punnett-square-notesgrade9-221107071845-2fbd9a64.ppt

science 9

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
1
st
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 Two: Complete the Punnett Square
T T
t

t

TtTt
TtTt

18. TT x tt
Step Three: Write the genotype and phenotype
T T
t

t

TtTt
TtTt
Genotype:
4 - Tt
Phenotype:
100% Tall
Remember: Each box is 25%

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

20. 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

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

t

Tt tt
Tt tt

22. 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%

23. Some Terminology
P
1 – Original parents
F
1
– First generation
F
2 – Second generation
P
1 X P
1 = F
1
F
1 X F
1 = F
2

24. 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.

25. 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).

+

26. 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.

27. 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.

28. Examples of Multiple Alleles
2.Blood Type – 3 alleles
exist (I
A
, I
B
, 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.

29. 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.

30. 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.