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Mendel’s law of segregation

The basic template of an organism’s genetic information is coded in the double-stranded DNA of its cells. In most
prokaryotes the double-stranded DNA is circular and together with associated proteins make up a rather jumbled mass of
a single chromosome found in the cell’s nucleoid region (remember: prokaryotes lack a nuclear membrane). In eukaryotes the DNA and associated protein exist in discrete chromosomes that are enclosed within the cell’s nucleus. Often the
chromosomes are paired—that is, each has a homologue, having identical size, shape, and genes coding for the same
kinds of traits. Eukaryotic cells can be haploid (n), diploid (2n), triploid (3n), or polyploid (>3n) depending on the number
of homologues for each chromosome. Diploid cells have paired chromosomes—one homologue of the pair was obtained
from the male parent and the other from the female parent—whereas, they are unpaired in haploid cells. For triploid cells,
each chromosome type has two other homologues, and polyploid cells have multiple homologues for each chromosome.
The total number of homologous chromosome pairs in a nucleus is usually species specific and varies widely across the
plant and animal kingdoms. However, very often one pair, the sex chromosomes, is involved in sex determination and the
remaining pairs, the autosomes, carry genes for traits other than sex determination. The table below contains some examples of the variation observed among species.
TABLE 25. Number of autosomes and sex chromosomes for a
variety of diploid organisms.
Species Autosome Pairs
Sex
Chromosome
Pairs Total Number
Penecillium –2
Garden Pea – – 14
Fruit Fly 3 1 8
Chickens 38 1 78
Dog 38 1 78
Humans 22 1 46