DNA is the code that makes… us. From cheetahs
to killer whales to Don Rickles. That code is our genotype, the order of the DNA sequence.
The traits that the code results in, what we see, is the phenotype.
DNA is coiled up into bunches called chromosomes. Humans have 23 pairs, or 46 difference chromosomes.
Why do we call them pairs of chromosomes? It’s because, for the most part, each chromosome
of a pair code for the same things and have the same genes on them.
A gene is some certain stretch of DNA that codes for a given protein or trait. Some traits
are controlled by a single gene like ear wax consistency, others are controlled by multiple
genes, like hair colour. But each chromosome of a pair, will code for
the same things. So like here and here will both be the gene
for ear wax consistency. But they’re different versions of that gene.
Maybe one allele makes your ear wax more liquid, and the other allele makes your ear wax more
solid. An allele is a different form of a given gene. So in this example we have a gene
for ear wax viscosity, with 2 versions or 2 alleles of that gene. A metaphor can help explain. Lets say your
genome, all your DNA, is a car. A car is made up of all these different parts. These parts
are your genes. A gene is just a section of DNA. Like a car part is a section of the car.
But there are different versions of each car part. Each specific version, is an allele.
This hubcap is different than this hubcap. But these are all hubcap parts. These are
gene’s for the same thing. These are the gene’s for ear wax consistency. And this allele leads
to solid ear wax. This metaphor would probably make more sense
if I compared your genome to the blueprints for a car, with a gene being a drawing for
a part. And then you are the car? But moving on.
One of each pair of chromosomes was from each of your parents. And your children will be
a mix of a your chromosomes and your mate’s chromosomes. A random assortment of 1 of each
pair from you, and a random assortment of 1 of each pair from your mate.
And then your grand children will be a mix of your child’s chromosomes and THEIR mate’s
chromosomes. They’ll have, on average, about a quarter of your chromosomes, a quarter of
your genes. “On average” because it’s a random assortment of your child’s gene’s in you grandchildren.
So it could be that your grandchild just happen to have none of your genes. Or it could just
so happen that your grandchild has half your genes, that is all the gene’s you gave your
child. So let’s say Barack Obama made a baby with
Halle Berry. They both have a parent of African decent, and a parent of white people decent.
There is a chance, although a very small chance, of them producing a sperm or an egg that only
contain the chromosomes of one of their parents. If they each made one and they happen to come
together, then they could produce an offspring of entirely African decent, or vise versa
an offspring of entirely white person decent. OK, back to your genes. For all intents and
purposes, both chromosomes are producing proteins at the same time, even though members of the
same pair code for the same things, they’re both active at the same time. But it may be
that one allele is conveying the phenotype, the trait. If those alleles have a complete
dominance relationship. For example, whether a pea seed is wrinkled
or smooth, is controlled by 1 gene with 2 different alleles. We’ll call them little the alleles
big S and little s. When you have 2 big S alleles, you get smooth
peas. And when you have 2 little s alleles, you get wrinkled peas. But when you get a
big S and little s together, you get smooth peas. So it seems that the big S allele is
dominant and the little s allele is recessive. Because the phenotype of the SS and Ss is
different from the ss. This is called complete dominance. Where the phenotype of the mixed
allele individual, is indistinguishable from the 2 dominant allele individual. It’s as
though the phenotypic effects of little s are being ignored or deleted.
It’s not that a dominant trait is always better or always worse, this is just one of the ways
the genes influences the trait. But what you get, is that the Ss genotype,
while expressing the smooth pea phenotype, is a carrier for the wrinkled phenotype. If
2 Ss individuals got together and got their pollination on, you could get any random mixture
of their chromosomes. Remember these alleles are on different chromosomes.
So when these parents breed you can get any combination of 1 of each of their pairs of
chromosomes. So you can get all these genotypes. Which lead to these phenotypes. So even though
the parents have the smooth phenotype, they are can produce offspring that have the wrinkled
phenotype. I don’t think wrinkled peas are any better
or worse at… being peas, but sometimes the recessive trait can be detrimental. This is
one of the reason why inbreeding, making offspring with close family, is often shunned or illegal.
These genes that might lead to hazardous traits that might not be very common in the population,
are expressed when close family produce offspring. The DNA sequence, the order of the nucleotides,
codes for all the proteins that makes up us and out cells. The sequence can change by
things like random mutations, mistakes in the way it’s copied that lead sto different
proteins being made or from mixing genomes like with sexual reproduction. These are some
of the ways our blueprints can change, this is HOW things evolve. To understand WHY things
evolve, let’s first look at….