Transcript The Dawn of Genetics

The Dawn of Genetics
Genetics – the study of heredity
which is the transmission of
characteristics from parents to
offspring
Gregor Johann Mendel
• born on July 22, 1822
• to peasant parents in a
small agrarian town in
Czechoslovakia
• In 1843 he entered an
Augustinian monastery in
Czechoslovakia
• He was later sent to the
University of Vienna to
study
The above photo is from http://www.open.cz/project/tourist/person/photo.htm.
The garden of the Augustinian
Convent in Brno.
part of the
foundations of the
greenhouse that
Mendel used
Mendel studied Pea Plants
Why?
• The pea flower
Pistil
Stamen
• The male part of the
flower (stamen) was
easily removed to
prevent self-pollination.
• Pea flowers are easy to
cross-pollinate
• They were easy to grow.
Mendel Isolated 7 pairs of
contrasting traits.
• Dominant Recessive
Round
wrinkled
Yellow
seed
Green
seed
Purple
flower
White
flower
Images from Purves et al., Life: The Science of Biology, 4th Edition, by
Sinauer Associates
In each case, one characteristic
was dominant and one was
recessive
Inflated
pod
Constricted
pod
Green pod Yellow pod
And the last 2 traits he selected:
Tall is dominant over short.
Flower postion:
Axial is dominant over terminal
• Mendel tested all 34 varieties of peas available to
him through seed dealers. The garden peas were
planted and studied for eight years.
• Mendel's experiments used some 28,000 pea
plants.
• So just imagine what life would have been like
living in the same monastery as Brother Gregor??
Genetic Terminology you will need
to know:
• Genotype
- The alleles that an organism has for a particular trait. Tt is hybrid
tall, TT is pure tall
• Phenotype
- The observable traits of an organism. Tall and short are
phenotypes.
• Allele
– Alternate forms of genes. Tall (T) and short (t) are the alleles for
height in pea plants.
• Dominant
–Tall is stronger over short so the tall allele will always be expressed
if it is present. We use capital letters to show dominant alleles.
• Recessive
– Short is the weaker allele and won’t show unless both alleles for
short are present. We use lower case for recessive alleles.
Chromosomes, Genes and
Alleles….Oh My!
More terms:
• Homozygous – a
genotype in which
both alleles are the
same…pure
• Heterozygous – a
genotype in which the
alleles are
different…hybrid
So what did Mendel find out about
his pea plants?
• Mendel first selected certain traits and
bred those plants so that they would be
purebred for that trait.
• He chose plants that were tall and bred
them together. He then took only the tall
offspring and bred those together. He did
this until only tall offspring were produced.
• Mendel produced pure breeding varieties
of all six traits.
What next?
• Mendel called his purebred plants the
parent generation or P generation.
• He then crossed a tall plant (TT) with a
short plant (tt).
• This is called a monohybrid cross
because only one trait, plant height, is
being tested.
• The offspring from this cross are called the
first filial generation or the F1 generation
What did the F1’s look like?
• Mendel found that 100% percent of the
F1’s were tall.
• This is where the terms dominant and
recessive came from. Mendel concluded
that tall was dominant over short.
• Mendel concluded that heredity is not just
a blending of traits. Instead one trait will
be dominant over another.
• This is called the Law of Dominance.
Law of Dominance
• When individuals that are pure for
contrasting traits are crossed, the offspring
will express only the dominant trait
• So, when a purebred tall plant is crossed
with a purebred short plant, all the
offspring will be tall
• This also means that when an organism is
heterozygous (Tt) the dominant trait will
always be expressed.
The Next Step
• Mendel’s next experiment
crossed the F1 generation with
itself to produce the F2
generation
• This is just like our fast plant
experiment
• What happened when we
crossed the F1 generation with
itself to produce the F2
generation?
How our findings compare to
Mendel’s
What happened?
• Mendel repeated this experiment many
times and each time found that the F2
generation had phenotypes like the
dominant trait 75% of the time and
phenotypes like the recessive trait 25% of
the time
• This 3:1 ratio is known as the Mendelian
ratio
Why did this happen?
• During Meiosis the alleles for a trait
separate into different gametes
• Offspring inherit one allele from each
parent
• If the dominant allele is present it will be
expressed (TT and Tt)
• The recessive allele will only be expressed
if both alleles are recessive (tt)
Law of Segregation
• Factors (alleles) that occur in pairs are
separated from each other during gamete
formation and recombined at fertilization.
• The new combination consists of one
allele from each parent, giving rise to new
combinations of alleles and possibly new
genotypes and phenotypes than the
parents
Punnett Square
• Diagram that helps organize
the results of a cross
T
between the gametes of two
individuals
• Used to predict phenotypes
and genotypes of offspring
t
• Genotypic ratio - 1:2:1
• Phenotypic ratio - 3:1
T
t
TT
Tt
Tt
tt
• Since the genotypes TT and Tt both appear tall this type
of inheritance is said to have complete dominance
because both dominant homozygotes and heterozygotes
have the same phenotype
Homozygous Dominant or
Heterozygous?
• In order to determine the genotype of
these individuals we must perform a test
cross
• Cross unknown (TT or Tt) with a
homozygous recessive (tt)
• If any offspring show the recessive trait
then the unknown must be heterozygous.
If all offspring show the dominant trait the
unknown must be homozygous.
Example Test Cross
TT x tt
Tt x tt
t
t
T
Tt
Tt
T
Tt
Tt
Offspring: all dominant phenotype
t
t
T
Tt
Tt
t
tt
tt
Offspring:1/2 dominant, ½ recessive
Mendel’s Second Experiment
• Does the inheritance of one trait affect the
inheritance of a different trait? Ex: pea
shape and pea color
• Produced purebreds for both
traits…round, yellow (dominant) and
wrinkled green (recessive)
• Dihybrid Cross - cross involving two traits
• P generation was (RRYY) x (rryy)
What was the F1 generation?
• Phenotype?
– 100% round yellow
• Genotype?
– RrYy
RY
ry
RrYy
Now, what about the F2
generation?
• Phenotype?
– 9 round yellow, 3 round green, 3 wrinkled
yellow, and 1 wrinkled green….9:3:3:1
• Genotype?
– Complicated!
(RrYy) x (RrYy)
What did Mendel do with this
information?
• Concluded that different traits were
inherited independently of one another
• Law of independent assortment: the
inheritance of alleles of one trait do not
affect the inheritance of alleles of another
trait
• Offspring may have new combinations of
alleles that are not present in either parent
Recap of Mendel’s Laws
• Law of Dominance
– When individuals that are pure for contrasting traits are
crossed, the offspring will express only the dominant trait
• Law of Segregation
– Factors (alleles) that occur in pairs are separated from each
other during gamete formation and recombined at fertilization.
• Law of Independent Assortment
– The inheritance of alleles of one trait do not affect the
inheritance of alleles of another trait if they are on different
chromosomes
Beyond Mendel
• Some traits do not show complete
dominance…ie dominant and recessive
• Some inheritance patterns we will discuss:
– Incomplete dominance
– Co-dominance
– Multiple alleles
Incomplete Dominance
• Mendel thought that inherited traits were
not simply blended but determined by
dominant and recessive relationships. We
now know that blending does occur in
nature…it is called incomplete
dominance
Example: If red snapdragons (RR)
are crossed with white
snapdragons (R1 R1) the F1
generation is 100% pink (R R1)
What about the F2 generation?
• Phenotype?
– 1 red, 2 pink and 1 white
– 1:2:1
• Genotype?
– 1 RR, 2 RR1 and 1 R1R1
Codominance
• Sometimes both alleles for a trait are
dominant and both dominant alleles can
be expressed at the same time in
heterozygotes
• Example: Roan coat color in horses and
cattle are codominant for the alleles R and
r. Homozygous R individuals are red (or
bay), homozygous r individuals are white,
while heterozygous Rr individuals are roan
and have both white and red hair, giving
them a lightened appearance.
What about the F2 generation?
• Phenotype?
– 1 red, 2 roan and 1 white
– 1:2:1
• Genotype?
– 1Hr Hr, 2 HrHw and 1Hw Hw
Multiple Alleles
• Many genes have more than two alleles
• An individual may not have more than two
alleles for each trait but different
individuals can have different pairs of
alleles when multiple alleles exist
Example of Multiple Alleles
• Human blood type is controlled by three alleles
– IA, IB and i. Alleles IA and IB are dominant over
I, but IA and IB are codominant
Phenotype (blood type)
Genotypes
A
B
AB
O
IA IA or IAi
IB IB or IBi
IA IB
ii