The production of haploid gametes in animals and plants, fertilisation as the fusion of two haploid gametes to form a diploid zygote, and how this restores the diploid chromosome number.

What this dot point is asking

The SQA wants you to describe the gametes (sex cells) in animals and plants and where they are made, explain that gametes are haploid, and describe fertilisation as the fusion of two haploid gametes to make a diploid zygote, restoring the diploid chromosome number.

Diploid and haploid

Gametes must be haploid for a clear reason: if two diploid cells joined, the offspring would have double the chromosome number, and it would keep doubling every generation. Having haploid gametes keeps the number constant.

Gametes in animals and plants

In both cases the gametes are haploid, so that fertilisation gives a diploid offspring. The gametes are also specialised for their job. In animals the sperm is a small, mobile cell with a tail to swim to the egg, while the egg is a larger cell that carries a food store for the early embryo. In plants, the male gamete is delivered inside a pollen grain that is carried by wind or animals to the female part of another flower, while the female gamete waits inside the ovule.

Why two parents matter

Because each offspring receives one set of chromosomes from each parent, it inherits a mixture of alleles from both. This is why offspring of the same parents are not identical to each other or to either parent: sexual reproduction shuffles the alleles. This variation is important, because it is the raw material on which natural selection acts (covered in the evolution key area). It is also the reason a monohybrid cross can give more than one possible genotype in the offspring (covered in variation and inheritance).

Fertilisation

The zygote is the first cell of the new organism. It then divides by mitosis (covered in producing new cells) to grow into the whole organism, every cell diploid.

Examples in context

Example 1. Human reproduction. A sperm with 23 chromosomes fertilises an egg with 23 chromosomes, giving a zygote with 46. That single diploid cell then divides by mitosis over and over to build a baby in which every body cell has 46 chromosomes, half from each parent.

Example 2. Flowering plants. In a flower, a pollen grain lands on the stigma and its haploid male gamete travels down to fuse with the haploid female gamete in the ovule. The diploid zygote that forms grows into the embryo inside a seed, ready to germinate into a new plant.

Try this

Q1. State how many chromosomes a human gamete contains. [1 mark]

• Cue. 23 (the haploid number).

Q2. Name the cell formed when two gametes fuse at fertilisation. [1 mark]

• Cue. A zygote.