Evolution__3rd_Edition

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Humans, and many other creatures,
have two sets of genes, one
inherited from each parent
Various technical terms apply to
diploid organisms
CHAPTER 2 / Molecular and Mendelian Genetics 33
The numbers we have looked at here are averages. Some regions of DNA have higher,
or lower, than average rates. For instance, we saw that short repeats (for example,
...ACCACCACC...) are vulnerable to slippage. These regions expand and contract by
mutation, such that a parent with three repeats of the unit sequence may have offspring
with two or four repeats of it. The mutation rates are high, up to 10 −2 (Jeffreys et al. 1988).
The high mutation rates make these regions of DNA useful in genetic fingerprinting.
2.7 Diploid organisms inherit a double set of genes
DNA is physically carried on chromosomes. Humans, as noted above, have 46 chromosomes.
However, the 46 consist of two sets of 23 distinct chromosomes. (To be exact, an
individual has a pair of sex chromosomes a which are similar (XX) in females, but
noticeably different (XY) in males a plus a double set of 22 non-sex chromosomes,
called autosomes.) The condition of having two sets of chromosomes (and therefore
two sets of the genes carried on them) is called diploidy. The figure given above of
3 × 10 9 nucleotides in the human genome is for only one of the sets of 23 chromosomes:
the total DNA library of a human cell has about 6 × 10 9 nucleotides (and 6.6 × 10 9 is a
more exact figure).
Diploidy is important in reproduction. An adult individual has two sets of chromosomes.
Its gametes (eggs in the female, sperm or pollen in the male) have only one set: a
human egg, for example, has only 23 chromosomes before it is fertilized. Gametes are
said to be haploid. They are formed by a special kind of cell division, called meiosis; in
meiosis, the double set of chromosomes is reduced to result in a gamete with only one
set. When male and female gametes fuse, at fertilization, the resulting zygote (the first
cell of the new organism) has the double chromosome set restored, and it develops to
produce a diploid adult. The cycle of genesis can then repeat itself. (In some species,
organisms are permanently haploid; but in this book we shall mainly be concerned with
diploid species. Most familiar, non-microscopic species are diploid.)
Because each individual possesses a double set of chromosomes, it also possesses a
double set of each of its genes. Any one gene is located at a particular place on a chromosome,
called its genetic locus. An individual is therefore said to have two genes at
each genetic locus in its DNA. One gene comes from its father and the other from its
mother. The two genes at a locus are called a genotype. The two copies of a gene in an
individual may be the same, or slightly different (i.e., the amino acid sequences of the
proteins encoded by the two copies may be identical or have one or two differences). If
they are the same the genotype is a homozygote; if they differ it is a heterozygote. The different
forms of the gene that can be present at a locus are called alleles. Genes and genotypes
are usually symbolized by alphabetic letters. For instance, if there are two alleles at
the genetic locus under consideration, we can call them A and a. An individual can then
have one of three genotypes: it can be AA, or Aa, or aa.
The genotype at a locus should be distinguished from the phenotype it produces. If
there are two alleles at a locus in a population, the two can combine into three possible
genotypes: AA, Aa, and aa. (If there are more than two alleles, there will be more than
three genotypes.) The genes will influence some property of the organism, and the