By Shrestha Das

Mendelian Disorders In Humans

The disorders whose inheritance is in accordance to the Mendels’ laws of inheritance.

They occur as a result of mutation.They are single gene disorders.They may be 4 types:1. Autosomal Dominant2. Autosomal Recessive3. X-linked Dominant4. X-linked Recessive5. Mitochondrial

What are Mendelian Disorders?

1st Law: Law of Dominance: For a group of genes, the one which expresses itself is called the dominant while the other is called the recessive gene.

Eg: In pea plant T and t are the genes for height of the pea plant. T (tall) is dominant while t (short) is recessive. Thus in a condition such as Tt, T will express itself and the pea plant will be tall.

A Recap of Mendels’ Laws of Inheritance

2nd Law: Law of Segregation: The two members of a gene pair (alleles) segregate (separate) from each other in the formation of gametes. Half the gametes carry one allele, and the other half carry the other allele.

3rd law: Law of independent assortment: Genes for different traits assort independently of one another in the formation of gametes.

Humans have 23 pairs of Chromosome out of which 21 are autosomes and 1 pair is the sex chromosome.

1. Autosomal Dominant: When the affected gene is located on an

autosome and the disorder gets expressed in both homozygous and heterozygous condition, the disorder is called autosomal dominant.

Affected person has an affected parent and occus in every generation.

Eg: Cardiomyopathy, hypercholesterolemia, skeletal dysplasia etc.

Types Of Mendelian Disorders

2. Autosomal Recessive: When the affected gene is located on an

autosome but the disorder gets expressed only in homozygous condition, the disorder is called autosomal recessive.

Both parents of an affected person are either carriers or affected themselves.

Eg: Sickle-celled anaemia, phenylketonuria, cystic fibrosis etc.

3. X-linked dominant: When the affected gene is located on the X sex

chromosome and the disorder gets expressed in both homozygous and heterozygous condition, the disorder is called X-linked dominant.

Females are more frequently affected but both males as well as females can be affected in one generation.

Eg: Hypophatemic rickets (vitamin D-resistant rickets), ornithine transcarbamylase deficiency

4. X-linked recessive: When the affected gene is located on the X

sex chromosome and the disorder gets expressed only in homozygous condition, the disorder is called X-linked recessive.

Males are more frequently and affected males are present in every generation.

Eg: Haemophilia, colour blindness, muscular dystrophy etc.

5. Mitochondrial Disorder:When the affected gene is coming from the

mitochondrial DNA it is called mitochondrial disorder.

It is transferred from one generation to the other only by females but can affect both males and females.

It can appear in every generation.Eg: Leber’s hereditary optic neuropathy,

Kearns-Sayre syndrome