male can carry X or Y.
- Fertilisation determines sex:
- X sperm + X egg → XX → Female
- Y sperm + X egg → XY → Male
- The male gamete decides the sex of the offspring.
Key Points
- Sex is determined at fertilisation.
- XX → female, XY → male.
- Male gamete is critical because it can carry either X or Y.
- Probability of male or female offspring = 50:50.
📌 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
🌱 Key Idea
The sex of the offspring is decided at the moment of fertilisation, depending on which sperm (X or Y) fuses with the egg.
Eggs always carry X chromosomes.
Sperm carry either X or Y chromosomes.
Genetic Principle
- XX → Female
- XY → Male
- The male gamete determines the sex of the child because it can contribute either an X or Y chromosome.
Genetic Diagram
| X (Egg) | |
|---|---|
| X (Sperm) | XX → Female |
| Y (Sperm) | XY → Male |
Probability of Female = 50%
Probability of Male = 50%
Stepwise Explanation
- Female produces eggs (X).
- Male produces sperm (X or Y).
- Fertilisation occurs when one sperm fuses with an egg.
- Resulting zygote:
- XX → develops into female
- XY → develops into male
Key Points
- Male sperm decides the sex of the offspring.
- Probability of male or female = 1:1.
- Can be illustrated clearly using a Punnett square.
📌 Quick Recap
– Egg = always X, Sperm = X or Y.
– XX = female, XY = male.
– Fertilisation determines sex.
– Male gamete is critical in sex determination.