1. Homologous chromosomes: 2 chromosomes with the same genetic information
The same genes, but different forms (alleles) of those genes
1 chromosome comes from the father, the other from the mother
Tetrad: a pair of
homologous
chromosomes

2. Meiosis Where does meiosis take place?
In the reproductive organs (testes, ovaries)
Only carried out by the sex cells to produce gametes
Gametes: ovum (egg) and sperm cells
Each contains 22 autosomes and 1 sex chromosome (X or Y)
In humans: gametes have a haploid number n=23, somatic cells have a diploid number 2n=46

3. Cells which are produced by meiosis are haploid – they only have half of the genetic material that is found in a somatic cell
They have 1 copy of each chromosome – the number of chromosomes is reduced during the first stage of meiosis
n = # of chromosomes that are genetically different
The gametes do not have homologous chromosomes

4. There are 2 stages of meiosis
Meiosis I
Meiosis II
Over the course of these 2 stages, the cell will divide 2 times

5. Genetic Diversity Produced by 2 different mechanisms:
Crossing over between homologous chromosomes
Independent assortment
Reorganization of genetic information, AKA genetic recombination
Diversity is important for a species because certain combinations of genes can better ensure the survival of individual organisms

6. Prophase I
Homologous chromosomes synapse (pair up) to form tetrads (2 chromosomes, 4 chromatids)
Crossing over may occur
An exchange of genes between the pairs
Chromatids overlap, break & exchange genetic information

7. Crossing Over

8. Metaphase I The tetrads line up on the equatorial plate
Spindle fibre attached to the centromere of each chromosome
Independent assortment happens here
No specific order in which the tetrads line up
DNA from the father could go to either pole, same for the DNA from the mother
Results in a random distribution of genetic information from the parents

9. Independent Assortment
Father XX
Mother xx
Mother XX
Father

10. Anaphase I
The tetrads are separated & each chromosome is pulled towards a different pole
Sister chromatids stay attached – there is no separation at the centromere

11. Telophase I Cytoplasmic division – like in mitosis
Some cells: cytokinesis does not fully occur & the cells pass straight to the 2nd divsion
Each new cell now has only 1 copy of each chromosome – haploid
The 2 daughter cells are not identical to each other or to the parent cell

12. Meiosis II
The stages of meiosis II are identical to the stages of mitosis
The cells at the end of the 2nd division are haploid
New cells are now called “gametes” or “spores” (plant cells)

13. Gametogenesis Formation of sex cells (gametes) during meiosis
Male & female gametes both undergo meiosis, but there are some differences
Spermatogenesis – formation of sperm cells
Oogenesis – formation of egg cells

14. Spermatogenesis Occurs in the testes
Males produce 1 billion sperm cells each day
At the end of meiosis II:
Cytoplasm is divided equally between the 4 daughter cells
Cells become specialized – streamlined for movement, develop flagella

17. Oogenesis Occurs in the ovaries
Females begin with 2 million oocytes (potential to develop in to egg cells)
~ remaining by time of sexual maturity
~400 – 500 actually go through meiosis (from ~ 12 years to ~ 50 years)
Meiosis occurs once each month to produce an egg cell
if the cell is not fertilized, the egg and other tissues that would be used for pregnancy are released from the body during menstruation

18. Each oocyte goes through meiosis to produce 4 daughter cells
Due to an unequal division of cytoplasm, only 1 becomes an ootid
Produces 3 polar bodies which are reabsorbed by the body
Egg cells need cytoplasm, organelles & nutrients for cell division in case of fertilization

21. Sex Chromosomes
Females have 22 pairs of homologous autosomes & 1 pair of double X sex chromosomes
One X chromosome came from her mother, one came from her father

23. Males have 22 pairs of homologous autosomes, 1 X sex chromosome and 1 Y sex chromosome
Although not the same shape, the XY pair acts as homologous during meiosis
The X chromosome came from his mother, the Y chromosome came from his father

26. Karyotype
A karyotype is a picture of an organism’s chromosomes, laid out so that the homologous pairs are side by side
Ordered by size
Sex chromosomes
are always last

27. Errors During meiosis – errors are possible
Mutation – error in the DNA
Can be transmitted from one generation to the next when 2 gametes come together to form a zygote
Cells with too much or too little genetic information will not function correctly

28. Nondisjunction
Nondisjunction – incorrect separation of chromosomes during Anaphase I or Anaphase II
If it happens during...
Anaphase I: results in 4 abnormal cells
Anaphase II: results in 2 normal cells, 2 abnormal cells

29. If a gamete with an extra chromosome is fertilized by a normal gamete...
Trisomy – one chromosome too many

30. Trisomy – the number refers to which homologous pair has an extra chromosome
Trisomy 21 – Down Syndrome
Small stature, large, shortened head, round face, mental deficiency
Trisomy 18 – Edwards Syndrome
Numerous physical and mental anomalies
Trisomy 13 – Patau Syndrom
Numerous physical anomalies

31. More commonly: nondisjunction involves the sex chromosomes

32. The union of abnormal eggs with normal sperm 
The union of abnormal sperm with normal eggs 

33. Sex Chromosome Trisomy
XXY – Klinefelter Syndrome
Male with the presence of feminine secondary sex characteristics
XXX – Triple X Syndrome
Normal female, not often diagnosed
XO – Turner Syndrome
Female, short, thick body, short neck, non-functioning ovaries Petite stature, corps massif, cou court, ovaires non fonctionnels
XYY Syndrome
Male, taller than average height

34. If the sister chromatids do not separate during Anaphase II, the gamete will be diploid
Diploid + haploid = triploid (3n)
3 sets of chromosomes
More than 2 sets of chromosomes - polyploid