1. How many chromosomes do humans have?
Warm-up # /23/15
Draw a chromosome and label the following parts: chromosome, sister chromatid, centromere
How many chromosomes do humans have?

2. Human Chromosomes Humans have 23 pairs of chromosomes (46 total)
22 pairs of autosomes
1 pair of sex chromosomes
Karyotype – A picture of all the chromosomes in a somatic cell arranged by size

3. One chromosome in the pair came from the male parent and one came from the female parent.
Homologous chromosomes – each pair has genes for the same traits

4. How can 2 cells join without doubling the amount of chromosomes?
Meiosis a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell

5. Meiosis: Where and Who?
For humans meiosis occurs in the ovaries and testes
The process of meiosis produces egg and sperm cells gametogenesis (spermatogenesis & oogenesis)
Two gametes come together by fertilization

6. A cell with only one of each kind of chromosome is called haploid (n).
Organisms produce gametes (sex cells) that contain one of each kind of chromosome.
A cell with only one of each kind of chromosome is called haploid (n).
Sex cells have one of each kind of chromosome so that when they combine
the resulting zygote is diploid

8. Meiosis: Why?
Mitosis divides one diploid cell to form two diploid cells
For example: A human cell with 46 chromosomes divides to form two cells with 46 chromosomes.
If each parent were to pass on a diploid cell to the offspring, that offspring would then have 4 copies of each chromosome
46 chromosomes from each parent would yield a 92 chromosome offspring
Meiosis allows for two divisions to divide one diploid cell into four haploid cells.

9. Label the diagram Homologous chromosomes Sister chromatids Centromere

10. Warm-up # /26/15
How are homologous chromosomes different from sister chromatids?
Sister chromatids are identical
Homologous chromosomes have the same genes, but may have different version of the gene
What makes a cell haploid (n) or diploid (2n)?
Haploid – 1 set of chromosomes
Diploid – 2 complete sets of chromosomes

11. Chromosomes Each chromosome can contain thousands of genes
Genes can have different versions  Alleles

12. Meiosis Phases Meiosis I - homologous chromosomes separate
Prophase I
Metaphase I
Anaphase I
Telophase I
Meiosis II - sister
chromatids separate
(more similar to mitosis)
Prophase II
Metaphase II
Anaphase II
Telophase II

13. Interphase
Before Meiosis (just like before Mitosis) the cell must prepare for division:
DNA and organelles are
replicated
During this phase, chromosomes are not yet visible.
chromatin

15. Law of Segregation
Each gamete only gets one allele of each gene

16. Independent Assortment
Mendel's law of independent assortment: Genes for different traits assort independently of each other
Organisms inherit two alleles for each trait when gametes are produced

17. Variation of Traits Crossing over during Prophase I
Exchange of genetic material between homologous chromosomes

18. Crossing Over

19. All Different
Humans have 223 different combination for gametes!

20. 2 1 4 2 Function Type of cells # of divisions # of daughter cells
Mitosis
Meiosis
Function
Type of cells
# of divisions
# of daughter cells
Results in Haploid/diploid cells?
Growth
Repair
Reproduction
Create new cells for the
purpose of reproduction
Somatic cells
(body cells, anything other
than gametes)
Germ line cells
(gametes/sex cells) 2 1 4 2
Haploid
Diploid

21. Warm-up # /27/15
Name & describe 2 events/processes that occur during meiosis that contribute to genetic variation
Crossing over – recombination of alleles
Independent assortment – genes of different traits assort independently of each other

22. Warm-up #10 1/28/15 Copy the diagram in your notebook.
Draw the possible gametes if crossing over DOES NOT occur
Draw the gametes if crossing over DOES occur
What affect does crossing over have on the variation of gametes?

23. Without Crossing Over

24. With crossing over

25. Law of Independent assortment
2n different combinations
If we have 3 different chromosomes, how many different combinations are possible?

26. n=3
23=8

27. Chromosomal Mutations
Errors can also occur during Meiosis.
Homologous chromosomes do not separate properly –nondisjunction
Gametes with either an extra copy of a chromosome or no copy
Normal Example
Nondisjunction Examples

28. Identifying Chromosomal Disorders
Karyotype
Photograph is taken of the paired chromosomes during metaphase
Arranged according to length
Easy to see if there are any extra or missing chromosomes
This individual has an extra Y chromosome

29. Monosomy Zygote gets only one chromosome  missing one chromosome
Most zygotes with monosomy do not survive

30. Trisomy
One copy of a chromosome from one parent and two copies from the other parent three copies

31. Downs Syndrome (Trisomy 21)
3 copies of the 21st chromosome
mental retardation, susceptibility to certain illness or diseases, and a shorter life span