DNA in a Normal Cell
(one chromosome from each parent)

F
Free
Earlobe Gene
f
Attached
Allele


Dominant

t
Toe Gene
Long
big toe
P
Extra
Fingers
Polydactyly Gene
♂
♀
t
Long
big toe
P
Extra
Fingers

Allele that may be masked by another
Heterozygous


Allele that may mask the presence of another
Recessive


A version of a gene
The 2 alleles are different
Homozygous

The 2 alleles are identical
DNA in a Normal Cell
(one chromosome from each parent)
F
Free
Earlobe Gene
f

Attached
Genotype

t
Toe Gene
Long
big toe
P
Extra
Fingers
Polydactyly Gene
♂
♀
t
Long
big toe
P
Extra
Fingers

The set of alleles an organism
possesses
Phenotype

The outcome resulting from the
genotype
Meiosis
f
f
f
f
f
F
F
f
Maternal/Paternal
alleles segregate
F
F
F
F
Genetic Crosses
Geneticists perform crosses to predict
what offspring will result from two
parents.
1. Determine parent genotypes.
2. Determine possible gametes.
3. Use a Punnett square to show possible
gamete combinations.
4. List possible offspring genotypes.
5. List possible offspring phenotypes.


A woman with attached
earlobes has a child with a
man who is heterozygous.
ff ♀ x Ff♂
f or f
F or f
Practice Problems

1.
2.
3.
4.
Y = yellow pea seeds
y = green pea seeds
Perform this cross:
Yy x Yy
What is the probability of the first offspring having green seeds?
What about the 5th?
What are the chances of the first 5 plants having green seeds?
Out of 1000 offspring, how many would you expect to have green
seeds?
Practice Problems
1. Cystic fibrosis is a recessive genetic disease caused by buildup
of mucus in the lungs, liver, and pancreas. If two healthy people
have a child with cystic fibrosis, what are the chances of their
next child having CF?
2. People with the nervous system disorder Huntington’s disease
(caused by a dominant allele) usually don’t show symptoms
until their 30’s. A 27-yr-old man’s family has no history of the
disease. His 26-yr-old wife’s grandmothers both had the
disease though her parents do not. What are the chances of
their 1st kid having HD?
Law of Independent Assortment
PpYy
P
p
P
p
P
P
p
p
Y
y
Y
Y
y
y
P
p
P
P
p
p
y
Y
y
y
Y
Y
OR
Y
y
Gametes
PY
py
Py
pY
Practice Problem
P: FfTt ♂ x Fftt♀
F = Free earlobes
f = attached earlobes
T = 2nd toe longest
t = great toe longest
F1 Genotypes
F1 Phenotypes