Meiosis to the Punnett Square
How are haploid gamete cells
produced from diploid cells?
Your parents have 46 chromosomes
each and you have 46 chromosomes.
 If you got half of your genes from each
parent, how did this happen?


How many chromosomes do a sperm and
an egg have?
Meiosis
 The
process of reduction division in
which the number of chromosomes per
cell is cut in half through the separation
of homologous chromosomes in a
diploid cell that become haploid cells.
 Involves two distinct divisions; Meiosis I
and Meiosis II
 One diploid cell becomes 4 haploid
cells
Crossing Over

Prophase I
 Homologous
chromosomes
exchange alleles
 Produces new
combinations of
alleles
 Causes randomness
in gamete formation
Meiosis I





Prophase I- Chromosomes pair with same
homologous chromosomes to form a tetrad.
Metaphase I- Homologous chromosomes meet
in the middle and spindle fibers attach.
Anaphase I- Homologous chromosomes are
pulled apart by spindle fibers
Telophase I- Nuclear membrane forms around
separated chromosomes to form two nuclei.
Cytokinesis- The diploid cell separates into two
haploid cells.
Meiosis II
starts with two haploid daughter cells




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Prophase II- haploid chromosomes with sister
chromatid are in the cytoplasm.
Metaphase II- chromosomes meet in the
middle of the cell
Anaphase II- Sister chromatid are pulled apart
by spindle fibers
Telophase II- Nuclear envelop forms around
the chromosomes
Cytokinesis- Results in 4 haploid daughter
cells
Meiosis animation:
Pay close attention to what “meets” in
each metaphase and what separates in
each of the two anaphases.
 http://www.cellsalive.com/meiosis.htm

Review
Mitosis
Both
Meiosis
Place these items into your Venn Diagram:

Produces diploid cells
 Produces haploid cells
 Type of cell division
 Creates identical cells
 Separates homologous
chromosomes
 Separates chromatids
 Creates “daughter” cells






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2 rounds of cell division
Only 1 round of cell
division
Creates unique cells
Begins with diploid cells
Creates gametes
Creates “body cells”
Has crossing over
Genes
Genes are the characteristics contained
in the DNA of every cell.
 Each gene is made up of 2 alleles- one
inherited from each parent.


For instance you may have one brown eye
allele inherited from your dad and one blue
eye allele inherited from your mom.
Kinds of alleles

Dominant


These alleles always determine the physical feature
of the individual.
These alleles are represented by a capital letter.
Recessive

These alleles only determine the physical feature of
the individual when there is no dominant allele.
These alleles are represented by a lower case
letter.
Genotype and Phenotype
The genotype of an individual is the
description of the actual genetic
information for a trait.
 The phenotype of an individual is the
physical feature observed.

Allele Combinations
Every gene is made up of two allelesone from each parent.
 If both alleles are the same, the gene is
called homozygous. (BB or bb)
 If the alleles are different, the gene is
called heterozygous. (Bb)

Label the following as homozygous dominant,
homozygous recessive, or heterozygous.
1.
2.
3.
4.
5.
6.
7.
8.
BB
Aa
nn
Kk
GG
ff
qq
Hh
Probability and Punnett Squares
Meiosis separates the alleles of a parent
into different sex cells (gametes).
 For instance: Dad is heterozygous for
Brown hair, with a blond hair allele as the
recessive. (Bb).


Through meiosis, these alleles are copied
and separated into 4 sperm.
B
B
b
b
Single Trait Crossing
What kind of offspring (kid) would two
parents have?
 A Punnett Square can help you find out!


Example: Dad is heterozygous (Bb) for
Brown hair over blond hair. Mom is also
heterozygous (Bb) for brown hair over blond
hair.
Set up the Square
B
B
Mom’s
alleles
b
b
Dad’s
Alleles
Set up the Square
B
B
b
BB
Bb
Mom’s
alleles
b
Bb
bb
Dad’s
Alleles
4
possible
offspring
What do these results mean?
¼ or 25% chance of an offspring with
homozygous dominant genes for brown
hair.
 ½ or 50% chance of an offspring with
heterozygous genes for brown hair.
 ¼ or 25% chance of an offspring with
homozygous recessive genes for blond
hair.

You try it!
1. Dad is heterozygous for Brown eyes
over blue eyes. (Ee) Mom is
homozygous recessive with blue eyes.
(ee) What are the probabilities for the
children’s genotypes. What are the
probabilities for the children’s
phenotypes?
Set up the Square
E
e
Mom’s
alleles
e
e
Dad’s
Alleles
e
E
e
Ee
ee
Mom’s
alleles
e
Ee
ee
Dad’s
Alleles
4
possible
offspring
½ or 50% of the children would inherit
heterozygous genes for Brown eyes (Ee)
 ½ or 50% of the children would inherit
homozygous recessive genes for Blue
eyes (ee)

Hmmm…
Is it possible for two Brown haired
parents to produce a blond child?
Explain.
 Is it possible for 2 blue eyed parents to
produce a brown eyed child? Explain.
 Can you figure out your genotype for hair
color? How do you know?

Review Questions
1.
2.
3.
How is meiosis related to heredity?
What is the difference between
heterozygous, homozygous dominant,
and homozygous recessive?
Is it possible for two brown haired
parents to produce a blond child?
Explain.
Review cont…
4.
5.
What is the difference between
genotype and phenotype?
What are the chances that two
heterozygous parents with Brown
hair(Bb) produce a child that is also
heterozygous?
Review cont…
6.
7.
In pea plants, round peas are dominant
over wrinkled peas. (R and r). What
would the offspring of a homozygous
dominant, and a homozygous
recessive cross look like?
If the offspring from question #6 were
to pollinate each other (as plants often
do), what would be the genotype
probabilities for the offspring?
Two Trait Crosses

When looking at two different genes in
the same cross, 4 alleles are dealt with.
For instance, a heterozygous haired(Bb),
and homozygous recessive eyed(ee) parent
would be Bbee.
 Through meiosis, there would be 4 possible
allele combinations in the sex cells.

FOIL!
Just like in math class, the first, outside,
inside, and last rule can help you
remember.
B b e e


These alleles would produce these four
combinations.

Be
Be
be
be
What does a two trait cross look
like?

Suppose two parents who are both
heterozygous haired and heterozygous
eyed produce a child. What are the
possible genotypes and phenotypes in
the offspring?
You try it!

A tall (TT) pea plant that produces yellow
colored peas (Yy) crosses with a short
(tt) pea plant with green colored peas
(yy). Create the Punnett square and list
the probabilities of each genotype.