Below is a pedigree for a neurological disease. The son is affected (solid squar

Question

Below is a pedigree for a neurological disease. The son is affected (solid square).

The identity of the disease gene is known. By PCR you amplify part of the disease gene from the affected son and get a 300 base pair PCR product. From the father’s DNA, you get a 330 base pair product; from the mother you get PCR products of 300 and 330 base pairs; and from the sister, you get PCR products of 300 and 330 base pairs. Is the sister a carrier? Explain your reasoning.

**I know that the sister is a carrier, the disease gene is represented by the 330 base pair DNA (the sister has this). But please explain this further.

Explanation / Answer

First part of question where son is affected with a disease gene which gave 300bp PCR product. Son gets only "Y" (recessive) chromosome from father and "X" (Dominant) chromosome from mother. Sister has two copied of "X' (both dominant) one from mother, one from father., but one chrmosome is silenced by formation of "Barr bodies". Sisters always have backup genes on silenced chromosome.

Sister contains both 300 & 330 bp PCR products.

When disease gene is represented by the 300 base pair DNA, sister becomes carrier as she has another nomal gene 330bp which works as a backup.

Second part when disease gene is represented by the 330 base pair DNA, still sister remains carrier as she is always going to have another backup gene copy of the normal gene. Mother is the reason behind this as she have both copies a) Diseased one b) Normal one and its obivious that both cannot be considered as diseased. Father's DNA has 330 base pair product, which predicts 100% son getting affected.

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