Genetics
Dominant versus Recessive Traits
Dominant traits can
mask or cover up
others.
 Dominant traits are
represented by capital
letters.
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Recessive traits are
masked or covered
up.
 Recessive traits are
represented by lowercase letters.
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Mendel’s Principles
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Heredity is not blending- there are discrete dominant
and recessive traits.
There are units or particles of heredity- we know now
that these are genes.
Every individual has a pair of these units for every traitwe have 2 alleles for every trait.
These pairs separate in gametes- this happens during
meiosis where one homologue goes to each daughter
cell.
Each gamete receives only one unit from each pair- they
are haploid and combine to become diploid.
Alleles
Homozygous versus Heterozygous
Homozygous means
that both alleles are
the same thus both
letters are the same.
 The letters may be
either both capital or
both lower-case.

Heterozygous means
that the alleles are
different meaning
that the letters are
different.
 One letter is capital
and the other letter is
lower-case.
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Genotype versus Phenotype
Heterozygous and
homozygous refer to
your genotype.
 Your genotype is what
alleles you have.
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Phenotype only refers to
what you look like.
Thus, if you show a
dominant phenotype you
can be heterozygous or
homozygous dominant.
If you show the recessive
phenotype, you must be
homozygous recessive.
To determine the
phenotypes and
genotypes of offspring,
You make a
Punnett Square!
Earlobe Attachment
Unattached Earlobe
Attached Earlobe
Dominant Phenotype
Recessive Phenotype
Genotypic and Phenotypic Ratios
Genotypic ratios
express the results of
the genotypes of the
offspring resulting
from a Punnett
Square.
 Therefore, they would
state how many
offspring would be
homozygous and
heterozygous.
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Phenotypic ratios
express the results of
the phenotypes of the
offspring resulting
from a Punnett
Square.
 Therefore, they would
state how many of
the offspring would
show a certain trait.

Punnett Squares Show
Theoretical Ratios and
Percentages
Recessive Traits
Tay-Sachs Disease
 Cystic Fibrosis
 PKU
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Dominant Traits
Huntington Disease
 Hypercholesterolemia
 Marfan Syndrome
 Achrondroplasia
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Non-Mendelian
Inheritance
Polygenic Inheritance
Black Lab
X
Yellow Lab
Chocolate Lab
Multiple Alleles
Co-Dominance
Blood Type Alleles
Blood Types and
Paternity Testing
This used to be all there was and
only allows you to exclude
someone as the father, not to
prove he is the father. Now we
have DNA fingerprinting!
Sickle Cell Genotypes and
Phenotypes
HbA HbA Normal
HbA HbS Sickle Cell Trait
HbS HbS Sickle Cell Anemia
Females Have 2 Copies of the
X
Chromosome, Males Only
Have 1.
The Male’s X Chromosome Comes
From His Mother.
Since Males Have Only 1 X
Chromosome, They Will
Show the Traits on that
Chromosome.
Sex Linked Traits
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Men Cannot Be Carriers of a Sex-Linked Trait
Sex Influenced Traits
Environmental Influence