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Ploidy

Polyploids and haploids

In humans, changes in chromosome number and structure have major impacts on health.  In plants – both wild and domesticated – variation in chromosome number is widespread and of evolutionary and economic importance. 

I. Euploidy:  Euploidy is the situation where an organism has exact multiples of a basic ("x") chromosome number.  Diploids and polyploids with exact multiples of an "x" number are all euploids. Aneuploidy is addressed after the section on polyploidy.

For example, barley in the sporophytic generation is 2n = 14 and n = 7 in the gametophytic generation. Since 7 x 2 = 14, barley is a diploid (euploid). The "base number (x)" is 7. In the case of a diploid, "x" is the same as the "n" number.

 

II. Polyploidy is the situation where an organism has other than two basic sets of chromosomes.  The two types of polyploidy are autopolyploidy and allopolyploidy.

Ploidy levels in the Triticeae (an allopolyploid series) where "x" = 7. Note: Based on a genome formula alone, you cannot tell if an organism is allopolyploid or autopolyploid.

Sporophytic generation

Gametophytic generation

Ploidy level

Full formula

2n = 14

n = 7

2x = Diploid

2n = 2x = 14

2n = 28

n = 14

4x = Tetraploid

2n = 4x = 28

2n = 42

n = 21

6x = Hexaploid

2n = 6x = 42

 

A. Autopolyploid: A cell or individual whose several chromosome sets, three or more, are all homologous. These sets arise within a species via a process of genome multiplication.

Autopolyploids can be fully fertile (potato (2n = 4x = 48), alfalfa (2n = 4x = 32). Even though there are multiple homologous chromosomes, there are even numbers of homologs, and pairs of homologs show bivalent pairing. Therefore meiosis is normal and balanced gametes are produced.

Newly synthesized autopolyploids are usually sterile due to meiotic irregularities.

Sterility in autotetraploids due to uneven numbers of homologous chromosomes can be useful for producing seedless fruits.

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