GENETICS
Medelian Genetics
• Gregor Mendel
• 1800’s
Medelian Genetics
Pollen transferred from white flower to
stigma of purple flower
all purple flowers result
self-pollinate
?
Looking closer at Mendel’s work
Parents
F1
true-breeding
purple-flower peas
true-breeding
X white-flower peas
100%
purple-flower peas
generation
(hybrids)
100%
self-pollinate
F2
generation
75%
purple-flower peas
25%
white-flower peas
3:1
We use letters to represent the
alleles or genes.
P = purple
p = white
Purple is dominant over white.
White is recessive.
Genotype
Phenotype
PP
Pp
Purple
Homozygous dominant
Purple
White
Heterozygous or hybrid
pp
Homozygous recessive
Predicting Inheritance
Let’s cross a totally dominant tall plant (TT) with a
short plant (tt).
Each plant will give only one of its’ two genes to
the offspring or F1 generation.
TT x tt
T
T
t
t
Predicting Inheritance
Let’s cross a totally dominant tall plant (TT) with a
short plant
(tt). “Law” of Segregation
Mendels’
Each plant will give only one of its’ two genes to
Eachoffspring
gene (allele)
from the other so that
the
or F1separates
generation.
the offspring get only one gene from each parent for a
given trait.
TT x tt
T
T
t
t
Punnett Squares
Tt
Tt
The genes from one parent go here.
The genes from the other parent go
here.
Tt
Tt
Punnett Squares
Tt
Tt
Tt
Tt
Aaaaah,
phenotype & genotype
can have different
ratios
Punnett squares
Pp x Pp
1st
generation
(hybrids)
%
genotype
male / sperm
P
p
PP
25%
75%
female / eggs
Pp
P
PP
%
phenotype
50%
Pp
Pp
p
Pp
pp
pp
25%
25%
1:2:1
3:1
What if you didn’t know the genotype
of an individual?
• TEST CROSS
Let’s say Gray is dominant over white in mice.
If we have a gray individual, how could
we determine if it is homozygous or
heterozygous gray?
Cross the gray mouse by a white
mouse.
What do you get?
If you get
100% all gray, then
the genotype must
have been
GG
1 white individual,
then the genotype
must have been
Gg
Dihybrids
More than 2 traits are crossed at a time.
T = tall
t = short
G = green
g = yellow
What is the genotype
for a heterozygous tall
and green pea plant?
TtGg
What gametes can it
produce?
First
Outer
TG Tg
TtGg
Inner
Last
tG tg
Dihybrid Cross
TG
TG
Tg
tG
tg
Tg
tG
tg