Genetics is the scientific study of heredity, focusing on traits and their variations driven by alleles, genes, and the principles established by Gregor Mendel. Key concepts include dominant and recessive traits, genotype (genetic makeup) vs. phenotype (physical appearance), Punnett squares for predicting trait inheritance, and the complexities of incomplete dominance, codominance, multiple alleles, and polygenic traits. Overall, genetics explains how traits are passed from one generation to the next and the variability present within populations.

2. What is Genetics?
Genetics is the scientific study
of heredity

3. What is a Trait?
 A trait is a specific characteristic that
varies from one individual to another.
 Examples: Brown hair, blue eyes, tall,
curly

4. 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

5. What are Genes?
Genes are the
sequence of
DNA that codes
for a protein
and thus
determines a
trait.

6. 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

7. 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

8. Dominant vs. Recessive
 Dominant - Masks the other trait; the trait that
shows if present
 Represented by a capital letter
 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
r

9. Dominant & Recessive Practice
 TT - Represent offspring with straight
hair
 Tt - Represent offspring with straight
hair
 tt - Represents offspring with curly hair
T – straight hair
t - curly hair

10. 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

11. 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

12. 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

13. Punnett Square
Parent
Parent Offspring

14. How to Complete a Punnett Square

15. Y-Yellow
y-white
Genotype:
1:2:1
(YY:Yy:yy)
Phenotype:
3 Yellow
1 White

16. You Try It Now!
 Give the genotype and phenotype for the
following cross: TT x tt (T = Tall and t = Short)

17. TT x tt
Step One: Set Up Punnett Square (put one parent on the
top and the other along the side)
T T
t
t

18. TT x tt
Step Two: Complete the Punnett Square
T T
t
t
Tt Tt
Tt Tt

19. TT x tt
Step Three: Write the genotype and phenotype
T T
t
t
Tt Tt
Tt Tt
Genotype:
4 - Tt
Phenotype:
100% Tall
Remember: Each box is 25%

20. You Try It Now!
 Give the genotype and phenotype for the
following cross: Tt x tt

21. Tt x tt
Step One: Set Up Punnett Square (put one parent on the
top and the other along the side)
T t
t
t

22. Tt x tt
Step Two: Complete the Punnett Square
T t
t
t
Tt tt
Tt tt

23. Tt x tt
Step Two: Complete the Punnett Square
T t
t
t
Tt tt
Tt tt
Genotype:
Tt - 2 (50%)
tt - 2 (50%)
Phenotype:
50% Tall
50% Short
Remember: Each box is 25%

24. Some Terminology
 P1 – Original parents
 F1 – First generation
 F2 – Second generation
 P1 X P1 = F1
 F1 X F1 = F2

25. 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.

26. 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).

+

27. 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.

28. 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.

29. 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.

30. Multiple Alleles
 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.

31. 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.