Lecture 3: Lethal Genes (PDF)

LECTURE 3 By Prof.Dr/ HEBA TAHA Professor of Genetics Genetics Department Faculty of agriculture LETHAL GENES One of the most important assumptions of inheritance of any trait is the equal survival of all gametes and zygotes produced as a result of segregation. Some genes affe...

LECTURE 3 By Prof.Dr/ HEBA TAHA Professor of Genetics Genetics Department Faculty of agriculture LETHAL GENES One of the most important assumptions of inheritance of any trait is the equal survival of all gametes and zygotes produced as a result of segregation. Some genes affect the survival of those zygotes or individuals in which they are present in the proper genotype and are called as lethal genes.  LETHAL GENES A Lethal genes B semi lethal genes Lethal genes C sub vital genes D Vital genes E super vital genes.  A. LETHAL GENES A lethal gene causes death of all the individuals carrying this gene in the proper genotype before these individuals reach adulthood. The proper genotype for an allele would depend on its dominance relationship with its other allele(s). For an allele producing a A a recessive effect on survival, A Aa AA the proper genotype would be the a Aa homozygous state. aa while for an allele having a A a dominant effect on survival A AA Aa both homozygous and heterozygous a aa states would be the proper genotype. Aa Lethal genes may be grouped into the following five categories: 1 Recessive lethals 4 Balanced lethals 2 Dominant lethals 5 Gametic lethals. 3 Conditional lethals  1 Recessive Lethals Most of the lethal genes are recessive lethals as their effect is expressed only when they are in the homozygous state. The survival of heterozygotes is unaffected. A recessive lethal affecting coat colour in mice the yellow coat colour in mice is produced by a dominant gene Y, while its recessive allele y determines the normal grey coat colour. Furthermore, all yellow coat colour mice were heterozygous (Yy) and no mice with a homozygous Y allele (YY) were found. Later in 1910, it was shown by Castle and Little that the dominant phynotypic allele Y is a recessive lethal effect and that it causes death of homozygous YY embryos at an early stage of development. They showed that approximately 25 % of the embryos of yellow females mated to yellow males are unviable and fail to develop in a very early stage of embryonic development. But many genes are recessive both in their phenotypic as well as lethal effects. In such cases, heterozygous individuals have normal phenotype as well as normal survival and they cannot be differentiated from normal homozygotes. An example of such a gene are the genes producing albino seedlings in barley. Albino character is governed by recessive alleles in barley. When anyone of these alleles is in homozygous state, the seedlings are near- white and totally devoid of chlorophyll. Albino seedlings survive only as long as the food stored in seeds is available since they are not able to carry out photosynthesis. The heterozygotes, however, are normal green and are identical with the normal homozygotes in their phenotype as well as survival. Segregation for such genes yields 3 green: 1 albino, if the seedlings are scored within a week from germination. However, if the plants are scored several weeks after germination or at maturity, there will be only green plants in the progeny. The lethal genes that reduce the survival of zygotes that carry them in the proper genotype are known as zygotic lethal. The stage of development at which a lethal gene produces its lethal effect varies considerably from one gene to the other. Some genes cause the death of embryo very early in development, e.g., the Y gene in mice, while others allow survival and development close to the reproductive age, e.g., Epiloia in man.  2 Dominant Lethal Dominant Lethal Dominant lethal allele kills both in homozygous and heterozygous states. An example of a dominant lethal is the epiloia and Huntington disease in human beings. Epiloia in man: This gene causes abnormal skin growths, severe mental defects and multiple tumors in the heterozygotes so that they die before reaching adulthood. Therefore Dominant lethals, cannot be maintained in the population. They have to be produced in every generation through mutation.  HUNTINGTON’S DISEASE Huntington disease is a progressive brain disorder that causes uncontrolled movements, emotional problems, and loss of thinking ability (cognition). Adults with Huntington disease, the most common form of this disorder, usually appears in a person's thirties or forties.  3 Conditional Lethals Conditional Lethals Some lethal genes act only under specified condition which is necessary for their lethal effect and such lethal genes are termed as conditional lethals. Otherwise, they behave as the normal allele. An example of a temperature sensitive lethal gene is the Kidney-eyed mutant of the wasp Bracon hebetor. This gene allows normal development and survival at lower temperatures, but is lethal at 30°C. Similarly, a chlorophyll mutant of barley permits normal chlorophyll development at a temperature of 19°C or above but produces albino seedlings at temperatures below 8°C. This conditional lethal in barley requires a lower temperature to exert its lethal effect.  4 Balanced Lethals Balanced Lethals In progeny from the matings between two heterozygote parents, about 1/2 individuals are homozygous for the normal allele, while the remaining 1/2 are heterozygous for this gene. But in a balanced lethal system all the surviving progeny are heterozygous for the lethal genes and the homozygotes for their normal alleles are not obtained. In such a system occurs due to linkage of two nonallelic recessive lethals in the same chromosome. In the example of a balanced lethal system shown, the recessive allele (l1) of the first lethal gene and the normal allele (L2) of the second lethal gene are present in one chromosome, while the homologous chromosome carries normal allele (L1) of the first gene and recessive lethal allele (l2) of the second gene. Lethal genes arranged in this manner are known as balanced lethal and the arrangement itself is called as balanced lethal system. Heterozygotes for both the recessive lethal are viable. When crossed, 25% of the zygotes will be homozygous for the recessive lethal l1 and will not survive. Another 25% will be homozygous for the other recessive lethal l2 and will die. Only heterozygotes will survive. Thus a balanced lethal system maintains the genes closely linked to the lethal genes in a perpetual heterozygous state.  5 Gametic Lethals Gametic Lethals Some genes lead to inviability of gametes, making them incapable of fertilization and such genes are termed as gametic lethals. Gametic lethals lead to a Complete disappearance from the typical ratios expected in segregating generations. This phenomenon is commonly known as segregation distortion or meiotic drive.  B. SEMILETHAL GENES Semilethal genes do not lead to death of all the individuals that carry them in proper genotype but cause death of more than 90% of the individuals. Example, xantha mutants of many plants are semilethal in the homozygous state. C. SUBVITAL GENES When the mutant genes reduce viability of the individuals and kill less than 90% of the individuals carrying them in proper genotype they are called as subvital genes. Most of the mutant genes are subvital in their effect. Example: miniature wings in Drosophila, Viridis mutants of barley (deficient in the aerobic cyclase reaction of chlorophyll biosynthesis)  D. VITAL GENES Those genes, which do not affect survival of the individuals in which they are present are known as vital genes. The vital genes neither enhance nor reduce the viability of individuals carrying them in appropriate genotype. Wild type alleles of all the genes of an organism are regarded as vital genes. They serve as the reference point in determining the effect of mutant alleles of these genes on survival. E. SUPERVITAL GENES Some mutant alleles enhance the survival of those individuals that carry them in proper genotype and such genes are known as supervital genes. Example: genes for disease resistance, genes conferring resistance/tolerance to the various abiotic stresses, e.g., salinity, alkalinity, high temperature, low temperature, drought etc., may be regarded as supervital genes as they enhance the fitness of plants in presence of the concerned stress.