1. Meiosis

2. Page 27, question 1
1) Sexual reproduction involves the combining of two cells from two parents. If meiosis didn’t happen, the chromosome number in the zygote after fertilisation would be too high. Meiosis makes sure each gamete has only half the normal chromosome number so that the new individual will inherit the right number when the gametes have combined.

3. Question 2 What is the purpose?
Mitosis
Meiosis
Where does it occur?
In all cells
In the sex organs
What is involved?
Chromosomes replicated, two identical cells are produced
Chromosomes are replicated and the cell divides twice to create four gametes with half the original chromosome number.
What is the purpose?
New cells for growth, repair and replacement
Gametes for reproduction
Unique individual with a new combination of alleles

4. Question 3
Meiosis would show some cells with only half the number of chromosomes and mitosis wouldn’t
Question 4
- Sex cells have only half the number of chromosomes so that when they combine at fertilisation, the zygote will have the normal number of chromosomes.
- If this didn’t happen, every generation would have twice the number of chromosomes as the last one.

5. Page 28, question 8:
A scientist wanted to produce a hybrid crab by mating a fast growing crab with another crab which had a good flavour. To do this, he injected the sperm from a crab which had 194 chromosomes in each body cell into the egg of a crab which had 100 chromosomes in each egg cell.
A) How many chromosomes would there be in a fertilised egg resulting from this technique?
194 chromosomes in a body cell
Halve this for a gamete = 97 chromosomes in sperm
Egg has 100 chromosomes
So a fertilised egg would contain = 197 chromosomes

6. Question 9, page 29
A crab, produced by artificial fertilisation, has 183 chromosomes in each body cell. In terms of cell division, explain why this crab would be unable to produce sex cells.
183 / 2 = 91.5 chromosomes in gametes
It would be infertile because you cannot have half a chromosome

7. Why is an individual produced by sexual reproduction not identical to its parents?
When a new individual is formed by sexual reproduction it gains half its chromosomes from one parent and a corresponding half from the other.
Therefore, the offspring has a unique set of chromosomes that code for its characteristics.
They will have a unique genotype and phenotype