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Punnett Squares
And Meiosis!
Meiosis:
A type of cell division in which a diploid cell
(two copies of each gene) divides to form a
haploid cell (one copy of each gene).
= SPERM and EGG!
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T
t
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Tt
Recessive t - not curved
allele
Dominant T - curved thumb/allele
T
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t
Tt
Dominant T - curved thumb
allele
Recessive t - not curved
allele
Punnett Square: A method to determine
the likely genotypes and phenotypes
of offspring
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T
T
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eggs
t
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TT
Tt
Curved thumb
Curved thumb
sperm
t
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Tt
Curved thumb
tt
Straight thumb
How to fill in a punnett square
Using the Punnett Square below, correctly
cross a heterozygous brown haired person
with a homozygous blonde haired person.
B
b
b
Bb
bb
b
Bb
bb
Bb x bb
B = brown hair allele
b = blonde hair allele
b
b
B
Bb
Bb
b
bb
bb
Phenotype: 2 brown hair and 2 blonde hair
Genotype: 2 Bb and 2 bb
Solve the following problems:
GIVE THE PHENOTYPE & GENOTYPE!
Cross EE x ee (brown eyed person with blue eyed)
Cross a wrinkled seed with a homozygous round
seed (W = wrinkled w = round)
Cross a heterozygous tall person with a
heterozygous tall person.
Cross a homozygous tall person with a short person
Mendel’s Laws
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Law of Segregation
Two alleles separate
from each other
during meiosis!
Each homologous
chromosome pair
separates during the
formation of the
sperm and egg
(meiosis!)
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Law of Segregation
Law of segregation
Incomplete Dominance

Hh (heterozygote)
where the
functioning allele
makes less protein
 Example: in
snapdragon
flowers, red is
dominant, but less
protein pigment is
made w/ Rr; the
flower is pink (Rr) =
pink
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Incomplete Dominance Practice

In the budgerigars
bird (“budgie”),
olive feather color is
dominant (GG),
dark green is
heterozygous (Gg),
and light green is
recessive (gg).
 Cross an olive bird
and a light green
bird.
 Cross two
heterozygous birds
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Codominance

When both alleles
(forms of a gene)
are expressed
 Blood type is an
example of
codominance
 Blood type A is
codominant with B
 Blood type O is
recessive to A and
B
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Codominance Practice

Cross a person with
type O blood (oo)
with a person who
has type A blood
(Ao).
 What are the
possible
genotypes and
phenotypes of their
children?
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G: 2 Ao: 2 oo P: 2: type A blood : 2 type o blood
Sex-Lined Traits

gene carried only
on the X
chromosome
 there is no
corresponding gene
on the y
chromosome
 Cross a noncolorblind male
(XBY) with a
“carrier” female
(XBXb)
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Law of Independent Assortment
The distribution of alleles for one
trait does not affect the distribution
for of alleles for other traits
Law of Independent Assortment
Law of Independent Assortment
Law of Independent Assortment