Question

A certain type of congenital deafness in humans is caused by a rare autosomal (not X-linked) dominant gene. (a) In a mating involving a deaf man and a deaf woman (both heterozygous), would you expect all the children to be deaf? Explain your answer. (b) In a mating involving a deaf man and a deaf woman (both heterozygous), could all the children have normal hearing? Explain your answer. (c) 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.

Short answer

Answer: The likelihood of deaf children being born depends on the inheritance pattern and the parents' genotypes. In an autosomal dominant pattern, where both parents are heterozygous for the gene, there is a 75% chance their offspring will be deaf. In an autosomal recessive pattern, if both parents are homozygous for the recessive gene, all of their offspring will be deaf, but if both parents are heterozygous, there is a 25% chance their children will be deaf and a 75% chance they will have normal hearing.

Step-by-step solution

Part (a)

In this scenario, both the deaf man and the deaf woman are heterozygous for the autosomal dominant gene. When two heterozygotes mate, the possible genotypes of the offspring are as follows: 1/4 homozygous dominant (deaf), 1/2 heterozygous (deaf), and 1/4 homozygous recessive (with normal hearing). Therefore, not all of their children would be deaf. Only 75% (1/4 + 1/2) of the offspring would be expected to be deaf.

Part (b)

In this situation, both parents are heterozygous for the autosomal dominant gene causing deafness, as stated in Part (a). The offspring have a 25% (1/4) chance of being homozygous recessive (with normal hearing). However, that does not imply all the children would have normal hearing; it only indicates the likelihood for an individual offspring. It is statistically very unlikely that all children would have normal hearing, as each offspring's genotype is an independent event with a 75% chance of being deaf.

Part (c)

In this part, the cause of deafness is a rare autosomal recessive gene. Consider the case where both parents are homozygous for the recessive gene, which means they have the genotype "rr." In this situation, all of their offspring will be deaf due to inheriting two recessive alleles. However, if both parents are heterozygous (a genotype of "Rr"), their offspring have a 25% chance of being homozygous dominant (with normal hearing), a 50% chance of being heterozygous (with normal hearing), and a 25% chance of being homozygous recessive (deaf). In such a case, their children could have normal hearing.

Key concepts

Autosomal Dominant

An autosomal dominant gene is one that is located on a non-sex chromosome, also known as an autosome, and only needs one copy to manifest its effect. This means that an individual with just one dominant allele will express the trait related to the gene.

For instance, in the case of congenital deafness caused by a rare autosomal dominant gene, a person would be deaf if they have just one copy of this dominant gene. If both parents are heterozygous (i.e., they each carry one dominant gene responsible for deafness and one normal recessive gene), their children could inherit the dominant allele from either parent.

The potential combinations for their children are:

• 1/4 homozygous dominant (carrying two dominant alleles: both dominant genes; deaf)
• 1/2 heterozygous (one dominant and one recessive allele; deaf)
• 1/4 homozygous recessive (two recessive alleles; normal hearing)

This means that around 75% of the children are expected to be deaf, with the presence of this dominant gene ensuring the expression of the trait.

Autosomal Recessive

An autosomal recessive gene, like a dominant one, is located on one of the autosomes, but for it to express its trait, two copies of the recessive allele must be present. If an individual has only one recessive allele, the trait will not be expressed, and they will be a carrier instead.

In cases of genetic conditions such as autosomal recessive deafness, a person will be deaf if they inherit two copies of the recessive gene. If both parents are heterozygous (i.e., carriers of one recessive gene), children have the following genotype possibilities:

• 1/4 homozygous dominant (normal hearing)
• 1/2 heterozygous (carriers with normal hearing but can pass on the recessive gene)
• 1/4 homozygous recessive (deafness)

The chance of having children with normal hearing is high if the parents carry the recessive allele but do not exhibit the trait because they also have a normal dominant allele that masks the recessive allele.

Heterozygous

The term heterozygous refers to having two different alleles for a particular gene — one inherited from each parent. In the context of genetic inheritance, heterozygous individuals possess one dominant allele and one recessive allele for a trait.

Using autosomal dominant traits as an example, a heterozygous person will express the trait because the dominant allele overshadows the effect of the recessive allele. In our exercise example, a man and a woman who are both heterozygous for an autosomal dominant deafness trait can pass either allele onto their children.

When considering autosomal recessive traits, these heterozygous individuals carry the recessive trait without showing any of its attributes. They have the potential to pass on the recessive allele to their offspring. It plays a crucial role in carrying traits through generations without physically manifesting them in every carrier.