Unit 7: Genes and inheritance
Gametes And Inheritance — HIA International - Grade 8 Revision Notes & Topical Questions
5 min read 8 parts Lesson 7 of 9 2 quizzes
Lesson summary
Gametes and inheritance — HIA INTERNATIONAL notes: Describe the structure of sperm and egg cells, what happens at fertilisation and how sex and other features are inherited.Gamete = a sex cell: the sperm cell is the male gamete and the egg cell is the female gamete.Every human be…
- Every human begins life as a single cell
- Gametes contain half the normal number of chromosomes
- Sperm are made in the testes
- The head of the sperm enters the egg
- The zygote has two sets of chromosomes
- The zygote divides again and again
Section 1 of 8Gametes
1Gamete
Gamete a sex cell: the sperm cell is the male gamete and the egg cell is the female gamete.
2Every human begins life as a single cell
- Every human begins life as a single cell formed when a sperm cell joined an egg cell.
3Gametes contain half the normal number of chromosomes
- 23 instead of 46 in humans.
4Sperm are made in the testes
- Sperm are made in the testes egg cells are made in the ovaries.
Section 2 of 8Sperm cell and egg cell compared
| Feature | Sperm cell | Egg cell |
|---|---|---|
| Size | Very small | Much bigger, about the size of a full stop |
| Movement | Swims actively using its tail | Cannot move by itself |
| Cytoplasm | Small amount | Large amount containing food reserves |
| Nucleus | 23 chromosomes | 23 chromosomes |
| Other adaptations | Many mitochondria for energy; acrosome with enzymes to digest into the egg; streamlined shape | Jelly coat or membrane that changes after one sperm enters, so no other sperm can enter |
Section 3 of 8Fertilisation
Fertilisation
Fertilisation
Fertilisation the joining of a sperm cell with an egg cell.
The head of the sperm enters the egg
- The head of the sperm enters the egg and the nucleus of the sperm fuses with the nucleus of the egg.
Zygote
Zygote the new cell formed when the two nuclei join.
The zygote has two sets of chromosomes
- 46 in humans, restoring the full number.
The zygote divides again and again
- The zygote divides again and again eventually producing all the cells of the body.
Section 4 of 8From gametes to a new individual
1
Sperm and egg cells are made with 23 chromosomes each.
2
The sperm swims to the egg in the oviduct.
3
The sperm head enters and the egg membrane changes to block other sperm.
4
The sperm and egg nuclei fuse (fertilisation).
5
The zygote has 46 chromosomes and divides to form an embryo.
Section 5 of 8Sex inheritance
Sex inheritance
Sex chromosomes
- X and Y chromosomes determine whether a person is male or female.
XX is female and XY is male
- XX is female and XY is male the Y chromosome is much smaller than the X chromosome.
All egg cells carry one X chromosome
- All egg cells carry one X chromosome because gametes have only one of each kind.
Half of the sperm cells carry an X chromosome and half carry a Y chromosome.
An X sperm fertilising an egg gives an XX zygote, which becomes a girl.
- A Y sperm fertilising an egg gives an XY zygote, which becomes a boy.
The chance of either is about equal
- The chance of either is about equal so approximately equal numbers of boys and girls are born.
The sperm decides the sex of the baby
- The sperm decides the sex of the baby because only sperm can carry the Y chromosome.
Section 6 of 8Inheritance
Inheritance
Inheritance
- Inheritance passing on DNA, as chromosomes carrying genes, from parents to offspring.
Offspring inherit half their genes from each parent
- Offspring inherit half their genes from each parent so they resemble both parents but are not identical to either.
Which chromosome of each pair goes into a gamete is random
- Which chromosome of each pair goes into a gamete is random and which sperm fertilises the egg is random, so offspring vary.
Inherited features
- Inherited features for example eye colour and blood group.
Features caused by the environment are not inherited
- Features caused by the environment are not inherited for example a scar.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Gamete | A sex cell: sperm in males and egg in females. |
| Fertilisation | The joining of a sperm nucleus with an egg nucleus. |
| Zygote | The single cell formed at fertilisation. |
| Sex inheritance | The passing on of X and Y chromosomes that decides whether a baby is a girl or boy. |
| Inheritance | The passing on of genes from parents to offspring. |
| Sperm cell | The male gamete, which swims using a tail. |
| Egg cell | The female gamete, which is large and contains food reserves. |
| Allele | One of the different versions of the same gene. |
Section 8 of 8Exam technique
1
Saying gametes have 46 chromosomes — gametes have 23 and fertilisation restores 46.
2
Saying the mother decides the sex of the baby — all eggs carry X, so the sperm (X or Y) decides.
3
Thinking offspring get all their genes from one parent — half come from each parent.
4
Saying egg cells are smaller than sperm cells — egg cells are much bigger because they hold food reserves.
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