Key Terms for Genetics
Key Terms for Genetics
Dominant alleles are bossy — if one is present, it shows. Capital letter (B).
Recessive alleles are shy — only show up when there's no bossy allele around. Lowercase (b).
Bb = Brown eyes (bossy B wins). bb = Blue eyes (both shy, so blue shows).
| Term | Meaning |
|---|---|
| Dominant | Allele that is always expressed when present (shown with CAPITAL letter, e.g., B) |
| Recessive | Allele only expressed when there's no dominant allele (lowercase, e.g., b) |
| Homozygous | Two identical alleles (BB or bb) |
| Heterozygous | Two different alleles (Bb) |
| Genotype | The alleles an organism has (e.g., Bb) |
| Phenotype | The physical characteristic (e.g., brown eyes) |
Biology glossary
- What is a dominant allele?
- An allele that is always expressed in the phenotype, even if only one copy is present. Represented by a capital letter (e.g. B). Think of it as the 'bossy' allele.
- What is a recessive allele?
- An allele that is only expressed when two copies are present (homozygous recessive). Represented by a lower-case letter (e.g. b). It is 'hidden' by a dominant allele.
Dominant alleles are bossy — if one is present, it shows. Capital letter (B).
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Memory Aids
Homozygous vs Heterozygous: "Homozygous = Homogeneous = Same" (HH or hh — both letters are the same). "Heterozygous = Different" (Hh — the letters are different). The prefixes homo (same) and hetero (different) are the key.
How to draw a Punnett square — 4 steps: (1) Write the parents' genotypes. (2) Identify the gametes (one allele each). (3) Place gametes along the top and side of the grid. (4) Fill in the grid by combining each pair. Always show your working — examiners award method marks.
Dominant allele rule: "Capital letter = Dominant = always shows." If you see a capital letter in the genotype, that characteristic is in the phenotype. No exceptions for simple inheritance.
Probability language: Practise converting fractions to percentages: 1/4 = 25%, 1/2 = 50%, 3/4 = 75%. Exam questions may ask for ratio (3:1), fraction (3/4), or percentage (75%).
Quick Check: Two parents, both with brown eyes, have a child with blue eyes. Use a Punnett square to show this is possible, and explain what it tells us about the parents' genotypes.
For a child to have blue eyes (recessive), they must have genotype bb. This means each parent must have contributed a b allele. Since both parents have brown eyes but carry a b allele, they must both be heterozygous (Bb). Punnett square for Bb x Bb: offspring = BB, Bb, Bb, bb. Probability of blue eyes (bb) = 1 in 4 = 25%. The fact that a blue-eyed child has appeared confirms that both parents must be carriers of the recessive allele, even though neither shows the trait.
Quick Check: Explain why two individuals with the same phenotype (e.g., both with brown eyes) may have different genotypes, and state what the consequences might be for their offspring.
Brown eyes result from having at least one dominant allele (B). An individual with genotype BB and an individual with genotype Bb both have the same phenotype (brown eyes), but carry different allele combinations. If two BB individuals have children, all offspring will be BB (brown eyes, no recessive allele passed on). If two Bb individuals have children, there is a 25% chance of a bb offspring (blue eyes). The phenotype does not reveal the genotype — genetic testing would be needed to distinguish BB from Bb.
Quick Check: A genetic cross gives a 1:1 ratio of phenotypes in the offspring. What does this tell you about the genotypes of the two parent organisms?
A 1:1 phenotype ratio in offspring arises from a cross between a heterozygous parent (Bb) and a homozygous recessive parent (bb). The Punnett square gives Bb and bb in equal proportions — half the offspring express the dominant phenotype and half express the recessive phenotype. This cross is called a test cross and is used by geneticists to determine whether an organism showing a dominant phenotype is homozygous dominant (BB) or heterozygous (Bb).
Now try it yourself
Quiz · Question 1 of 26
What is the term for an allele that is always expressed when present?
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This topic in real past papers
Every real exam question we've found on genetic inheritance, with a full worked answer.