We must reduce our chromosomes ……or else!!
Chromosome Number
 Each human baby gets a copy of every single gene from its
parents in an egg and sperm cell…



23 chromosomes from mom
23 chromosomes from dad
46 Total
 When organisms make gametes (sex cells), they need to be able
to reduce the number of chromosomes in half.
 This process is called Meiosis – reduction division.
46
92
Specialized
sex cell.
Meiosis begins
with a specialized
sex cell containing
46 chromosmes.
46
23
46 in the baby!
•We do this in two divisions called meiosis I and meiosis II
•Each division reduces the chromosomes in half.
Meiosis I and Meiosis II
In order to get the right number of chromosomes we must go
through P, M, A, T….TWICE!
Crossing over
 Sometimes during
Meiosis 1, doubled
chromosomes
called tetrads get
tangled up and
exchange genes.
 This is why you
don’t look exactly
like your parents.
Vocabulary
 Diploid cell – A cell that has the full number of
chromosomes.


For example, heart cells, liver cells, skin cells, etc.
We call these cells 2N cells.
 Haploid cell – A cell that has half the number of
chromosomes.


For example egg and sperm cells
We call these cells 1N cells.
Males vs. Females
 Males produce 4
sperm for each
specialized sex cell.
 Women only produce
one egg for each
specialized sex cell.
The other three are
called polar bodies
and disintegrate.
How it works………
 Let’s suppose that
A= curly hair
a = straight hair.
 You get one gene from
Mom in an egg cell. (A)
 You get one gene from
dad in the sperm cell (a).
 What kind of hair would
you have?