This document discusses Mendel's laws of inheritance and the different types of single gene disorders. It explains the three laws of inheritance: dominance, segregation, and independent assortment. The five types of single gene disorders are then defined: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, and mitochondrial. Each type is described in terms of inheritance pattern, affected individuals, and examples.

1. By
Shrestha Das

2.  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 Dominant
2. Autosomal Recessive
3. X-linked Dominant
4. X-linked Recessive
5. Mitochondrial

3.  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.

4.  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.

5.  3rd law: Law of independent assortment:
Genes for different traits assort independently of one
another in the formation of gametes.

6. 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.

8. 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.

10. 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

13. 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.

15. 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