Question

Which of the following is a chromosome variation important for the evolution of new genes?

                        allopolyploidy

                        chromosome inversion

                        Robertsonian translocations

                        aneuploidy

                        chromosome duplications

Answer 1

Gene duplication (or chromosomal duplication or gene amplification) is a major mechanism through which new genetic material is generated during molecular evolution. It can be defined as any duplication of a region of DNA that contains a gene. Gene duplications can arise as products of several types of errors in DNA replication and repair machinery as well as through fortuitous capture by selfish genetic elements. Common sources of gene duplications include ectopic recombination, retrotransposition event, aneuploidy, polyploidy, and replication slippage.

Evolutionary fate of duplicate genes

Allopolyploidy

• Normally different species cannot interbreed because they have different chromosome numbers and cannot form diploid pairs
• Offsprings produced by such unions are typically sterile due to this uneven chromosome pairing
• If cytokinesis fails to occur in one of the gametes, the hybrid offspring will have paired chromosomes from that parent species
• If the hybrid interbreeds with a member of the other parent species, all chromosomes from both parent species will be paired
• The resulting offspring will now be fertile and have the combined chromosome composition of both parental species

Allopolyploids are more prevalent than autopolyploids as they do not show polysomic inheritance and have better fertility rates

Allopolyploidy

The high prevalence of polyploids in some groups clearly suggests that this must somehow confer a competitive advantage for the taxon. In plants, the allopolyploid progeny may come out healthier and fitter than either parent. This is usually a problem from a biodiversity standpoint because it means reduced diversity should the hybrid threaten the prevalence of either or both parents. When a hybrid is stronger that the parental species this is referred to as hybrid vigor or heterosis. Ideally, you want the hybrid to be just as fit as either parent, as this means increased diversity and a possibility of another evolutionary trajectory. Indeed, this is one of the ways explosive radiation of some lineages can arise.

Allopolyploids are more genetically diverse.

Where , Aneuploidy is presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more complete sets of chromosomes. A cell with any number of complete chromosome sets is called a euploid cell.

Gene duplication is an important mechanism for acquiring new genes and creating genetic novelty in organisms. Many new gene functions have evolved through gene duplication and it has contributed tremendously to the evolution of developmental programmes in various organisms.

An inversion is a chromosome rearrangement in which a segment of a chromosome is reversed end to end. An inversion occurs when a single chromosome undergoes breakage and rearrangement within itself. Inversions are of two types: paracentric and pericentric.

Paracentric inversions do not include the centromere and both breaks occur in one arm of the chromosome. Pericentric inversions include the centromere and there is a break point in each arm.

Inversions usually do not cause any abnormalities in carriers as long as the rearrangement is balanced with no extra or missing DNA. However, in individuals which are heterozygous for an inversion, there is an increased production of abnormal chromatids (this occurs when crossing-over occurs within the span of the inversion). This leads to lowered fertility due to production of unbalanced gametes.

Chromosomal inversions are widely thought to be favored by natural selection because they suppress recombination between alleles that have higher fitness on the same genetic background or in similar environments.

Robertsonian translocation (ROB) is a chromosomal abnormality wherein a certain type of a chromosome becomes attached to another. It is the most common form of chromosomal translocation in humans, affecting 1 out of every 1,000 babies born. It does not usually cause health difficulties, but can in some cases result in genetic disorders such as Down syndrome and Patau syndrome.Robertsonian translocations result in a reduction in the number of chromosomes.

Robertsonian translocation may provide material for evolution. Long term isolation of a group of individuals who are homozygous for a particular Robertsonian translocation chromosome could theoretically lead to the establishment of a new human subspecies having a full genetic complement in 44 chromosomes.

Among all discussion , concluded that gene or chromosome duplications are important for evolution of new genes. It mostly provided new genetic material during evolution.