Video Transcript
A gene is a section of DNA that
controls a certain characteristic. Which of the following statements
about genes is true? (A) All living organisms have
exactly the same number of genes. (B) There are usually two copies of
each gene, and one allele may dominate over the other. Or (C) each gene contains thousands
of chromosomes.
What’s the difference between
humans and fruit flies? The answer is a lot. There are many characteristics that
humans and fruit flies do not share with each other. What determines these
characteristics are the different genes contained in a DNA of human cells compared
to fruit fly cells. Humans have different kinds of
genes and a different number of genes compared to fruit flies or other organisms for
that matter. We have a lot of DNA in our
cells. If we were to take it out of a
single cell and stretch it out, it would be about two meters long. This isn’t one continuous piece of
DNA. It’s actually broken up and
organized into 46 structures that are called chromosomes.
There’s actually 23 different
chromosomes, but we have two copies of each to get a total of 46. We get 23 from our biological
mother and 23 from our biological father. These 23 chromosomes are nearly
identical to these 23 chromosomes, but there are some differences. Let’s take a closer look at a
chromosome pair. The different chromosomes are
numbered, and this is chromosome one. As mentioned, we get one from our
mother and one from our father, so we have two copies of chromosome one. Each of these chromosomes are a
single long piece of compacted DNA that contains different genes shown here in
orange.
Remember, genes contain the
information needed for different characteristics. In fact, chromosome one has about
2000 genes on it, but we’re just showing a few here. And we have two copies of these
genes because we have two copies of the chromosome. The 2000 genes on this chromosome
are nearly identical to the 2000 genes on this chromosome. They’re nearly identical because
there’re small differences in these copies that makes humans a bit different from
each other. So this gene here is nearly
identical to this gene here.
These different versions of genes
are called alleles, and sometimes one version can dominate over another version. For instance, the alleles that
control dark hair are dominant over the alleles that control light hair. This is why there are so many
people with dark hair instead of lighter hair. Now getting back to our question,
the statement about genes that is true is (B). There are usually two copies of
each gene, and one allele may dominate over the other.
