Inheritance and genetic crossesIB MYP Biology: Subtopic test

1. 1
   A student in Nairobi studies inheritance in pea plants. The gene for height has two alleles: the allele for tall stems (T) is dominant and the allele for short stems (t) is recessive.

   (a)

   A pea plant has the genotype Tt. Which term describes this genotype?

   [1 mark]

   • AHeterozygous
   • BHomozygous dominant
   • CHomozygous recessive
   • DRecessive

   (b)

   What is the phenotype of a pea plant with the genotype Tt?

   [1 mark]

   • AShort
   • BBoth tall and short
   • CMedium height
   • DTall

   (c)

   Define the terms allele and dominant.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A breeder in Lisbon keeps guinea pigs. Black fur is controlled by a dominant allele (B) and white fur by a recessive allele (b). She has two heterozygous black guinea pigs and one white guinea pig.

   (a)

   Which gametes can a heterozygous black guinea pig (Bb) produce?

   [1 mark]

   • AOnly B
   • BB or b, in equal numbers
   • COnly Bb
   • DOnly b

   (b)

   Two heterozygous black guinea pigs (Bb × Bb) are bred. What is the probability that an offspring is white?

   [1 mark]

   • A0%
   • B50%
   • C25%
   • D75%

   (c)

   The breeder crosses a heterozygous black guinea pig with the white one. State the expected ratio of black to white offspring and the genotype of the black offspring.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Cystic fibrosis is an inherited disorder caused by a recessive allele (f). A healthy couple, Amara and Kwame, have a first child who has cystic fibrosis. Both parents carry one copy of the allele.

   (a)

   Explain how two healthy parents can have a child with cystic fibrosis.

   [3 marks]

   (b)

   Use a genetic cross to find the probability that the next child has cystic fibrosis, and the probability that the child is a carrier.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A university in Seoul breeds mice to study inheritance. Black fur is dominant (B) and white fur is recessive (b). Two heterozygous black mice were crossed repeatedly and produced 80 offspring in total: 58 black and 22 white. Researchers use work like this to understand inherited human disorders such as Huntington's disease, which is caused by a dominant allele and usually starts to cause symptoms after the age of 35. A healthy 25-year-old woman, whose father has Huntington's disease (with one copy of the allele) and whose mother does not carry the allele, is considering a genetic test to find out whether she has inherited the allele.

   (a)

   Evaluate the hypothesis that both parent mice are heterozygous and that black is dominant, using the data.

   [6 marks]

   (b)

   Discuss whether the 25-year-old woman should have the genetic test for Huntington's disease.

   [6 marks]

   Total for question 4: 12 marks

End of questions