The document explains the concept of Punnet squares, a tool used in genetics to predict the genotype and phenotype outcomes of genetic crosses. It covers monohybrid, dihybrid, and trihybrid crosses, providing examples and calculations for each type. The document also discusses genetic terminology, including alleles, homozygous, heterozygous, and ratios of genotypes and phenotypes.

1. PUNNET SQUARE
Monohybrid, and Dihybrid,Cross
JENABELLE A. ADORO
BSED Science 2A

2. WHAT IS A PUNNET SQUARE?
When Johann Gregor Mendel, the Father of Genetics, published
his revolutionary research on heredity and genetics in 1865, his
findings were initially overlooked and underappreciated by the
scientific community. It wasn’t until 1901 that Mendel’s research was
“rediscovered’ and appreciated. In 1905, Dr. Reginald Punnet, a
British scientist, published the first genetics textbook, called
Mendelism. In it, Punnet unveiled his unique tool for calculating
possible genotypes and phenotypes in offspring — Punnet Square

3. PUNNET SQUARE DEFINITION
A Punnet Square is a graphical way of determining the
possible outcomes for a genetic cross between two
individuals with known genotypes are given.

4. Alleles. Alternative forms of a gene. Each of which produces a
different traits.
Dominant Allele. Has a higher probability to be observed.
Recessive Allele. Has a lower probability to be observed. It is
always almost masked by the dominant allele.
Homozygous. Both alleles are the same. Either both are
dominant or both are recessive.
Heterozygous. Alleles differ from each other. Either one is
dominant and the other is recessive or one is recessive and the
other is dominant.

5. BB is a homozygous dominant. Both genes are the same and
dominant.The dominant is expressed.
Bb is heterozygous.The organism has dominant gene and
recessive gene. The dominant trait usually masked the
recessive, so the dominant trait is expressed.
bb is homozygous recessive. Both genes are the same and
recessive.The recessive trait is expressed.
Genotype.The genetic make up of an organism
Phenotype.The physical make up of an organism

6. HOW TO MAKE A PUNNET SQUARE?
1. Identify the trait to be studied and assigned a letter that represent it.
2. Draw the square. If you are only investigating one trait and each trait has two
genes, then the punnet square should have four boxes.
3. Place each parent’s genes on the outside of each square.
4. Just like in math, cross- multiply. Do this by putting the first letter of the
vertical parent in the first box.Then you add to that the first letter of the
horizontal parent.Then you fill in the next box with first letter of the vertical
parent with the second letter of the horizontal parent.
5. Do the same thing with the second letter of the vertical parent.
6. Once you have filled in all the boxes, it’s time to look at what the possible
outcomes are.

7. A = tall
a = short
AA = Tall, homozygous dominant
Aa = Tall, heterozygous dominant
aa = Short, homozygous recessive
Genotype Ratio
1 AA : 2 Aa : 1 aa
1 : 2 : 1
Phenotype Ratio:
Tall : Short
3 : 1

8. MONOHYBRID CROSS
A monohybrid cross refers to a genetic cross design to
study the inheritance of a single trait or gene.
Example:
Two heterozygous cats with brown eyes mate together. (a)
What is the probability that they will produce a cat with brown
eyes? (b) What is the probability that the baby cat will be
homozygous? (c) Calculate the phenotype and genotype ratio.

9. BB Bb
Bb bb
B b
B
b
B = Brown eyes
B = blue eyes
A. P = or 75%
B. P = or 50%
C. Phenotype:
Brown eyes : Blue eyes
3 : 1
Genotype:
BB : Bb : bb
1 : 2 : 1

10. DIHYBRID CROSS
A Dihybrid Cross refers to a genetic cross between two organisms who are
both heterozygous for two different traits.
Example:
A bear(Ffbb) with black fur(F) and blue eyes(b) mates with another
bear(ffBb) that has white fur(f) and brown eyes(B). (a) What is the probability that
the baby bear will have black fur and brown eyes? (b) What is the probability that
the baby bear will have white fur and blue eyes? (c) What is the probability that
the baby bear will be homozygous dominant for atleast one trait? (d)
Heterozygous for both traits? (e) Calculate the genotype and phenotype ratio.

11. FfBb FfBb FfBb ffBb
Ffbb Ffbb ffbb ffbb
FfBb FfBb ffBb ffBb
Ffbb Ffbb ffbb ffbb
Fb Fb
fb fb
fB
fb
fB
fb
F= black fur f= white fur
B= brown eyes b= blue eyes
a) P= = or 25%
b) P= or 25%
c) P= 0%
d) P= or 25%
e) Genotypic Ratio
FfBb : ffBb : Ffbb : ffbb
4 : 4 : 4 : 4
1 : 1 : 1 : 1
Phenotypic Ratio
Black fur,brown eyes : white
fur,brown eyes : black fur,blue eyes :
white fur,blue eyes
1 : 1 : 1 : 1

12. TRIHYBRID CROSS
A Trihybrid Cross refers to a genetic cross between two
organisms. It deals with the study of three different traits. For
example: height, skin color, and eye color.
Height: A= tall and a=short
Skin color: B=brown and b=white
Eye color: C=brown eyes c= blue eyes

13. AaBbCc X AaBbCc

14. 27:9:9:9:3:3:3:1
27/64 or 42% :tall,brown skin, brown eyes
9/64 or 14%:short,brown skin,brown eyes
9/64 0r 14%:tall,white skin,brown eyes
9/64 or 14%:tall,brown skin, blue eyes
3/64 or 4.68%:tall,white skin,blue eyes
3/64 or 4.68%: short,brown skin,blue eyes
3/64 or 4.68%: short,white skin, brown eyes
1/64 or 1.56%: short,white skin,blue eyes

15. THANK YOU!