Question

1. another form of deafness is caused by a rare autosomal recessive gene. in a mating involving a deaf man and a deaf woman, could some of the children have normal hearing? explain your answer.
2. below is a pedigree of a fairly common human hereditary trait where the boxes represent males and the circles represent females. shading symbolizes the abnormal phenotype. given that one gene - pair is involved, (a) is the inheritance pattern x - linked or autosomal, recessive or dominant? (b) give the genotype of each individual in the pedigree. if more than one genotypic possibility exists, present all possible alternatives.
3. in the mouse, gene a allows pigmentation to be deposited in the individual coat hairs, while its allele a prevents such deposition of pigment, resulting in an albino. gene b gives agouti (wild - type fur), while its allele b gives black fur.

(a) diagram the cross between a doubly heterozygous agouti mouse mated with a doubly homozygous recessive white mouse.

Explanation:

Step1: Analyze the deaf - inheritance case

If deafness is caused by a rare autosomal recessive gene, let the normal allele be \(A\) and the recessive deaf - causing allele be \(a\). A deaf person can have a genotype of \(aa\). If both the deaf man and the deaf woman have the genotype \(aa\), all of their children will have the genotype \(aa\) and be deaf. However, if one or both of them are deaf due to a different genetic cause (e.g., a different autosomal recessive gene at another locus, or a non - genetic cause), or if there is genetic heterogeneity, it is possible that some of their children could have normal hearing. For example, if the deafness in the parents is due to mutations in different non - interacting autosomal recessive genes (a phenomenon called complementation), their children could inherit a normal allele from each parent and have normal hearing.

Step2: Analyze the pedigree

(a) To determine if the inheritance pattern is X - linked or autosomal, recessive or dominant:

• If it were X - linked recessive, an affected female would have to inherit the recessive allele from both parents. Looking at the pedigree, if it were X - linked recessive, an affected male would pass the recessive allele to all of his daughters. Since we see affected daughters with unaffected fathers in some cases, it is not X - linked recessive. If it were X - linked dominant, an affected male would pass the dominant allele to all of his daughters and none of his sons. This does not match the pedigree pattern. So, it is autosomal. Since two unaffected parents can have affected children, it is recessive.
• (b) Let the normal allele be \(A\) and the recessive abnormal allele be \(a\). Unaffected individuals can have genotypes \(AA\) or \(Aa\). Affected individuals have the genotype \(aa\). For unaffected parents of affected children, they must be carriers (\(Aa\)).

Step3: Analyze the mouse - gene cross

The doubly heterozygous agouti mouse has the genotype \(AaBb\) and the doubly homozygous recessive white mouse has the genotype \(aabb\). The possible gametes from the \(AaBb\) mouse are \(AB\), \(Ab\), \(aB\), \(ab\) and the only gamete from the \(aabb\) mouse is \(ab\). Using a Punnett - square analysis:

	\(AB\)	\(Ab\)	\(aB\)	\(ab\)

The phenotypic ratio of agouti : albino is 1:3.

Answer:

For the deafness question: It is possible if there is genetic heterogeneity (e.g., mutations in different non - interacting autosomal recessive genes).
For the pedigree question: (a) Autosomal recessive. (b) Unaffected: \(AA\) or \(Aa\), Affected: \(aa\), Parents of affected: \(Aa\)
For the mouse - gene cross question: The phenotypic ratio of agouti : albino is 1:3.