Download Mendelian Genetics

A
homozygous recessive blonde
dog is crossed with a
heterozygous dominant brown
dog.
 What percentage of the offspring
do you expect to be brown?
◦ Blonde?



“Since chromosomes possess many
different genes they are polygenic.”
“There is a mutation in the MC1R gene that
causes an enzyme to be created which turns
pheomelanin into Eumelanin…”
“The…shade of your hair is controlled by
melanins which are produced by
melanocytes, a skin pigment.”
Phelan Ch 7
Krogh Ch 11
Male gamete
SPERM
Female gamete
EGG
Black
Box
Offspring

“Garden pea”
◦ How might peas be better
model organisms than our
previous example, Labrador
retrievers?

Most pea traits have only
two phenotypes:
◦ Tall vs. Dwarf
◦ Purple vs. White
◦ Wrinkled vs. smooth seeds

Reproduces sexually
 Female
 Male
(eggs)
(pollen)
 “If
you were researching pea plant
genetics, what factors would be
important to control?
 How
would you control these
factors in the pea plant?”
Yellow vs. Green seeds
Yellow vs.
Green seeds


No blending of characteristics.
Plants retained the potential for
passing on recessive phenotypes.
This led him to conclude that these
plants retained a recessive element
(green seed allele).


Dominant allele = expressed gene (ex.
Purple pigment  purple flower)
Recessive allele = can be hidden/not
expressed (ex. No pigment  white flower)
Why did the recessive trait show up again in
the F2-gen?
◦
 “Based
on the sum of his results,
Mendel determined:
1. Genes are material elements
2. These elements come in pairs.
3. These elements can retain their
character through many generations
(heritable).
4. That these element pairs separate
during the formation of gametes.”
◦Differing characters displayed by
organisms result from two
genetic elements (alleles) that
separate in gamete formation,
such that each gamete gets only
one of the 2 alleles.
◦

“So far Mendel had looked at pea plants
that only differ in one character.
 smooth,
yellow seeds vs.
wrinkled, green seeds?”
◦ monohybrid cross.
◦ dihybrid cross.
Actual results??
?
“Why do you think
these specific ratios
were observed?
What might these
results tell you
about the genetics
of pea plants?”
Why??



“What conclusions can be made based on this
data?”
This suggested to Mendel that the characters
were transmitted independently of one
another.
Law of Independent Assortment (during
gamete formation, gene pairs assort
independently of one another.)

Used a large number of plants

Recorded and drew everything


Studied more than one generation of
offspring
Picked traits that were easy to distinguish
“There is a saying that it doesn’t
pay to be more than 10 minutes
ahead of your time. Gregor
Mendel had the misfortune of
being about 34 years ahead of
his.”
~ David Krogh