Lethal genes

A SIMPLE POWERPOINT FOR EASY UNDERSTANDING OF THE TERM LETHAL GENES.

1. LETHAL GENES SUBMITTED BY ; SATISH KANNA . M 1 ST MSc. Botany Bharathidasan University Tiruchirappalli – 620 024 SUBMITTED TO ; Dr . T. SENTHIL KUMAR M. Sc .,M.Phil., Ph.D Department of Botany Bharathidasan University Tiruchirappalli – 620 024

2. TABLE OF CONTENTS Defination ,History, Observers and their Observations a quick overview. What Are Lethal Genes ? 01 An overview of major types of lethal genes or lethal alleles Types of Lethal Genes 02 An overview Lethal gene in mouse 04 An overview. Lethal gene in Snapdragon 03

3. INTRODUCTION

4. WHAT ARE LETHAL GENES ? Lethal Gene - a gene that leads to the death of an individual. Certain genes are absolutely essential for survival. Mutation in these genes creates lethal allele / lethal genes. It can be either dominant or recessive in nature or alleles that cause an organism to die only when present in homozygous condition are called lethal alleles. The gene involved is considered as an essential gene . Essential genes are those indispensable for the survival of an organism, and therefore are considered a foundation of life. Lethal alleles are dominant or recessive. Fully dominant lethal allele kills organism in both homozygous and heterozygous condition . Certain lethal alleles kills organisms in homozygous condition only.

5. 1905 E. Baur (1907) observed lethal gene in Snapdragon (Antirrhinum) and found that it is characterized by variegated leaves 1910 HISTORY 1907 A French geneticist L. Cuenot (1905) reported lethal gene on the inheritance of mouse body colour. W. E. Castle and  C. C. Little  confirmed Cuénot's work and the first documented example of a recessive lethal allele was recorded.

6. 01 02 Discovered the concept of lethal gene first in animals . Lucien Cuenot (1905 ) Observed the lethal gene first in plants . E. Baur (1907) Lethal gene in mouse body coat(colour) L ethal gene in Snapdragon (Antirrhinum) E. Baur (1907) observed lethal gene in Snapdragon (Antirrhinum) and found that it is characterized by variegated leaves. The “golden” variety on selfing gives rise to 2 types of offsprings , golden and green in the ratio of 2:1 instead of 3: 1. The golden ones are heterozygous and the green ones breed true being recessive homozygous. A French geneticist L. Cuenot (1905) reported on the inheritance of mouse body colour. He found that “yellow” body colour was dominant over normal “brown” colour and was governed by single gene “Y”. It was observed that yellow mice could never be obtained in homozygous condition.

7. TYPES OF LETHAL ALLELES LATE ONSET EARLY ONSET CONDITIONAL SEMI - LETHAL Lethal alleles which result in early death of an organism ,during embryogenesis . 1) EARLY ONSET Lethal alleles which kill organism under certain environment conditions only . For example, a temperature sensitive lethal allele may kill organism at high temperature, but not at low temperature. 3) CONDITIONAL Lethal genes which have delayed effects so that the organism can live for some time but eventually succumb to the disease. 2) LATE ONSET Lethal alleles which kills only some individuals in the population but not all. 4) SEMI LETHAL 4 MAJOR TYPES

8. TYPES OF LETHAL ALLELES Dominant Lethal Genes Scientists studying the fruit fly observed that pairwise combinations of some mutant alleles were not viable, whereas singly, the same mutant alleles did not cause death (Boone  et al ., 2007 ). In other words, some mutations are only lethal when paired with a second mutation. These genes are called synthetic lethal genes. When the functions of the two affected genes are not fully understood, scientists can create and study synthetic lethal mutants and their phenotypes to identify a gene's function. Dominant lethal genes are expressed in both homozygotes and heterozygotes . These are rarely detected due to their rapid elimination from populations . Example ; i )coat colour inheritance of mouse [ L. Cuenot (1905) ] ii) Huntington’s disease in H uman Beings. Synthetic Lethal Genes

9. Lethal gene in Snapdragon ( Antirrhinum majus ) In 1907, Edwin Baur began his work with the snapdragon plant  Antirrhinum  and found that it is characterized by variegated leaves(characterized the condition  aurea ) , in which plants had golden instead of green leaves. When two  aurea  snapdragon plants were crossed, Baur observed a 2:1 ratio of golden seedlings to green seedlings. Homozygous   aurea  plants lacked normal chlorophyll development and died either during the embryonic stage or when the plant seedlings were two to three days old. Examples of human diseases :- cystic fibrosis and sickle cell anemia .

10. Lethal gene in Mouse A French geneticist L. Cuenot (1905) reported on the inheritance of mouse body colour. He found that “yellow” body colour was dominant over normal “brown” colour and was governed by single gene “ Y ”. It was observed that yellow mice could never be obtained in homozygous condition . When yellow coated mice was crossed with another yellow coated mice, segregation for yellow and brown body colour was obtained in 2:1 ratio. The brown individuals were pure and homozygous where as yellow individuals were heterozygous. These results may be explained on the assumption that the dominant allele for yellow body colour is lethal in homozygous condition.

11. QUERIES AND QUESTIONS

12. REFERENCES https ://www.nature.com/scitable/topicpage/mendelian-ratios-and-lethal-genes-557/ https://www.biologydiscussion.com/genetics/modified-monohybrid-ratios/lethal-genes-meaning-and-types-genetics/35328 BIOLOGY DISCUSSION (WEBSITE) LIFE SCIENCES Fundamentals and Practices II By: Usha Mina and Pranav Kumar

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