03 Inheritance and Variation - part 09 - Autosomal Inheritance

03 Inheritance and Variation - part 09 - Autosomal Inheritance


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, 
  1. Autosomal dominant traits like Widow's peak and Huntington's disease, etc.
  2. 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.

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