Hillary is 8 years old and has neuronal ceroid lipofuscinosis,also known as Batten disease.It is autosomal recessive.She was fine until age five,when she began to lose her vision and stumble.She had learning difficulties in school and was diagnosed at age six.Hillary may only live into her twenties,experiencing dementia and seizures.Her younger brother Jaden,age six,is healthy,as are her parents.The probability that Jaden is a heterozygote is

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Since Batten disease is autosomal recessive, both parents must be carriers (heterozygotes) for Hillary to be affected. Jaden, being healthy, could either be a carrier or not. The probability of him being a carrier (heterozygote) is 2/3, given that he is not affected.

Hillary Is 8 Years Old and Has Neuronal Ceroid Lipofuscinosis,also