4.8.2Additional lessons
Inheritance: Terminology — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 69 of 101
Part 1 of 5Genes, Alleles and Gametes
Characteristics are passed on through gametes, which fuse at fertilisation to restore the diploid number.
Gene
- A length of DNA on a chromosome that codes for a polypeptide or functional RNA and so influences a particular characteristic.
Allele
- One of several alternative forms of a gene, differing from the others by one or a few bases in the DNA sequence.
Gamete
- A haploid sex cell such as a sperm or egg, which carries one allele of each gene after meiosis.
Zygote
- The diploid cell formed when two gametes fuse, so it holds two alleles of every gene, one from each parent.
Locus
- The fixed position on a chromosome occupied by a gene, which is the same on both chromosomes of a homologous pair.
Part 2 of 5Genotype and Phenotype
Genotype and Phenotype
Genotype
- The combination of alleles an individual carries for a gene, written with letters such as Tt or tt.
Phenotype
- The observable characteristics of an individual, produced by the genotype together with the effect of the environment.
Homozygous
- Having two identical alleles of a gene, for example TT (homozygous dominant) or tt (homozygous recessive).
Heterozygous
- Having two different alleles of a gene, for example Tt, so the dominant allele usually decides the phenotype.
Carrier
- A heterozygous individual who has one recessive allele that is not shown in the phenotype but can be passed on to offspring.
Part 3 of 5Patterns of Dominance
Different pairs of alleles interact in different ways when they are both present.
Complete dominance
- The dominant allele is expressed in both the homozygous and heterozygous states, hiding the recessive allele in a heterozygote.
Recessive allele
- An allele that affects the phenotype only when no dominant allele is present, which means the genotype must be homozygous.
Codominance
- Both alleles are fully and separately expressed in a heterozygote, as in the speckled black and white feathers of some chickens.
Incomplete dominance
- The heterozygote shows an intermediate phenotype that blends the two alleles, such as pink flowers from red and white parents.
| Pattern | Genotype | Phenotype |
|---|---|---|
| Complete dominance | Tt | Same as TT, the dominant form |
| Codominance | CᴮCᵂ | Both traits visible together |
| Incomplete dominance | CᴿCᵂ | Intermediate, new appearance |
Part 4 of 5Worked Example
Question
- In four o'clock plants red flowers (CᴿCᴿ) crossed with white flowers (CᵂCᵂ) give pink F₁ plants.
- Two pink F₁ plants are crossed.
- Predict the genotype and phenotype ratios of the F₂.
- Gametes of the F₁ crosseach pink plant (CᴿCᵂ) makes gametes Cᴿ and Cᵂ in equal numbers.
- Combine the gametes in a Punnett grid: CᴿCᴿ, CᴿCᵂ, CᵂCᴿ, CᵂCᵂ.
- Count genotypes1 CᴿCᴿ : 2 CᴿCᵂ : 1 CᵂCᵂ.
- Because the heterozygote is intermediate, each genotype has its own phenotype.
Answer Genotype ratio 1 : 2 : 1, phenotype ratio 1 red : 2 pink : 1 white (a 1 : 2 : 1 ratio, not 3 : 1).
Part 5 of 5Exam Checklist & Tips
4.8.2 – Key Points for the Exam
- Allele An alternative form of a gene that occupies the same locus.
- Genotype The alleles an organism carries for a gene.
- Phenotype The observable characteristics resulting from genotype and environment.
- Codominance A pattern in which both alleles are expressed fully in the heterozygote.
- Carrier A heterozygote with one copy of a recessive allele that is not expressed.
Common Mistakes
- Using 'gene' and 'allele' as if they meant the same thing; a gene has several possible alleles.
- Writing that a heterozygote has two dominant alleles instead of one dominant and one recessive.
- Mixing up codominance with incomplete dominance;
- in codominance both traits appear separately, in incomplete dominance a blend appears.
- Forgetting that phenotype is influenced by the environment as well as the genotype.
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