0610 · 16.2Sexual reproduction

Fertilisation and sexual reproduction

0:00Imagine a farmer growing a crop of beans. Differences between the plants might be useful if some survive a disease, but inconvenient if the farmer wants every plant ready for harvesting together. Sexual reproduction helps explain why offspring differ. By the end of this video, you will understand what happens during fertilisation, why chromosome sets matter, and why genetic variation can be both an advantage and a disadvantage. We need a little background about genetic information.

0:26The nucleus of a cell contains chromosomes, which carry genes. Genes are sections of genetic material that influence characteristics, such as flower colour. Offspring inherit genetic information from their parents, so understanding reproduction means following what happens to that information, not just noticing that a new organism has appeared. Sexual reproduction is a process involving the fusion of the nuclei of two gametes to form a zygote and the production of offspring that are genetically different from each other.

0:54A gamete is a reproductive cell, such as a sperm cell or an egg cell. Fusion means joining together. The zygote is the cell formed when the two gamete nuclei fuse, and it can develop into a new organism. Fertilisation is the fusion of the nuclei of gametes. Notice how this definition fits inside the larger description of sexual reproduction. Fertilisation is the particular event that brings the genetic information together.

1:18Sexual reproduction includes this event and the production of genetically different offspring, so the two terms are closely connected but do not mean exactly the same thing. The important event is not simply two cells touching. Their nuclei must fuse, bringing together the chromosomes carried by the gametes. This gives the zygote genetic information from both gametes. Usually, these gametes come from two parents, but some organisms can supply both gametes themselves, so the defining feature is two gamete nuclei, not necessarily two separate individuals.

Haploid and diploid chromosome sets

1:50Now consider how much genetic information each nucleus contains. Gamete nuclei are haploid, which means they contain one set of chromosomes. The nucleus of a zygote is diploid, which means it contains two sets of chromosomes. At fertilisation, one set from each gamete comes together, giving the zygote two sets in total. Think of a chromosome set as a complete collection containing one chromosome of each type. Haploid does not mean that there is only one chromosome altogether.

2:17It means there is one complete set. Likewise, diploid means two complete sets, not just two chromosomes, and the number of chromosomes in a set depends on the species. Why does this matter? If each gamete supplied two sets, their fusion would produce a cell with four sets instead of restoring the usual two. Gametes carry one set each so that fertilisation produces a diploid zygote. How cells produce haploid gametes is explained through meiosis, which you will see in the video on inheritance.

2:44Let us work through an example using humans. A human sperm nucleus contains twenty-three chromosomes, and an egg nucleus also contains twenty-three chromosomes. To find the number in the zygote nucleus, we add the chromosomes brought by the two gametes. Twenty-three plus twenty-three gives forty-six chromosomes, arranged as two sets, so the zygote nucleus is diploid. Now suppose an unfamiliar species has twelve chromosomes in its diploid zygote nucleus. We first recognise that twelve represents two sets, rather than one.

3:15Each gamete supplies one set, so we divide twelve by two and obtain six chromosomes per gamete nucleus. Checking the answer, six from one gamete plus six from the other restores twelve in the zygote.

Genetic variation in populations and crops

3:26Chromosome number is only part of the story. Gametes can carry different combinations of genetic information, and fertilisation brings these combinations together. As a result, offspring produced by sexual reproduction are genetically different from each other. They can inherit characteristics from the same parents without having exactly the same combination, which is why siblings can resemble each other without being identical. For a population in the wild, this genetic variation can be a major advantage.

3:53A population is a group of organisms of the same species living in an area. If conditions change, perhaps because a disease spreads or water becomes less available, some individuals may have inherited characteristics that help them survive. Those individuals may reproduce and pass these characteristics to their offspring. Variation therefore increases the chance that some members of a population survive a change. It does not guarantee that every individual survives, or that every offspring is better suited to its surroundings.

4:22Nor do organisms deliberately produce whatever characteristic they need. The useful point is that existing differences give the population a greater chance of containing individuals suited to new conditions. Sexual reproduction also has costs in the wild. Where separate parents are needed, finding a mate takes time and energy, and an isolated individual may fail to reproduce. Gametes must be produced and brought together, and reproduction is often slower than asexual reproduction.

4:48For plants that depend on pollinators, a shortage of pollinators can reduce the chance of sexual reproduction succeeding. In crop production, variation can be useful because breeders want new combinations of desirable characteristics. Imagine one crop variety has good disease resistance, while another produces a high yield. Sexual reproduction between these varieties can produce offspring with different combinations of their inherited characteristics. Breeders can select suitable offspring, including those combining useful characteristics, to help develop improved varieties.

5:19But the combination is not guaranteed in every offspring. Some may have a lower yield, less disease resistance, or other unwanted characteristics. A farmer growing offspring from an excellent parent therefore cannot assume that all its desirable features will be preserved. Sexual reproduction can create useful new combinations, but it can also break up combinations that were already useful. A crop grown from seed is also less uniform. The plants may differ in size or quality, and they may be ready for harvest at different times.

5:49This makes farming less predictable, and breeding a new variety takes many years. In many crops, seeds only form after pollination. So variation helps breeders develop new varieties, but it is a problem when a farmer wants every plant to be the same.

Exam mistakes and recap

6:03Watch out for three common mistakes. First, saying fertilisation is the joining of gametes misses the precise requirement that their nuclei fuse. Second, calling a gamete diploid confuses its one set of chromosomes with the two sets formed in the zygote. Third, saying variation always improves offspring is wrong because a different combination may be helpful, harmful or make little difference in the conditions concerned. An exam question might ask you to describe fertilisation and state whether the zygote nucleus is haploid or diploid.

6:35Keep the answer precise rather than describing an entire reproductive system. Fertilisation is the fusion of the nuclei of gametes. The zygote nucleus is diploid because it contains two sets of chromosomes, one supplied by each gamete. Another question might ask you to discuss the advantages and disadvantages of sexual reproduction in crop production. Discuss means considering both benefits and drawbacks, so connect each point to the farmer’s situation. Sexual reproduction produces genetic variation that breeders can use to develop improved crop varieties.

7:08Desirable combinations of characteristics may be lost in offspring. Variation can make the crop less uniform, making harvesting less predictable. Let us bring the ideas together. Sexual reproduction involves two gamete nuclei fusing to form a zygote, followed by the production of genetically different offspring. Each gamete nucleus is haploid, and fertilisation brings two chromosome sets together in a diploid zygote nucleus. Genetic variation can help wild populations survive changing conditions and allow breeders to develop new crop varieties, but reproduction has costs and desirable characteristics are not always preserved.