Inheritance and Probability
- Biology
- Ages 14–18 · Grades 9–12
What you’ll find out
- Alleles, genotype and phenotype; dominant and recessive alleles; homozygous and heterozygous individuals.
- Punnett squares predict probabilities, such as the 3:1 ratio from a Tt × Tt cross, not guaranteed outcomes.
- Incomplete dominance, codominance, sex-linkage and polygenic inheritance extend Mendel’s patterns.
No sign-up and no real names. In the lesson you check your understanding as you read, and practise until it sticks.
Alleles, dominance and Punnett squares
Most genes exist in different versions, called alleles, and you inherit one allele of each gene from each parent. The pair of alleles you carry is your genotype. The trait that results is your phenotype. Some alleles are dominant, meaning one copy is enough to show the trait. Others are recessive and only show when two copies are present. In pea plants, the allele for tall stems (T) is dominant over the allele for short stems (t). If two Tt plants are crossed, a Punnett square shows the possible offspring: TT, Tt, Tt and tt. On average, three quarters are tall and one quarter short.
Beyond simple dominance
Many traits do not follow simple dominant and recessive patterns. In incomplete dominance, neither allele fully masks the other. So a red snapdragon crossed with a white one produces pink offspring. In codominance, both alleles are expressed at the same time. Human blood group AB, for example, results from inheriting both the A and B alleles. Some genes are carried on the X chromosome. So conditions such as red–green colour blindness are more common in males. Most traits, such as height, are polygenic. They are influenced by many genes as well as by the environment, so they vary continuously.
Curriculum links: NGSS HS-LS3-3 · England KS4: Biology · Cambridge IGCSE: Biology · NCERT Biology, Class 12