I.
Genetics
A. The work of
Gregor Mendel
1. Monohybrid
crosses
2. Dominant and
recessive alleles
3. Law of
Segregation
B. Mendel and
meiosis
Who was this Mendel and what the heck is
he doing in a monastery?
• born in 1822
• trained himself to be a
naturalist early in life
• worked as a substitute
science teacher
• failed the qualifying exams to be
a regular high school teacher!
• joined a monastery in Brunn,
Austria
• sent to Vienna U. to study
science and math
MendelWeb
Mendel’s first published work:
"Versuche über Pflanzen-Hybriden“
or Experiments in Plant Hybridization
was a landmark in clarity and insight!
Trained as a mathematician
and a biologist, he figured
out the laws of inheritance…
mathematically!!
The work of Gregor Mendel
• worked with pea plants…
…he called
them his
children!
Why pea plants???
There was a long-standing tradition of breeding pea
plants at the monastery where Mendel lived and worked
So…they were readily available and they come in lots
of varieties!
…there were plants with different flower colors, seed
color, flower position etc..
‘Brother Greg... We grow tired of peas again!!!’
And best of all…
Pea plants flowers can
reproduce by themselves
This allowed Mendel to see if
strains were true breeding and to
produce hybrids
How Mendel made hybrids…
He’d then tie little
bags around the
flowers to prevent
contact with stray
pollen.
Mendel’s hybridization experiments…
Monohybrid crosses:
Parental
Generation
F1 generation
True-breeding
purple flower
x
True-breeding
white flower
All purple flowers (the hybrids)
Allowed F1 offspring
to self-fertilize
F2 generation
705 purple
224 white
The results of Mendel’s monohybrid
crosses led him to propose…
1. All organisms contain two “units of
heredity” for each trait (alleles).
2. Dominant and recessive alleles…
…and organisms can have any combination of
the two alleles (2 dominants, 2 recessives
or a mixture 1 dominant and 1 recessive).
3. The Law of Segregation – during
gamete formation, alleles separate
randomly into separate gametes.
A bit of genetic jargon…
phenotype vs. genotype
What the
organism
looks like
What alleles
the organism
has - its
genetic makeup
More jargon…
homozygous vs. heterozygous
P
2 of the
same alleles:
PP or pp
2 different
alleles:
Pp
p
A Punnett
square
A Punnett square…
Gametes from
one parent
p
P
Gametes from
other parent
P
PP
purple
p
Pp
purple
Pp
purple
pp
white
Ratio: 3:1 or ¾ purple, ¼ white
Let’s relate Mendel’s findings to what
we now know about gamete formation
True-breeding
purple flower
P
all purple
P
P
P
P
P
True-breeding
white flower
x
P
p
p
p
p
p
p
p
F1 generation
purple hybrid
P
F2
p
P
purple hybrid
p
P P
P
x
p
P
p
P
p
p
p
P
PP (purple) Pp (purple) Pp - purple
pp - white
p
Join to your partner and together, work
on the following…
Determine the phenotypic and genotypic ratios for
each of the following monohybrid crosses.
Aa x Aa
Phenotypic
ratio
3:1 (dom:rec)
Genotypic
ratio
1:2:1 (hd:h:hr)
AA x Aa
1:0 (dom:rec)
1:1:0 (hd:h:hr)
or all dominant
aa x Aa
1:1 (dom:rec)
0:1:1 (hd:h:hr)
AA x aa
1:0 (dom:rec)
0:1:0 (hd:h:hr)
or all dominant
hd = homozygous dominant; h = heterozygous; hr = homozygous recessive
SO, NOW IT’S YOUR TURN TO WORK ALONE
Click on the site below
http://www.mendel-museum.com/eng/1online/experiment.htm