Punnett
Squares
What is Genetics?
Genetics is the scientific study of heredity
What is a Trait?
•A trait is a specific
characteristic that
varies from one
individual to
another.
•Examples:
Brown hair, blue
eyes, tall, curly
What is an Allele?
 Alleles
are the
different possibilities
for a given trait.
 Every
trait has at least
two alleles (one from
the mother and one
from the father)
 Example: Eye color –
Brown, blue, green,
hazel
Examples of Alleles:
A = Brown Eyes
a = Blue Eyes
B = Green Eyes
b = Hazel Eyes
What are Genes?
•Genes are the
sequence of DNA
that codes for a
protein and thus
determines a
trait.
Gregor Mendel
• Father of Genetics
• 1st important studies of
heredity
• Identified specific traits
in the garden pea and
studied them from one
generation to another
Mendel’s Conclusions
1.Law of Segregation – Two
alleles for each trait
separate when gametes
form; Parents pass only one
allele for each trait to each
offspring
2.Law of Independent
Assortment – Genes for
different traits are inherited
independently of each
other
Dominant vs. Recessive
Dominant - Masks the other trait; the trait that
shows if present
◦ Represented by a capital letter
R
Recessive – An organism with a recessive allele for a
particular trait will only exhibit that trait when the
dominant allele is not present; Will only show if both
alleles are present
◦ Represented by a lower case letter
r
• TT - Represent
offspring with
straight hair
• Tt - Represent
offspring with
straight hair
• tt - Represents
offspring with curly
hair
Dominant &
Recessive Practice
• T – straight
hair
• t - curly
hair
Genotype vs. Phenotype
• Genotype – The genetic
makeup of an organism;
The gene (or allele)
combination an organism
has.
• Example: Tt, ss, GG, Ww
• Phenotype – The physical
characteristics of an
organism; The way an
organism looks
• Example: Curly hair,
straight hair, blue eyes,
tall, green
Homozygous vs. Heterozygous
Homozygous – Term used to refer to
an organism that has two identical
alleles for a particular trait (TT or tt)
Heterozygous - Term used to refer to
an organism that has two different
alleles for the same trait (Tt)
RR
rr
Rr
Punnett Squares
• Punnett Square –
Diagram showing the
gene combinations that
might result from a
genetic cross
• Used to calculate the
probability of inheriting
a particular trait
• Probability – The chance
that a given event will
occur
Punnett Square
Parent
Parent
Offspring
How to Complete a Punnett Square
• Y-Yellow
• y-white
• Genotype:
• 1:2:1
• (YY:Yy:yy)
• Phenotype:
• 3 Yellow
• 1 White
Incomplete
Dominance
• Incomplete
Dominance - Situation
in which one allele is
not completely
dominant over
another.
• Example – Red and
white flowers are
crossed and pink
flowers are
produced.
Codominance
Codominance - Situation in which both alleles
of a gene contribute to the phenotype of the
organism.
• Example – A solid white cow is crossed with a solid brown
cow and the resulting offspring are spotted brown and
white (called roan).
+
Multiple
Alleles
• Multiple Alleles- Three or more alleles of the
same gene.
• Even though three or more alleles exist for
a particular trait, an individual can only
have two alleles - one from the mother
and one from the father.
Examples of
Multiple
Alleles
1.
Coat color in rabbits is determined by a single
gene that has at least four different alleles.
Different combinations of alleles result in the
four colors you see here.
Examples of Multiple
Alleles
2. Blood Type – 3 alleles
exist (IA, IB, and i),
which results in four
different possible blood
types
3. Hair Color – Too many
alleles exist to count
• There are over 20
different shades of
hair color.
• There Are Always Multiple Alleles!
• Genetic inheritance is often presented with
straightforward examples involving only two alleles
with clear-cut dominance. This makes inheritance
patterns easy to see.
• But very few traits actually only have two alleles with
clear-cut dominance. As we learn more about genetics,
we have found that there are often hundreds of alleles
for any particular gene.
• We probably know this already - as we look around
at other people, we see infinite variation.
Multiple Alleles
Polygenic Trait
• Polygenic Trait - Trait controlled
by two or more genes.
• Polygenic traits often show a
wide range of phenotypes.
• Example: The wide range of
skin color in humans comes
about partly because more
than four different genes
probably control this trait.