4.7.2Additional lessons
Inheritance: Terminology — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 124 of 142
Section 1 of 5Genes and Alleles
Gene
- A section of DNA that codes for a polypeptide or functional RNA and so influences a characteristic.
Locus
- The fixed position of a gene on a particular chromosome, the same in all members of the species.
Allele
- A variant of a gene; alleles of the same gene differ slightly in their base sequence.
Homologous pair
- Two chromosomes with the same genes in the same order, one from each parent, which may carry different alleles.
Section 2 of 5Genotype and Phenotype
Genotype and Phenotype
Genotype
- The alleles an individual has for a gene, shown with letters such as Bb.
Phenotype
- The observable characteristic produced by the genotype together with the environment.
Homozygous
- Both alleles at the locus are identical, such as BB or bb.
Heterozygous
- The two alleles differ, such as Bb, so only one may be expressed.
Carrier
- A heterozygous person who carries a recessive allele for a condition without showing it.
Section 3 of 5Dominance Relationships
Dominant allele
- It is expressed in the phenotype even when only one copy is present, and is written with a capital letter.
Recessive allele
- It is expressed only when two copies are present and is written with a lower-case letter.
Codominant alleles
- Both alleles are fully expressed in a heterozygote, giving a combined phenotype, as in the AB blood group.
Incomplete dominance
- The heterozygote has an intermediate phenotype, as with red, pink and white flowers in some plants.
| Genotype | Dominant/recessive | Codominance |
|---|---|---|
| Heterozygous | Shows only the dominant trait | Shows both traits |
| Homozygous recessive | Shows the recessive trait | Shows that allele's trait |
Section 4 of 5Notation
1
Choose a letter to represent the gene, using the capital for a dominant allele and the lower case for a recessive allele.
2
For codominance, use a main letter with superscripts, such as Iᴬ and Iᴮ, rather than capital and lower-case letters.
3
Write genotypes as pairs of alleles, with homozygous dominant first and homozygous recessive last.
4
Link each genotype to a phenotype using a key, so that the meaning of each symbol is clear in your answer.
5
State whether individuals are homozygous or heterozygous when explaining how a recessive trait can skip a generation.
Section 5 of 5Exam Checklist & Tips
4.7.2 – Key Points for the Exam
- Gene A heritable factor consisting of a length of DNA that influences a specific characteristic.
- Locus The position of a gene on a chromosome.
- Dominant An allele that determines the phenotype when present in one copy.
- Recessive An allele that is expressed only when homozygous.
- Codominant Alleles that are both expressed in the phenotype of a heterozygote.
- Heterozygous Having two different alleles of a gene.
Common Mistakes
- Using the word gene when allele is meant; a gene has alleles, not the other way round.
- Assuming dominant alleles are always more common in a population;
- dominance describes expression, not frequency.
- Writing codominant alleles as capital and lower-case letters, which implies one is recessive.
- Saying heterozygous means that the person shows a blend; that is true only for incomplete dominance.
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