This document discusses different types of gene interactions and single gene disorders. It describes how gene expression can be affected by other genes, either through allelic or non-allelic interaction. Epistasis occurs when a gene's effect depends on the presence or absence of other genes. Single gene disorders can result from mutations in dominant, recessive, or X-linked genes. X-linked disorders particularly affect males since they only have one X chromosome.

1. Gene interaction
• The phenomenon of two or more genes affecting the
expression of each other in various ways in the
development of a single character of an organism is
known as gene interaction.
• In gene interaction, expression of one gene depends
on expression(presence or absence)of another gene.
• Most of the characters of living organisms are
controlled/influenced/governed by a collaboration of
several different genes.

2. Types of gene interaction
• Allelic/non epistatic gene interaction – this type of
interaction gives the classical ratio of 3:1 or 9:3:3:1
• Non -allelic/epistatic gene interaction – in this type of
gene interaction genes located on same or different
chromosome interact with each other for their
expression .

3. Epistasis
• Epistasis is a phenomenon in genetics in which the effect of
a gene mutation is dependent on the presence or absence of
mutations in one or more other genes, respectively termed
modifier genes. In other words, the effect of the mutation is
dependent on the genetic background in which it appears.
• In mice, the expression of a specific fur color by one gene is
dependent upon the production of hair pigment by another
gene.
• Black fur (B) is dominant to brown fur (b), but in the
absence of hair pigment (cc) mice will appear albino.

4. Epistasis – Mouse Coat Colour

5. Single gene disorders In humans
• Single gene disorders are caused by DNA changes in
one particular gene, and often have predictable
inheritance patterns.
• Single gene disorders can be divided into different
categories:
 dominant
 recessive and
 X-linked.

6. Dominant diseases
• Dominant diseases are single gene disorders that
occur in the heterozygous state – when an individual
has one mutant copy of the relevant gene and one
healthy copy.
• Dominant disorders tend to crop up in every
generation of an affected family because everyone
carrying a dominant mutant allele shows the
symptoms of the disease.
• Dominant disorders spread vertically down family
trees, from parent to child.

7. Example of a dominant single gene
disorder

8. Recessive Diseases
• Recessive diseases are single gene disorders that only occur in
the homozygous state - when an individual carries two mutant
versions (alleles) of the relevant gene.
• Recessive diseases are more difficult to trace through family
trees because carriers of a mutant allele do not show symptoms
of the disease. It therefore appears that the disease has skipped
a generation when it is seen in groups of children within a
family.
• The risk of an individual having a recessive disorder increases
when two people who are closely related have a child together
(consanguinity). This is because there is a much greater chance
that the same mutant allele will be present in related parents.

9. Example of recessive diseases

10. X-Linked disorders
• X-linked disorders are single gene disorders that
result from the presence of a mutated gene on the
X chromosome.
• Because females (XX) have two copies of the
X chromosome but males (XY) only have one copy,
X-linked disorders are more common in males. If a
male’s single copy on the X chromosome is mutated
he has no healthy copy to restore healthy function.
• Like other single gene disorders, X-linked disorders
can be either recessive or dominant.

11. X-linked recessive diseases
• Examples of X-linked recessive disorders include
red-green colour blindness, haemophilia and the
Duchenne and Becker forms of muscular dystrophy.
• X-linked recessive disorders are much more common
in males than females because two copies of the
mutant allele are required for the disorder to occur in
females, while only one copy is required in males.

12. The overall pattern of the disease is characterised by
the transmission of the disease from a carrier mother,
who inherited a copy of the mutant gene from her
affected father (this is sometimes described as a
‘knight’s move’).

13. Males always pass their X chromosome to their daughters
but never their sons (who receive their Y chromosome). These
daughters are described as obligate carriers. They generally
show no disease symptoms as they have one copy of the
mutant gene but also one copy of the healthy gene.

14. Female carriers pass the defective X chromosome to
half of their daughters (who are carriers) and half of
their sons (who will be affected by the disease). Their
other children will inherit the healthy copy of the gene.

15. X-linked dominant disorders
• X-linked dominant disorders are very uncommon.
Examples include Rett syndrome (a condition found
almost exclusively in girls that seriously affects brain
development, causing severe disabilities) and some
inherited forms of rickets (slowed growth and skeletal
development due to vitamin D deficiency).
• Unlike other dominant diseases, X-linked dominant
disorders cannot be transmitted from father to son
because fathers do not pass their X chromosome to
their sons.