Question

Leigh's disease is a mitochondrially inherited disease with symptoms that include seizures, fatigue, impaired reflexes, breathing problems, and ataxia. The pedigree shows the presence of Leigh's disease in three generations. Individual 4 had two daughters and one son, and individual 6 had one daughter and two sons. Which of the individuals indicated are affected by Leigh's disease

Answer 1

About the question:

You will find the pedigree in the attached files

Answer:

• individual 4 → Affected woman → Black circle
• individual 5 → Affected woman or man → Black circle or square
• individual 7 → Healthy woman → Empty circle
• individual 10 → Affected boy → Black Square

Explanation:      

Mitochondrial inheritance is the transmission of a disease or a trait from the maternal line.

Most of the DNI is in the nucleus, but there is also DNI in mitochondria. Sperm cells hardly transfer mitochondrial genes to the progeny, so mitochondrial DNI is mostly inherited from the maternal side. If there exists any mutation in this mitochondrial DNI, the whole progeny of the mutated woman will be affected, as they will get the mother genotype carrying the mutation. On the contrary, if an affected man is carrying a mutation in mitochondrial DNI, non of their descendants will get the disease.

Before answering the question, let us remember the pedigree symbols.

• Squares represent Males/Men
• Circles represent Females/Women
• Empty symbols represent healthy/non-affected individuals
• Solid black symbols represent sick/affected individuals

In the exposed pedigree, we can see that the mother is affected by the disease (individual number 2), so all her children are also affected (individuals 4, 5, and 6) because the disease is mitochondrially inherited.

Individual 3 is a healthy man, so individual 4 must be an affected woman (Black circle). As she is the one affected, then all her children will also be affected. This couple had one boy and two girls. Individuals 8 and 9 are girls (circle), so individual 10 must be the affected boy (black square).  

On the other hand, individual 6 is an affected man (black square), son of individuals 1 and 2. This man couples with a woman, and they have all healthy children. So this woman (individual 7) must be healthy. Even though the man is affected, all their children are not because their mother (7) is not. Remember that sperm cells do not transmit the mitochondrial genes to the progeny.      

And finally, individual number 5 might be either a man or a woman. In any case, this person is also affected by Leigh's disease.