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....