Chapter 7, Problem 1TQ

Summary Introduction

The human genome consists of 46 chromosomes (23 pairs) and they carry genetic information from one generation to another. The 22 pairs of chromosomes are known as autosomes, each pair in the autosomes carries the same genes or genetic information except the sex chromosomes, such as X and Y chromosomes. Chromosomes contain genes and alleles are different versions of a gene. Two autosomal recessive alleles cause recessive genetic disorders. Several thousands of genetic disorders in human are inherited due to recessive traits on autosomes (for example, sickle-cell disease, cystic fibrosis, and Tay-Sachs disease).

Answer to Problem 1TQ

Recessive genetic disorders are more common when compared to sex-linked disorders because, autosomes contain more genes than the sex chromosomes. If carriers carry an allele for a particular genetic disorder, they do not express any symptoms of the disease.

Therefore, options (b) and (c) are correct.

Explanation of Solution

Justify the reasons for the correct statement:

The presence of genes on the X or Y chromosomes is known as sex-linked. Merely 1,240 genes are sex-linked. However, when compared to the autosomes, the sex-linked genes and their genetic disorders are very low. Humans are affected by several thousands of recessive genetic disorders. Therefore, recessive genetic disorders (due to mutations on autosomes) are more common than sex-linked disorders.

An individual has two alleles of a gene. Carriers contain only one copy of the recessive allele and another copy of a dominant allele. The recessive allele codes for the genetic disease. Generally, the dominant allele masks the expression of the recessive allele, therefore, carriers do not express that trait or symptoms of the genetic disorder. If a person contains only a single copy of a recessive allele for a particular disease, the person can transfer the disorder allele (recessive allele) to the next generation. However, the person can be completely normal and does not have the disease due to containing only one copy of the recessive allele.

Option (b) is given as, “They are more common than sex-linked genetic disorders because there are more genes on autosomes than on sex chromosomes”.

Option (c) is given as, “Carriers inherit an allele for the disorder but do not display symptoms of the disorder themselves”.

Hence, options (b) and (c) are correct.

Justify the reasons for the incorrect statements:

Option (a) is given as, “They are less common than dominant genetic disorders”.

The given statement is not true. Dominant genetic disorders are less common than recessive genetic disorders. Hence, it is a wrong answer.

Option (d) is given as, “Only individuals with the disorders can pass it on to their offspring”.

Both individuals with disorders and carriers can transfer genetic disorders to their offspring. Hence, it is a wrong answer.

Hence, options (a) and (d) are incorrect.

Conclusion

Sex-linked recessive genetic disorders are less common than recessive genetic disorders. Carriers for a particular recessive genetic disorder can pass the disease to their offspring, however, carriers do not express symptoms of the genetic disorder.