Heredity is the transmission of genetic information from parents to offspring. Genes are regions of DNA associated with biological functions, and alleles are alternative versions of a gene. Simple Mendelian models help explain some inheritance patterns, but many real traits involve multiple genes and environmental influences.
Genotype and phenotype
A genotype describes the allele combination in the model, such as TT, Tt or tt. A phenotype is an observable characteristic, such as tall or dwarf in a simplified pea-plant example. A heterozygous organism has different alleles; a homozygous organism has matching alleles at the locus considered.
Under complete dominance, Tt shows the dominant phenotype. Recessive does not mean absent: the t allele can be passed on even when the recessive phenotype is not visible.
- Dominant does not mean stronger, healthier or more common.
- Do not apply a one-gene pea-plant model directly to complex human traits such as height.
Use a cross to find probabilities
In a Tt × Tt cross, each parent can contribute T or t. Combining these possibilities gives TT, Tt, Tt and tt. The expected genotype ratio is 1 : 2 : 1. With complete dominance, the expected phenotype ratio is 3 : 1.
These are probabilities across many outcomes, not a promise that every group of four offspring will contain exactly three of one type and one of another. Keep genotype and phenotype ratios separate.
Put it into practice
A worked example
In the simplified model Tt × tt, what is the chance of a dwarf offspring?
- The Tt parent can contribute T or t; the tt parent contributes t.
- The possible offspring genotypes are Tt and tt with equal probability.
- Only tt has the recessive dwarf phenotype in this model.
Answer: 1/2, or 50%, for each offspring under the model’s assumptions.
Try it yourself
Have a go before revealing the answer.
Is Tt homozygous or heterozygous?
Show answer
Heterozygous, because the two alleles differ.
Answer: Heterozygous, because the two alleles differ.
In Tt × Tt, what is the probability of TT?
Show answer
1/4, or 25%.
Answer: 1/4, or 25%.
Watch out for these
- Do not confuse a 1 : 2 : 1 genotype ratio with a 3 : 1 phenotype ratio.
- An expected ratio is not a guaranteed outcome for a small family or sample.
Continue with official resources
Match these concept notes to your current textbook and syllabus. They are independent explanations, not official board publications.