1. Li-Fraumeni syndrome is a rare inherited condition. lt makes someone much more likely to develop cancer at an early age. Li-Fraumeni syndrome is caused by the dominant allele of a gene (F).
Fig 1.1 shows the family history of a family affected by Li-Fraumeni syndrome. Parents A and B have 2 children, G and D.
(a) Explain the terms gene and allele.
(b) Using info from Fig 1 .1, write down the genotype for the following people:
(c) Persons A and B have a third child, Person E.
Using a genetic diagram, deduce the probability that person E does not have Li-Fraumeni syndrome.
What is the probability that person E does not have Li-Fraumeni syndrome?
It was recently discovered that a mutation in the gene coding for a particular protein is responsible for development of the Li-Fraumeni syndrome.
Apart from gene mutation, mutations may cause a change in the chromosome number.
Fig. 1.2 shows the chromosomes of an individual affected by Down’s syndrome.
(d) With reference to Fig. 1.2, how can we tell that the individual’s suffering from Down’s syndrome?
(e) What is the sex of the individual? Explain your answer.
2. Marfan syndrome is a genetic disorder of the connective tissue. People with Marfan syndrome tend to be unusually tall, with long limbs and long, thin fingers.
Fig. 1.2 shows a family tree.
(a) Marfan syndrome is caused by a dominant allele. Jane and John have two children, Adam and Amy. Adam has Marfan syndrome but Amy does not. Use a genetic diagram to explain the difference. lndicate on the diagram who may be Adam and who may be Amy.
(b) Adam marries May who does not have Marfan syndrome. They have two children, Sandy and Danny. Danny does not have Marfan syndrome. What is the
chance of Sandy having Marfan syndrome? You may use the genetic diagram in Fig 2.2 or draw your genetic diagram.
Use M to represent the dominant allele and m to represent the recessive allele.Figure 2.2
(c) A couple has ten children. Eight of these are boys and two are girls.
(i) Describe how the sex of each child is determined.
(ii) Suggest why the numbers of boys and girls in this family are not equal. Use these ideas about the chance of a baby being a boy or girr in your answer.
1. (a) Gene is a small segment of DNA that contains information to make a protein OR a small segment of DNA where a specific genetic information is stored. Alleles are alternative/different forms of a gene.
(b) C: ff, A: Ff, B: Ff
(d) There are three copies of chromosome 21 OR an extra chromosome 21.
(e) The individual is a male. The chromosome for male is XY. The last pair/23rd pair is the sex chromosome and the chromosomes are different in size.
(c)(i) The sex is determined by sex chromosome. If the child has XX, she is a female, else if the child has XY, he is a male.
(ii) The expected ratio of boy or girl is 0.5 or 50%. The expected ratios only indicate the probability/chance of producing the genotypes/phenotypes in offspring. Fertilisation of the sperms with ova is a random event. with small number of offspring, the observed ratios often differ from expected ratios.