Genetics
Terminology
• Allele – contrasting
form of a gene
– Ex: T = tall; t = short
– Ex: G = green; g =
yellow
– CAPITAL LETTERS –
DOMINANT TRAIT
– lowercase letters –
recessive trait
More Terms
• Genotype
– Genetic makeup of an organism
• Phenotype
– Physical appearance
More Terms
• Homozygous Alleles
– when the alleles of a
pair are the same
– Ex: GG, TT, rr, nn
• Heterozygous Alleles
– when the alleles are
not the same
– Ex: Gg, Tt, Rr, Nn
Probability
• The likelihood that a specific event will
occur
# of one kind of event
Probability = ----------------------------# of all events
Coin Toss
• Probability of a coin landing heads up?
• The more times you flip the coin, the
closer to 50% the probability will get
Punnett Squares
• Device used in predicting possible
offspring
Monohybrid Cross
• A cross that involves only one trait with
two phenotypes
• Ex: Seed color
– GG x gg;
– G = green, g = yellow
Making a Punnett Square
• Draw a box
• Put one set of traits on
top
– Male parent
• Put the other on the
side
– Female parent
Parent Generation:
P1
GG x gg
G
G
g
Gg
Gg
g
Gg
Gg
Cross F1 Generation
• Gg x Gg
• Genotype?
• Phenotype?
Parent Generation:
F1
G
Gg x Gg
g
G GG Gg
g
Gg
gg
Results
• Genotypes
– 25% GG
– 25% gg
– 50% Gg
• Phenotypes
– 75% green
– 25% yellow
G
g
G
GG
Gg
g
Gg
gg
You Try
•TT x Tt
(T = tall;
t = short)
TT x Tt
(T = tall; t = short)
• Genotypes
– 50% TT
– 50% Tt
– 1:1
• Phenotypes
– 100% tall
T
T
T
TT
TT
t
Tt
Tt
You
Try
You Try
• Bb x bb (B = black; b = white)
Bb x bb (B = black; b = white)
B
b
b
Bb
bb
b
Bb
bb
• Genotypes
– 50% Bb
– 50% bb
– 1:1
• Phenotypes
– 50% black
– 50% white
– 1:1
Testcross
• A cross between a
homozygous
recessive individual
and an unknown
• Used to determine
whether an
individual is
homozygous
dominant or
heterozygous
Testcross
• Mendel developed what is known as a test
cross.
• He took a homozygous recessive
individual and mated it with a pea plant
showing the dominant trait.
bb x
Bb or BB
Testcross
• If in the F1 generation all the offspring
showed the dominant trait, then the
dominant parent was probably BB.
b
?
b
B Bb Bb
B Bb Bb
Testcross
• If there were any recessives in the F1,
then the dominant parent had to be Bb.
b
b
B Bb Bb
?
b bb
bb
What if we are looking
at two traits at a time?
What if we
are looking
at Tall plants
with Axial
flowers
(TTAA) and
cross it with
a short plant
with terminal
flowers
(ttaa)?
If we look at meiosis what does that
tell us?
T
T
T
A
A
T
A
Parent TTAA
A
• Parent TTAA can only pass on the alleles
TA to the offspring
• What about Parent ttaa?
• Can only pass on the alleles ta to the
offspring
Parents: TTAA x ttaa
Parent TTAA produces only one kind of
gamete.
Parent ttaa produces only one kind of
gamete.
Thus our punnett square is very simple.
TA
ta
TtAa
What about the F1 generation?
T
T
A
t
t
A
T
A
a
Parent TtAa
a
t
a
F1 generation:
TtAa x TtAa
TA
Ta
tA
ta
TA
TTAA
TTAa
TtAA
TtAa
Ta
TTAa
TTaa
TtAa
Ttaa
tA
TtAA
TtAa
ttAA
ttAa
ta
TtAa
Ttaa
ttAa
ttaa
Dihybrid Cross
• A cross with two traits
• Ex: seed color & seed shape
– R = round, r = wrinkled; Y = yellow, y = green
– RRYY x rryy
– YYRr x yyRR
Dihybrid Cross
You try
RRYY x rryy
R = round, r = wrinkled; Y = yellow, y = green
Parents: RRYY x rryy
All offspring will be heterozygous
RY
ry
RrYy
Dihybrid Cross
You try
F1 generation: RrYy x RrYy
R = round, r = wrinkled; Y = yellow, y = green
F1 generation RrYy x RrYy
F1 generation:
RrYy x RrYy
• Phenotypes
– Round: Yellow
– Round: Green
– Wrinkled: Yellow
– Wrinkled: Green
– 9:3:3:1 ratio
9
3
3
1
Sex-linked Genes
• Genes carried on the X
and Y chromosomes are
called sex-linked genes.
• Traits that are controlled
by these genes occur
more often in one sex
than the other.
• Can you explain why?
Sex-linked Genes
• Colorblindness is one example of a trait
controlled by sex-linked genes.
Color blindness
• The normal human retina's color receptors are
tuned to green, blue, and red. Working
together, the three give us our colorful view of
the world. When one or more of those color
receptors is missing the result is color-blindness.
The genes for our red and green color receptors
are located on the X-chromosome, giving
women a redundant set of receptor genes. This
is why men are far more prone to colorblindness than women.
Sex-linked Genes
Recessive gene for
color blindness
XCY x XX
Sex-linked Genes
XC
Y
X
XCX
XY
X
C
X X
XY