Polyploidy

In eukaryotic organisms, chromosomes come in sets. The somatic cells, called soma, usually have a diploid chromosome number, which in scientific notation is abbreviated as 2N. The diploid state contains two sets of chromosomes, one set of which has been contributed by each parent. A single set of chromosomes composes the haploid chromosome number, which is abbreviated as N. The haploid set is found in reproductive cells or gametes (also called the germplasm). In humans the diploid number is 46, and is represented as 2N = 46. Human sperm or eggs, however, have a haploid number of 23, which is represented as N = 23. In some circumstances, however, an organism can have more than two chromosomal sets. This occurrence is called polyploidy.

One cause of polyploidy is polyspermy. If two sperm fertilize an egg, the resulting zygote or fertilized egg will have three sets of chromosomes, somatic nonreproduc-tive; not an egg or sperm diploid possessing pairs of chromosomes, one member of each pair derived from each parent haploid possessing only one copy of each chromosome

Ploidy refers to the number of chromosomes in an organism's genome. A haploid organism has just one copy of each chromosome (A, B, C, and D). Diploids have two of each, and may have sex chromosomes (X and Y). Triploids have three, which prevents proper pairing and separation of chromosomes in meiosis.

chromosomes

A B C D

A A B B C C D D A A A B B B C C C D D D

haploid

diploid

triploid

A A B B C C X X

A A B B C C X Y

diploid female

diploid male

meiosis

A A A B B B C C C X X X

-AABCCX + ABBCXX

triploid female

Incorrect chromosome

numbers leads to

few offspring

three sets of chromosomes triptoM possessing and thus have a triploid number (3N). When this occurs in humans, 3N =

96. Triploidy in humans and most other animals is incompatible with life. Triploid individuals abort or fail to survive the first days of life after birth. Polyploidy is more common in plants, and polyploid forms often survive to produce much larger cells and plant organs. Ferns, which may have up to 1,500 chromosomes, are frequently polyploid, as are varieties of domesticated cereal plants. Most often, polyploids run in sets of three to eight (triploid to octoploid).

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