2. Objectives:
⚫Explain Gregor Mendel’s laws of inheritance
⚫Discuss the difference between genotypes and
phenotypes
⚫Label the three types of genotypes
⚫Draw and label a Punnett square
⚫Predict outcomes of a Punnett square
3. ● Genetics is the science
which deals with how
genes control the
biological characteristics
● Gregor Mendel initiated
the study of this particular
branch of biology.
Gregor Mendel (Johann Mendel)
4. ⚫ Modern genetics had its beginnings in The Abbey garden of St
Thomas, where a monk named Gregor Mendel documented a
particulate mechanism of inheritance.
⚫ He discovered the basic principles of heredity by breeding
garden peas in carefully planned experiments.
⚫ His approach to science had been influenced at the University of
Vienna by one of his professors: the physicist Doppler.
5. Terminologies
● Alleles: variation of the genes
● Phenotype: the expressed and observable traits of organisms.
● Genotype: the genetic makeup or allelic set of an individual.
● Homozygous: a condition wherein the alleles for a gene are identical.
When using letters, it can be written as RR or rr. This is an alternative
term to the true-breeding condition.
● Heterozygous: a condition wherein the alleles for a gene are not
identical. When using letters, it can be represented as Rr.
● Dominant allele: the allele that masks the presence of another type of
allele (stronger)
● Recessive allele: the allele in which expression is masked (weaker)
6. Punnett Square
● In a testcross, the results are
shown in a box known as the
Punnett square.
● It was devised by geneticist
Reginald Punnett. This
organizes the genotypes of
the individuals to be crossed.
7. Punnett Square
Problem: A homozygous recessive mother dog has brown fur and heterozygous father has black fur.
a. What is the probability of having puppies with brown fur and with black fur?
Answer: Black : 2 = 50%
Brown : 2 = 50%
a. Phenotype ratio:
Brown : Black
2 : 2
1 : 1
a. Genotype ratio:
Ff : ff
2 : 2
1 : 1
F f
f Ff ff
f Ff ff
Note: Phenotype: Brown and Black Fur
Genotype: Ff and ff
