Lethal Genes PDF

# Lethal Genes ## What is a lethal allele? - Lethal alleles occur when a mutation results in an allele that produces a non-functional version of an essential protein. - If an individual inherits a lethal combination of mutated alleles, it will die before or shortly after birth. ## What are Lethal...

# Lethal Genes ## What is a lethal allele? - Lethal alleles occur when a mutation results in an allele that produces a non-functional version of an essential protein. - If an individual inherits a lethal combination of mutated alleles, it will die before or shortly after birth. ## 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 the organism in both homozygous and heterozygous condition. - Certain lethal alleles kill organisms in homozygous condition only. ## Types of Lethal Alleles ### Dominant Lethal Genes - Dominant lethal genes are expressed in both homozygotes and heterozygotes. - These are rarely detected due to their rapid elimination from populations. - Example: -Coat colour inheritance of mouse [L. Cuenot (1905)] -Huntington's disease in Human Beings. ### Synthetic 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. ## Types of Lethal Alleles - **1) Early Onset:** Lethal alleles which result in early death of an organism during embryogenesis. - **2) Late Onset:** Lethal genes which have delayed effects so that the organism can live for some time but eventually succumb to the disease. - **3) Conditional:** Lethal alleles which kill the organism under certain environment conditions only. For example, a temperature-sensitive lethal allele may kill the organism at a high temperature, but not at a low temperature. - **4) Semi Lethal:** Lethal alleles which kill only some individuals in the population but not all. ## History - 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. - E. Baur (1907) observed a lethal gene in Snapdragon (Antirrhinum) and found that it is characterized by variegated leaves. ## Lethal 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. ## Lethal gene in mouse body coat(colour) - 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. ## 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 were 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 whereas 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. ## Examples of Lethal Alleles - When lethal genes prevent full term development of the embryo, offspring are produced in a 2:1 ratio (2 heterozygotes to one normal). - In the inheritance of coat colour in yellow mice, offspring phenotype ratios depart from the expected Mendelian 3:1 when yellow mice are mated together. - About two thirds of the offspring are yellow, and one third are non-yellow (right). A testcross reveals the yellow coloured mice to be heterozygotes.. ## Thank you - The page ends with a thank you note with an image of a DNA molecule and two other chemical structures.