Segregation diagrams are useful because they allow geneticist
to make predictions.
Example: What is the chance that this brown-eyed couple will
have a blue-eyed baby?
Bb
BB
B
b
B
B
BB
BB
Bb
Bb
0% chance
The ability to make predictions becomes important when
dealing with genetic disorders/diseases.
The man and the woman below have achondroplasia
(dwarfism). If this couple has children, will the children also
have achondroplasia?
To answer this question, we have to figure out if the disorder
is dominant or recessive.
The FGFR3 gene codes for cartilage protein
FGFR3
mutation
FGFR3
ONE COPY
ACHONDROPLASIA
IS DOMINANT
From Dad
From Mom
Achondroplasia = A; regular height = a
AA: fatal
Aa: dwarf
aa: regular
FGFR3
mutation
FGFR3
mutation
From Dad
From Mom
A
a
a
A
If these two people have a baby, how will the baby turn out?
Aa
Aa
AA: 25%
Aa: 50%
A
a
A
a
AA
Aa
Aa
aa
aa: 25%
This explains the family below:
AA: 25%
Aa: 50%
aa: 25%
However, we cannot say with absolute certainty what the
outcome of this cross will be. We can only determine the
probability.
Probability
50% chance of heads
50% chance of tails
C
The chance
that an event will occur; expressed as a
percentage.
P
Probability Practice
Solid: 4/8 = 1/2 = 50%
Female: 6/8 = 3/4 = 75%
Black/solid: 2/8 = 1/4 = 25%
Black nose: 8/8 = 100%
male
male
female
female
female
female
female
female
Segregation diagrams are not the only way to determine
probability. You can also use a Punnett square.
Punnett Square
P
Diagram that shows the probable
outcome of a genetic cross;
used by geneticists to make predictions.
P
gametes
Offspring
Curly = H; straight = h
HH
Gametes (one allele)
hh
H
H
h
Hh
Hh
h
Hh
Hh
Offspring (two alleles)
Probabilities:
HH: 0%
Hh: 100%
hh: 0%
Curly: 100%
Straight: 0%
Make a blank Punnett square in your notes. Include the
probabilities section:
Probabilities:
HH:
Hh:
hh:
Curly:
Straight:
Use the Punnett square to solve the genetics problem below.
Hh
hh
H
h
h
Hh
hh
h
Hh
hh
Probabilities:
HH: 0%
Hh: 50%
hh: 50%
Curly: 50%
Straight: 50%
#48 in SG: Mendel’s P generation cross.
t
t
T
Tt
Tt
T
Tt
Tt
FI generation
Probabilities:
TT: 0%
Tt: 100%
tt: 0%
Tall: 100%
Short: 0%
In genetics, an organism can be described by its genotype or
its phenotype.
Tall
Tall
Short
TT
Tt
tt
Genotype
Tall
Tall
Short
TT
Tt
tt
A
G
An organism’s allele
combination. Geno = gene.
Phenotype
Tall
Tall
Short
TT
Tt
tt
L
P
The way the organism looks.
Pheno = physical.
What is the genotype of person A?
A
B
C
Brown
Brown
Blue
BB
Bb
What is the phenotype of person B?
A
B
C
Brown
Brown
Blue
BB
Bb
What is the phenotype of person C?
A
B
C
Brown
Brown
Blue
BB
Bb
What is the only possible genotype for a blue-eye phenotype?
A
B
C
Brown
Brown
Blue
BB
Bb
bb
Which two people have the same phenotype?
A
B
C
Brown
Brown
Blue
BB
Bb
bb
Do they also have the same genotype?
No!
A
B
C
Brown
Brown
Blue
BB
Bb
bb
Two organisms can have the same phenotype, but different
genotypes.
Tall
Tall
TT
Tt
Short
tt
What is the phenotype of dog #2?
Black
1
2
3
4
5
6
Brown (B) is dominant; black (b) is recessive
What is the genotype of dog #2?
bb
1
2
3
4
5
6
Brown (B) is dominant; black (b) is recessive
What is the phenotype of dog #6?
Brown
1
2
3
4
5
6
Brown (B) is dominant; black (b) is recessive
What are two possible genotypes for dog #6?
BB or Bb
1
2
3
4
5
6
Brown (B) is dominant; black (b) is recessive
A genotype may either be homozygous or heterozygous
TT
Tt
tt
Homozygous
TT
Tt
tt
S
A
S
Genotype with the same
alleles.
Homo = same.
Heterozygous
TT
Tt
tt
D
A
D
Genotype with different
alleles.
Hetero = different.
Which people have a homozygous genotype?
A
B
C
Brown
Brown
Blue
BB
Bb
bb
What word describes person B’s genotype?
Heterozygous
A
B
C
Brown
Brown
Blue
BB
Bb
bb
Which person is homozygous dominant?
A
B
C
Brown
Brown
Blue
BB
Bb
bb
Which person is homozygous recessive?
A
B
C
Brown
Brown
Blue
BB
Bb
bb
To have a recessive phenotype, the genotype must be
homozygous.
Short
Blue
tt
bb
Genetics Problem #3: Black Parents, White Baby
Dark skin (D) is dominant; light skin (d) is recessive
Create a Punnett square showing how this type of inheritance
is possible.
D
d
D
DD
Dd
d
Dd
dd
Probabilities:
DD: 25%
Dd: 50%
dd: 25%
Dark: 75%
Light: 25%
45. If this couple decides to have
another baby, what is the
probability that the baby will have
light skin? 25%
46. If this couple decides to have
another baby, what is the
probability that the baby will have
dark skin? 75%
47. If this couple decides to have
another baby, what is the
probability that the baby will be a
carrier? 50%