Chapter 11: Introduction to
Genetics
SECTION 11.2
Probability
Probability is the likelihood that something will
happen
Example: Flipping a coin
Example: Choosing the right question on a multiple
choice test
Probability can be used to predict traits
Dominant vs. Recessive
 Alleles are represented by letters
 Dominant alleles are represented by a capital letter

R
 Recessive traits are represented by a lower case letter

r
 Dominant alleles are expressed over recessive alleles
 If you have to alleles together the dominant one will always be
expressed or “visable”
 Example: Height
 Example: Brown hair
Key Words
 Homozygous: organisms that have 2 identical alleles
for a particular gene

Homozygous dominant (2 dominant alleles)


RR
Homozygous recessive (2 recessive alleles)

rr
 Heterzgous: organisms that have 2 different alleles
 Have a dominant allele (written first) and a recessive allele

Rr
Practice
 Label the following as either homozygous or
heterozygous








EE
Tt
Zz
YY
Yy
pp
bb
NN
Key Words
 Phenotype: physical expression of alleles, the traits,
“what you see”
 Genotype: genetic make-up, you cannot see it, this is
the letter representation of the phenotype

YY, Ff, ff
Example
Phenotype: Brown
Genotype
Bb
Practice
 In people, brown eyes (B) are dominant to blue (b),
from the following phenotypes identify possible
genotypes:
BB or Bb
bb
BB or Bb
bb
What are your traits?
 Curley (H) hair is dominant to straight (h)
 Widow’s peak (W) is dominant to no widow’s peak
(w)
 Free ear lobes (F) are dominant to attached ear
 lobes (f)
Hair
Widow’s
Peak
Ear lobes
Phenotype
Straight
Yes
Free
Possible
Genotypes
hh
WW or Ww
FF or Ff
Punnet Squares
 Punnet Squares: use probability to help predict
genotypes and phenotype combinations (ratios) in
genetic crosses
 Monohybrid Cross: examine the inheritance of only
one specific trait.
Completeing Punnet Squares
1.
2.
3.
4.
5.
Start with the Parents: Find the genotype of the
parents
Figure out the Gametes: Determine the alleles that
each parent could pass on to their offspring
Set-up the Table: Draw the Punnet Square and
place the alleles and parents on the top and the left
side of the square
Complete the Punnet Square: Fill in the table with
offspring’s possible genotypes
Interpret the results
Step 1: Start with Parents
 Long fins (L) in fish are dominant to short (l). You
are breeding a homozygous dominant long fin male
to a homozygous recessive short fin female. What are
the possible genotypes of the F1 offspring?

What are the genotypes of the parents
Male: LL
 Female: ll

Step 2: Figure out the gametes
 What alleles can the male and female possibly give to
their offspring?

LL
Male Genotype: LL

Can give an L allele or an L allele
L

L
Female Genotype: ll

Can give an l allele or an l allele
ll
l
l
Step 3: Set up the Punnet Square
L
l
l
L
Step 4: Fill in Punnet Square
L
l
l
L
Step 4: Completed Punnet Square
L
l
l
L
Ll
Ll
Ll
Ll
Step 5: Interpret Results
 What were the possible genotypes from the cross?
 Ll
(heterozygous dominant)
 What are the possible phenotypes from the cross?
 All
offspring will have long fins
Step 5: Interpret Results
 What is the probability that the genotype will be Ll?
4/4
= 100%
 What is the probability that the genotype will be ll?
 0/4
= 0%
Step 5: Interpret the Results
 What is the probability that the offspring will have
long fins?
 4/4
= 100%
 What is the probability that the offspring will have
short fins
 0/4=0%
Testcross
 A testcross is a cross between an organism with an
unknown genotype and an organism with the
recessive phenotype
 If you have an organism with the dominant phenotype and
you need to determine the possible genotype you can
perform a test cross.
 If you cross your Dominant organism with a recessive and
produce recessive offspring you know the unknown
organism must be heterozygous.
 If no recessive offspring are produced can you be sure of
the genotype of the unknown organism?
 NO
Testcross Example
 Cattle are either horned or polled (no horns)
 Horns are dominant to polled
 Polled cattle are worth more so want genotypes that
carry the polled allele
 Ranchers will test cross cattle with horns to
determine if they are heterozygous or homozygous
dominant
Questions??????