MELAS syndrome is an inherited disease caused by a mutation in a gene located in mitochondrial DNA (mtDNA). All mtDNA is inherited from the mother. [Figure 6.1] shows four generations of a family where several individuals are affected by MELAS syndrome.

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2019). The mark or mark type is shown in brackets.

1. [4] any four from: inheritance is through mtDNA: 1 MELAS syndrome is not inherited from affected males ; 2 all offspring of a female with MELAS syndrome also have MELAS syndrome ; 3 ref. to numbered individuals to support mp1 or mp2; if X-linked: 4 males more likely to be affected; 5 (because) males inherit only one X chromosome (from mother) ; 6 (however) more females affected than males; 7 there are no, heterozygous / carrier, females ;
2. [3] any three from: in mitochondrial DNA: 1 mutations occur at constant rate; 2 mutations occur at faster rate than, nuclear / chromosomal, DNA ; 3 not protected by histone proteins; 4 no enzymes to repair DNA mutations ; 5 many copies of mtDNA per cell; 6 no mixing of DNA at fertilisation (as only inherited from mother) or circular DNA, so no crossing over (all sequence changes are mutations);