MENDEL’S LAWS, MONOHYBRID
AND DIHYBRID CROSSES
Mrs. Stewart
Honors Biology
Bell work
 Describe the relationship between genotype and phenotype
STANDARDS
CLE 3210.4.1 Investigate how genetic
information is encoded in nucleic acids.
CLE 3210.4.3 Predict the outcome of
monohybrid and dihybrid crosses.
OBJECTIVES
Analyze the law of segregation
Create a punnett square using the genotypes
of parents
Predict the outcome and probability of
monohybrid crosses
DECIDE WITH YOUR PARTNER
REVIEW: WHAT IS HEREDITY?
 Why do children look like their parents?
 Why do brothers and sisters resemble each other?
 We inherit traits from our parents
 Heredity = the passing of genetic traits from parents to offspring
TRAITS AND GENES
 Genes carry the instructions that define our traits
 Genes = segments of the DNA sequence that code for a
particular trait
 Traits = genetically determined variations of characteristics
(qualities)
 Example: natural hair color, eye color, skin tone, etc.
 The environment we live in can also help define our traits
 Example: a person’s genes may code for a certain hair color, but
exposure to dyes, chemicals, sunlight, etc can change that color
 Characteristic = can be altered by the environment
DOGS
Tell your CAT how to differentiate between a
characteristic and a trait
HOW DO WE
INHERIT
TRAITS FROM
OUR
PARENTS?
 Remember Meiosis?
HOW DO WE INHERIT TRAITS FROM OUR
PARENTS?
 Human body cells (somatic
cells) have 2 complete sets of
23 chromosomes
 2 x 23 = 46 chromosomes
 One set of 23 comes from
sperm (Dad)
 One set of 23 comes from egg
(Mom)
 Each parent contributes one
complete set to the child,
giving the child a “mix” of
genes
FERTILIZATION
 Fertilization – one
sperm fuses with an egg to
form a zygote
 The zygote now has 2 sets
of 23 chromosomes (46
total)
 This cell will begin dividing
and will ultimately become
a child.
Zygote
CATS
Tell your DOG how we inherit “traits” from our
parents
PARENTS CONTRIBUTE ONE OF EACH
CHROMOSOME PAIR TO THE CHILD
 On a karyotype, there are 2
chromosomes at each site.
 These represents the 2
chromosomes received from
the parents. One from mom,
one from dad.
 During meiosis, these will
separate into different
gametes (sex cells).
MENDEL PROPOSED TWO LAWS
These laws explain how the homologous
chromosome pairs for each parent will separate
into the gametes during meiosis.
Law of segregation
Law of Independent Assortment
LAW OF SEGREGATION
Homologous chromosomes separate during the
formation of gametes
DOGS
Explain the law of segregation to your CAT
LAW OF INDEPENDENT ASSORTMENT
 Allele pairs
separate
independently
during gamete
formation -which
means that the
transmission of
traits to offspring
are independent
to one another.
CATS
Explain the law of independent assortment
to your DOG
SIBLINGS
 Since parents contribute chromosomes randomly, every child
inherits a “unique” combination of traits.
 Some may resemble mom; some may resemble dad; others
will be completely unique
 They may be resemble each other or be totally different.
THINK – PAIR – SHARE
CATS AND DOGS
How can we predict the inheritance of
traits?
Punnett Squares
PUNNETT SQUARES?
 Punnett Squares use genotypes to predict inheritance
 Punnett Squares show the law of segregation in action
M
o
m
Dad
What do the
letters on the
outside of the
punnett square
represent?
What do each of the
squares inside the
punnett square
represent?
DOES IT MATTER WHICH SIDE YOU PUT
THE PARENT’S GENOTYPE ON?
b
b
B
Bb
Bb
B
Bb
Bb
No, the results are the same.
MONOHYBRID CROSS
Monohybrid = a cross between two organisms
that predicts the inheritance pattern/probability of
only one characteristic at a time
PRACTICE TOGETHER
fur color
B = brown fur
b = white fur
Which trait is dominant? Brown Fur = B
Cross a homozygous dominant with a heterozygous
dominant
What are the genotypes of the parents? BB and Bb
What percentage of the offspring will have white fur? 0
B
B
B
b
BB
Bb
BB
Bb
CHECK FOR UNDERSTANDING
 A one-eyed purple people eater is crossed with a two-
eyed purple people eater. All of their offspring have
two eyes. Which trait is dominant? Two-eyed
 Use the letter E or e to represent the alleles
(variations) for this gene. What is the genotype of the
offspring if you cross a purebred one-eyed purple
people eater with a homozygous two-eyed purple
people eater?
Ee
 What generation are the offspring of this cross part
of?
F1
 If you crossed the offspring with each other, how many
of the resulting offspring would have two eyes?
3 out of 4 or 75 %
RATIOS
Ratio – how much of one thing there is in
comparison to another
Genotypic ratio = ratio of possible genotypes in
offspring of a cross
Phenotypic ratio = ration of possible
phenotypes in offspring of a cross
KNOWN SHORTCUTS
 Homozygous dominant crossed with a homozygous recessive
 100% of offspring will be heterozygous
 4:0 ratio of heterozygous to other genotypes
 100% of offspring will show dominant trait
 4:0 ratio of dominant to recessive
 Example: Cross a BB x bb
B = black fur; b = white fur
b
b
B
Bb
Bb
B
Bb
Bb
HETEROZYGOUS X HETEROZYGOUS
 Complete this cross - Bb x Bb
 Genotypic ratio:
 BB : Bb: bb = 1:2:1 Always!
 Phenotypic ratio:
 Dominant trait shown: Recessive
Trait shown = 3:1
Always!
B
b
B BB
Bb
b Bb
bb
TEST CROSS
 Guinea Pig Fur Color
B = Black fur
b = White fur
 What are the possible genotypes for a Guinea Pig with black fur?
 How can we determine which genotype is correct?
 Test Cross = cross an individual that expresses the dominant trait
with a homozygous recessive individual to determine the genotype
TEST CROSS
If a black guinea pig is crossed with a homozgous
recessive white guinea pig and even one of the offspring
is white, what is the only possible genotype for the black
guinea pig?
b
b
b
b
B
Bb
Bb
B
Bb
Bb
B
Bb
Bb
b
bb
bb
THREE TYPES OF DOMINANT
RELATIONSHIPS FOR ALLELES
Complete dominance
Incomplete dominance
Codominance
COMPLETE DOMINANCE
Standard dominant vs recessive relationship
INCOMPLETE DOMINANCE
One is not completely dominant over the other
Instead, heterozygotes will have a blending of the
dominant and recessive traits
CODOMINANCE
Both alleles for a gene are expressed in the
heterozygotes – simultaneously
Neither allele is dominant or recessive, nor do they
blend together
DIHYBRID CROSSES
A cross in which TWO characteristics are
tracked
USE F.O.I.L. METHOD TO DETERMINE
GAMETES
 F – pair the first two alleles of each characteristic
 O – pair the outer two alleles
 I – pair the inner two alleles
 L – Pair the last two alleles for each characteristic
 F – BL
O – Bl
I – bL
 You do the next one on your worksheet
L – bl
HETEROZYGOUS X HETEROZYGOUS
 What are the gamete combinations for each guinea pig?
 Offspring Shortcut:
9:3:3:1 phenotypic ratio
 9 – Two dominant traits
 3 – One dominant one recessive trait
 3 – The other dominant and recessive combo
 1 – both recessive traits
Always!