Key Genetic Terminology
This topic covers the fundamental vocabulary of genetics, explaining how traits are encoded in DNA and passed from one generation to the next. Mastering these terms is crucial for interpreting genetic diagrams and solving inheritance problems.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- A gene is a section of DNA on a chromosome that codes for a trait, while alleles are the different versions of that gene.
- An organism's genotype is its specific combination of alleles (e.g., Tt), which determines its phenotype, the observable characteristic (e.g., tall).
- A dominant allele is expressed if at least one copy is present, whereas a recessive allele is only expressed when two copies are present.
- Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes.
- A standard monohybrid cross between two heterozygous parents (e.g., Aa x Aa) reliably produces offspring with a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio.
Definitions
- Gene
- A segment of DNA that contains the instructions for building a specific protein or functional RNA molecule, thereby determining a particular trait.
- Allele
- One of the alternative forms of a single gene. For example, the gene for eye colour has alleles for blue, brown, etc.
- Dominant
- An allele whose characteristic is always expressed in the phenotype, even when only one copy is present in the genotype.
- Recessive
- An allele whose characteristic is only expressed in the phenotype if two copies are present; it is masked by a dominant allele.
- Heterozygous
- Having two different alleles for a particular gene, for example, 'Aa'.
- Homozygous
- Having two identical alleles for a particular gene, for example, 'AA' (homozygous dominant) or 'aa' (homozygous recessive).
- Phenotype
- The observable physical, physiological, and biochemical characteristics of an organism, resulting from its genotype and environmental influences.
- Genotype
- The genetic constitution of an organism, specifically referring to the combination of alleles for a particular gene.
- Chromosome
- A structure within the cell nucleus composed of tightly coiled DNA, carrying the organism's genetic information.
- Autosome
- Any chromosome that is not a sex chromosome (X or Y). Humans have 22 pairs of autosomes.
Worked example
In a species of plant, the allele for tall stems (T) is dominant to the allele for short stems (t). A heterozygous tall plant is crossed with a short-stemmed plant. If this cross produces 120 offspring, what is the expected number of plants with a heterozygous genotype?
- 1
Identify the genotypes of the parent plants.
The heterozygous tall plant is Tt.
The short-stemmed plant must be homozygous recessive, so its genotype is tt.
- 2
Set up the Punnett square for the cross:
Tt x tt.
- 3
Determine the genotypes of the offspring.
The possible combinations are Tt and tt.
- 4
Calculate the probability of each genotype.
The Punnett square shows that 2 out of 4 squares are Tt and 2 out of 4 are tt.
So, the probability of a heterozygous genotype (Tt) is 2/4 = 1/2.
- 5
Calculate the expected number of heterozygous offspring from the total of 120.
Expected number = Probability × Total offspring = (1/2) × 120.
- 6
The final calculation is 120 / 2 = 60.
Answer: 60
Common mistakes
- ×Confusing genotype with phenotype. 'Tt' is a genotype; 'tall' is a phenotype. Answering with the wrong category is a common domain confusion error.
- ×Incorrectly identifying homozygous individuals. 'Homozygous' refers to both the dominant (AA) and recessive (aa) pure-breeding states. Questions asking for the 'number of homozygous offspring' require you to sum both.
- ×Mixing up 'allele' and 'gene'. An individual has two alleles for each gene (one on each homologous chromosome). A common error is to miscount the number of possibilities based on this.
- ×Forgetting that a recessive phenotype always reveals a homozygous recessive genotype (e.g., a short plant must be 'tt').
No-calculator tips
- ✓Most ESAT genetics problems involve simple ratios (1:1, 3:1, 1:2:1). Mentally calculating outcomes using fractions (1/4, 1/2, 3/4) is much faster than using percentages.
- ✓Memorise the standard outcomes for a heterozygous cross (Aa x Aa): the genotypic ratio is 1 AA : 2 Aa : 1 aa, and the phenotypic ratio is 3 dominant : 1 recessive.
- ✓To find an expected number, multiply by the fraction. For example, to find 3/4 of 80, first find 1/4 (80/4 = 20), then multiply by 3 (20*3 = 60).