1. Occurs in reproductive organs during gamete formation
Meiosis
Meiosis is a reduction division of a diploid nucleus to form haploid nuclei.
Occurs in reproductive organs during gamete formation

2. Homologous chromosomes
homologous chromosomes = chromosomes with the same gene loci in the same sequence which are capable of pairing up to form bivalents during the first prophase of meiosis.

3. 1.pairing of homologous chromosomes
2.crossing over
3.two divisions
4.results in four haploid cells

4. Stages of meiosis

5. Meiosis Prophase 1 chromosomes condense nucleolus disappears
spindle forms
synapsis of bivalent: pairing of homologous chromosomes
crossing over occurs between non-sister chromatids
nuclear membrane disappears

6. Crossing over Homologous chromosomes pair forming bivalents
Portions of non-sister chromatids overlap, break at chiasmata, reattach to new chromatid in a system of reciprocal exchange
Chromosomes with new combinations are known as recombinants

8. chromosomes continue to condense
Meiosis Metaphase 1
chromosomes continue to condense
spindle microtubules attach to centromeres
bivalents line up at equator

9. separation of homologous pairs; chromosomes moved to poles by spindle
Meiosis Anaphase 1
separation of homologous pairs;
chromosomes moved to poles by spindle

10. chromosomes at poles spindle disappears chromosomes partially uncoil
Meiosis Telophase 1
chromosomes at poles
spindle disappears
chromosomes partially uncoil

11. Meiosis prophase 2 chromosomes condense again
Newspindle forms (at right angles to previous spindle)

12. Meiosis metaphase 2 spindle microtubules attach to centromeres
chromosomes move to the equator

13. Meiosis Anaphase 2 sister chromatids separate
spindle microtubules move chromatids to opposite poles

14. Meiosis Telophase 2 chromosomes (=chromatids) arrive at poles
spindle disappears
nuclear membrane reappears
nucleolus reappears
chromosomes decondense into chromatin

15. Genetic variety in gametes
Crossing over in prophase 1
Random orientation in metaphase 1
maternal and paternal chromosomes assort to daughter cells randomly
possible arrangements of chromosomes in haploid daughter cells = (2)nth, where n = number of homologous pairs
in humans, n = 23, and possible arrangements = (2)23 = about 8 million
Chance fusion of gametes

16. https://highered. mcgraw-hill

17. https://highered. mcgraw-hill

18. Here is the link to the McGraw Hill Animation

19. Mendel’s law of independent assortment and meiosis.
Independent assortment occurs as a result of the alignment of homologs during metaphase I, determining which maternal and paternal chromosomes assort to each daughter cell
each pair of alleles separates independently of every other pair of unlinked alleles

21. non-disjunction can lead to changes in chromosome number
Down syndrome:
in either oogenesis or spermatogenesis, chromosome 21moves to pole as a pair instead of singly;
egg or sperm contain an extra copy of chromosome 21, and fertilized egg has 3 copies of chromosome 21
Down syndrome is also known as trisomy 21

22. non-disjunction can lead to changes in chromosome number.
which is why Down syndrome is also known as trisomy 21

23. Here is the link to this video

24. Trisomy
A trisomy occurs when there are three copies of a chromosome in a cell. This is the result of non-disjunction during meiosis ,
Followed by fertilization with another gamete

25. Down Syndrome Trisomy 21 Physical Characteristics
Low muscle tone (babies appear "floppy")
Flat facial features, with a small nose
Upward slant to the eyes
Small skin folds on the inner corner of the eyes
Small, abnormally shaped ears
Single deep crease across the center of the palm
Hyperflexibility (excessive ability to extend joints)
Fifth finger has only one flexion furrow instead of two
Extra space between the big toe and the second toe
Enlarged tongue that tends to stick out

26. Health Problems associated with Down Syndrome
Heart Defects
Visual Problems
crossed eyes nearsightedness farsightedness cataracts
15-20 times greater risk of developing leukemia
hormonal problems
50% of adults with Down syndrome have thyroid disease
Approximately 10%-12% of babies born with Down syndrome also have abnormalities in the gastrointestinal tract that require surgery for correction.
Life expectancy for people with Down syndrome has increased dramatically in recent decades - from 25 in 1983 to 60 today
The incidence of births of children with Down syndrome increases with the age of the mother. But due to higher fertility rates in younger women, 80% of children with Down syndrome are born to women under 35 years of age. - See more at:

31. What’s the difference? Chorionic Villus Sampling Amniocentesis
Takes a small sample of cells from the developing placenta
Draws a small sample of amniotic fluid from the uterus (amniotic fluid)
May take place during 1st trimester (Earlier diagnosis)
Completed during 2nd trimester
CVS thought to be more risky for miscarriage but a 20 year study at the University of California, San Francisco concluded that the miscarriage rate for either test averages less than 2 percent

32. Karyotyping
Chromosomes are arranged in pairs according to their size, structure, banding pattern.
Karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.
Created during metaphase of MITOSIS?
Why does this make sense?

33. Karyotyping Video