16.1.3Inheritance
Control of Gene Expression — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 4 of 7
Part 1 of 4Chromosomes in human cells
Chromosome sets
- Chromosome sets Human cells contain 23 different types of chromosome.
- Most cells hold two copies of each type, one inherited from each parent.
- Two copies of 23 types gives a total of 46 chromosomes in the nucleus.
Body cells
- Body cells Almost all body cells, such as skin, muscle and nerve cells, contain the full 46 chromosomes arranged in 23 matching pairs.
Gametes
- Gametes Sex cells, which are sperm in males and egg cells in females, hold only one copy of each chromosome, giving 23 in total.
Part 2 of 4Diploid compared with haploid
| Feature | Diploid nucleus | Haploid nucleus |
|---|---|---|
| Sets of chromosomes | Two sets | One set |
| Human chromosome number | 46 | 23 |
| Chromosomes in pairs? | Yes, in matching pairs | No, they are unpaired |
| Where found in humans | Body cells | Sperm and egg cells |
Part 3 of 4Why gametes are haploid
1Gametes are made
- Gametes are made Special cell division in the reproductive organs halves the chromosome number, so each gamete gets one chromosome from every pair.
2Fertilisation
- Fertilisation A haploid sperm nucleus fuses with a haploid egg nucleus.
3Zygote forms
- Zygote forms The fertilised egg has 23 + 23 = 46 chromosomes, so the diploid number is restored.
4Number stays constant
- Number stays constant If gametes were diploid, the chromosome number would double every generation.
- Haploid gametes keep it the same.
Part 4 of 4Working out chromosome numbers
1
Read the question to find which species it is about. Do not assume the number is always 46, because other species have different numbers.
2
Decide whether the cell is a body cell or a gamete. Body cells are diploid and gametes are haploid.
3
For a diploid cell use the number given. For a haploid cell divide that number by two.
4
To find the number in a fertilised egg, add the two gamete numbers together. This should equal the diploid number.
5
Check your answer: a haploid number must be exactly half of the diploid number for the same species.
Key terms
| Term | Meaning |
|---|---|
| Diploid | A nucleus with two sets of chromosomes, so the chromosomes occur in pairs. |
| Haploid | A nucleus with a single, unpaired set of chromosomes, which is half the normal number. |
| Gamete | A sex cell, such as a sperm or an egg, that fuses with another gamete at fertilisation. |
| Chromosome | A thread-like structure in the nucleus that carries genes made of DNA. |
| Zygote | The cell formed when two gametes fuse; it is diploid. |
Avoid these mistakes
1
Stating that gametes have 46 chromosomes. In humans they have 23, which is haploid.
2
Forgetting that other species have different chromosome numbers, then using 46 in every question.
3
Mixing up the wordshaploid means half the normal number, diploid means two sets.
4
Writing that the egg and sperm have different numbers of chromosomes. Both are haploid and carry 23 in humans.
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