Mendelian Genetics
Chapter 6
“Father of Genetics”: Gregor Johann
Mendel
• Austrian monk
• Studied the inheritance
of traits in pea plants
• Developed the laws of
inheritance
• Mendel’s work was not
recognized until the
turn of the 20th century
Gregor Johann Mendel continued..
• Between 1856 and 1863
Mendel cultivated and
tested some 28,000 pea
plants
• He found that the
plants’ offspring
retained traits of the
parents
• Mendel was called the
“Father of Genetics”
Particulate Inheritance
- Mendel stated that
physical traits are
inherited as “particles”
- Mendel did not know
that these “particles”
were actually genes on
chromosomes.
- Remember genes are
segments of DNA that
code for a particular
protein!
Genetic Terminology
Trait- any characteristic
that can be passed from
parent to offspring
Heredity- passing of traits
from parent to offspring
Genetics- study of heredity
Punnett Squares
Used to help solve
genetics problems
They show all the
possible genotypes &
phenotypes of the
offspring from a test
cross of two specific
parents
Types of Genetic Test Crosses
Monohybrid cross- cross
involving a single trait
Example: Flower color
Dihybrid cross- cross involving
two traits
Example: flower color and
plant height
Designer “Genes”
Alleles- two forms of a gene (dominant and
recessive)
Dominant- most common of the two genes and
expressed in a hybrid (represented by a
capital letter)
Recessive- shows up less often in a cross and is
hidden when a dominant gene is present
(represented by a lower case letter)
More Terminology
Genotype- gene combination
for a trait
(Example: RR, Rr, or rr)
Phenotype- the physical
feature resulting from a
genotype
(Example: Red or white)
Genotype and Phenotype in Flowers
Genotypes of alleles:
R = red flower
r = yellow flower
All genes occur in pairs, so 2 alleles affect a
characteristic
Possible combinations are:
Genotypes:
RR
Rr
Phenotypes:
RED
RED
rr
YELLOW
Genotypes
Homozygous
Heterozygous
genotype: gene
genotype: gene
combination involving combination of one
2 dominant or 2
dominant and one
recessive alleles (RR
recessive allele (Rr);
or rr); also called
also called hybrid
purebred
Mendel’s Experiments: Why peas,
Pisum sativum?
•Peas can be grown in
a small area
•Produce lots of
offspring
•Produce pure plants
when allowed to
self-pollinate
•Can be artificially
cross-pollinated
Reproduction in Flowering Plants
Pollen contains sperm
Ovary contains eggs
produced by the stamen found inside the flower
Mendel’s Experimental Methods
- Mendel hand-pollinated
flowers using a paintbrush
- He snipped the stamens
(male parts) to prevent
self-pollination
- Covered each flower with
a cloth bag
Generation “Gap”
P1 Generation- the parental
generation in a breeding
experiment
F1 Generation- the first
generation of offspring in a
breeding experiment
F2 Generation- the second
generation of offspring in a
breeding experiment (from
breeding individuals from
the F1 generation)
Following the Generations
Cross 2 pure plants TT
and ss
Results in all hybrids
(Ts)
Cross two hybrids and
get 3 tall and one
short
(TT, Ts, and ss)
Law of Dominance
In a cross of parents that are
pure for different traits, only
one form of the trait will
appear in the next generation
All the offspring will be
heterozygous and express
only the dominant trait.
GG x gg = all Gg
Mendel’s Laws
Law of Segregationduring the formation of
gametes (sex cells; aka
eggs or sperm), the two
alleles responsible for a
trait separate from each
other
Alleles for a trait are then
“recombined” at
fertilization to produce
the genotype of the
offspring
bb
b
b
2. A pure female white flower (rr)
rr
Yy
• Male red flower (Rr)
• Female white flower
Rr
Sperm 1
rr
Sperm 2
Egg 1
Egg 2
Genotype
+
1
=
1
+
1
=
2
+
2
1
+
2
=
2
=
Phenotype
Law of Independent Assortment
Alleles for different traits
are distributed to sex
cells independently of
one another.
This law can be illustrated
using dihybrid crosses.
Bellringers Week of
12/14/2015
Jenny has genotype “tt”. Her genotype would best
be described as:
a.
b.
c.
d.
heterozygous dominant
homozygous recessive
homozygous dominant
heterozygous recessive
The idea that for any particular trait, the pair
of alleles of each parent separate and only
one allele from each parent passes to an
offspring is Mendel's principle of:
a) independent assortment
b) hybridization
c) segregation
d) dependent assortment
BB
A. 25%
B. 50%
C. 75%
D. 100%
When crossing a homozygous recessive with a
heterozygote, what is the chance of getting an
offspring with the homozygous recessive phenotype?
A.
B.
C.
D.
100%
75%
50%
25%
If you had two guinea pigs of opposite sex, both homozygous, one black and one brown, but
you didn't know which was the dominant characteristic, how could you be certain that the
guinea pigs are truly homozygous?
The guinea pigs would be homozygous for black (or
brown) coat
color if each strain could be bred for
many generations and only black (or brown) colored
offspring were produced.
B) If the immediate parents of the black (or brown) guinea
pigs were
both of that color, it proves they are
homozygous.
C) If a cross between the black and brown guinea pig
produced four
all black offspring, the black guinea pig
would have to be homozygous for black coat color.
D) Any of the results would prove the black guinea pig was
homozygous.
E) Only microscopic examination of the guinea pig's genes
could absolutely confirm homozygosity.
A)