Genetics

Environmental

Influence on Gene

Expression

Gene Expression

The gene expression is determined by

two features called as penetrance and

expressivity of the genes. Penetrance

is the ratio of individuals exhibiting

expected phenotype and expressivity is

the extent of gene expression in an

individual.

Environmental Factor

Temperature in Gene Expression

- The body of the rabbit which is generally

exposed to high temperature (>34 degrees)

expressed white color whereas the other parts like

ears, nose , tail and paws which are little exposed

to temperature expressed black color. Keeping the

rabbit under cold climate resulted in the

expression of fully black colored skin.

Environmental Factor

Light in Gene Expression

- Maize plant carrying the homozygous gene

for pigmentation when exposed to sun light

developed bright red color and when the light was

retarded by covering the area of the plant prone to

pigmentation, the bright red phenotype was not

observed.

Environmental Factor

Chemicals in gene expression

- Normally, F. heteroclitus embryos feature

two eyes; however, half of the eggs placed in the

magnesium chloride mixture gave rise to one-eyed

embryos.

Environmental Factor

Nutrition in Gene Expression

- The deficient nutrient supplement alters the

genetic expression. What a pregnant woman eats

determines the health of her offspring.

Lethal

Alleles

Lethal Alleles

-Genes which result in viability reduction of

individual or become a cause for death of

individuals carrying them.

Types of Lethal Allele

- Recessive - Sex-linked

- Dominant - Conditional

Recessive Lethal Allele

Recessive lethal genes can code for either

dominant or recessive traits, but they do not

actually cause death unless an organism carries

two copies of the lethal allele.

Examples of human diseases caused by recessive

lethal alleles include cystic fibrosis, sickle-cell

anemia, and achondroplasia

Punnett Square

Punnett Square

Dominant Lethal Alleles

Dominant lethal genes are expressed in both

homozygotes and heterozygotes

Dominant lethal genes are rarely detected due to

their rapid elimination from populations. One

example of a disease caused by a dominant lethal

allele is Huntington's disease

Conditional Lethal Allele

An organism lives normally under one set of

conditions, but when certain changes are

introduced in its environment, lethality results.

Favism is a sex-linked, inherited condition that

results from deficiency in an enzyme called

glucose-6-phosphate dehydrogenase.

The lethal gene is carried on the sex chromosome,

usually X.

Hemophilia is a hereditary disease caused by

deficiencies in clotting factors, which results in

impaired blood clotting and coagulation.

Because the allele responsible for hemophilia is

carried on the X chromosome, affected individuals are

predominantly males, and they inherit the allele from

their mothers.

Sex-Linked Lethal Allele

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