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Elementary Teacher
Education
GENETICS &
INHERITANCE

The passing of physical characteristics from characteristics from parents
to offspring
What is heredity?
What is genetics?
• The study of how traits are passed from parent to offspring by
looking at genes
• Genes are small sections of DNA on a chromosomes that has
information about a trait
What is trait?
Physical characteristics of an organism

6

Sex cells have 23 chromosomes and the two sex
cells combine to form a zygote with 46
chromosomes
During fertilization the offspring receives half of
its genetic information from its mother and the
other half from its father.

• The sex chromosomes carry genes
that determine whether a person
is male or female.
• also carry genes that determine
other traits.
XX = female
XY = male

• Gregor Mendel (1822-1884) laid
down the basic principles of
heredity.
• He explored the ways physical traits
could be expressed in plant
hybrids.
• A hybrid is the offspring of
individuals that differ with regard to
certain traits or certain aspects of
genetic makeup.

Phenotype: the observed traits of an organism. Example: hair color
Genotype: the genetic makeup of an organism. Example: the gene that says
what your hair color will be.
Gene: a region of DNA that describes a trait of an organism.
Example: eye color gene
Allele: a different form of a gene
Example: brown eye color allele, blue eye color allele

Dominant allele: Traits that mask the effect of other traits
Recessive allele: Traits that are masked by dominant traits
Examples of dominant and recessive traits
Heterozygous: the organism has one dominant allele and one recessive allele.
Example: Bb
Homozygous: the organism has either 2 dominant alleles or 2 recessive
alleles. Example: BB or bb

Monohybrid cross
• Parents differ by a single trait.
• Crossing two pea plants that differ in stem size,
one tall one short
T = allele for Tall
t = allele for dwarf
TT = homozygous tall plant
t t = homozygous dwarf plant
T T  t t

Monohybrid cross for stem length:
T T  t t
(tall) (dwarf)
P = parentals
true breeding,
homozygous plants:
F1 generation
is heterozygous:
T t
(all tall plants)

• A useful tool to do genetic crosses
• For a monohybrid cross, you need a square divided by
four….
• Looks like
a window
pane…
We use the
Punnett square
to predict the
genotypes and phenotypes of
the offspring.

Using a Punnett Square
STEPS:
1. determine the genotypes of the parent organisms
2. write down your "cross" (mating)
3. draw a p-square
Parent genotypes:
TT and t t
Cross
T T  t t

Punnett square
4. "split" the letters of the genotype for each parent & put
them "outside" the p-square
5. determine the possible genotypes of the offspring by filling
in the p-square
6. summarize results (genotypes & phenotypes of offspring)
T t T t
T t T t
T T
t
t
Genotypes:
100% T t
Phenotypes:
100% Tall plants
T T  t t
T T t t
The gametes are:

Monohybrid cross: F2 generation
• If you let the F1 generation self-fertilize, the next
monohybrid cross would be:
T t  T t
(tall) (tall)
T T T t
T t t t
T t
T
t
Genotypes:
1 TT= Tall
2 Tt = Tall
1 tt = dwarf
Genotypic ratio= 1:2:1
Phenotype:
3 Tall
1 dwarf
Phenotypic ratio= 3:1

A man who is heterozygous for brown eyes marries a woman
who is homozygous recessive for blue eyes. What are the
phenotypes, genotypes of their 4 children?

 Geneticist use
pedigrees to
follow a human
trait to learn how
the trait was
inherited
 A pedigree is a
chart or “family
tree” that tracks
the members of a
family that have a
certain trait

The chart below follows hemophilia in a family. Hemophilia is
a genetic disorder that does not allow the blood to clot
normally. How many males have hemophilia?

Blood type is determined by a single gene with three alleles.