Cambridge O Level Biology · Syllabus 5090 · Inheritance
Gene
What is Gene?
A length of DNA that codes for a protein. The sequence of bases along a gene determines the sequence of amino acids in the protein made from it; that amino-acid sequence determines how the protein chain folds and therefore its three-dimensional shape; and the shape determines what the protein can do. Because enzymes are proteins, and enzymes control the chemical reactions of a cell, a change in the base sequence of a gene can change the function of a protein and so change the phenotype of the organism.
This definition is part of the Inheritance chapter in Cambridge O Level Biology.
Gene in context
Inheritance is the transmission of genetic information from one generation to the next. A gene is a length of DNA that codes for a protein; an allele is one of the alternative forms that gene can take. The order of the bases A, T, C and G along a strand of DNA sets the order of amino acids in a protein, which sets the protein’s shape, which sets what the protein can do — and that is how a molecule you cannot see decides a feature you can. Because gametes carry one allele of each gene, a cross between two known genotypes has a predictable outcome: Tt × Tt is expected to give 3 tall : 1 short, and Tt × tt is expected to give 1 tall : 1 short. Those are probabilities, not guarantees. Mutation makes new alleles at random, meiosis and fertilisation shuffle them into new combinations, and selection — by the environment in natural selection, by people in artificial selection — changes how common each allele becomes over generations. That change is evolution.
Common mistakes with Gene
- Changing symbols part-way through. Defining T and t and then writing S for short in the square. Use one letter for the gene, capital for the dominant allele, lower case for the recessive, from the first line to the last. Fix Write the symbol definition first and never look away from it.
- “Continuous variation is controlled only by genes.” It is usually influenced by many genes and by the environment, and that combination is what produces a smooth range rather than distinct groups. Write “Continuous variation is usually caused by many genes acting together with the environment.”
- “Discontinuous variation cannot be inherited.” The reverse: the required examples are controlled mainly by genes and are inherited very reliably. Your blood group was fixed at fertilisation. Write “Discontinuous variation in these examples is controlled mainly by genes, with little environmental effect.”
- “A gene is a chromosome.” A chromosome is the whole structure; a gene is a length of the DNA in it, and one chromosome carries many genes. Write “A gene is a length of DNA that codes for a protein.”
- “The gene makes the characteristic.” It codes for a protein; the protein does something in a cell; the effect of that is the characteristic. Skipping the middle costs most of the marks in an explain question. Write the whole chain: base sequence, amino-acid sequence, protein shape, protein function, cell activity, phenotype.
- “A gamete can carry both alleles.” Gametes are haploid: meiosis separates the pair, so each gamete gets one allele of the gene. Write gametes as single circled letters.
- “Artificial selection is genetic modification.” Artificial selection chooses which existing organisms breed. Genetic modification changes genetic material directly. Write “Artificial selection selects parents; it does not alter their genes.”
Questions students ask about Gene
What is the difference between a gene and an allele?
A gene is a length of DNA that codes for a protein. An allele is one particular version of that gene, differing from other versions in its base sequence. Every pea plant has the gene for stem height; what differs between plants is which alleles of it they carry. Genes occupy a fixed position on a chromosome, and the two chromosomes of a homologous pair carry the same gene at that position — but not necessarily the same allele.
How many chromosomes does a person with Down’s syndrome have?
47, rather than the usual 46, because there is an additional copy of chromosome 21. It is classified as a chromosome mutation because what has changed is the number of whole chromosomes, not the base sequence within a gene. It arises at random during the formation of gametes.
Is artificial selection the same as genetic modification?
No. Artificial selection chooses which existing individuals are allowed to breed, and relies entirely on normal inheritance of alleles that are already present in the species. Genetic modification changes an organism’s genetic material directly and can introduce a gene from a different species. Artificial selection is Topic 17; genetic modification belongs to Chapter 18.