Download Principles of Genetics

The Basics of Genetics
1. Traits are passed or inherited
from one generation to the next.
What part of your cells contain your
genetic instructions?

Your genetic instructions are located in the nucleus of
every cell, stored within chromosomes.
How will your traits be passed on to
your future children?

Fathers pass genetic
instructions to offspring
through sperm cells.

Mothers pass genetic
instructions to offspring
through egg cells.
Chromosomes: How many?
Where are they?


A human body cell
contains 46
Chromosomes, which
are stored inside every
cell’s nucleus.
Human sex cells (sperm
or egg) contain 23
chromosomes each.
2. Traits of an organism are
controlled by genes.


A gene is a section of a
chromosome, that
codes for a specific trait.
Chromosomes are
made of tightly wound
strands of DNA and
proteins.
gene
3. Organisms inherit genes in pairs,
one from each parent.


Human sex cells (sperm
or egg) contain 23
chromosomes each.
When the egg is
fertilized, the new baby
will have 46
chromosomes in each of
its cells, which is the
correct number of
chromosomes for a
human.
Sexual Reproduction
(2 parents) creates unique offspring.


In plants, male
chromosomes are
passed through pollen
grains to female egg
cells. Once fertilized,
these eggs will develop
into seeds.
The new plants will
receive a combination
of traits from both
parents.
4. Some genes are dominant,
while others are recessive.


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“Stronger” traits are
called dominant.
“Weaker” traits are
called recessive.
Geneticists use
symbols (letters) to
represent the
different forms of a
gene.
Dominant traits are
represented by a capital letter.
Yellow seeds are
dominant……..
Y
Recessive genes (for the same trait)
are represented by THE
SAME lower case letter.
Green seeds are
recessive……….
y
Dominant (yellow) = Y
Recessive (green) = y
In pea plants,
tall is dominant
over short. The
letter used to
represent the tall
gene is T.
 The short gene is
represented by t.

Tall =T
Short = t
In humans, brown eyes are
dominant over blue.
Brown = B
Blue = b
5. Dominant genes hide recessive
genes when both are inherited by an
organism.


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Y + y = yellow seeds
(yellow is dominant)
T + t = tall plant
(tall is dominant)
B+ b = brown eyes
(brown is dominant)
A PUREBRED organism has two of
the same genes for a trait.
TT = purebred TALL
tt = purebred SHORT
A HYBRID organism has two
different genes for a trait.
Tt = hybrid TALL
(tall is dominant.)
Bb = hybrid for black
feathers (black is
dominant.)
Will a hybrid human with the genes
Bb have brown or blue eyes?


Bb = Brown eyes
The dominant gene (brown), will be expressed
in a hybrid.
Why must all blue eyed people be
PUREBRED for that trait?


If the dominant gene is present, it will always be
expressed.
The only possible gene combination for blue eyes is
bb.
6. Some genes are neither dominant
nor recessive. These genes show
incomplete dominance.

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If an organism shows
INCOMPLETE
DOMINANCE for a trait, the
hybrid will show a blend of
the two traits.
In humans, hair texture is
such a trait.
Because neither curly or
straight hair is recessive, no
lower case letters are used.
CC = curly hair
CS = wavy hair
SS = straight hair
These basic laws of inheritance
control the traits of all living
organisms on Earth.