Download History of Genetics

And Probability of Inheritance
 Considered
the father of genetics
 He was a monk who enjoyed caring for
a garden of pea plants
 These pea plants can self-pollinate
(fertilize one of its own eggs with its own pollen) or
cross-pollinate (allow pollen from another pea plant
to fertilize its eggs)
 Mendel created stocks of true-breeding pea plants
 True-breeding means that if these plants selfpollinate they will always create offspring with the
same traits as the parent plant
 Mendel
looked at several traits
in his pea plants:






Tall or short
Yellow or green seeds
Green or yellow pods
Round or wrinkled seeds
Gray or white seed coat
Smooth or constricted pod shape
 He
noticed when he crossed
true-breeds of each there was
always the same outcome.
 So,
every time he fertilized a true-breeding
tall plant with pollen from a true-breeding
short plant, the offspring were tall (no
matter which way the fertilization occurred)
 Mendel
realized that something was being passed
on from generation to generation, today we
recognized that as genes
 Each gene has different alleles, or different
genetic make-up that leads to a different
expression of that gene
 We each get two alleles, one from each parent
 This lead Mendel to create the law of dominance:

Some alleles are dominant and some are recessive


Tall = dominant pea plants
Short = recessive pea plants
F1 Generation
 Mendel
was still curious
about whether or not the
recessive alleles had
disappeared in the offspring
 So, Mendel took the
offspring (F1 generation) of
the true tall and true short
and bred them
 After breeding several F1
plants with other F1 plants
Mendel noticed that the
recessive allele reappeared
in about ¼ of the F1’s
offspring (F2 generation)
F2 Generation
 Mendel
concluded that the dominant allele in the
F1 generation masked the recessive allele, but
these alleles could be separated again in the next
generation
 Mendel presumed that the alleles separated from
one another during formation of the gametes (sex
cells)
 Mendel came up with the Law of Segregation:

When each organism produces sex cells, the two alleles
separate so that each gamete only carries only one
single copy of each gene
 Therefore,
the F1 generation were all heterozygous
 Heterozygotes carry two different alleles for the
same gene
 Homozygotes carry the two of the same allele
True breeding
tall plants
A
True breeding
short plants
Hybrid tall
plants
capital letter always represents the dominant
allele, and a lower-case of the same letter is the
recessive
 The
fact that ¼ of the pea plants in
the F2 generation were short was
important, because probability
plays a role in genetics
 Probability is the likelihood that a
certain event will occur (like
getting a child with blue eyes)
 The easiest way to figure out
genetic probability is using Punnett
squares
 Heterozygous
– having two different alleles for a
gene

Ex:
 Homozygous

– having the same allele for a gene
Ex:
 Genotype
– the genetic make-up or two alleles that
are present

Ex:
 Phenotype
alleles

Ex:
– the physical expression of a set of
A
punnett square is simply a way to visualize
Mendel’s Law of Segregation
 One parent goes on top and one on the right,
then just fill in
the two alleles
they have in the
boxes and
check your
outcome.
T
t
t
T
 Now
 Now
try it with the F1 generation plants (hybrids)
you can see why ¼ of the F2 generation of pea
plants ended up short
 Now
Mendel was curious to see what would happen
if he crossed a plant with two true-breeding traits
and a plant with the two recessive true-breeding
alleles of that gene.
 So, he took a true-breeding round seed, yellow
seed and crossed it with a true-breeding wrinkled
green seed pea plant.
 We’ll need a bigger punnett square!
 Mendel
found that when he crossed the F1
generation F2 plants developed with traits
that were not there in either parent
(example: wrinkled yellow seeds)
 This lead Mendel to come up with the Law of
Independent Assortment:

Genes for different traits can segregate
independently during the formation of gametes.
 What
this means is that the inheritance of
one gene does not necessarily affect other
genes because they may be located on
separate chromosomes.
 Law
of Dominance – Some alleles are
dominant and some are recessive
 Law of Segregation - When each organism
produces sex cells, the two alleles separate
so that each gamete only carries only one
single copy of each gene
 Law of Independent Assortment - Genes for
different traits can segregate independently
during the formation of gametes