Alleles, monohybrid inheritance, pedigrees and sex determination – IGCSE Biology exam questions with mark scheme

1Grade 4–54 marks

In pea plants the allele for tall stems (T) is dominant to the allele for short stems (t). A heterozygous tall plant is crossed with a short plant.

(a) Use a genetic diagram to show the possible offspring of this cross. [3]

(b) State the probability that an offspring plant will be short. [1]

(a) parental genotypes Tt and tt(1)
(a) gametes T and t (from Tt) and t (from tt)(1)
(a) offspring Tt and tt with phenotypes tall and short identified(1)
(b) 0.5; accept 50% / ½ / 1 in 2; reject 1 : 1 / 50:50(1)
Answer: (a) Tt × tt → Tt (tall) and tt (short) (b) 0.5
Examiner tip: A probability must be a fraction, decimal or percentage — '1 : 1' is a ratio and is rejected.

2Grade 8–95 marks

The diagram shows the inheritance of cystic fibrosis in a family. Cystic fibrosis is caused by a recessive allele, f. The dominant allele is F.

(a) Explain how the diagram shows that cystic fibrosis is caused by a recessive allele. [2]

(b) Persons 4 and 5 are planning another child. State the probability that this child will have cystic fibrosis. [1]

(a) persons 1 and 2 (or 4 and 5) are unaffected but have a child (3 or 7) who is affected(1)
(a) so both parents must be carriers / heterozygous (Ff) and the affected child is ff / received a recessive allele from each parent; allow if the allele were dominant an affected child would need an affected parent(1)
(b) 0.25; accept 25% / ¼ / 1 in 4; reject 1 : 3 / 1 : 4(1)
(c) 4 and 5 are both Ff, so offspring FF, Ff, Ff, ff and person 6 is unaffected so cannot be ff (only 3 possibilities left)(1)
(c) ⅔; accept 0.67 / 66.7% / 67% / 2 in 3; reject ½ / 0.5(1)
Answer: (a) unaffected parents have an affected child, so parents are carriers (b) 0.25 (c) 2/3
Examiner tip: In (c) you already know person 6 is unaffected, so only three genotypes remain — the answer is 2/3, not 1/2.

3Grade 4–54 marks

(a) Use a genetic diagram to explain why there is an equal chance of a child being a boy or a girl. [3]

(b) A couple already have three daughters. State the probability that their next child will be a boy. [1]

(a) parents XX (female) and XY (male)(1)
(a) gametes: X (egg) and X, Y (sperm)(1)
(a) offspring XX, XX, XY, XY / 2 XX : 2 XY so 50% (each) girl and boy(1)
(b) 0.5 / 50% / ½ — each pregnancy is independent(1)
Answer: (a) XX × XY → XX, XY in 1 : 1 (b) 0.5
Examiner tip: Previous children do not change the odds — each fertilisation is an independent event.

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