Balanced lethal system
- 1. Balanced Lethal System Submitted By : Akshita Awasthi (A-2018-30-038) Submitted To: Dr R.K.Mittal &Dr Swaranlata
- 2. LETHAL GENES Some genes in an organism are known to cause the death of the organism. These genes can be either dominant or recessive, but the organism must be homozygous for them. Hence, alleles of a lethal gene show a deviation from the normal Mendelian inheritance. For instance, the Mendelian inheritance shows a phenotypic ratio of 3:1, whereas in the case of expression of lethal alleles there is a deviation from this ratio. Lethal alleles are produced when mutation in a usual allele distorts the function of an essential gene. This in turn results in a phenotype, which when expressed, is fatal to the organism carrying them.
- 4. When the allele is fully dominant it can cause the death of the organism in both heterozygous and homozygous conditions. But when the allele is recessive in nature the organism dies only when the lethal allele is in the homozygous condition. Types of lethal alleles Lethal alleles can be further classified into the following: recessive lethals dominant lethals conditional lethals balanced lethals gametic lethals.
- 5. Balanced lethals Two different lethal systems can operate in an organism, and balance each other's effect. Both genes are tightly linked in the repulsion phase of linkage where a dominant allele of one gene is linked to a recessive allele of another gene. In such cases, one parent contributes the dominant allele of gene one and recessive allele of gene two, while the other parent contributes the recessive allele of gene one and dominant allele of gene two. This arrangement of lethal alleles maintains a heterozygous combination, and ensures that homozygotes for lethal chromosomes do not occur.
- 6. Balanced lethal system Balanced lethal system was first reported by Muller (1917) in Drosophila where: if dominant alleles of two traits, each associated with a recessive lethal effect, are present in heterozygous repulsion phase, homozygotes will not survive and permanent hybridity will be maintained. A similar balanced lethal system operates in Euoenotheras, so that the heterozygous of different races is maintained in progeny over generations. In Oenothera lamarckiana, the lethality is zygotic .
- 7. Balanced lethal systems of Drosophila would therefore, lead to 50% ovule abortion or seed set. In most species of Oenothera, two gametic complexes are found and are called alpha(α) and beta (β) the former being present in all functional egg cells and the latter in all functional pollen grains. While beta is eliminated in eggs, alpha is eliminated in all functional pollen, due to failure of development of gametes containing these respective gametic complexes. This leads to 50% pollen abortion but a full seed set. The full ovule fertility or seed set is due to the fact that,
- 8. the two types of gametes will function irrespective of its position in linear tetrad,only that megaspore functions which carries alpha complex. Since alpha complex in the egg always unites with beta complex in pollen, plants breed true for heterozygous condition. Thus there are two balanced lethal mechanisms,one having zygotic lethality and the other involving gametic lethality. The gametic and zygotic lethality leads to survival of only heterozygotes. It may be noticed that in gametic lethality,only one of the male side, the other type being functional
- 9. on the female side, thus giving rise to only one type of progeny, which will be heterozygous. In zygotic lethality on the other hand, both type of gametes will function on male as well as on female side, but the homozygotes progeny due to recessive lethal genes will not survive.
- 10. Balanced Lethal system involving curly(Cy), and plum (Pm) genes Drosophila Two principles involved in evolving balanced lethal system in Oenothera. (A) gametic lethality, (B) zygotic lethality.
- 11. Interchange heterozygosity in Oenothera Subgenus Euoenothera of genus Oenothera has been studied during 1920-1930 & cytogenetic structure leading to evolution in this group was examined. This group has 2n=14 & all 7 chromosomes of a haploid complement have median centromeres. Different species in the subgenus Euoenothera, can be classified in three groups:
- 12. i) first group is represented by species showing bivalents or small rings at meiosis. e.g. O.hookeri, O.grandiflora ii) Second group is represented by species forming rings of various sizes at meiosis indicating the presence of interchanges. These rings are not permanent but are maintained due to their superiority in adaptive value. e.g. O.irrigua iii) the third group is represented by those having permanent translocation heterozygosity involving all chromosome so that a ring of 14 chromosome is regularly formed .
- 13. In O.lamarckiana, produces one bivalent and a ring of only 12 instead of a ring of 14 chromosomes is observed. In O.lamarckiana alternate segregation in the ring of 12 gave two complexes: 3.4, 12.11, 7.6, 5.8, 14.13, 10.9, and 4.12, 11.7, 6.5, 8.14, 13.10, 9.3, each also bearing the 1.2 chromosome of the segregating bivalent. Thus each complex of six chromosomes in this case would be considered as a single linkage group. According to terminology invented by Renner, the two complexes above were named velans and gaudens, respectively.
- 15. The interesting and surprising, however, was that O.lamarckiana bred true to these complexes and did not produce either velans/velans or gaudens/gaudens, although both homozygous are chromosomally balanced. The answer to this puzzle came from Muller’s observation of balanced lethal genes in Drosophila. Apparently, recessive lethals were maintained in both the velans and gaudens complexes, so that homozygous combinations were lethal. In lamarckiana this lethality affects the zygotes,
- 16. so that half the seeds did not germinate.In other Oenothera complexes, gametophytes were affected so that only the male gametophytes of one complex were viable in the female gametophytes of the other, i.e., only translocation heterozygous combinations could be produced .
- 17. REFERENCES Cytology Genetics and Evolution :P.K.Gupta Genetics : M.W.Strickberger