Presentation on lethal gene by Sazzad Mahmud ,Department of Pharmacy, PUST

1. WELCOME TO LETHALITY

3. Definition: Genes which result in viability
reduction of individual or become a cause for
death of individuals carrying them are called lethal
genes.
Some lethal genes cause death of zygote or the early
embryonic stage while some express their effect in
later stages of development.
Certain genes are absolutely essential for
survival. Mutation in these genes creates lethal
allele.

4. History
Lethal genes was first discovered by Lucien
Cuenot while studying the inheritance of coat
colour in mice.
He expected a phenotype ratio from a cross of 3
yellow: 1 white, but the observed ratio was 2:1.
Allele was lethal in homozygous dominant
condition.

5. Lethal
Gene
1.
Recessive
lethals
3.
Conditional
lethals
2.
Dominant
lethals

6. 1. Recessive lethal:
Most of the lethal genes are recessive lethals. It is expressed only when
they are in homozygous condition. The survival of heterozygotes is not
affected e.g., coat colour in mice. According to Cuenot, Castle and Little,
the dominant allele Y is a recessive lethal and it causes death of
homozygous YY embryos at an early stage of development.
2. Dominant lethal:
There are some lethal genes which reduce viability even in heterozygotes,
are said as dominant lethals. e.g., epiloia gene in human beings. This
cause mental defects, abnormal skin growth and tumors in heterozygotes,
therefore, they die before reaching adulthood. The dominant lethals may
be produced in every generation through mutation.
Explanation : TT Tt tt

7. 3. Conditional lethal:
The lethal genes require a definite or specific condition for their lethal
action are said as conditional lethals. Many mutants of barley, maize,
Neurospora, Drosophila and many other organisms are termed as
temperature sensitive mutations. Each of them needs a definite,
generally high temperature to express their lethal action.
A chlorophyll mutant of barley allows normal
chlorophyll development at a temperature of
19°C or above, but it develops albina or abnormal
white seedlings at temperature below 8°C.
Temperature is not only responsible to bring out
conditional lethals. Some conditional lethals
require light, nutrition etc.

8. Lethal Gene in Mouse
Y gene in mouse: Y gene is responsible for yellow coat color
in mouse. In recessive condition it produces white color.
Allele Y is lethal only when it present in homozygous
dominant condition (YY).
But this allele is not lethal in heterozygous dominant (Yy)
conditions.

10. Feel free to ask any questions