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DNA and Inheritance
Biology 30
Genes & Heredity

Your biological traits are
controlled by genes, which are
located on the chromosomes
that are found in every cell of
your body. Only one copy of
each gene is on a chromosome.
Genes are like books; they may
or may not be read by the
chemical machinery of the cell.
• Your traits are a result of the interactions of the
genes from both parents.
• Although you contain half the genes from each
parent, your genes and traits are uniquely your
own.
• There are over eight million combinations
possible from the 23 chromosomes you inherit
from your mom and 23 you inherit from your dad.
• The passing of traits from parents to offspring is
called heredity.
Pioneer of Genetics: Gregor Mendel
Gregor Mendel, an Austrian monk, conducted
some simple but significant experiments in
his monastery garden. (1822-1884).
Mendel’s work with garden peas
not only explained the mechanism
of gene inheritance for plants, but
also provided a basis for
understanding heredity.
Why did Mendel work with the garden pea?
1) Garden peas have a number of
different characteristics that can be
expressed in one of two ways.
(ex. yellow vs. green pea)
2) It was easy for Mendel to go about
his experiments, because of the way the
plant reproduces.
Mendel fertilized
plants by
cross-pollination
rather than
self-fertilization.
Mendel’s Characters
The seven characteristics that Mendel studied
in peas
Mendel’s Experiments
Crossed the pollen from a plant that produced
round seeds with the eggs of the one that
produced wrinkled seeds, the offspring were
round.
Crossed the pollen from a plant that produced
wrinkled seeds with the eggs of the one that
produced round seeds, the offspring were
round.
Mendel then crossed the F1 hybrids
together to produce an F2 generation.
The recessive trait “reappeared”.
F1
F2
He found a 3:1 ratio of dominant
to recessive phenotypes.
3 round : 1 wrinkled
1/
4
homozygous dominant
1/
2
heterozygous
1/
4
homozygous recessive
Rr
r
R
1/
4
homozygous dominant
R
RR
Rr
Rr
1/
4
homozygous recessive
r
Rr
rr
He repeated the procedure for other
characteristics (eg. flower colour).
He discovered that one trait dominated
another, whether the sex cell came from
a female or a male part of the plant.
Mendel reasoned that things called
“factors” controlled the traits of a plant.
These factors were later termed genes.
Dominant genes –
Determine the expression of the genetic
trait in the offspring.
(Capitalize this letter).
Recessive genes –
Are “masked” by dominant genes.
(Lower case letter).
Mendel’s Laws of Heredity
First Law (Law of Unit Characters)
Inheritance is governed by genes that exist
in the individual and are passed on to
offspring. These factors (genes) occur in
pairs, one gene comes from the female and
one gene comes from the male. The
alternate forms of the same genes are known
as alleles.
Second Law (Law of Dominance)
One factor, or gene, masks the effect of
another. This process is known as the
principle of dominance. The dominant
expression is seen and the recessive gene is
not seen (remains hidden).
Third Law (Law of Segregation &
Recombination)
A pair of factors (genes) segregate/separate
during the formation of sex cells (meiosis).
As a result, each parent can only contribute
one member (allele) of a pair of genes to their
offspring.
Genetic Terms

Define the following terms:
Genotype –

Phenotype –

Homozygous (“purebred”) –

Heterozygous –

Alleles –

Monohybrid Cross
The combining of single
contrasting traits:
eg. Crossing a tall plant (TT) with a
short plant (tt)


a special chart called a Punnett
square, helps geneticists organize
the results of a cross between the
sex cells of two individuals

from a Punnett square, we can
predict the genotypes & phenotypes
of the offspring:
Draw a Punnett square for a cross between a
heterozygous round-seed plant and a wrinkledseed plant.
R
r
r
Rr
rr
r
Rr
rr
In the F1 generation ½ are round ½ are wrinkled
Rr
r
R
1/
4
homozygous dominant
R
RR
Rr
Rr
1/
4
homozygous recessive
r
Rr
rr
Draw a Punnett square for a cross between
two heterozygous round-seed plants.
R
r
R
RR
Rr
r
Rr
rr
In F1 generation ¾ are round ¼ wrinkled
Draw a Punnett square for the cross between a
heterozygous purple flower with a white flower.
W
w
w
Ww
ww
w
Ww
ww
In the F1 generation ½ are purple ½ white
Phenotypic Ratio:
50% of flowers are purple
50% of flowers are white
Genotypic Ratio:
50% of flowers are Ww
50% of flowers are ww
Test Cross
A test cross is often performed to determine the
genotype of a dominant phenotype. When would
we need to know this?
There would
eg. A sheep farmer wants to
be some
ensure that all of his flock
offspring
will have white hair (black
produced
that
wool is brittle & hard to
have
white
dye). He chooses a white
wool and
ram to mate with the flock.
some with
What if the ram is
heterozygous?
black.
To ensure that the ram is not, a test cross can be
performed to ensure that the ram is homozygous
for the white phenotype
A test cross is always
performed between the
unknown genotype and a
homozygous recessive
(known) genotype.
Possible outcomes:
If 50% of the offspring are black and the
other half white, the unknown genotype must
have been heterozygous (Ww).
If 100% of the offspring are white, then the
unknown genotype must be homozygous
dominant (WW).
Mendel showed that by performing a
test cross with the homozygous
recessive, the genotype could be
determined.
YY x yy
Yy
Yy x yy
Y
y
Yy
yy
Yy
yy
yy
y
y
This test cross
produces
1/2
dominant and
1/2
recessive
phenotypes