How Chromosomes are passed from parent to offspring
Each human cell (aside from red blood cells and gametes) contains a full set of 46 chromosomes. Clearly havoc would
result if a
cell each containing 46 human
chromosomes were to fuse! Not only would the resulting offspring
have 98 chromosomes in each cell but the number would keep on doubling
with each successive generation. For this reason a process other
which produces cells with a
number of chromosomes is necessary to produce the sperm and egg cells.
The process by which the chromosome number is halved and chromosomes are
sorted and packaged to be passed on to an organism’s offspring is called
Each of the
resulting reproductive cells, or gametes (sperm and egg), has only a single set of 22
autosomes plus a single sex
chromosome, either an X or a Y.
A cell with a single chromosome set is
called a haploid cell.
By means of sexual
intercourse, a sperm cell carrying one 23 chromosome set from the father
reaches and fuses with an egg cell carrying a corresponding set of 23
chromosomes from the mother.
fertilized egg, or zygote,
contains the two haploid sets of chromosomes bearing genes originating in
both the maternal and paternal family lines.
As a human develops
from a zygote to a sexually mature adult, the new combination of genes in
the zygote are passed on with precision to all
somatic cells of the body by the process of
process of meiosis has many similarities to the process of
mitosis--chromosomes replicate before the process begins and shorten and
thicken to look like the chromosomes at the beginning of mitosis. A
spindle with fibers appears and the nuclear membrane dissolves.
However, as meiosis begins, each chromosome is mysteriously attracted to
its special homologous partner. The two #1 chromosomes--one from the
paternal set and one from the maternal set--wrap tightly about each other
in a process called
Since each #1
chromosome is already doubled, a
of four chromosomes is created. The same thing happens with the
other chromosomes. Each homologous pair forms its own tetrad. All the tetrads
arrange themselves on the spindle. Two quick divisions follow and
the chromosomes are pulled apart. The four chromosomes of each
tetrad are first separated into twos and then into ones.
Before meiosis begins each
chromosome in the pair has already doubled. The double maternal
chromosome (black) and the double paternal chromosome (white) attach
to the spindle.
As the cell divides, the
double maternal chromosome and the double paternal chromosome move
toward opposite poles.
The pair in each daughter
cell starts to pull apart in the direction of the arrows.
Another cell division occurs.
The final result is four
cells, each containing the haploid number of chromosomes. Two of
the cells contain a chromosome from the maternal set and two contain a
chromosome from the paternal set.
diagram above shows only a single chromosome pair.
everything that is happening to this tetrad is happening simultaneously to
the other 22 tetrads.
The result is always four
cells, each having a single #1 chromosome, and one #2 and one #3 and so on
up to one each of 23 chromosomes.
Thus each cell has
one complete set of chromosomes
and is ready to become either a sperm or egg cell.
The chromosomes in each sperm or egg are
a random mix from the maternal
set and paternal set of the original cell.
each of the 23 pairs of chromosomes can line up in two different ways one
person can produce more than 8 million (223)
different kinds of eggs or sperm. When fertilization occurs, 223
70 trillion different zygotes are possible!
genealogist, this means that any one of his or her chromosomes could have
come from any one of eight great-grandparents or, going back say twenty
generations, from any one of over a million potential ancestors (in
reality, many of these potential ancestors may be the same individuals as
lines cross and intermarry over generations.) In the same light,
after so many generations of shuffling and dividing, contributions from
some ancestors may have been lost completely. With
one exception, it is impossible to trace the path of individual
chromosomes in the nucleus from generation to generation.
Cross Over Animation
complicate matters even further, the chromatids in a tetrad pair so
tightly at the beginning of meiosis that non-sister chromatids from
homologous chromosomes actually exchange genetic material in a
process known as
This further shuffles
the ancestral genes so that a
chromosome in a gamete may contain genes from both maternal and paternal
Crossing over can occur
at any location on a chromosome, and it can occur at several locations at
the same time. It is estimated that during meiosis in humans, there
is an average of two to three crossovers for each pair of homologous
Differences Between Sperm and Egg Formation
What they mean to the
The process of meiosis and gamete formation is fundamentally the same in
males and females. However, whereas gametogenesis (formations of
gametes) results in four functional sperm cells for each meiotic division
in males, the same process in females gives rise to only a single
functional egg capable of being fertilized and developing into an embryo.
mature sperm has a head, which contains a nucleus with its haploid set of
chromosomes, a long tail which propels the sperm through its fluid
surroundings, and between these a midpiece containing several hundred
mitochondria which supply the energy necessary for the long journey
in search of the egg.
other hand, the
development of the egg cell involves an unequal cell division.
It produces one relatively large primary egg cell which receives all of the
thousands of mitochondria--from
the starting cell
while the other three cells that result from the meiotic process form
small polar bodies that, at first, attach themselves to the surface of the
primary egg but eventually deteriorate.
fertilization, the entire sperm does not enter the egg. Practically
the only contribution that the sperm cell makes to the zygote is its
haploid nucleus with its set of 23 chromosomes. Importantly to the
genealogist, the tail and midsection of the sperm drop off outside the egg
virtually no mitochondria from the male parent enter the zygote. All
of the mitochondria (with its own DNA you will recall) in the developing
embryo come from the mother.
The diagram below
illustrates the differences between the formation of sperm and egg cells.
cross-over animation above is a modification of an animation by
Hironao NUMABE, M.D, Tokyo Medical University, Department of Pediatrics
Genetic Study Group
I have also
included an exceptionally nice movie of cells undergoing meiosis which I
and use with
permission though the webmaster was uncertain of the origin.
Unfortunately, the size of this file is 11.8 MB meaning that unless you
have a high speed Internet connection the download time could be as much
as 18 minutes! For the fortunate with cable, DSL or satellite