Genetics
Mendelian inheritance and Punnett squares.
Genetics explains how traits pass from parents to offspring. Genes are stretches of DNA, and each gene can come in different versions called alleles. You inherit one allele of each gene from each parent, and the combination, your genotype, determines the trait you show, your phenotype.
A dominant allele masks a recessive one, so a person with genotype looks the same as . Punnett squares predict the probabilities of offspring genotypes. Crossing two heterozygous parents () gives the classic genotype ratio and phenotype ratio.
Key terms
- Monohybrid cross
- A cross of two heterozygotes gives a 3:1 phenotype ratio for a dominant trait.
Key ideas
- Homozygous: two identical alleles ( or ); heterozygous: two different ().
- Each parent passes on one allele at random, which is why outcomes are probabilities.
- A test cross with reveals whether a dominant-looking individual is or .
- Independent genes multiply: a dihybrid cross gives .
Worked example
Two heterozygous pea plants (, tall dominant) are crossed. What fraction of offspring are short?
- Each parent gives or with probability .
- Punnett square outcomes: , , , .
- Only is short: .
- Phenotype ratio: 3 tall : 1 short.
Answer:
Common mistakes
- Expecting exact ratios in small families rather than probabilities.
- Confusing genotype with phenotype.
- Thinking dominant alleles are more common in the population.
Practice questions
Try each one, then open the answer. Numerical answers on this page are verified by an automated calculation check.
1.In , what fraction of offspring show the recessive trait?
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2.In , what fraction are heterozygous?
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3.In with independent genes, what fraction show both recessive traits?
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