INTRO TO GENETICS
Your DNA is organized into chromosomes. There are 46 chromosomes in each
cell of your body. You get one chromosome of each pair from your father,
and one of each pair from your mother. Because you have two of each
chromosome in your body cells, they are known as diploid cells (2n)
In meiosis, you create sex cells, also known as gametes (sperm in men and eggs
in women). These cells only contain one of each pair of chromosomes in them
and are known as haploid cells (n). When a haploid sperm fertilizes a haploid
egg, it creates a diploid cell (the zygote), the first of a new person. This cell
will divide trillions of times through mitosis to create a person.
Your chromosomes can be divided into genes. You have 46 chromosomes but
more than 30,000 genes. Let’s say the gene for whether you have brown eyes
or blue eyes is on chromosome 12, for example. This means that you have
two of this gene- one on the chromosome 12 you received from your mother
and one on the chromosome 12 you received from your father.
The different forms of a gene- brown eyes or blue eyes, for example- are
called alleles. The genes on the chromosomes could be the same (both brown
or both blue, for example), which is called homozygous, or they could be
different (one brown and one blue, for example), which is called
heterozygous.
In basic genetics, one allele is dominant over the other, which is referred to as
recessive, although this isn’t always the case. For example, let’s say the gene
for brown eyes is dominant over the gene for blue eyes, which is recessive.
What this means is that the only way a person could have blue eyes, the
recessive trait, is if they have two blue eyes genes, one on each chromosome
number 12. If a person had one brown eyes gene and one blue eyes gene,
the person would have brown eyes, because brown eyes is dominant.
Let’s use a different example. Let’s say that we are looking at the genes on
pea plants for whether the peas are green or yellow. We know that green
color gene is dominant, so we will symbolize it with a capital G. Thus, yellow
color would be recessive, so we will symbolize it with a lower-case g.
The word genotype refers to the genetic makeup for a particular trait: GG,
for example.
GG
The word phenotype refers to the physical appearance of a particular trait:
Green peas, for example.
So, the pea plant can be one of three things:
Genotype
Symbolized Phenotype
as…
Homozygous
Dominant
Heterozygous
GG
Green peas
Gg
Green peas (because green is dominant)
Homozygous
recessive
gg
Yellow peas (because no dominant allele is
present)
It is possible to tell the probability of different traits of two organisms who
have children by performing a Punnett Square (also called a monohybrid
cross), in which all of the possible gametes a parent could make are crossed
with all of the possible gametes a second parent could make.
- The gametes of a homozygous dominant plant would all have the gene for
green peas (G).
- Half of the gametes of a heterozygous plant would have the gene for green
peas (G) and half would have the gene for yellow peas (g).
- The gametes of a homozygous recessive plant would all have the gene for
yellow peas (g).