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**1. Definition and Types of Ploidy:**
Ploidy term derived from haploidy and diploidy.
– Haploid refers to gametes with half the number of somatic cell chromosomes.
– Tetraploid organisms produce gametes with two sets of chromosomes.
– Eukaryotic cells may be considered haploid with one set of chromosomes.
– Humans are diploid with 46 chromosomes in somatic cells.
– Diploid cells have two homologous copies of each chromosome.
– Most mammals are diploid, with rare exceptions.
Polyploidy involves multiple sets of chromosomes beyond the basic set.
– Higher ploidy levels include triploid, tetraploid, and hexaploid.
– Allopolyploids are formed from hybridization of two species.

**2. Polyploidy in Nature:**
– Majority of sexually reproducing organisms are diploid or polyploid.
Ploidy level varies between organisms and tissues.
– Half of plant genera contain polyploid species.
Polyploidy common in invertebrates, reptiles, and amphibians.
– Changes in ploidy drive speciation in plants and fungi.
Polyploidy is rare in animals, but somatic cells can be polyploid.
Polyploidy can occur in plants through meiotically unreduced gametes.
Polyploidy is seen in Deinococcus radiodurans and Halobacterium salinarum.

**3. Human Ploidy and Genetic Variations:**
– Humans have 46 chromosomes in somatic cells.
– Each homologous pair contains one paternal and one maternal chromosome.
– Germ cells undergo meiosis to form haploid gametes.
– Fusion of gametes during fertilization restores diploid state.
– Most human cells have 46 chromosomes.
– Examples of aneuploidy in humans: Down syndrome, Turner syndrome.

**4. Genetic and Evolutionary Significance of Ploidy:**
Polyploidy is associated with increased vigor and adaptability.
– Selection favors diploidy in host species and haploidy in parasite species.
– Polyploidization increases transposable element content.
– Triploid organisms are usually sterile.
Nutrient limitation in unicellular organisms may encourage haploidy.

**5. Mechanisms and Implications of Ploidy:**
– Investigation of ancient whole genome duplications in organisms.
– Understanding the genetic basis of polyploidy in fungi and other organisms.
– Implications of polyploidy on evolutionary success and genetic diversity.
– Factors influencing the rarity of polyploidy in animals compared to plants.
– Historical contributions to understanding ploidy and chromosome numbers.

Ploidy (Wikipedia)

Ploidy (/ˈplɔɪdi/) is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively, in each homologous chromosome pair, which chromosomes naturally exist as. Somatic cells, tissues, and individual organisms can be described according to the number of sets of chromosomes present (the "ploidy level"): monoploid (1 set), diploid (2 sets), triploid (3 sets), tetraploid (4 sets), pentaploid (5 sets), hexaploid (6 sets), heptaploid or septaploid (7 sets), etc. The generic term polyploid is often used to describe cells with three or more sets of chromosomes.

A haploid set that consists of a single complete set of chromosomes (equal to the monoploid set), as shown in the picture above, must belong to a diploid species. If a haploid set consists of two sets, it must be of a tetraploid (four sets) species.

Virtually all sexually reproducing organisms are made up of somatic cells that are diploid or greater, but ploidy level may vary widely between different organisms, between different tissues within the same organism, and at different stages in an organism's life cycle. Half of all known plant genera contain polyploid species, and about two-thirds of all grasses are polyploid. Many animals are uniformly diploid, though polyploidy is common in invertebrates, reptiles, and amphibians. In some species, ploidy varies between individuals of the same species (as in the social insects), and in others entire tissues and organ systems may be polyploid despite the rest of the body being diploid (as in the mammalian liver). For many organisms, especially plants and fungi, changes in ploidy level between generations are major drivers of speciation. In mammals and birds, ploidy changes are typically fatal. There is, however, evidence of polyploidy in organisms now considered to be diploid, suggesting that polyploidy has contributed to evolutionary diversification in plants and animals through successive rounds of polyploidization and rediploidization.

Humans are diploid organisms, normally carrying two complete sets of chromosomes in their somatic cells: one copy of paternal and maternal chromosomes, respectively, in each of the 23 homologous pairs of chromosomes that humans normally have. This results in two homologous pairs within each of the 23 homologous pairs, providing a full complement of 46 chromosomes. This total number of individual chromosomes (counting all complete sets) is called the chromosome number or chromosome complement. The number of chromosomes found in a single complete set of chromosomes is called the monoploid number (x). The haploid number (n) refers to the total number of chromosomes found in a gamete (a sperm or egg cell produced by meiosis in preparation for sexual reproduction). Under normal conditions, the haploid number is exactly half the total number of chromosomes present in the organism's somatic cells, with one paternal and maternal copy in each chromosome pair. For diploid organisms, the monoploid number and haploid number are equal; in humans, both are equal to 23. When a human germ cell undergoes meiosis, the diploid 46 chromosome complement is split in half to form haploid gametes. After fusion of a male and a female gamete (each containing 1 set of 23 chromosomes) during fertilization, the resulting zygote again has the full complement of 46 chromosomes: 2 sets of 23 chromosomes. Euploidy and aneuploidy describe having a number of chromosomes that is an exact multiple of the number of chromosomes in a normal gamete; and having any other number, respectively. For example, a person with Turner syndrome may be missing one sex chromosome (X or Y), resulting in a (45,X) karyotype instead of the usual (46,XX) or (46,XY). This is a type of aneuploidy and cells from the person may be said to be aneuploid with a (diploid) chromosome complement of 45.

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