O-LevelBiologyGenetics & Variation5090/22 · May/June 2018 · Q710 Marks

Describe the cause of each of the following: Down's syndrome, sickle cell anaemia.

(a)Describe the cause of each of the following: Down's syndrome, sickle cell anaemia.[3]
(b)(i)Parents, one with blood group B and the other with blood group A, have a child with blood group O. The parents decide to have another child. Complete the genetic diagram to show the possible blood groups for the second child of these parents. genotypes of parents .................................................................................................... X .................................................................................................... gametes [Figure: Four empty circles for gametes] possible genotypes of child ................................................................................................................................................................................................................ possible blood groups of child ................................................................................................................................................................................................................ [4]
(b)(ii)State the probability of each of the following for the second child of these parents: being the same sex as the first child having the same blood group as the first child[2]
(b)(iii)Name the type of inheritance shown by the alleles that produce the blood group AB.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[3] 1 inherited / genetic AW ;
(Down's syndrome)
2 mutation + chromosome ;
3 reference to chromosome 21;
4 one extra or 3 instead of 2 or 47 not 46 or 24 not 23 in + gamete / egg / sperm ;
(sickle cell anaemia)
5 mutation + gene ;
6 homozygous recessive / two recessive alleles ;
[4] 1 I^B I^O + I^A I^O or B + O + A + O or BO + AO ;
2 I^B + I^O + I^A + I^O or B + O + A + O or gametes correct for stated parent genotypes ;
3 I^B I^A + I^B I^O + I^A I^O + I^O I^O or AB + BO + AO + OO or possible genotypes of child correct for stated gametes ;
4 AB + B + A + O or possible blood groups of child correct for stated genotypes ;
[2] (same sex) 50% / half / ½ / 0.5 / 2 in 4 / 1 in 2 / 1:1;
(same blood group) 25% / quarter / ¼ / 0.25 / 1 in 4 / 1:3;
[1] codominance / codominant ; R incomplete dominance

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