The Basics of Genetics
6 Laws about Heredity
1. Traits are passed or inherited
from one generation to the next.
• 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.
2. Genes control the traits of an
organism.
•No, not
THAT
Gene!
2. Genes control the traits of an
organism.
• A gene is a section of
a chromosome, that
codes for a specific
trait.
• Chromosomes are
made of tightly wound
strands of DNA
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 embryo
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.
• “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 letter, but it is lower case .
Green seeds are
recessive……….
y
Dominant (yellow) = Y
Recessive (green) = y
Pea Plant Genotype
Tall =T
Short = t
• In pea plants, tall
is dominant over
short. The letter
used to represent
the tall gene is T
(BIG T).
• The short gene is
represented by t
(little t).
In humans, brown eyes are dominant
over blue.
Brown = B
Blue = b
5. Dominant genes hide recessive genes
when an organism inherits both.
•
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
• This genotype is also
called
HOMOZYGOUS
A HYBRID organism has two
different genes for a trait.
• Tt = hybrid TALL
(tall is dominant)
• This genotype is also
called
HETEROZYGOUS
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.
• If an organism shows
INCOMPLETE
DOMINANCE for a
trait, the hybrid will
show a blend of the
two traits.
Human Hair Texture
• Because neither curly
nor 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.