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Date	November 2020	Marks available	1	Reference code	20N.2.HL.TZ0.5
Level	Higher level	Paper	Paper 2	Time zone	TZ0 / no time zone
Command term	Identify	Question number	5	Adapted from	N/A

Question

Dachshunds have three basic coat types: wire-, smooth- or long-haired. These are affected by two genes, W and K. The presence of W always results in wire hair.

[Source: American Kennel Club, 2007. Available at:
https://www.slideshare.net/ghinck/dachshund-power-point.]

When long-haired dogs are crossed among themselves, they always produce long-haired puppies. When dogs heterozygous for both genes are crossed, they produce offspring in the ratio 12 wire-haired:3 smooth-haired:1 long-haired.

A male wire-haired dog was crossed with several long-haired females and the phenotypic ratio of the puppies was approximately 2 wire-haired:1 smooth-haired:1 long-haired.

Identify the genotype of the male wire-haired dog.

[1]

Using a Punnett square, determine how a smooth-haired puppy could be produced in the offspring.

[2]

Markscheme

WwKk

WK Wk wK and wk for male gamete genotypes and wk for female;

wwKk shown in Punnett square and identified as smooth;

Examiners report

Only 40 % of candidates answered this correctly. A surprisingly high number of candidates gave a genotype with only one allele for each gene, suggesting that they were not familiar with dihybrid crosses. In many cases it was hard to distinguish between upper and lower case letters -W/w and K/k. Candidates should be encouraged to make letters used for alleles as clear as possible.

There were some complaints from teachers that epistasis is not included in the program. It was not expected that candidates would have studied ratios derived from interaction between genes and instead it was intended that they work out the pattern of inheritance themselves from the information provided. About a third of candidates successfully did this. The Punnett squares drawn up by other candidates often revealed fundamental misunderstandings of dihybrid inheritance.

Syllabus sections

Additional higher level » Topic 10: Genetics and evolution » 10.2 Inheritance

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