IB Biology SL D3.2 Inheritance questions

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Type
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Question 1

Multiple choiceMedium1 mark

A mother with XX sex chromosomes and a father with XY sex chromosomes have a child. The father has allele R on his X chromosome, and R is absent from his Y chromosome. Which statement(s) are correct?

I. A sperm carrying Y results in an XY zygote, associated with certain male-typical physical characteristics.
II. A child with XY sex chromosomes inherits allele R from the father.
III. The X chromosome carries more genes than the Y chromosome.

Show answer and explanation

Answer: C. The egg contributes X, while a Y-bearing sperm results in an XY zygote, associated with certain male-typical physical characteristics. An XY child receives the father’s Y chromosome, not his X chromosome, so cannot inherit R from him. The X chromosome carries more genes than the Y chromosome.

A: assumes the father passes his X-linked allele R to an XY child.
B: identifies the Y-sperm outcome but omits the true comparison of gene numbers.
D: assumes R can pass from father to an XY child and omits the sperm’s role in determining the zygote’s sex chromosomes.

Remember: an XY child receives its X chromosome from the egg and its Y chromosome from the sperm.

Question 2

Paper 2Hard16 marks

A family has several members diagnosed with phenylketonuria (PKU). Figure 1 shows their pedigree. No other family members are shown.

IIIIIII-1I-2II-1II-2II-3III-1III-2malefemalediagnosed with PKUnot diagnosed with PKU

Use P for the allele not associated with PKU and p for the PKU-associated allele.

(a)

Using Figure 1, state two observations that support PKU having an autosomal recessive inheritance pattern in this family.

[2]
(b)

Deduce the genotypes of I-1, I-2, II-2 and II-3.

[3]
(c)

Show the genetic cross for a child of II-2 and II-3, and state the probability that this child has PKU.

[3]
(d)

III-2 does not have PKU. Determine the probability that III-2 is a carrier of the p allele. Show your reasoning.

[3]
(e)

Suppose III-2 has a child with a person known to be a Pp carrier. Determine the probability that their child will have PKU. Use your answer to part (d).

[2]
(f)

Explain how inheritance of the p allele can result in PKU and affect the conversion of phenylalanine to tyrosine.

[3]

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