LETHAL GENE By Dr H B Jaruhar assist

LETHAL GENE By : Dr. H. B. Jaruhar assist. Prof. , Dept. of Botany , S. P. D. College, Garhwa Programme name: B. Sc. Botany Course name: Genetics Title: Mendelian genetics and its extension

Introduction • Unlike the other gene which produce visible phenotypes, lethal genes seldom appear in phenotypes. • It is however, observed by either disappearance in phenotypic production in offspring when it was expected. • Lethal phenotypes not visible because such gene causes unviable phenotypes to appear which may die during embryogenesis or just after birth depending upon the metabolic response of the gene action. Since these genes causes death of the offspring it is called lethal gene.

Early Discovery In 1905, A French geneticist, Cuenot working on a Mouse which have yellow skin body that did not appear to fit expected 3: 1 ratio of mendelian segregation

When he crossed two yellow mouse , they produced 2/3 yellow and 1 Agauti mouse. Cuenot found when yellow offspring back crossed with normal agauti all yellow were heterozygous and none were homozygous yellow.

Lethal gene Yellow body colour is usually have dominant effect. It can not breed as pure strain because the allele for homozygous yellow YY shall have lethal effect on mouse.

Types of lethal gene 1. Dominant lethal 2. Recessive lethal 3. Sex linked lethal and 4. Conditional lethal

Dominant lethal gene: • In dominant lethal gene the homozygous dominant traits disappeared in the population and only heterozygous phenotypes present in population. Example- yellow body colour of mouse.

Recessive lethal gene • Such gene rarely identified in the population and because they are producing lethal action in homozygous phenotype but can be carried out through heterozygous phenotypes in the population.

Recessive lethal gene • In Human a genetic disorder called Tay sachs disease responsible for recessive lethal gene. • The homozygous individual for this gene lack one of the enzyme needed for normal metabolism of fatty substances. The phenotype of the disease become visible after the first year one year when fatty substances accumulate in nerve sheaths. The transmission of nerve impulse become affected leading to loss of muscular control and mental deficiencies within few years the individual die.

Sex linked lethal gene • Some lethal genes located on sex chromosome usually on X chromosome are called sex linked lethal gene. • Sex linked lethal gene may be dominant or recessive. • It is usually expressed itself in hemizygous condition because it can be expressed even it is present as single allele in the offspring.

Sex linked lethal gene • Presence of sex lethal gene alter the sex ratio in the population so that more female are born instead of the expected ratio of 1 male : 1 female. • Thus a female carrying a recessive lethal gene will produce a progeny in which one half of the male offspring would not be viable. • The sex linked lethal gene are applied however largely to detect mutations in Drosophila and other organism.

Sex linked lethal gene • In human muscular dystrophy is due to an X linked recessive gene which shows a visible phenotype many years after birth. Boys having this gene are normal for about 10 years after which there is failure of muscular control and death results.

Conditional lethal gene • Some time an organism live under one set of condition, but when certain changes are introduced in its environment , lethality occurs. Such conditional lethality is called conditional lethal. • In D. pseudoobscura, Dobzhannsky, recognized conditional lethal. • The fly live normally at temp. 16. 50 C but at 250 C it dies.

Conditional lethal gene • In 1970, Suzuki, a Japanese geneticist, described conditional lethal in D. melanogaster. • Certain mutant fly become lethal when they were exposed to high temp. only at larval stage. If the larva was kept at low temp. the fly metamorphose to young fly and live even at high temp. throughout the life cycle. • Probably a specific gene product ( enzyme or protein) become altered , causing death if the larva are exposed to high temp. during critical period. • With this perception microorganism are extensively studied for analyzing genes for such enzymes and proteins.

conclusions • Lethal gene in the organism are common occurrence and it is one of the principal regulatory mechanism to control the number as well as sex ratio in the population. • Most of the lethal gene are mutant of the normal/ wild type gene. • Lethal gene are also important tool for study of inheritance. • Several genetic disorders are lethal in human beings which are considered were lethal in nature but treated to elongate survival rate. • All the lethal gene however one of the cause of alteration in Normal mendelian Segregation Ratio of 3: 1.