Four words matter most in inheritance answers: gene, allele, genotype and phenotype. This lesson defines each with one example, then shows the swaps that lose marks.
This is the first lesson in inheritance, and it prepares you for completing monohybrid crosses.
What does each term mean?
One example can hold all four terms. Suppose a plant has a gene for flower colour, and the gene has two alleles: R for red petals and r for white petals.
- Gene: the section of DNA that controls flower colour.
- Allele: one version of that gene, either R or r.
- Genotype: the pair of alleles in the plant, such as Rr.
- Phenotype: what you can see, such as red petals.
A plant carries two alleles for each gene, one from each parent. That is why every genotype has two letters.
What do homozygous and heterozygous mean?
If the two alleles are the same, the genotype is homozygous. RR is homozygous dominant and rr is homozygous recessive.
If the two alleles differ, the genotype is heterozygous, written Rr. Because R is dominant, Rr shows red petals, the same phenotype as RR.
| Genotype | Type | Phenotype |
|---|---|---|
| RR | homozygous dominant | red |
| Rr | heterozygous | red |
| rr | homozygous recessive | white |
Two different genotypes can give one phenotype, so you cannot always work backwards from appearance to genotype.
Worked example
A gardener crosses a red-flowered plant with a white-flowered plant. All 10 offspring have red flowers, so the red parent must be RR.
Question: Give the genotype of the white parent and explain why every offspring is red.
The white parent shows the recessive phenotype, so its genotype must be rr. It can give only r gametes.
Every offspring receives r from the white parent. The offspring are red, so each one also received R from the red parent. Their genotype is Rr, and R is dominant, so the flower is red.
A safe answer reads: “The white parent is rr. The offspring are Rr, and the dominant allele R masks the recessive allele r.”
Which swaps lose marks?
These swaps cost marks because the term names the wrong thing. Compare each one with the correct wording.
| Student wrote | Problem | Correct wording |
|---|---|---|
| “The genotype is red” | Red is a phenotype | “The phenotype is red” |
| “The gene for red is R” | R is an allele | “The allele for red is R” |
| “The plant has one allele” | Organisms carry two | “The plant has two alleles, Rr” |
| “rr is dominant” | rr is homozygous recessive | “R is the dominant allele” |
| “Red is heterozygous” | Genotype word on a phenotype | “The red plant is heterozygous, Rr” |
Genotype words belong to letters, and phenotype words belong to appearance.
How should you write symbols?
Use the same letter for both alleles of one gene. A capital letter marks the dominant allele and a small letter marks the recessive one.
Choose a letter from the dominant trait: R for red, T for tall. Do not mix letters such as R and w for one gene. State your key at the start, for example “R = red, r = white”, so the marker can follow your working.
Check yourself
In a species of beetle, a gene for wing case colour has two alleles: B for black and b for brown. A beetle has the genotype Bb. State its phenotype, say whether it is homozygous or heterozygous, and name the allele that is not visible.
Answer
The phenotype is black wing cases, because B is dominant. The genotype Bb is heterozygous, since the two alleles differ.
The allele that is not visible is b, the recessive allele. It is still present in the beetle and can be passed to offspring.
What to study next
Use these terms in completing monohybrid crosses. The simple inheritance cross explorer lets you change the parent genotypes and see the offspring ratios.
If you want a teacher to hear your explanations and correct the terms as you use them, see online one-to-one Biology tuition or the one-hour trial class (from RM50).