Calculating Probabilities of
Monohybrid Crosses
Notes #6
Principles of Probability
What is it?
ANS: It is used to predict the outcome of
a genetic cross.
How to Determine the Sex of the baby?
●
●
●
Human cells have a total of _44__ somatic
(body) chromosomes.
Each human cell has a total of __2___
sex chromosomes
Total # of Chromosomes in a typical single
human cell is ____46___.
What makes girls different from guys?
ANS: Girls have two “X” sex chromosomes;
Boys have only one “X”
Female Sex Chr. vs Male Sex Chr.
XX
XY
Story 1 (Starting a family)
What is the probability that the newly
weds, Kevin & Connie will have a boy as
their first born child?
Story 1
P
Gamete
Cells
Kevin
X
Y
___
___
X
Y
x
Meiosis I
& II
Connie
X
X
___
___
X
X
Punnett Square
What is the probability that the newly-weds, will have a boy as
their first born child?
X
_______
X
_______
X X
XX
X
_______
Total Possible Outcome: 4
Total Possible Boys: 2
Y
_______
XY
XY
XY possibility:
ANS: The chance that Kevin & Connie will have a boy is: 50%
Trivia Question
●
Which parent determines the sex of the
baby?
●
ANS: Father, because he either
contributes a “Y” or “X” to the baby.
Story 2 (The Expected Couple)
Congratulations! Kevin & Connie are
expecting their first child. Both parents
have brown eyes, and they are curious
about what color eyes their son will
have.
Your prediction:
Arrival of Kevin Jr.
Kevin Jr’s Eye Color: Blue
The Dominant Eye Color is: Brown
B
Symbol:___
The Recessive Eye Color is: blue
Symbol: ___
b
Phenotype vs. Genotype
1. Phenotype: Organism’s physical
appearance
2. Genotype: Organism’s gene type
Quick Check
Stating that she has brown eyes tells you
which of the following?
a. genotype
ANS: B (phenotype)
b. phenotype
Homozygous vs Heterozygous
1. Homozygous: Organism that has 2
identical genes (aka. Pure breed)
Ex. BB or bb
2. Heterozygous: Organism that has 2
different genes (aka. Hybrid)
Ex. Bb
Quick Check
●
Given the following genotypes, indicate
what category they fall under
1.
BB
Homozygous
a.
b. Heterozygous
2. Bb
a.
Homozygous
1. A;
2. B
b. Heterozygous
Trait Analyzed: Eye Color (Brown vs
Blue)
1.
List the possible Genotype(s) for
BB or Bb
brown eyes: ________
2.
List possible Genotype(s) for blue
bb
eyes: ________
Fill out the chart below
Kevin
Phenotype
Genotype
Brown
Bb
Connie
Brown
Bb
Q1: Is Kevin Jr considered a Heterozygous or Homozygous
individual? ______________
Homozygous
Kevin Jr.
Blue
bb
Q2: What is the probability that their
second child will have brown eyes?
Show work w/ a Punnett Square
B
b
BB
Bb
B
b
b b
Bb
3
Phenotypic Ratio: ____ Brown:
____Blue
Genotypic Ratio: _____:______:______
1BB
2Bb
1
1bb
75% or 3/4
chance of
having brown
eyes