Identical twins form when 1 sperm fertilizes one egg to become 2 babies and fraternal twins
form when 2 sperm fertilize 2 eggs to become 2 babies.
..... hm..looks like we have more time. When a man and a woman meet, and they love
each other very much they may decide to have a baby. If they don't plan on having the baby,
but have one, we call that baby a "mistake". Or the politically correct term "unplanned".
Twins are produced mostly randomly. There is only a 1-2 % chance of birthing twins.
Because of this couples typically expect 1 baby, so if you are a twin, 1 of you is always
an "unplanned mistake". For normal birth, the man injects sperm into
the female's uteris to fertilize her egg. After 8-10 months of gestation a baby is born.
But with twins, 2 babies burst from the womb. So what's going on?
Let's look closer at cells. Your cells contain the information that it takes to make you
as DNA within chromosomes. You have 23 pairs, or 46, chromosomes. 1 of each pair of chromosome
is from your mother, the other from your father. They are different but code for the same things.
One has your mothers traits, the other your father's. Each chromosome is made up of 2
chromatids. These chromatids are identical to one another.
There is one pair of chromosomes that are not the same and they determine gender. This
is the X and the Y. If you have an X and an X you are a female, if you have an X and a
Y you are a male. Most of your cells, like hair, skin, brain,
or heart cells, the somatic cells, undergo mitosis to reproduce. In mitosis, each chromosome's
chromatids are separated, one going to each of the new cells. Then they replicate themselves
to form a new chromosome. This results in two cells that are identical to the first
one. each with the full 46 chromosomes But the sex cells, your sperm or your eggs,
undergo a different process called meiosis. There are 2 important differences. Before
the chromosomes split apart they swap some of the genes. This is called crossing over.
It creates chromosomes different from the parent chromosome.
The other difference is they cell splits twice making 4 cells. Each cell with only 1 chromatid
from each of the chromosome pairs. So each of these cells only has 23 chromosomes instead
of 46. When they meet the sex cells of another person, the new cell they form has will have
46 chromosomes, 23 from each parent. Then they'll replicate like in mitosis to form
46 full chromosomes and eventually a new person. What's the point of this?
Let's say this represents genetic diversity. A lot of organisms don't reproduce sexually
like us. They reproduce through mitosis which creates offspring that's usually very close
genetic copies of the parent. But sexual reproduction produces offspring which are a combination
of the parents allowing for a mixture of their genes. This creates more genetic diversity
allowing for a greater chance of getting more fit individuals. Ignoring crossing over, there
are 70 trillion different combinations of the 2 parents. That's like a 7.... followed
by a trillion zeroes. Crossing over creates even more diversity. The more diversity the
greater the chance of having an offspring that will have something to let it do well
enough to breed again. While males produce a whole load of sperm.
The female produces only produces about 1 egg a month each ovary taking a turn every
other month. Sometimes the ovaries each produce an egg at the same time. If both eggs get
fertilized then you can get fraternal twins. Because these twins are formed from totally
different sperms and totally different eggs, these twins are no different than any other
siblings. They are each their own separate genome compiled from their parents and can
even be different genders. When the sperm and the egg meet it becomes
what is called a zygote. Sometimes a zygote will spontaneously form into 2 zygotes. Since
each zygote has the same DNA, they will form 2 people that are genetically identical, identical
twins. But, there is something different in females.
Every cell in any female's body really only uses 1 of the X chromosomes, the other is
deactivated. This happens pretty early on in development. So any trait that the X chromosome
codes for will be different in any given area of cells. Some cells will have the father's
traits, and some the mothers. This can lead to some interesting likes like one identical
twin female having colour blindness or hemophilia and the other not.
This is most easy to see in calico cats. The colour of their fur is coded by the x chromosome.
So whether orange or black remains depends on which parent the trait is coming from.
So identical twin calicos will look different. Sometimes after the zygote splits the zygote
will conjoin. This is how conjoined or Siamese twins are
formed.
The grand banks cod was one of the biggest on the planet. So what happened? Well it turned out cod, was made of meat. And because of
advances in technology, cod catch reacheded record levels. The researchers and local fishermen
warned about the declines in population, so why didn't it stop? the way the system was
set up, the department of fisheries and oceans....