DNA, genes and chromosomes
The instructions for building and running your body are stored as a chemical called DNA. It is easiest to picture as three nested levels:
- DNA is a long molecule shaped like a twisted ladder (a double helix).
- A gene is a short section of DNA that codes for one feature, such as eye colour.
- A chromosome is a very long, coiled-up thread of DNA carrying many genes.
Human body cells contain 23 pairs of chromosomes, 46 in total. You inherit one chromosome of each pair from each parent, which is why you show a mixture of both parents' features.
- Gene
- A section of DNA that carries the code for making a particular protein and so controls a characteristic.
The key terms
Genetics has a lot of vocabulary, but the words come in pairs that make sense together. Learn these and the crosses become easy to read.
- Allele
- A different version of the same gene, for example a tall allele and a short allele of a height gene.
- Dominant
- An allele that shows its effect even when only one copy is present. Written as a capital letter, for example T.
- Recessive
- An allele that only shows its effect when two copies are present. Written as a small letter, for example t.
- Genotype
- The alleles an organism has, for example Tt.
- Phenotype
- The characteristic you can actually see, for example "tall".
- Homozygous
- Two identical alleles for a gene, for example TT or tt.
- Heterozygous
- Two different alleles for a gene, for example Tt.
Monohybrid crosses and Punnett squares
A monohybrid cross follows a single gene from parents to offspring. A Punnett square is a grid that shows every way the parents' alleles can combine, so you can predict the ratios.
Tall (T) is dominant to short (t). Two heterozygous tall pea plants (Tt) are crossed. What are the expected genotype and phenotype ratios of the offspring?
| T | t | |
|---|---|---|
| T | TT (tall) | Tt (tall) |
| t | Tt (tall) | tt (short) |
A ratio is a probability, not a promise
A 3:1 ratio means each offspring has a 3 in 4 chance of being tall, not that exactly three of every four will be. With small numbers of offspring the real result can differ, just like flipping a coin a few times.
Pedigree charts
A pedigree chart is a family tree that tracks a characteristic through the generations. It lets you work out how a feature is inherited and predict the chances for future children. The standard symbols are:
| Symbol | Meaning |
|---|---|
| Square | Male |
| Circle | Female |
| Shaded shape | Shows the characteristic (affected) |
| Unshaded shape | Does not show it (unaffected) |
| Horizontal line joining two shapes | Parents |
| Vertical line down | Leads to their children |
A useful clue: if two unaffected parents have an affected child, the allele must be recessive, because both parents were carriers (heterozygous) without showing it themselves.
Codominance
Sometimes neither allele is fully dominant, and both show up in the phenotype at once. This is codominance. A clear example is coat colour in some cattle: a red allele (CR) and a white allele (CW). A cow with CRCW is not pink, it is roan, a coat with both red and white hairs mixed together, so both alleles are clearly expressed.
- Codominance
- When both alleles in a heterozygous genotype are expressed fully and at the same time, so the phenotype shows both, not a blend.
Notice how the notation changes: with codominance we use a capital letter for the gene (C) and superscripts for the alleles (CR, CW), because neither is recessive to the other. Crossing a red cow (CRCR) with a white bull (CWCW) gives all roan (CRCW) calves.
Where this is assessed
Setting out a Punnett square clearly and stating ratios correctly is Criterion A (knowing and understanding) and Criterion C (processing and evaluating). Examiners want to see the parent alleles, the grid, and both genotype and phenotype ratios written out, so never skip straight to the answer.
Check yourself
1. Explain the difference between genotype and phenotype, and between homozygous and heterozygous. +
Genotype is the alleles present (for example Tt); phenotype is the visible characteristic (for example tall). Homozygous means two identical alleles (TT or tt); heterozygous means two different alleles (Tt).
2. A cross between two heterozygous plants (Bb) is carried out, where B is dominant. What phenotype ratio is expected? +
The Punnett square gives BB, Bb, Bb, bb, that is a genotype ratio 1 BB : 2 Bb : 1 bb. Since B is dominant, BB and both Bb show the dominant phenotype and only bb shows the recessive one. Phenotype ratio 3 dominant : 1 recessive.
3. Two red flowers and two white flowers can produce pink offspring under codominance in snapdragons. In codominant cattle, what does the heterozygous roan phenotype tell you about the alleles? +
In roan cattle the coat shows both red and white hairs at the same time, so both alleles (CR and CW) are fully and equally expressed. Neither allele is dominant over the other, which is the meaning of codominance.
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