1) Meiosis produces haploid sex cells from diploid body cells, separating homologous chromosome pairs and sister chromatids in two steps. 2) Nondisjunction during meiosis can result in gametes with an abnormal number of chromosomes, potentially leading to conditions like Down syndrome. 3) Complete nondisjunction produces gametes with a full extra set of chromosomes, which if fertilized could result in conditions like triploid syndrome, though these are often fatal in humans. Polyploidy has occurred and been beneficial in some plant species.

1. PLOIDIES
MEIOSIS AND MENDEL
HS BIOLOGY 30.3.13

2. PLOIDY AND YOU
DIPLOID CELLS (2N) HAPLOID CELLS (N)
Mitosis produces Meiosis produces
diploid cells, which haploid cells, which
in humans have 46 in humans have 23
chromosomes. chromosomes.

3. The normal process of meiosis turns diploid body
cells (2n) into haploid sex cells (n), in two steps. First,
homologous chromosome pairs separate in meiosis 1.
Then, sister chromatids separate in meiosis 2.

4. SOMETIMES, CHROMOSOMES GET STUCK
DURING MEIOSIS. THIS IS CALLED
CHROMOSOMAL NONDISJUNCTION

5. Nondisjunction leaves us
with messed up gametes.
Instead of having 23
chromosomes, they may
have 22, 24, or even
another number.
Your body will often
purge these germ cells
before they ever get a
chance to fertilize. But if
they do…

6. (A FEW TOO MANY OR TOO FEW CHROMOSOMES)
ANEUPLOIDY

7. DOWN SYNDROME
(TRISOMY 21) Total chromosomes: 47
Either the sperm or egg cell has an extra copy of chromosome 23 due to
nondisjunction during meiosis. The child can be a boy or a girl.
Symptoms include: short stature, weak muscles, distinct facial features, intellectual
disability, heart defects, eye conditions, hearing problems, dental problems, ADHD.

8. TURNER SYNDROME
(MONOSOMY X) Total chromosomes: 45
Either the sperm was missing
an X or Y chromosome, or the
egg was missing the X
chromosome and the sperm
carried the Y chromosome.
The baby is always female.
Symptoms include: wide neck and small
jaw, distinct facial features, small stature,
learning disabilities, autism, lack of
puberty, early menopause, infertility.

9. Sometimes, meiosis
goes catastrophically
wrong, and the
gametes end up with a
complete double set of
chromosomes (or a
complete missing set).
This is called complete
nondisjunction, and
the body usually
identifies and purges
these gametes.
However, if a 2n
gamete goes on to
fertilize another normal
gamete…

10. (FULL SETS OF EXTRA CHROMOSOMES)
POLYPLOIDY

11. TRIPLOID SYNDROME
Total chromosomes: 69
Either the sperm or the egg has two full sets of chromosomes,
leading to a triploid (3n) baby. In humans, triploid babies will
either miscarry or die within the first year of life

12. SUCCESSFUL POLYPLOIDY
Polyploidy in non-human species is not always fatal.
Plants, especially, can survive and even thrive with extra sets
of chromosomes.
One reason for this is self-fertilization: plants can combine
their own gametes, even after nondisjunction. For
example, two messed up diploid gametes (2n) can combine
to make a tetraploid (4n) plant, which will continue to have
diploid gametes in the next generation.
That doubles a plant species’
chromosomes in one generation!

13. Most plants we eat are either a result of (1) ancient, accidentally
polyploidy or (2) intentional polyploidy due to modern agricultural
techniques. Here are some relatively dramatic examples:
Diploid (2n) & Tetraploid (4n) grapes

14. Monoploid (n) and triploid (3n) plant lines are usually sterile,
and sometimes seedless.