Human somatic cells, with
their full set of 46 chromosomes, have what geneticists refer to as a diploid
number of
chromosomes. Gametes have a haploid
number
(23). When conception occurs, a human sperm and ovum combine their chromosomes to
make a zygote
(fertilized egg) with 46
chromosomes. This is the same number that the parents each had in their somatic
cells. In doing this, nature is acting conservatively. Each generation
inherits the same number of chromosomes. Without reducing their number
by half in meiosis
first, each new generation would have double the number of chromosomes in their cells as
the previous one. Within only 15 generations, humans would have over 1½
million
chromosomes per cell and would be a radically different kind of animal.
In fact, when a zygote has an extra set of chromosomes, it usually is
spontaneously aborted by the mother's reproductive system--it is a lethal
condition.
The complete meiosis process
in human males takes about 74 hours. Spermatogenesis usually begins at 12-13
years of age and continues throughout life. Several hundred million sperm cells are
produced daily by healthy young adult males. Between 200 and 600 million sperm cells
are normally released in each ejaculation. Since only one sperm cell is required for
conception, this huge number would seem to be an extreme overkill. However, as many
as 20% of sperm cells are likely to be defective and the female reproductive tract is
hostile even to healthy ones--it is acidic and contains antibodies that seek
out and destroy the sperm cells. Ejaculating large numbers of sperm at the same time is
nature's way of overcoming these difficulties and increasing the likelihood that
conception will take place. The number of sperm cells produced can be significantly
diminished by psychological and physiological stress. Sperm count also
progressively declines with age after reaching a peak, usually in the early 20's.
In addition, the percentage of sperm that move randomly rather than in a
straight line generally increases in older men. The result is a decrease
in male fertility. The genes that are responsible for sperm production
are in the Y sex chromosome. Unfortunately, the mutation rate for the Y chromosome is thought to be thousands of times
higher than for those in other chromosomes. This may be a major cause of
male infertility. As a result, genetic testing is beginning to be used
to diagnose it.
|
|
 |
| |
Human female
reproductive system
|
Meiosis in human females
is more complex. By the 5th month after conception, immature sex cells begin to develop in the fetal ovaries but stop at an early
stage of meiosis (after prophase I). They remain in this precursor egg
cell, or primary oöcyte
, phase until puberty when hormones cause a resumption of meiosis for one to
several cells each month. They proceed to the 1st and 2nd reduction divisions and
once again stop developing. At this stage they are
secondary oöcytes.
When a secondary oöcyte is finally released from the ovaries into the fallopian
tube (during ovulation
), the egg
still has not completed the last stage of meiosis. That happens only at conception
as a result of chemical changes that occur when the main part of a sperm cell enters the
ovum.
Virtually all (99.9%)
sex cells in a woman's ovaries never develop beyond the primary oöcyte stage and eventually are reabsorbed by
her body. By 20 weeks after conception, there are approximately
7,000,000 primary oöcytes. All but about 1,200,000 are lost by birth.
At puberty, there are only around 400,000 of
them remaining. Throughout life, there is a constant decline in the
number of potential eggs. Each time one is successfully ovulated, as
many as 2000 are lost. Normally, women have on average 11-14 ovulations per year for 33-36
years. This means that less than 500 secondary oöcytes usually are produced out of
the store of hundreds of thousands of primary oöcytes. The actual number of
ovulations is highly variable and often much lower since the process is governed
by hormones and ultimately other factors including psychological stress, nutrition,
physical activity, and pathological conditions. The fact that women rarely have more
than a few children is evidence that only a small fraction of the successfully ovulated
eggs are fertilized and become viable zygotes. Beginning about age 27, a
woman's fertility progressively declines. Around 35-37 years old, the
decline becomes much steeper and the chances of conception significantly
lower. By the early 50's, most women begin the transition
to menopause when they stop ovulating altogether. The temporary
cessation of ovulation and subsequent infertility can occur much earlier in
life as a result of reduced blood estrogen levels caused by excessive
physical activity. This is very likely the reason that roughly a
quarter of American women athletes in high school and college cease having
menstrual periods. It also results in a significant decrease in their
bone density.
NOTE:
Humans may be the only
animal species in which females now normally live for many years following
menopause. Freed from having more children themselves, human
grandmothers are in a position to assist their own daughters and sons in
rearing their offspring. This potentially increases the chances that
grandchildren will survive, thereby giving our species an advantage over
other animals in the competition for survival. However, people rarely lived
beyond menopause until a little over a century ago when modern medicine and
other technological advances made it possible. Prior to that time, we
were more often like other animals in that most of us succumbed to disease,
accidents, or predators before middle age and menopause.
NOTE:
On-going
research suggests that it may be possible within a few years to return
fertility to post menopausal women by stimulating stem cells in their
ovaries to produce new eggs.
When human sexual intercourse occurs, it
takes about 5 minutes for sperm to reach the upper end of the
fallopian tubes where conception usually takes place. Of the several
hundred million sperm cells that enter a vagina, rarely do more than a few hundred successfully pass through the
cervix and uterus. Only about 100 reach the upper end of the fallopian
tubes. Sperm cells are guided over
this path mostly by heat sensing. The upper ends of the fallopian tubes
are about two degrees warmer than the lower ends. Secondary oöcytes
secrete chemicals that also may guide sperm cells to them when they are in
close proximity. Usually, only the most viable sperm cells reach the
secondary oöcytes
and play their part in conception. Those sperm that fail in this
competition are often genetically abnormal. The endurance test that
they must go through in the female reproductive tract is nature's way of
eliminating these poorer specimens.
Among humans, fertilization usually occurs within a
day after ovulation. It takes
about 4 days for secondary oöcytes to
pass through the fallopian tube in their journey to the uterus.
Conception must occur early in this process. Sperm usually can remain
viable for up to 48 hours in the female reproductive tract, but secondary oöcytes
remain viable for only about 24 hours after they have left the ovaries.
This means that sexual intercourse must occur from a few days before to one
day after ovulation if conception is desired. In most non-human
mammals, birds, reptiles, fish, and insects, fertilization is made more
likely by the fact that females are sexually receptive only around the time
of ovulation. This period of female sexual receptivity is called
estrus. In most species, it is common for all females to have
their ovulations around the same time of year. This reproductive
synchronization results in a common mating season. In humans and some
other primates, sexuality is far less related to the timing of ovulation.
Among these species, there is no mating season. More precisely, the
entire year is a mating season since they have a more or less chronic interest in sex.
This is another way in which nature has chosen to increase the likelihood of
conception.
Half of the sperm
cells produced normally carry the X-chromosome and half have the
Y-chromosome. Subsequently,
we would expect that 50% of human babies would be males and 50% females, but this
usually is not the case. The ratio of male to female newborns in the
U.S. and most of the world is 105-110 males to 100 females. The ratio of
males to females at conception is generally even higher than it is at birth.
This is fortunate in the long run for society because male spontaneous
abortions and infant mortality rates are higher. In addition, males are
more likely to die from accidents and combat as teenagers and young men.
Males in developed nations who survive to adulthood also can expect to die at a
younger age than women.
|
Male-Female Ratios in
the United States
|
| |
|
Males |
|
Females |
| |
|
|
|
|
|
at conception |
|
130-150 |
|
100 |
|
at birth |
|
105-110 |
|
100 |
|
at age 20 |
|
98 |
|
100 |
|
at age 65+ |
|
68 |
|
100 |
Current cultural
practices in China and India result in even higher rates of male births.
Since 1979, China has had a national policy of allowing parents to have only
one child in order to reduce population growth. A traditional
preference for male children and the ready availability of ultrasound
technology has led to large numbers of abortions of female fetuses.
This occurs despite the fact that abortions for the purpose of preventing
the birth of female children is illegal.
There are now 119 boys born for every 100 girls in China, and in some regions the
ratio is as high as 144 to 100. A consequence of this practice is the
growing scarcity of marriageable women. As of the 2005 census, there
were 32 million more young men than women in China. While India does
not have a one child policy, it is facing a similar problem because of the
selective abortion of female fetuses, especially in the more prosperous
states of North India. This is driven by the economic difficulty of
raising large families and the preference for male children. The huge
excess of marriageable men in China and India could be socially and
politically explosive in the near future.
In societies that encourage a form of marriage in which
one man has more than one wife at the same time (polygyny), higher numbers
of female children are usually born than in the predominantly monogamous
nations. Why this reverse birth ratio pattern occurs is not
entirely understood. However, it is very likely connected with the
fact that each wife has less frequent sexual intercourse. Girls are
more likely to be conceived when conception is close to the time of
ovulation. When there is intercourse at other times as well, the sperm
cells are more likely to be waiting for ovulation at the upper end of the
fallopian tubes. They do not have as far to go to reach the egg.
Life's
Greatest Miracle--PBS Nova series video on conception,
gestation, and birth
This link takes
you to an external website. To return here, you must click the "back" button
on your browser program.
(8 parts of 4-10 mins each) |
Why Twins?
Multiple births at the same time are rare for humans
and most other primate species. Having fraternal, but not identical,
twins apparently runs in family lines, and is also somewhat more likely for
women over age 30.
Fraternal twins may
look similar but are not genetically identical. In fact they are no
more identical than any brother or sister. They share their mother's
uterus during gestation but come from two different eggs fertilized with
different sperm. Subsequently, they are called
dizygotic
twins
. In contrast,
identical twins are mostly identical genetically because they result from
one zygote splitting into two or more separate ones within a few days after
conception. As a consequence, they are called
monozygotic twins
.
If the division of the original zygote does not occur until the 9th to the 12th
day after conception, the monozygotic twins are likely to be
mirror twins.
That is, they will have small mirror image differences internally and
externally. For instance, one may be left handed and the other right
handed. Likewise, the cowlick in their hair at the back of the head
will be on opposite sides. If the division of the zygote occurs after
day 13, the monozygotic twins are likely to be born conjoined.
Any differences
between monozygotic twins later in life are mostly the result of
environmental influences rather than genetic inheritance. However, monozygotic twins may
not share all of the same sequences of
mitochondrial DNA.
This is due to the fact that the
mitochondria in a cell may
have somewhat different versions of DNA, and the mitochondria can be
dispersed unequally when a zygote fissions. Female monozygotic twins can
also differ because of differences between them in X-chromosome
inactivation. Subsequently, one female twin can have an
X-linked condition such as
muscular dystrophy
and the other twin can be free of it.
NOTE:
X-chromosome inactivation in females was described at the end of the first
topic section of this tutorial ("Basic Cell Structures") and mitochondrial DNA
is described in the
last topic section ("Molecular Level of Genetics").
There has been at
least one recorded instance of twins who are identical on their mother's
side but share only half of their father's genes. These
"semi-identical" twins result from two sperm cells fertilizing the same egg.
This double fertilization of an egg apparently occurs in about 1% of human
conceptions. In most cases, the embryo is not viable and dies.
Dizygotic
twins can also be produced when a woman has sexual intercourse with more
than one man around the time she is ovulating. If multiple viable eggs
are released from her ovaries, each can be fertilized by sperm from a
different man. This is referred to as heteropaternity.
Why Intersex?
Normally in humans one sperm cell combines its chromosomes with those of one ovum at
conception and that in turn develops into a single embryo that will become a
fetus. Very rarely, however, two zygotes fuse and become a single embryo. If
it survives gestation
,
a baby will be born who is a true
chimera
--it
is genetically two "people" in one body. If those two "people" are not the same
gender, the baby will likely be an
intersex individual (also
known as hermaphrodite
)--it
will have both male and female sex organs and other body tissues. Some
researchers believe that the frequency of chimeras being born will be
increasing as in vitro fertilization
becomes more common since two or more embryos are usually placed in the uterus
with this procedure.
NEWS:
On January 16, 2005, a 66 year old Romanian woman named Adriana Iliescu gave
birth to a daughter. This makes her the oldest woman to become
pregnant and deliver a live baby. She was implanted with a fertilized
egg from a healthy younger woman. The baby was 6 weeks premature and
weighed only 1.4 kilograms (3.1 pounds), which is less than half the normal
newborn weight. Two other fetuses died during the pregnancy (News at
Nature.com, January 17, 2005).
NEWS: In the August 23, 2009 issue of
Biology Letters, Kristen Navara of the University of Georgia presented
the results of a 202 nation survey of data on male to female ratios at
birth. She noted that there is a skewing of sex ratios that
corresponds to latitude. In temperate and subarctic latitudes, there
are slightly more boys born compared to tropical latitudes. Navara
said that these differences were independent of cultural practices and the
socio-economic status of families.
