3.9 Autosomal Inheritance :

Human somatic (2n) cell contains 23 pairs of chromosomes. They can be divided functionally as autosomes and sex chromosomes. A single pair of chromosomes is involved in sex determination and remaining 22 pairs are called autosomes.

Autosomes control a variety of traits other than sex. These traits are called autosome linked traits. Transmission of body characters other than the sex linked traits from parents to their offsprings through autosomes, is called autosomal inheritance.

Some characters are influenced by dominant genes while some other are by recessive genes, present on autosomes.

For example,

• Autosomal dominant traits like Widow's peak and Huntington's disease, etc.

• Autosomal recessive traits like Phenyl ketonuria (PKU), Cystic fibrosis and Sickle cell anaemia.

a. Widow's peak :

A prominent "V" shaped hairline on forehead is described as widow's peak.

It is determined by autosomal dominant gene. Widow's peak occurs in homozygous dominant (WW) and also heterozygous (Ww) individuals.

Individuals with homozygous recessive (ww) genotype have a straight hair line (no widows peak). Both males and females have equal chance of inheritance.

b. Phenylketonuria (PKU):-

It is an inborn metabolic disorder caused due to recessive autosomal genes.

When recessive genes are present in homozygous condition, phenylalanine hydroxylase enzyme is not produced. This enzyme is essential for conversion of amino acid phenylalanine into tyrosine. Due to absence of this enzyme, phenylalanine is not converted into tyrosine. Hence, phenylalanine and its derivatives are accumulated in blood and cerebrospinal fluid (CSF). It affects development of brain and causes mental retardation. Excess phenylalanine is excreted in urine, hence this disease is called phenylketonuria.

Autosomal recessive traits appear in both sexes with equal frequency. These traits tend to skip generations.