A2 BIOLOGY
GRADED WORKSHEET
GENETIC INHERITANCE
TOTAL MARKS = 30
2
1 Huntington’s Disease (HD) is a severe neurological disorder in which symptoms usually
appear after the person has reached sexual maturity. Symptoms include memory loss and
changes in personality and mood.
HD is caused by a gene mutation on chromosome 4 in which the triplet code CAG is repeated
many times. The resulting allele is dominant.
(a) Explain what is meant by the terms Phenotype and Homozygous.
Phenotype .... ................................................................................................................. ..
..........................................................................................................................................
..........................................................................................................................................
..........................................................................................................................................
......
Homozygous ..................................................................................................................
..........................................................................................................................................
..........................................................................................................................................
...................................................................................................................................... [ 2]
(b) A couple wish to start a family. The man does not have HD but the woman does have
the disease. The woman’s father does not have the disease.
Complete the genetic diagram below to show the probability of the couple’s first child
having HD.
key
Huntington allele = T
normal allele = t
parental phenotypes man without HD woman with HD
parental genotypes ............................ ............................
gametes .......................................... .........................................
offspring genotypes ........................................................................................
offspring phenotypes ........................................................................................
probability of first child having HD ............................................................................ [5]
[Total: 7]
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2 (a) Explain what is meant by the term heterozygous genotype.
heterozygous ...................................................................................................................
..........................................................................................................................................
genotype ..........................................................................................................................
...................................................................................................................................... [2]
(b) The budgerigar, Melopsittacus undulatus, is a small type of parrot that is native to
Australia.
Fig. 2.1 shows a budgerigar.
Fig. 2.1
A budgerigar can have blue, green, yellow or white feathers.
Two genes, A/a and D/d, are involved in the inheritance of feather colour in budgerigars.
• A bird which has at least one dominant allele A but is homozygous for d has blue
feathers.
• A bird which has at least one dominant allele D but is homozygous for a has yellow
feathers.
• A bird with at least one dominant A allele and one dominant D allele has green
feathers.
• A bird that is homozygous for a and d has white feathers.
4
(c) Two green-feathered budgerigars, heterozygous at both gene loci, were crossed.
Draw a genetic diagram of this cross to show the probability of producing offspring with
yellow feathers.
[6]
[Total: 8]
5
3 (a) Explain what is meant by the term autosomal linkage.
..........................................................................................................................................
..........................................................................................................................................
..........................................................................................................................................
.................................................................................................................................... [2]
(b) Rickets is a childhood disorder involving the softening and weakening of bones. It is
usually caused by a lack of vitamin D, calcium ions or phosphate ions. A rare form of
rickets that cannot be successfully treated with vitamin D therapy is caused by a mutant
allele on the X chromosome.
Fig. 3.1 shows a pedigree chart for a family that has a history of this condition.
1 2
3 4 5 6 7
8 9 10 11
key:
male without male with
rickets rickets
female without female with
rickets rickets
Fig. 3.1
Using the symbols
XR for the mutant allele on the X chromosome
Xr for the non-mutant allele on the X chromosome
state the genotypes of the following individuals.
1 ..................................................................................................................................
3 ..................................................................................................................................
9 ..................................................................................................................................
10 ............................................................................................................................ [4]
[Total: 6]
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4 The fruit fly, Drosophila melanogaster, has many phenotypic variations and has been used in
experiments to demonstrate the principles of inheritance.
(a) The majority of fruit flies have red eyes but there is a variant with white eyes.
Fig. 4.1 shows the red-eyed and white-eyed variants of the fruit fly.
Fig. 4.1
The gene for eye colour is located on the X chromosome.
Using suitable symbols, draw a genetic diagram to show the possible offspring of a
cross between a heterozygous red-eyed female fruit fly with a white-eyed male fruit fly.
key to symbols:
........................................................
.......................................................
parental
phenotypes red-eyed female white-eyed male
parental
genotypes
gametes
offspring
genotypes
offspring
phenotypes
[5]
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(b) One of the genes controlling the clotting of blood in humans is also located on the X
chromosome. A rare variation of the gene, a recessive allele for haemophilia, can lead
to a condition where the blood fails to clot properly.
(i) State why a man who has haemophilia is unable to pass the condition on to his
son.
..................................................................................................................................
.............................................................................................................................. [2]
(ii) Queen Victoria of Great Britain in the 19th century was a carrier of haemophilia, but
did not have the condition.
State the term used to describe the genotype of a carrier.
.............................................................................................................................. [1]
(iii) Neither of Queen Victoria’s parents carried the allele for haemophilia.
Suggest how Queen Victoria could have become a carrier.
..................................................................................................................................
.............................................................................................................................. [1]
[Total: 9]
© UCLES 2012 9700/41/M/J/12 [Turn over
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5 In humans a rare, sex-linked, recessive allele results in a change in the shape of the iris in
the eye. This condition is know as cleft iris (CI).
(a) Explain what is meant by the term sex linkage.
..........................................................................................................................................
..........................................................................................................................................
..........................................................................................................................................
.................................................................................................................................... [2]
(b) Using suitable symbols complete the genetic diagram below.
Key to symbols
recessive allele ..........................................
dominant allele ...........................................
parental phenotypes male with CI X normal female
parental phenotypes .......................... ..........................
gametes .............................................. ..............................................
offspring genotypes .........................................................................................................
offspring phenotypes ................................................................................................. [5]
(c) A woman who is heterozygous for CI becomes pregnant by a man with a normal iris.
State the probability that their child will have CI.
.................................................................................................................................... [1]
[Total: 8]
