Guest
Polyploidy is the heritable condition of possessing more than two complete sets of chromosomes. Polyploids are common among plants, as well as among certain groups of fish and amphibians. For instance, some salamanders, frogs, and leeches are polyploids. Many of these polyploid organisms are fit and well-adapted to their environments. In fact, recent findings in genome research indicate that many species that are currently diploid, including humans, were derived from polyploid ancestors. These species that have experienced ancient genome duplications and then genome reduction are referred to as paleopolyploids. This article discusses the mechanisms underlying polyploidy, and both the advantages and disadvantages of having multiple sets of chromosomes. How does an organism become polyploid? Polyploids arise when a rare mitotic or meiotic catastrophe, such as nondisjunction, causes the formation of gametes that have a complete set of duplicate chromosomes. Diploid gametes are frequently formed in this way. When a diploid gamete fuses with a haploid gamete, a triploid zygote forms, although these triploids are generally unstable and can often be sterile. If a diploid gamete fuses with another diploid gamete, however, this gives rise to a tetraploid zygote, which is potentially stable. Many types of polyploids are found in nature, including tetraploids (four sets of chromosomes), hexaploids (six sets of chromosomes), and other chromosome-pair multiples Researchers usually make a distinction between polyploids that arise within a species and those that arise due to the hybridization of two distinct species. The former are known as autopolyploids, while the latter are referred to as allopolyploids. Autopolyploids are essentially homozygous at every locus in the genome. However, allopolyploids may have varying degrees of heterozygosity depending on the divergence of the parental genomes. Heterozygosity is apparent in the gametes that polyploids produce. Allopolyploids can generally be distinguished from autopolyploids because they produce a more diverse set of gametes. Different species exhibit different levels of tolerance for polyploidy. For example, polyploids form at relatively high frequency in flowering plants (1 per 100,000 individuals), suggesting that plants have a remarkably high tolerance for polyploidy. This is also the case for some species of fish and frogs. However, higher vertebrates do not appear to tolerate polyploidy very well; in fact, it is believed that 10% of spontaneous abortions in humans are due to the formation of polyploid zygotes. Due to the high incidence of polyploidy in some taxa, such as plants, fish, and frogs, there clearly must be some advantages to being polyploid. A common example in plants is the observation of hybrid vigor, or heterosis, whereby the polyploid offspring of two diploid progenitors is more vigorous and healthy than either of the two diploid parents. There are several possible explanations for this observation. One is that the enforced pairing of homologous chromosomes within an allotetraploid prevents recombination between the genomes of the original progenitors, effectively maintaining heterozygosity throughout generations. This heterozygosity prevents the accumulation of recessive mutations in the genomes of later generations, thereby maintaining hybrid vigor. Another important factor is gene redundancy. Because the polyploid offspring now have twice as many copies of any particular gene, the offspring are shielded from the deleterious effects of recessive mutations. This is particularly important during the gametophyte life stage. One might envision that, during the haploid stage of the life cycle, any allele that is recessive for a deleterious mutation will not be masked by the presence of a dominant, normally functioning allele, allowing the mutation to cause developmental failure in the pollen or the egg sac. Conversely, a diploid gamete permits the masking of this deleterious allele by the presence of the dominant normal allele, thus protecting the pollen or egg sac from developmental dysfunction. This protective effect of polyploidy might be important when small, isolated populations are forced to inbreed. Problem: The histogram below summarizes the outcomes of simulations of random mating in population. This is a two allele system with B and b. N=5 A) How many populations are fixed B) How many populations have a B allele frequency of 0.5 after 10 generations? C) How many populations have a b allele frequency of 0.8 after 10 generations? #NikolaysGeneticsLessosns #biology #apBiology #Genetics #ploidy #DNA #chromosomes #teaching #tutorial #haploidNumber #Chromosome #cell #NumericalChromosomalAberrations #Euploidy #diploidy #polyploidy #Autopolyploidy #Allopolyploidy #Aneuploidy #Hypoploidy #Hyperploidy #SignificanceOfPloidy #TriticumDurum #SecaleCereale #Monoploidy #monosomy #trisomy #NULLISOMY #TETRASOMY #Education #TuscaroraHighSchool #cellDi
