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Mendel, Gregor
of the fertilized egg or young embryo, often due to chromosomal
abnormalities such as nondisjunction. In plants, complete
nondisjunction sometimes occurs, resulting in a diploid
egg cell that survives.
In animals, the cells that will ultimately become gametes
arise from germ cells that assume their specialized function
very early in the development of the embryo. The nonsexual
cells are somatic cells. Some of the somatic cells differentiate
into gonads or reproductive organs, into which the germ cells
migrate during embryonic development. Male gonads (testes)
contain male germ cells, and female gonads (ovaries) contain
female germ cells. In most animals, male and female germ
cells occur in separate male and female individuals. The germ
cells are all diploid at this stage.
• Male germ cells undergo mitosis, producing more male germ
cells. Then many of them become spermatocytes which
undergo meiosis and produce sperm (see figure). Sperm cells
are very small, and swim with a flagellum. At puberty, human
testes contain millions of cells that produce sperm throughout
the adult life of the male. Human testes can deliver hundreds
of millions of sperm cells at a time during sexual activity.
• Female germ cells undergo mitosis, producing more female
germ cells, during the fetal development of the human
female. These cells, inside the ovaries, begin the first division
of meiosis, becoming oocytes. Oocytes are much larger
than sperm and are not motile. Almost all of the oocytes
a human female will ever have, about 800,000, are present
at birth. The oocytes begin the first division of meiosis
prior to birth but do not complete it until puberty. Oocytes
are released, usually one each month, throughout the woman’s
reproductive life. When the oocyte is released from
the ovary, the second division of meiosis begins, but is not
completed until the oocyte is fertilized by a sperm cell.
Normally, when a sperm fertilizes an egg, the two haploid
gametes become one diploid zygote. However, in plants,
if a normal, haploid sperm nucleus fertilizes a diploid egg
nucleus produced by nondisjunction, the result is a zygote
that develops into a triploid (3N) organism. Such a plant will
have chromosomes in groups of three rather than in pairs.
The same thing would happen if an unusual diploid sperm
nucleus fertilized a normal haploid egg nucleus. If a diploid
sperm nucleus fertilizes a diploid egg nucleus, the zygote
grows into a tetraploid (4N) plant, with chromosomes in
groups of four. Nondisjunction in plants can produce gametes
that will result in zygotes with chromosomes in groups of five
(5N, or pentaploid), six (6N, or hexaploid), or even higher
numbers. These zygotes usually develop into perfectly healthy
plants; indeed, plants with doubled chromosome numbers
can be especially vigorous. Organisms with chromosomes in
groups greater than two are called polyploids. In contrast to
plants, polyploid animals are very rare, because a polyploid
animal zygote usually fails to develop. However, in some
animals such as some amphibians, polyploidy has occurred,
resulting in very large chromosome numbers.
Because chromosomes must form pairs during meiosis,
polyploids that have an odd number of chromosomes (3N,
5N, etc.) cannot complete meiosis; they are sterile. Dandelions
(Taraxacum oficinale), despite their abundant flower production,
cannot carry out sexual reproduction, because they are
triploid. They produce triploid egg cells, then triploid seeds,
that develop without fertilization. As everyone who mows
lawns would conclude, the triploid condition of the dandelion
does not reduce its vigor. Likewise, cultivated bananas
are triploid and must be propagated by cuttings rather than
by seed. Polyploid plants that have an even number of chromosomes
(4N, 6N, etc.) can often carry out normal meiosis,
because no homologs remain unpaired during meiosis.
Occasionally, a sperm and an egg come together whose
chromosomes are so different that they cannot function
as homologous pairs. The sperm or egg may come from a
mutant individual within a species, or may come from two
different species. Although the chromosomes cannot function
as pairs, they are frequently able to carry out normal gene
expression. Hybrid organisms, whose chromosomes do not
match up precisely, may develop into fully healthy organisms
(see hybridization). However, because in many cases their
chromosomes do not form homologous pairs, the process of
meiosis cannot be completed. Therefore many hybrid animals
(as the mule, which is a cross between a horse and a donkey)
are sterile. In plants, chromosome doubling can allow cells of
a sterile hybrid 2N to produce a 4N cell that can produce fertile
4N plants. Since these 4N plants cannot cross-breed with
the 2N plants that produced them, these 4N plants function
as a new species. Speciation by polyploidy can occur within a
single generation (see speciation).
Meiosis is the essential process that allows almost all
individuals in all species to produce genetically variable offspring,
which is essential to the continued evolution of each
lineage (see sex, evolution of).
Mendel, Gregor (1822–1884) Austrian Monk, Geneticist
Raising peas in a garden was not the main responsibility of
Gregor Mendel, a monk in the monastery at Brünn, now Brno
in the Czech Republic. But it was Mendel’s close observations
of and experiments with these peas that led him to discover
some basic patterns of inheritance of physical characteristics
that have become the foundation of the modern sciences of
genetics and evolution (see figure). Mendel’s work was not
recognized by leading scientists of his day. In particular, at
a time when even the leading scientists believed in blending
inheritance, Mendel realized that traits were passed on in a
particulate fashion, which was the clue that was needed to
connect natural selection with genetics. He is one of the few
people in history whose name has become an adjective (see
Mendelian genetics).
Johann Mendel was born July 27, 1822, into a peasant
family in Heinzendorf (now in Austria) in Silesia (most of
which is now part of Poland). He learned gardening and grafting
from his father, which was to prove valuable to him, and
to the future of science. He was doing well in school when his
father was permanently injured by a falling tree. His father,
however, believed in his abilities and sold the farm so he could
pay for his son to finish school and go to Olmütz University.
Johann Mendel entered the priesthood (where he took on the
name Gregor) to continue his education. He tried being a par-