Genetics & Inheritance
Reproduction and Inheritance
Inheritance is a topic where the right words matter. Most exam questions on genetics rely on you knowing what a gene, an allele, a genotype and a phenotype are, and being able to use these terms precisely.
The big eight
| Term | What it means |
|---|---|
| DNA | The chemical that carries all the genetic information of an organism. Stands for deoxyribonucleic acid |
| Chromosome | A long, tightly-coiled molecule of DNA found in the nucleus. Humans have 23 pairs in every body cell |
| Gene | A short section of DNA on a chromosome. Each gene codes for one specific protein |
| Allele | One of the different versions of the same gene. A gene might have two or more alleles |
| Genotype | The combination of alleles an individual has for a particular gene (e.g. BB, Bb, bb) |
| Phenotype | The observable characteristic that results from a particular genotype (e.g. brown eyes, blood group A) |
| Dominant | An allele whose effect is seen in the phenotype even when only one copy is present. Written as a capital letter (e.g. B) |
| Recessive | An allele whose effect is only seen in the phenotype when both copies are present. Written as a lowercase letter (e.g. b) |
Common exam question
Defining a key genetic term
Question: State what is meant by the term gene, allele, genome, recessive allele, codominant or mutation (1 mark).
Asked in 10 of the 23 papers, usually as the first part of a longer genetics question. Each definition is one mark. A gene is a length of DNA coding for one protein. An allele is a different form or version of a gene. The genome is all the DNA, or all the genes, of an organism; when a question asks how a gene differs from a genome, both definitions are needed for the single mark. A recessive allele is one that is only expressed when two copies are present (in a homozygote), or when no dominant allele is present: either wording is credited. Codominant alleles are both expressed in the phenotype of a heterozygote, which shows a third phenotype (blood group AB, or roan cattle). A mutation is a rare, random change to the DNA or to a gene.
Homozygous and heterozygous
The two alleles you carry for a given gene can be the same or different:
- Homozygous: both alleles are the same (e.g. BB or bb)
- Heterozygous: the two alleles are different (e.g. Bb)
If you are heterozygous and one allele is dominant, the dominant version is the one you show in your phenotype. The recessive allele is still there in your genotype, but it is masked by the dominant one.
An example: the PTC tasting gene. Some people can taste a bitter chemical called phenylthiocarbamide (PTC, found naturally in some vegetables like sprouts); others can't taste it at all. The ability to taste PTC is controlled by one gene with two alleles:
- T = can taste PTC (dominant)
- t = cannot taste PTC (recessive)
| Genotype | Possible alleles | Phenotype |
|---|---|---|
| TT | T from each parent | Taster (homozygous dominant) |
| Tt | T from one parent, t from the other | Taster (heterozygous; T masks t) |
| tt | t from each parent | Non-taster (homozygous recessive) |
Note that two different genotypes (TT and Tt) produce the same phenotype (taster). The only way to be a non-taster is to have two copies of the recessive allele.
Common exam question
Genotypes behind a dominant phenotype
Question: Give the possible genotypes of an individual with the dominant characteristic, or explain how a cross would show which genotype it has (1–2 marks).
Set in 3 of the 23 papers. For the possible genotypes give both: homozygous dominant and heterozygous (TT and Tt). Adding the homozygous recessive genotype to that list loses the mark. To tell the two apart, breeders use a test cross with a homozygous recessive partner (tt): if every offspring shows the dominant characteristic the parent was homozygous, while any recessive offspring proves it was heterozygous, because a recessive offspring needs a recessive allele from both parents. The two marks come from any two of those three points: the cross with the homozygous recessive, and what each outcome shows.
Other terms you should know
- Genome: the whole of an organism's DNA, taken together. Every chromosome, every gene, every base. Sequencing the entire human genome was completed in 2003.
- Diploid: a cell with two complete sets of chromosomes (e.g. a human body cell with 23 pairs = 46 chromosomes). Written as 2n.
- Haploid: a cell with one complete set of chromosomes (e.g. a human sperm or egg with 23 chromosomes). Written as n.
- Variation: differences between individuals of the same species. Variation can be genetic (caused by different alleles), environmental (caused by lifestyle, diet, accidents), or a mix of both.