Download Gregor Mendel

Gregor Mendel
Father of Genetics
Genetics
Branch of biology which deals with
principles of variations in traits
(distinguishing characteristics) and
inheritance
Allows us to predict patterns of
inheritance within family lines and
determine the probability of inheriting
certain traits
So why do family members
look alike anyway?
So why do family members
look alike anyway?
Gregor Mendel
Augustinian Monk at
Brno monastery in
Austria (now Czech
Republic)
Well-trained in math,
statistics,
probability, and
physics
Interested in plants
and heredity
Mendel worked with Peas
(Pisum sativum)
Commercially available
throughout Europe
Easy to grow and
mature quickly
Sexual organs of plant
enclosed in flower (so
the plants selfpollinate), allowing
Mendel to control which
plants reproduced
Different varieties have
different observable
traits
Mendel’s Peas
Mendel’s Experiments
Obtained purebred plants for the trait he
wanted to study (ex. height)
Produced through selective breeding
Designated “parent”, or P generation
Crossed purebred tall pea plant with purebred
short pea plant
Offspring designated “first filial”, or F1 generation
Also called “hybrid”, as they were the result of a
cross between two different purebred plants
This is a monohybrid cross, because it involved
only one trait (height)
Results of the Monohybrid
Cross (F1 generation)
Expected to obtain plants of
medium height
Instead, found 100% of the
plants were tall
Concluded that trait for tall
plants is dominant
Dominant traits are always
expressed (or always appear)
in an individual
Recessive traits (ex.
shortness in pea plants) are
present, but usually inactive
Principle of Dominance
When individuals with contrasting traits
are crossed, the offspring will express
only the dominant trait.
The Punnet Square
Organizes the “factors” or alleles of a particular gene
to determine possible combinations of alleles in
offspring
Helps us calculate the probability of inheriting a
particular trait
Mendel’s Experiments (Part 2)
Allowed hybrid F1 plants to cross
This produced the “Second Filial”, or F2
generation
Law of Segregation
Each parent in F1 generation starts with
two “factors” (alleles) - one is dominant
and the other is recessive
The alleles separate,
and only one allele
from each parent is
contributed to the
offspring
Law of Segregation
Each offspring (F2
generation) inherits one
allele from each parent.
If the dominant factor is
present, it will be
expressed even if the
recessive factor is also
present
The recessive factor will
be expressed only if two
recessive factors are
present
Solve the Punnet Square for
the F2 Generation
TT
Tt
Tt
tt
Gametes formed by
each parent are
listed along the top
and side
Possible genotypes
are determined by
combining gametes
from each row and
column in the
square
So the genotypes are…
TT
Tt
Tt
tt
25% TT
(homozygous tall)
50% Tt
(heterozygous tall)
25% tt (homozygous
short)
And the phenotypes are:
75% (3/4) tall
TT
Tt
25% (1/4) short
Tt
tt
Since tall is dominant,
any time there is a “T”
in the genotype, the
offspring will be tall
Since short is
recessive, it is only
expressed when the
genotype is
homozygous “tt”
3 tall : 1 short
Another Example of Mendel’s
Work
P
Green seeds
gg
x
Yellow seeds
GG
Homozygous
recessive
Homozygous
dominant
All Yellow
Gg
F1
Phenotype
Genotype
Phenotype
Genotype
Heterozygous
Phenotype
Genotype
F2
G
g
G
GG
Gg
g
Gg
gg
3/4 Yellow
1/4 Green
Mendel as a Scientist
Mendel repeated his
crosses over and
over, experimenting
with different traits the results held true
The ratio of 3:1 is
known as the
Mendelian Ratio
Remember:
Phenotype vs. Genotype
Phenotype
Genotype
P TT x tt
F1 All Tt
F2 TT, Tt, Tt, tt
Genotype vs. Phenotype
Trait
Fur colour
Presence of
horns
Dominant/
Recessive
Black/
White
Horned/
No horns
Phenotype
Genotype
Homozygous black
BB
Homozygous white
bb
Heterozygous black
Bb
Homozygous horned
HH
Homozygous no horns
hh
Heterozygous horns
Hh
Solving Genetics Problems 1
S
s
S
s
SS
Ss
Ss
ss
In pea plants, spherical seeds
(S) are dominant to dented
seeds (s). In a genetic cross of
two plants that are
heterozygous for the seed
shape trait, what fraction of
the offspring should have
spherical seeds?
3/4 of the offspring should
have spherical seeds.
Solving Genetics Problems 2
The ability to taste the
bitter chemical PTC is
determined by the
dominant allele T and
inability to taste by the
recessive allele t.
Suppose two
heterozygous tasters
(Tt) have a large family.
Solving Genetics Problems 2
a)
Predict the
proportion of their
children who will be
tasters and
nontasters. Use a
Punnett square to
illustrate how you
make these
predictions.
T
t
T
TT
Tt
t
Tt
tt
3/4 of their
children will be
tasters
Solving Genetics Problems 2
b)
c)
What is the likelihood
that their first child will
be a taster?
3/4 or 75%
What is the likelihood
that their fourth child
will be a taster?
3/4 or 75%
In these questions,
each child is
independent of the
other
d)
What is the likelihood
that all of the first three
children of this couple
will be nontasters?
1/4 for each child
1/4 x 1/4 x 1/4 = 1/64
chance that all three
will be non-tasters
In this question, the
probability of the event
is dependent on each
of the children being
nontasters
Pedigrees
We can’t do experimental crosses on humans, so
how can we study patterns of inheritance?
We use
medical,
historical, and
family records
to study
crosses that
have already
occurred
The Test Cross
Used to determine the genotype of an
individual
Cross an individual of unknown genotype
with a homozygous recessive individual
Offspring’s phenotypes will allow you to
determine if the parent of unknown
genotype was homozygous dominant or
heterozygous
The Test Cross
In dogs, there is a hereditary deafness caused by
a recessive gene, “d.”
A kennel owner has a male dog that she wants to
use for breeding purposes. The dog can hear, so
the owner knows his genotype is either DD or Dd.
If the dog’s genotype is Dd, the
owner does not wish to use
him for breeding so that the
deafness gene will not be
passed on. This can be tested
by breeding the dog to a deaf
female (dd).
The Test Cross
Draw the Punnet squares
to represent the two
possible crosses
If the male dog is
homozygous dominant
(DD), all the offspring will
be normal (Dd)
If the male dog is
heterozygous, half the
offspring will be
heterozygous normal
(Dd), and half will be deaf
(dd)
d
d
d
d
D
D
Dd
Dd
Dd
Dd
D
d
Dd
dd
Dd
dd
Practice
Page 128 “Practice Problems” #3 & 4 (you
may want to read the section entitled
“Probability and Genetics”)
Page 130 “Practice Problems” #1 - 3
Practice Problems from Handout and
Text… to be announced!