1. Genetic Variability

2. Homologous Chromosomes23 23 +
Mom (egg)
Dad (sperm) 1 2 3 4 5… 22 X
X or Y 46
zygote

3. Homologous Chromosome
The 23 chromosomes from mom correspond to the 23 chromosomes from dad. These chromosomes are called homologous chromosomes.

4. Homologous Chromosomes
Each homologous chromosome contains a set of alleles for the same gene. T t

5. Homologous Chromosomes & Meiosis
Each chromosome is replicated before cell division.
Each chromosome now has two sister chromatids.
In meiosis, homologous chromosomes pair up to form a tetrad.

6. Homologous Chromosomes & Meiosis
In meiosis, homologous chromosomes pair up to form a tetrad.
Each homologous chromosome has been replicated into two sister chromatids.
Altogether, there are 4 sister chromatids in a tetrad.

7. Homologous Chromosome & Meiosis
During metaphase 1, tetrads line up across the middle of the cell.
How the tetrads line up determines what each gamete will look like.

8. Meiosis
Metaphase 1
End of Meiosis 1
End of Meiosis 2

9. Meiosis
Metaphase 1
End of Meiosis 1
End of Meiosis 2

10. Genetic Variability
After the chromosomes separate, they are packaged into individual gametes.
Each chromosome moves independently of all the others.
This is called INDEPENDENT ASSORTMENT.

11. Gamete Variability
The order of the tetrads in metaphase 1 determines what the gametes will look like.

12. Genetic Variability
Humans have 23 homologous chromosomes (46 total) that sort independently during gamete production.
The total number of possible chromosome combinations is over 8 million!
You have over 8 million different sperm/eggs.

13. Genetic Variability
With those kinds of odds, it is extremely unlikely that two sperm with the same assortment of chromosomes will meet two eggs with the same assortment of chromosomes.
Thus, each individual (with the exception of identical twins) is unique!

14. CROSSING OVER
During prophase 1, the homologous chromosomes pair up inside the cell and form tetrads.
While the homologous chromosomes are paired together, they might exchange a portion of their chromatids.

15. Crossing Over
Each chromosome may swap a portion of its genetic material for the matching portion on its homologue.

16. Chromosome

17. Reproduction! Asexual Reproduction Sexual Reproduction
involves only 1 parent
offspring genetically identical to parent
involves regular body cells
Sexual Reproduction
involves 2 parents
offspring genetic mix of both parents
involves specialized sex cells

18. Types of Asexual Reproduction
Binary fission
happens in bacteria, amoeba, some algae
one parent cell splits into 2 identical daughter cells
Budding
happens in yeast, hydra, corals
parent produces a bud
bud gets detached and develops into offspring which is identical to parent

19. 2 daughter cells are identical to parent
Binary Fission
Rod-Shaped Bacterium,
hemorrhagic E. coli
2 daughter cells are identical to parent

20. Budding

21. Asexual Reproduction contd.
Spore Formation
happens in fungi, green algae, molds and non-flowering plants (e.g. ferns)
spores are produced and each spore develops into offspring which are identical to parent
Vegetative Reproduction
does not involve seeds
some offspring can grow from cuttings (e.g. coleus), runners (e.g. strawberries), tubers (e.g. potatoes) or bulbs (e.g. tulips)… which are part of the parent plant

22. Spore Formation
Fern
Fungi

23. Vegetative Reproduction

24. Sexual Reproduction in Animals
involves specialized sex cells called gametes
the union of a male and female gamete results in the formation of a zygote that develops into a new individual

25. Sexual Reproduction in Plants
Female Parts
Male Parts
(Pistil)
pollen (male) + ovule (female) → single-celled zygote → multi-celled embryo (contained in a seed) → new individual

26. Sexual Reproduction in Plants
stamen is the male part and contains pollen
carpels or pistil is the female part and contains ovule (eggs)
pollen grains from the anther are transferred to the stigma by the process of pollination
self pollination (plant pollinates its own eggs)
cross pollination (pollen from one plant pollinates another plants eggs)

27. Pollination
flowers are designed to lure insects to help with the pollination process
also wind, animals, birds can transport pollen

28. Sexual Reproduction Summary
Male Gamete
Female Gamete
Type of Union
Result of Union
Final Result
Plants
pollen
ovule (egg)
pollination
single cell zygote
multi-cell embryo
(in seed)
Animals
sperm
egg
fertilization

29. Some Organisms do Both
most plants that produce seeds (sexual reproduction) can also reproduce asexually by things like cuttings or runners
this gives them an advantage for survival
mosses
sponges and hydra
Mosses produce asexual spores in the early part of their life cycle and then egg and sperm cells are produced in a later part of the same cycle.

30. Which is Better? It depends!
Asexual Reproduction
advantages
does not require special cells or a lot of energy
can produce offspring quickly
in a stable environment creates large, thriving population
disadvantages
limited ability to adapt
face massive die-off if environment changes
Sexual Reproduction
advantages
lots of variation within a species
able to live in a variety of environmental settings
able to adapt to changes in the environment
disadvantages
needs time & energy
produce small populations