Hillary is 8 years old and has neuronal ceroid lipofuscinosis also known as Batten disease

Question

Hillary is 8 years old and has neuronal ceroid lipofuscinosis also known as Batten disease it is autosomal recessive she was fine until age 5 when she began to lose her vision and stumble she had learning difficulties in school and was diagnosed at age six Hillary grey only live in our 20s experiencing dementia and seizures her younger brother Jaden H6 is healthy as are her parents calculate the probability that Jaden is a heterozygote

Answer 1

If the disease is autosomal recessive, then Hillary must have two recessive alleles for the trait. Since she got one allele from her father and one from her mother, then each parent must have at least one allele.

Since both of her parents are healthy, then neither of them has two recessive alleles. So we know both of Jaden's parents are heterozygotes.

Your question seems to have gotten a little garbled in the voice-to-text process. If Jaden is healthy, but not old enough to express the trait, then there is a 2/4 chance he is a heterozygote (the four options are AA, Aa, aA, aa; and the middle two are heterozygotes).

However, if Jaden is healthy and past the age of onset, then we know he can't be aa, double recessive (if he did, he wouldn't be healthy). In that case, there are three options, AA, Aa, and aA, two of which are heterozygotes, so the probability is 2/3.