Answer 1:
In biology a lot of technical terms are used, and
sometimes because the words sound so strange it's
easy to get confused. Personally when I was
studying things like genetics and reproduction, I
found that oftentimes I would have difficulty
understanding concepts because I didn't really
have a firm grasp of the little differences
between things. For example, you will learn (if
you haven't already) the terms gene, locus,
allele, and chromosome. All of these deal with
DNA, but they all refer to different
things! One
thing I've learned is that a good way to keep from
getting confused is to really focus on
understanding what each term means.
After
reading your question, I think that maybe you are
having this same problem! All the talk of
chromosomes and homologous pairs can be very
confusing, but if you really understand the
difference between what each of these means you
will find that it will be a lot easier to
understand what happens during meiosis.
To
get a better understanding of what chromosomes and
homologous chromosome pairs are, let's first talk
a little about shoes. Let's say that you have 26
shoes in your closet. Of course, not all of the
shoes are totally different. If you take all of
the shoes and match them up, you will find that
while you have 26 shoes, you really only have 13
different types of shoe. Each type has a mate of
the same type, forming 13 pairs of shoes. The 13
pairs are very different-- for example, one pair
is made by Skechers and looks very sporty, while
another pair is very smooth and dressy and made by
Ralph Lauren. However, within each pair are two
shoes that are basically the same
design.
This is how the chromosomes in your
cells are. You have 26 different chromosomes.
However, just like with shoes, the chromosomes are
arranged in pairs, meaning that you only have 13
different types of chromosomes. In order to
distinguish between just general chromosomes and
the specific similar chromosomes that match up to
form a pair, the term "homologous
chromosomes" is
used. ("Homologous" can be thought of to just
mean "the same type" in this case). Thus you have
13 different homologous chromosome pairs, with
each pair made up of two homologous chromosomes.
(In the shoe analogy, you have two different shoes
by Skechers. The left shoe is one homologous
shoe, and the right shoe is the other homologous
shoe, together forming a homologous pair. These
two shoes have to go together-- none of the other
24 remaining shoes match. They are just not
homologous-- they aren't "the same type"! You
would never wear one Skechers shoe and one Ralph
Lauren shoe!)
If you match up all of the
chromosomes in your cell into their proper pairs,
they look different under the microscope, as you
have observed. This is because they ARE
different! Each different chromosome type carries
different genes, and different numbers of genes.
Therefore, in order to have all of the different
genes, you need to have all of the different types
of chromosomes! Happily, the way meiosis occurs
guarantees that you will get all of the different
types of chromosomes.
You inherited your
chromosomes from your parents. Each of your
parents gave you 13 chromosomes, so you have a
total of 26. However, these chromosomes weren't
just handed out haphazardly, or else you could be
in trouble, because then you might not be lucky
enough to get the right ones! Instead, to make
sure that you get exactly the right chromosomes,
the homologous pairs split apart during meiosis.
(The homologous pairs do NOT stay together!) One
half of each pair goes into one gamete, and the
other half goes into another gamete. This way,
each gamete has 13 chromosomes-- one of each
different type. (So you see, the gametes do not
contain any homologous pairs at all-- only one of
each homologous chromosome). In this way, you get
one of each type from your mom, and one of each
type from your dad. When they come together, you
have two of each type, making the proper total of
13 homologous chromosome pairs, with none of them
(and the important genetic information they carry)
being lost. |
Answer 2:
This question brings up a lot of interesting
concepts about genetics. We have 23 pair of
chromosomes. A picture of a person's chromosomes
is called a karyotype. You can see one at
this site:
human karyotype
Take a look at chromosome #1. This person inherited
one copy of chromosome #1 from his dad and one
from his mom. If you look at #3, the two
chromosomes look different, but that's just
because one was bending when the picture was
taken. Now look at the last (23rd) pair,
after #22. These two are really different. One
is called an X chromosome, the other is a
Y.
That's how I knew this was a male. Females have 2
X chromosomes. The X chromosome has a lot of
genes with a lot of information. The Y has about
9 working genes that basically tell the body to
make this person male.
You might be wondering how a male gets by with
fewer genes than
a female. Most of the time it doesn't make a big
difference in health. In fact, in each one of a
female's cells, one X chromosome is 'switched off'
and only one set of X genes is used. You could see
this from a picture of my cat, Katie Lee.
She's a "tortoiseshell" cat, meaning that she has
splotches of black and orange. (Calico cats have
more white, but also have black and orange
patches.) What does this have to do with
genes?
The genes for both orange and black are dominant
and on the X chromosome. If a cat gets a gene for
black fur from one parent, and a gene for orange
fur from the other, about half of her fur cells
will make orange fur and half will make black fur,
depending on which X chromosome is being used by
that cell.
Why can't male cats be tortoiseshell
or calico? Sometimes males have problems
due to having only one X chromosome. If there is
a "bad" gene on their only X, they don't have a
chance of having a good copy. Females are less
likely to have certain diseases or problems
because if they have a bad gene on one X, they
probably have a good copy on the other
X.
Back to meiosis, remember that you have
to divide the number of chromosomes in half (from
23 pair to 23) when making egg or sperm cells or
the baby would have too many chromosomes. Each
cell made by meiosis will have one complete set,
but will be different because of the source of the
genes. Chromosomes will be patchworks of the
chromosomes inherited from the parents. But if
all goes well, each one will look the same outside
and have a complete set of genes.
For
more on karyotypes, check out this
site:
karyotyping
Click Here to return to the search form.
|