Heredity is the transmission of characteristics from parents to offspring through genes carried on chromosomes. Gregor Mendel’s experiments on pea plants laid the foundations: by tracking single traits (monohybrid crosses) and pairs of traits (dihybrid crosses), he showed that traits are controlled by factors (genes) that come in dominant and recessive forms. A dominant trait appears even when only one copy is present, while a recessive trait shows only when both copies are recessive. Mendel’s ratios — 3:1 in the F₂ generation of a monohybrid cross and 9:3:3:1 in a dihybrid cross — reveal how traits separate and recombine independently. The chapter also explains sex determination in humans, where the father’s X or Y chromosome decides the sex of the child, while the mother always contributes an X chromosome.
Mendel’s experimentsDominant and recessive traitsMonohybrid and dihybrid crossesSex determination in humansGenes and inheritance
Key terms
Gene
A unit of heredity that controls a particular trait.
Dominant trait
A trait expressed even when only one allele is present.
Recessive trait
A trait expressed only when both alleles are recessive.
Sex determination
How the sex of offspring is decided by sex chromosomes.
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Activity 8.2
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Who is regarded as the “Father of Genetics” for his work on pea plants?
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