Genetics Unit Notes
Term/
Main Idea
1. Chromosome
Definition/Detail
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2. gene
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3. allele
Name ____________ Per ___
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Example
“X”-shaped, made of DNA,
found in the nucleus of cells.
you inherit 1/2 of your
chromosomes from Mom and
1/2 from Dad
come in pairs – humans have 23
pairs of chromosomes in each
cell
location of genes
specific location on a
chromosome
contains exact instructions for
creating a trait
genes occur in pairs on
chromosomes
form of a gene
2 possible alleles – dominant and
recessive
capital letter represents
dominant allele
lower case letter represents
recessive allele
F – dominant allele
f – recessive allele
4. genotype

two letters representing a gene
pair
GG
rr
Bb
5. phenotype
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physical appearance
what the trait will look like
(based on the genotype)
purple flower color
green eyes
no widow’s peak
Term/
Main Idea
6.
complete
dominance
Definition/Detail
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7. incomplete
dominance
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dominant allele completely
masks (covers up) the recessive
allele
if even one dominant allele is
present in a genotype, the
dominant trait will show
a recessive trait only shows
when there are no dominant
alleles in the genotype
dominant allele does not
completely mask (cover up) the
recessive allele
Example
EE – dominant trait shows
Ee – dominant trait shows
ee – recessive trait shows
EE – dominant trait shows
Ee – part dominant/part
recessive trait shows
ee – recessive trait shows
8. Punnett
square
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9. probability
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a tool used to predict the
probability of offspring having
a particular trait
the parent’s genotypes are
written on the top and left
side
the offspring possibilities are
written inside the squares
an allele from each parent is
combined inside the squares
likelihood that a particular
genotype or phenotype will
occur
can be shown as a percentage or
a ratio
in a 2x2 Punnett square, each
offspring square is equal to
25%.
A
A
A
AA
AA
a
Aa
Aa
In the Punnett square
shown above, the
probability of
having a particular
genotype is 50% for
AA, and 50% for Aa.
OR
Mendel’s F2 generation
had a ratio of 3 : 1
purple to white flower
colors
Term/
Main Idea
10. homozygous
synonyms:
purebred
true-breeding
Definition/Detail
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11. heterozygous
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synonym:
hybrid
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12. co-dominance
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prefix homo- means “same”
both alleles in a genotype are
the same – both are dominant,
or both are recessive
Mendel called plants with these
genotypes “true-breeding, or
purebred”
the P1 generation in Mendel’s
experiments were homozygous
plants.
 purple flowers were P P
 white flowers were p p
prefix hetero- means
“different”
the alleles in a genotype are
different – one is dominant and
the other is recessive
the F1 generation in Mendel’s
experiments were heterozygous
plants.
 all purple F1 generation
were P p
two equally dominant alleles
Either (or both) will completely
mask (cover up) the recessive
allele
if both dominant alleles are
present in a genotype, both
forms will show
recessive form only shows when
neither dominant allele is
present in the gene pair
Example
EE – homozygous dominant
ee – homozygous recessive
In Mendel’s day…
EE – purebred (dominant)
ee – purebred (recessive)
Ee – heterozygous
In Mendel’s day…
Ee – hybrid
See blood types (below)
for an example.
Term/
Main Idea
13. blood types
Definition/Detail
 Three alleles: A, B, and O
 A & B are co-dominant
 O is recessive
 Six possible genotypes
 Four basic blood types
(phenotypes): A, B, AB, and O
Genotype
Phenotype
AA
A
AO
A
BB
B
BO
B
AB
AB
OO
O
Example
Mom is type B; Dad is O
Mom could be genotype
BB or BO; Dad
must be OO
Two possibilities for
Punnett squares:
B
B
O
BO
BO
O
BO
BO
B
O
O
BO
OO
O
BO
OO