Why offspring inherit different combinations

By UK A Level Biology · independent study · myAudience

Crossing over exchanges corresponding segments between nonsister chromatids, producing two recombinant and two unchanged chromatids in the selected example. The separate three-pair calculation gives eight possible chromosome combinations across gamete production, ignoring crossing over. The pictured single meiosis uses two pairs: homologues separate in meiosis I, sisters in meiosis II, yielding four cells each with two chromosomes. DNA is not copied between divisions. Mutation creates new alleles; segregation and crossing over rearrange existing ones.

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Mutations can create new alleles. Meiosis then reshuffles existing variation: homologous chromosomes can exchange corresponding DNA segments, and different chromosome pairs orient independently. Fertilisation combines two gametes, adding another source of different combinations.

Meiosis separates homologous chromosomes and later sister chromatids, normally producing haploid cells. Non-disjunction is a failure of separation that can change chromosome number. Crossing over changes combinations along chromosomes, so a calculation that ignores it gives only part of the possible variation.

A base substitution may leave amino-acid identity unchanged because the code is degenerate, while a deletion not divisible by three can change the downstream reading frame. Chromosome non-disjunction changes number rather than merely replacing one base. For n chromosome pairs, independent assortment supplies 2ⁿ combinations ignoring crossing over; combining two such gametes yields 2²ⁿ pairings under the same simplified assumptions. Crossing over adds variation beyond that count.

Worked example: An organism has three chromosome pairs. Ignoring crossing over, how many combinations can independent assortment produce in its gametes?

Use 2 to the power of the number of pairs.

2³ = 8.

Answer: Eight combinations.

Check your understanding: Does meiosis normally create new alleles through independent assortment?

Answer: No; it rearranges existing alleles.

Why: Mutation creates new sequence variants, while assortment distributes chromosomes carrying variants already present.

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