Cambridge O Level Biology · Syllabus 5090 · Inheritance
Allele
What is Allele?
An alternative form of a gene. Alleles of the same gene occupy the same position on a pair of homologous chromosomes and differ in their base sequence, so they code for slightly different versions of the same protein. A diploid organism carries two alleles of each gene, one inherited from each parent; if the two are identical the organism is homozygous for that gene, and if they differ it is heterozygous.
This definition is part of the Inheritance chapter in Cambridge O Level Biology.
Allele 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 Allele
- 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.
- Putting two alleles in one gamete. A gamete row reading “Tt and Tt”. Gametes are made by meiosis and are haploid: one allele each. Fix Circle each gamete as you write it — the circle only fits one letter.
- “Dominant means common.” Dominant describes what happens in a heterozygote, nothing else. Polydactyly is dominant and rare; blood group O is recessive and common. Write “A dominant allele is expressed in the heterozygote.”
- “Dominant means stronger or better.” There is no contest. A dominant allele produces its effect with one copy present; a recessive one needs two. Neither is superior. Write “The recessive allele is not expressed when a dominant allele is present.”
- “Recessive means weak or rare.” A recessive allele can be extremely common in a population, and is passed on just as faithfully as a dominant one. Write “A recessive allele is expressed only when no dominant allele is present.”
- “Genotype and phenotype mean the same thing.” Genotype is the alleles carried; phenotype is what is observed. They have different ratios from the same cross, which is why questions ask for one specifically. Write genotypes as symbols, phenotypes as words.
- “A heterozygote has two identical alleles.” That is homozygous. Heterozygous means different: one of each. Write “Tt is heterozygous; TT and tt are homozygous.”
- “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.
- “Codominance means the alleles blend.” Blending is incomplete dominance. Codominance means both alleles are expressed: a person with genotype IAIB has both A and B antigens, not something halfway. Write “Both alleles are expressed in the heterozygote.”
- “Blood group O has two possible genotypes.” Only IOIO, because any dominant allele present would give group A, B or AB. Groups A and B are the ones with two genotypes each. Write the four-row genotype table from 17.4 B.
- “The antibiotic made the bacteria resistant.” The resistance allele arose earlier, by random mutation. The antibiotic selected the bacteria that already had it. Write “A resistant bacterium was already present; the antibiotic killed the others, so the resistant one survived and reproduced.”
Examiner tips on Allele
- Notation matters here. With complete dominance, T and t are ideal, because the case of the letter records which allele is dominant. With codominance there is no dominant partner to record, so alleles are written as the same base letter with different superscripts — IA and IB in the ABO system. Using A and a for two codominant alleles states something false about them before you have written a single line of the cross.
Questions students ask about Allele
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.
Does dominant mean the allele is more common?
No. Dominant means only that the allele is expressed in the phenotype of a heterozygote — that one copy is enough to show its effect. How common an allele is in a population is a completely separate question, decided by inheritance and selection over generations. Polydactyly is caused by a dominant allele and is rare; blood group O is caused by a recessive allele and is the most common group in many populations.
Can two parents who both have brown eyes have a blue-eyed child?
Yes, if both parents are heterozygous. Each carries one allele for the dominant phenotype and one recessive allele that is present in the genotype but not expressed in the phenotype. If both pass on the recessive allele, the child is homozygous recessive and shows the recessive phenotype. This is the same logic as Tt × Tt producing a tt offspring, and it is why a recessive condition can appear in a family with no previous history of it.
Did the antibiotic cause the bacteria to become resistant?
No. The resistance allele arose earlier, by random mutation, in bacteria that had never encountered the drug. The antibiotic then acted as a selection pressure: it killed the bacteria that lacked the allele, leaving the resistant ones to survive and reproduce. The proportion of resistant bacteria in the population rose. The antibiotic selected variation that already existed — it did not create it.
Is a mutation always harmful?
No. Most mutations are neutral, having no detectable effect. Some are harmful. Occasionally one is beneficial in a particular environment, and those are the source of the new variation that natural selection can act on. Whether a mutation is harmful or helpful can also depend on the environment: the same allele may be a disadvantage in one place and an advantage in another.
Can an individual organism evolve?
No. Evolution is the change in the inherited features of a population over generations. An individual’s genotype is fixed at fertilisation and does not change during its life. What changes during evolution is how common each allele is among the members of the population, and that can only happen as one generation is replaced by the next.
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.