Chapter 12, Problem 17STP

Summary Introduction

To describe:

The inheritance pattern of the disease in the given pedigree

Introduction:

Genetic disorders result from inheritance of traits which are either dominant or recessive. The basic patterns of human inheritance follow Mendel’s law of inheritance. Traits that are controlled by genes located on sex chromosomes; X chromosomes in particular are called sex- linked traits or X- linked traits.

Answer to Problem 17STP

The pedigree given below is an example of X linked recessive inheritance in which females are carriers of the trait/ disorder but do not express it while males may or may not carry the trait. If males carry the trait, they will express it.

Example of such an inheritance pattern is hemophilia.

Explanation of Solution

Traits that are controlled by genes located on sex chromosomes; X chromosomes in particular are called sex- linked traits or X- linked traits. Since males have only X chromosome, they are affected by recessive X- linked traits more often than females. Females usually do not express the recessive trait as other X chromosome will mask the effect of the recessive trait.

The pedigree given below is an example of X linked recessive inheritance.

According to the given pedigree, father does not carry the trait but mother is a carrier of the trait. In the first generation, one son carries the gene while another one does not carry the gene. In the second generation, daughters carry the gene and sons may or may not carry the gene.

When a mother is a carrier of the gene,

• There is 50% chance that daughters will carry the gene and transfer it to next generation.
• There is 50% chance that a daughter will not carry the gene and therefore cannot transfer it to next generation.
• There is 50% chance that sons will also carry the gene and express it.
• There is 50% chance that sons will not carry the gene and will remain healthy.

There is no transmission of trait/ disorder from male to male.