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Inheritance Patterns
Simple dominance, incomplete
dominance, codominance, sex-linkage
polygenic inheritance
Applying Mendel’s Laws
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We can use Mendel’s Law of Segregation to
predict the outcome of possible crosses of
individuals
First, we need some terminology
Genotype: the alleles an organism has (e.g. I
have the genotype bb because I have two
copies of the blue eyes allele)
Phenotype: the characteristics an organism
displays (e.g. my phenotype is blue eyes)
Terms
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Homozygous – a genotype that has two
identical alleles (bb is a homozygous
genotype because both alleles are the same)
Heterozygous – a genotype that has two
different alleles (Bb is a heterozygous
genotype because the alleles are different)
Mendel & Inheritance Patterns
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Mendel observed
only a particular type
of inheritance pattern
His pea plants had
an inheritance
pattern of simple
dominance.
Mendel & Inheritance Patterns
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Type of alleles in simple dominance:
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Dominant allele: always appears in the phenotype, whether
it is part of a homozygous or heterozygous genotype
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e.g. if a person is heterozygous for polydactyly (having more
than 5 fingers) – Pp – they will have an extra finger
Recessive allele: two copies of the allele must appear in
the genotype (it must be homozygous) for the trait to
appear in the phenotype
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tongue rolling – you will only not be able to roll your tongue if
you have genotype rr
Simple Dominance: Using the terms
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We use capital letters
for dominant alleles
We use lower case
letters for recessive
alleles
The letters for the
alleles should be the
same (e.g. F for purple
flowers allele, f for
white flowers allele)
Monohybrid crosses
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If we are looking at the inheritance of only
one trait, we are looking at a monohybrid
cross
We can predict the outcome of a monohybrid
cross when there is simple dominantrecessive inheritance
A Punnett grid helps us to do this.
Monohybrid crosses, continued
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We know from Mendel’s
Law of Segregation, that
gametes contain only one
allele for each trait
So, if a purple pea plant is
heterozygous for flower
color, its gametes will either
have a F allele (purple) or a
f allele (white)
A homozygous white pea
plant’s gametes can only
have a f allele (white)
We can show this in a
Punnett grid
Monohybrid crosses, continued
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When we fill in the grid, the
center boxes show us the
possible genotypes of the
offspring
Monohybrid crosses, continued
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The genotypic ratio for
the outcome of this
cross is 1:1, Ff:ff
The phenotypic ratio is
also 1:1, purple:white
Incomplete Dominance
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For some genes,
neither gene is fully
dominant over
another.
In incomplete
dominance, the F1
phenotype will be
somewhere in
between the two
parental phenotypes.
Incomplete Dominance
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In this type of inheritance
pattern, the heterozygote
produces less red
pigment, so the F1
generation flowers are
pink.
If the F1 offspring are
crossed, what would you
predict the outcome
would be?
Codominance
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Codominance is similar to incomplete
dominance.
In a heterozygous individual, however,
each of the alleles will produce their own
distinct product.
Example: Blood type
Codominance: Blood type
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Blood types differ due to
proteins that are present
on the outside of red
blood cells.
In a person who is a
heterozygote for the A
and B alleles, both the
protein for type A and the
protein for type B will be
produced.
Codominance: Blood type
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Predict what the possible
outcomes are for a child
who has one parent with
type A blood and one
parent with type AB
blood.
Sex-linkage
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When genes are carried
on the sex chromosomes,
unique patterns of
inheritance are created.
Example: Hemophilia is a
recessive trait carried on
the X chromosome.
Predict the outcome of a
cross between a normal
man and his wife, who is
a carrier (heterozygous)
for hemophilia
Dihybrid Crosses
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If we want to predict
the outcome of a
cross involving two
genes, we use a
dihybrid cross.
Example: round
yellow seeds x
green wrinkled
seeds.
Polygenic Inheritance
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Traits aren’t always
controlled by only one
gene.
Polygenic Inheritance
happens when more than
one gene affect a single
phenotypic characteristic.
Example: Skin color in
humans is controlled by
at least three different
genes (and probably
more)
Review:
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Follow the link from Edmodo to the
Mendelian Genetics problem sets from the
Biology Project
Do the monohybrid crosses, dihybrid crosses,
and the first sex-linked inheritance problem
set.
Use the tutorials to help you understand
problems with which you have difficulty.