Practice Set 1 — Mendel and Inheritance
Monohybrid and dihybrid crosses, dominant and recessive traits.
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Practice Set 2 — Sex Determination and Variation
Human sex chromosomes and the origin of variation.
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Quick Q&A Before You Revise
How do Mendel's experiments show that traits may be dominant or recessive?
When Mendel crossed pure tall and pure short pea plants, all F₁ plants were tall — the short trait seemed to vanish. But in F₂, short plants reappeared. So the short factor was present in F₁ but not expressed: tall is dominant and short is recessive.
How do Mendel's experiments show that traits are inherited independently?
In a dihybrid cross the F₂ generation contained parental combinations and also new combinations (e.g. round-green, wrinkled-yellow) in a 9 : 3 : 3 : 1 ratio, which is only possible if each pair of traits is passed on independently of the other.
A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits — A or O — is dominant?
Yes. The daughter has O, so she received an O factor from each parent, meaning the father is AO. He shows blood group A even though he carries O, so A is dominant over O.
How is the sex of a newborn determined in human beings?
By the sex chromosome contributed by the father's sperm. An X-bearing sperm plus the egg's X gives a girl (XX); a Y-bearing sperm plus the egg's X gives a boy (XY).