Download Genes and Inheritance

Genes and Inheritance

We know from previous
discussions that an egg
contains half of the
information needed to
make a baby, and a
sperm contains the other
half.

This means that when
the egg is fertilized by
the sperm, it results in a
cell that has the exact
number of chromosomes
required to make a new
human being.

It also means that each
baby gets half of its
genetic information from
its mother, and half from
the father.


A trait is a particular
feature that you can see
in a person. Some
examples of physical
traits are hair color and
eye color.
Your physical traits are
determined by your
genetic information.

Each trait has two
genes associated
with it – one from the
mother and one from
the father.


Most genes are one of
two types. Dominant
genes are the ones
that take over.
If there is a dominant
gene present in a pair,
then that is the trait
you will see. It is like
the boss.



Recessive genes are
the ones that are
easily hidden.
They take a back seat
to the dominant ones.
The only time you will
see a recessive trait is
if there is no dominant
one to take over.


When we write about
traits, we pick a letter.
Then we use the
capital letter to
represent a dominant
gene, and a lower
case letter to
represent a recessive
gene.

The genotype of a person is the
combination of which two genes they have
for a particular triait. For example, for eyes
a person could be BB, Bb, or bb.



This would be called the person’s
genotype.
If the two letters are the same case, we call
the person homozygous for that trait.
If the two letters are different, we call the
person heterozygous for that trait.


The phenotype of a person is what you
actually see because of their genes.
So for eyes, the phenotype would be blue
eyed or brown eyed.
Note:


The genes that determine sex are a little different
than normal. Females have two matching genes,
usually called X. So every female is homozygous
for the sex gene, genotype XX.
Males have two different genes called X and Y, so
they are heterozygous for the sex gene, genotype
XY. This means each sperm can carry either X of
a Y chromosome. In this way, the sex of the baby
is determined by the male parent.