Level: JHS, Basic 9 | Subject: Integrated Science | Strand: Diversity of Matter | Topic: Genetics and Heredity (Living Cells)
Curriculum reference: B9.1.2.1 Understand genetics and heredity basics. Indicator B9.1.2.1.1: Explain basic concepts of heredity and gene transmission. All the exemplars are merged into one set of notes.
1. Heredity
Heredity is the passing of characteristics from parents to their children through the genes. Genetics is the study of heredity. This is why children look like their parents.
2. Chromosomes, Genes and DNA
- Nucleus: the control centre of the cell. It holds the chromosomes.
- Chromosomes: thread-like structures in the nucleus made of DNA and protein. Humans have 46 chromosomes in 23 pairs. One chromosome of each pair comes from the mother and the other from the father.
- DNA (deoxyribonucleic acid): the chemical that carries genetic information. It is shaped like a twisted ladder called a double helix.
- Gene: a short section of DNA that carries the instruction for one characteristic, such as blood group.
The sides of the DNA ladder are made of sugar and phosphate. The steps (rungs) are pairs of bases: Adenine pairs with Thymine (A with T), and Cytosine pairs with Guanine (C with G). The order of the bases is the genetic code.
Order of size: cell, nucleus, chromosome, gene, DNA base.
3. Sex Determination
The 23rd pair of chromosomes are the sex chromosomes. Females have XX and males have XY. The mother always gives an X. The father gives either X or Y, so the father decides the sex of the baby. If a sperm with X meets the egg, the baby is a girl (XX). If a sperm with Y meets the egg, the baby is a boy (XY).
4. Dominant and Recessive Genes
A gene can have different forms called alleles. A dominant allele shows its effect even when only one copy is present (written with a capital letter, e.g. T for tall). A recessive allele shows only when two copies are present (small letter, e.g. t for short).
| Genotype | What the plant shows |
|---|---|
| TT | Tall |
| Tt | Tall (T is dominant) |
| tt | Short |
Cross of Tt x Tt gives TT, Tt, Tt and tt, so three tall plants for every one short plant (3:1). Gregor Mendel found these patterns by studying pea plants.
5. Inherited and Acquired Characteristics
| Inherited (from genes) | Acquired (from the environment) |
|---|---|
| Eye colour | A scar from a wound |
| Skin colour | Ability to swim or play football |
| Blood group | Knowing French or Twi |
| Sickle cell trait | Muscles built by exercise |
| Height range | A tattoo or pierced ear |
Acquired characteristics are not passed on to children through the genes. Plants also inherit characteristics, such as the colour of maize grain, and acquire others, such as a bent stem caused by wind.
6. Variation and Why It Matters
Differences between members of the same species are called variation. It comes from different gene combinations and from the environment. Farmers use heredity when they select and breed the best plants and animals, such as high-yielding maize and fast-growing cattle.
Remember: Genes are on chromosomes, and chromosomes are in the nucleus. Inherited traits come from genes, but acquired traits come from experience.
Ready to practise?
Education Always (education.inspiringalways.com). Original notes written to follow the NaCCA Basic 9 Integrated Science curriculum.