How traits are expressed: genes, proteins and chromosomes
A gene is a section of DNA that codes for one protein. Proteins, such as enzymes, produce traits like plant height. Genes lie on chromosomes.
Draw it: from gene to trait
How a gene decides the height of a pea plant
- 1A gene in the DNA carries the information to make an enzyme.
- 2The enzyme helps make a plant hormone that triggers growth.
- 3★If the enzyme works efficiently, a lot of hormone is made and the plant grows tall.
- 4★If the gene has a change that makes the enzyme less efficient, less hormone is made and the plant stays short.
- 5So genes control traits through the proteins they code for.
★ key step
How the two copies of each gene are kept
- Each parent gives one copy of every gene, so every pea plant has two sets of genes.
- Genes are not on one long thread of DNA; they are on separate pieces called chromosomes.
- Each body cell has two copies of each chromosome, one from the male parent and one from the female parent.
- Each germ cell takes one chromosome from each pair, so it has only one set; that chromosome may come from either parent.
- When two germ cells join, the zygote again has two copies of each chromosome, so the DNA of the species stays stable across generations.
- Because each pair is shared out on its own, genes on different chromosomes can be inherited independently, as in the dihybrid cross.
DNA, gene and chromosome
A gene works like one recipe in a family cookbook: the cell reads it and makes a protein. Two plants grown from the same packet of seeds can differ in height if one carries a changed recipe for a growth enzyme.
2 marks: “What is a gene? How do genes control traits?” A section of DNA that codes for one protein. The protein, often an enzyme, controls a process, such as making a plant hormone for growth, which decides the trait.
3 marks: “Explain how a gene decides whether a pea plant is tall or short.” The gene codes for an enzyme that helps make a plant hormone for growth; an efficient enzyme gives a lot of hormone and a tall plant; a changed, less efficient enzyme gives less hormone and a short plant.
2 marks: “How is the number of chromosomes kept the same from parents to offspring?” Germ cells get one chromosome from each pair. At fertilisation, two germ cells join and the pairs are restored.
Gene → protein → trait: DNA is the recipe book, a gene is one recipe, the protein is the dish, and the trait is how it turns out.