If you took Biology in middle school, you probably remember that human cells contain 46 chromosomes, or 23 chromosome pairs (23 chromosomes inherited from the father plus 23 chromosomes inherited from the mother). Cells containing two sets of chromosomes, and therefore two copies of each gene, are called diploid cells, and most of the cells in your body are indeed diploid. However, egg cells and sperm cells are haploid, meaning that they only contain 23 chromosomes (one copy of each gene). This is the result of meiosis – a special kind of ovary/testis-specific cell division in which each egg/sperm cell is allocated just one randomly selected chromosome from each chromosome pair. Consequently, when an egg cell is fertilized by a sperm cell, a new diploid individual is created with a unique combination of chromosomes.
Halving your genetic information and mixing it with half of your partner’s genetic information every generation is a sustainable reproductive method. And yet, many plants, fish and frogs are polyploid, meaning that their cells contain more than two sets of chromosomes! Polyploidy occurs due to abnormal chromosome segregation during cell division and can result in anything from the three sets of chromosomes banana cells contain to the twelve sets of chromosomes found in the Uganda clawed frog.
But if polyploidy is abnormal, why is it so common in organisms such as the banana plant? The answer is that polyploidy appears to be advantageous because the polyploid offspring of diploid parents are often healthier than their parents. This can be explained by the special protection polyploidy provides from recessive mutations. In diploid organisms, an individual with just one copy of a gene with a recessive mutation is merely a carrier, while an individual unfortunate enough to have been dealt two copies of the mutated gene is afflicted. But a triploid organism like the banana would still be a carrier at two copies of the mutated gene and would require three copies of the mutated gene to be afflicted, rendering this outcome much more improbable. This shielding from the potentially devastating effects of recessive mutations is vital when organisms are forced to inbreed, as virtually all crops are. Another advantage to polyploidy is that gene redundancy results in decreased selective pressure to preserve the original sequence of genes, since preserving the sequence of just one copy of a gene suffices to ensure the production of a functional protein. Consequently, most of the copies of a gene are “free” to mutate into new genes encoding proteins with new functions, driving evolutionary innovation.
If polyploidy is so great, why aren’t we all polyploids? As it turns out, having too many sets of chromosomes can be problematic for cell division. In diploids, meiosis is pretty straightforward – just align the chromosome pairs and split them down the middle to yield haploid cells. But when it comes to polyploid cells, equal division is much more complicated and therefore harder to execute perfectly, which is why meiosis in polyploids often yields cells with abnormal numbers of chromosomes. Moreover, dividing an odd number of chromosomes equally between two cells is impossible. Ever wonder why bananas are seedless? The original bananas were diploid and contained seeds, but grocery stores sell a hybrid, triploid variety of bananas. If these bananas, with their three chromosome sets, were to attempt meiosis, the result would be one diploid cell and one haploid cell. This is clearly unsustainable, which is why the bananas we consume are sterile, seedless snacks.
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