A-LevelBiologyInherited change9700/41 · Oct/Nov 2017 · Q79 Marks

(a) Galactosaemia is a rare genetic disease in which the build-up of the monosaccharide galactose can result in an enlarged liver, kidney failure and brain damage. Galactose is produced in the body from the digestion of the sugar lactose, found in milk. Galactosaemia is caused by a recessive mutation of the GALT gene. The normal dominant allele codes for an enzyme which converts galactose to glucose. (i) Suggest how a person with galactosaemia can minimise damage to the liver, kidney and brain. (ii) Explain how a mutation in the GALT gene could result in a change in the enzyme responsible for the metabolism of galactose. (b) If the phenotypes of parents are known, the probabilities of having a child with galactosaemia, an unaffected child (healthy, not a carrier) or a child who is a carrier can be calculated. Complete [Table 7.1] to show the results of these calculations. (c) Testing for galactosaemia is usually carried out on newborn babies, although it is possible to carry out the test on a foetus. Explain how the presence of galactosaemia in a foetus may be determined.

(a)(i)Suggest how a person with galactosaemia can minimise damage to the liver, kidney and brain.[1]
(a)(ii)Explain how a mutation in the GALT gene could result in a change in the enzyme responsible for the metabolism of galactose.[4]
(b)Complete Table 7.1 to show the results of these calculations. parent 1 | parent 2 | percentage probability of having a child with galactosaemia | percentage probability of having an unaffected child | percentage probability of having a child who is a carrier unaffected | carrier | 0 | 50 | 50 carrier | carrier | unaffected | has galactosaemia | carrier | has galactosaemia | 50 | 0 | 50[2]
(c)Testing for galactosaemia is usually carried out on newborn babies, although it is possible to carry out the test on a foetus. Explain how the presence of galactosaemia in a foetus may be determined.[2]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

[1] 7(a)(i) consume, less / no, milk / lactose / (named) dairy products ;
[4] 7(a)(ii) four from: 1 changes / different, base / nucleotide (sequence), in, DNA / gene ; 2 changes / different, mRNA, codon / triplet ; 3 changes / different, primary structure of, polypeptide / protein / enzyme ; 4 changes / different, tertiary structure (of, polypeptide / protein / enzyme) ; 5 changes / different, allosteric / active, site ; 6 enzyme, non-functional / does not convert galactose (to glucose) ;
[2] 7(b) (table) carrier | carrier | 25 | 25 | 50 ; unaffected | affected | 0 | 0 | 100 ;
[2] 7(c) two from: genetic screening ; obtain fetal, cells / DNA ; by, amniocentesis / chorionic villus sampling ; electrophoresis + probe ;

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