Inheritance

Traits are inherited in various ways, and certain patterns or rules governing inheritance have been worked out. These rules, known as Mendel’s Laws, are named after the first person to describe these patterns of inheritance, Gregor Mendel.

 

The architecture of the genetic origin of traits is very diverse. For some traits, a single change in a single gene can have an effect; these are known as monogenic inherited traits. For the expression of other traits, changes in several genes must be present; these are polygenic traits. Still other traits are due to genetic alterations of many genes as well as environmental factors; these traits are described as being multifactorial.

 

Table 1: Inheritance of monogenic traits Inheritance

 

Chromosome with altered gene copy

Number of altered gene copies for expression of trait

Autosomal dominant

1-22

1

Autosomal recessive

1-22

2

X-chromosomal dominant

X

1

X-chromosomal recessive

X

1 (in men)

2 (in women)

Y-chromosomal

Y

1 (expression only in men)

 

The inheritance of monogenic traits is according to Mendel’s Laws (Table 1). If a trait is expressed when only one gene copy on one chromosome of a chromosome pair is altered, it is referred to as being dominant (Fig. 1). If both copies of a gene on a chromosome pair have to be altered before a trait is expressed, it is known as a recessive trait (Fig. 2). Depending on whether an altered gene is on an autosomal (chromosomes 1-22) or sex-determining (X or Y) chromosome, it is referred to as being linked to an autosomal or sex-linked trait. In the latter case, only the X-chromosome plays a significant role (Fig. 3).

The various modes of inheritance can be deduced from various combinations that can be passed on to the offspring via the transfer of individual chromosomes.

 

Figure 1: Autosomal-dominant inheritance: In this example the father is the carrier of an alteration in a gene copy on chromosome 2.

Figure 2: Autosomal-recessive inheritance: In this example both parents are carriers of an alteration in a gene copy on chromosome 7.

 

Figure 3: X-Chromosome-recessive inheritance: In this example the mother is the carrier of an alteration in a gene copy on the X-chromosome.