IB Biology HL D3.2 Inheritance questions

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Question 1

Multiple choiceEasy1 mark

In pea plants, the P generation is YY × yy. F1 plants are self-pollinated to produce F2 plants; assume the gamete combinations are equally likely. The figure shows the F2 Punnett grid. What fraction of F2 zygotes have genotype Yy?

Allele in pollen gameteYyAllele in ovule gameteYyYYYyYyyy
Show answer and explanation

Answer: B. Each zygote combines one allele from a pollen gamete with one allele from an ovule gamete. Two of the four equally likely grid outcomes are Yy, so the fraction is 2/4.

A: counts only one Yy cell, missing the second combination of these alleles.
C: counts the three cells containing at least one Y, rather than only the heterozygous cells.
D: treats every fertilization as producing a heterozygote, overlooking the YY and yy cells.
Remember: each gamete contributes one allele to the zygote.

Question 2

Paper 2Medium16 marks

In a beetle species, allele A gives a dark body and allele a gives a pale body. Allele B gives long antennae and allele b gives short antennae. A and B are completely dominant to a and b, respectively.

A homozygous dark-bodied, long-antennaed beetle (AABB) is crossed with a homozygous pale-bodied, short-antennaed beetle (aabb). An F₁ beetle is testcrossed with an aabb beetle. The following offspring are recorded.

Offspring phenotypeNumber
Dark body, long antennae416
Pale body, short antennae404
Dark body, short antennae92
Pale body, long antennae88
(a)

State the genotype of the F₁ beetle and the arrangement of its alleles on its homologous chromosomes.

[2]
(b)

Assuming the genes assort independently, determine the gamete types produced by the F₁ beetle and the expected offspring genotypes and phenotypes from the testcross. State the expected phenotypic ratio.

[6]
(c)

Using the observed offspring, identify the recombinant gamete types from the F₁ beetle and their corresponding offspring genotypes and phenotypes.

[4]
(d)

Identify the parental offspring phenotype and genotype classes. Compare the observed results with the expected outcome if the genes assort independently, and infer whether the genes are linked.

[4]

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