1. Meiosis Review

2. Terminology
What is the difference between a chromosome, homologous chromosomes, sister chromatids, and a tetrad?

4. GAMETE FORMATION

5. GAMETOGENESIS
Gametogenesis: the production of gametes (sex cells) through the process of meiosis.

6. SPERMATOGENESIS Meiosis in males occurs in the testes
Starts with a diploid cell called a spermatogonium
produces four non-identical haploid sperm cells (spermatids)
Following meiosis the sperm cells develop into mature sperm (spermatozoa) –the nucleus forms into a head, and a flagellum is formed for locomotion.

7. SPERMATOGENESIS
Occurs all the time from puberty until death. It takes approximately hours for a sperm to be created. Meiosis produces approximately sperm every day in males!

8. OOGENESIS Meiosis in females Occurs in the ovaries and oviducts
Starts with a diploid cell called an oogonium.
Egg production starts before a female is born, but pauses in Meiosis I before the cells (primary oocyte) divide.
The meiotic process resumes at puberty with ovulation (and fertilization), for 1 cell (ovum) every month.

9. OOGENESIS
Cytokinesis is not equal in meiosis of egg cells, one of the cells receives the majority of the cytoplasm, resulting in one egg cell and three polar bodies.
The purpose of the unequal division is to allow the egg cell to have sufficient nutrients to support a zygote immediately after fertilization.

10. ERRORS IN MEIOSIS Changes in chromosomes are known as mutations.
Mutations can be beneficial, neutral or harmful.
Because chromosomes are copied during interphase, all daughter cells (sperm or egg) will carry the mutation.
If that sperm or egg is part of fertilization, the new organism will carry that error in all of its cells.

11. Atypical Chromosome Numbers
Atypical chromosome numbers occur when too many, or too few, chromosomes appear in an embryo’s cells.
Effects depend on which chromosome is affected.
Usually atypical numbers result in miscarriage.
For example:
Trisomy = 3 identical chromosomes, instead of pair
Trisomy 21 = Down syndrome

12. Normal vs non-disjunction
The failure of chromosomes or tetrads to separate properly during anaphase is called non-disjunction. This results in the addition or deletion of a chromosome in a gamete.
one gamete has an extra chromosome, and the other is missing a chromosome.
If one of these joins normal gamete in fertilization, diploid zygote will have an atypical number of chromosomes.
Meiosis I:
Normal vs non-disjunction
Meiosis II:

13. NON-DISJUNCTION
If it happens in meiosis I, all the resulting cells will be affected. If it happens in meiosis II, only half will be affected.

14. NON-DISJUNCTION Incidence increases greatly with age of mother
Eggs formed at birth but do not complete meiosis until released, 1 per month
Delay in release can cause cell damage

15. NON-DISJUNCTION
If a gamete with an extra chromosome is fertilized by a normal gamete, the zygote will have an extra chromosome, called trisomy.
If a gamete missing a chromosome is fertilized by a normal gamete, the zygote will have only one copy of a chromosome, called monosomy.

16. TRISOMY Trisomy 21 results in Down Syndrome.
Some effects include mental delay, weakened cardiovascular system, shortened limbs, widely-spaced eyes, receding forehead/chin, and protruding tongue.

17. TRISOMY
Klinefelter’s Syndrome occurs when an individual receives two X chromosomes and a Y chromosome.
Result: infertile male with varying degrees of femininity.

18. MONOSOMY
Turner Syndrome: the individual only gets one sex chromosome, an X, from their mother.
Result: infertile female with a broad chest, poor breast development, low set ears, short stature and poor hearing amongst other things.

19. Chromosome Number
Condition
Traits
Trisomy 13
Patau syndrome
1 in live births.
severe physical and intellectual disability
have heart defects, brain or spinal cord abnormalities, extra fingers and/or toes, a cleft lip,
Only 5 –10% of babies survive past their first year.
Trisomy 18
Edwards syndrome
1 in 5000 live births
severe intellectual disability and low birth weight
small, head, jaw and mouth; clenched fists; heart and organ defects
Trisomy 21
Down syndrome
1 in 800 live births.
mild to moderate intellectual disability,
XXY
Klinefelter syndrome
1 in 500 to 1 in 1000 males.
affects male sexual development
small testes that do not produce enough testosterone.
Testosterone injections help treat the condition.

20. Patau syndrome Edwards syndrome Klinefelter syndrome Down syndrome
Cracking the Code: Understanding Rare Chromosome Disorders - YouTube

21. CHROMOSOME ABNORMALITIES

22. Damage to Chromosome Structure
A mutation is any damage that occurs to a chromosome
Can occur spontaneously, or by radiation or exposure to certain chemicals
Changes may be as small as a few base pairs or as large as the entire structure of the chromosome.
There are 4 different ways chromosome structure can be damaged:

23. DELETION
In deletion, part of the chromosome is actually lost. Viruses, radiation and chemicals can cause a piece of a chromosome to become dislocated.
This piece may carry a specific gene which may have a large effect on the host.

24. DUPLICATION
In duplication, a gene sequence is repeated one or more times within a chromosome. At some point, too many repeats can affect the function of the gene.

25. INVERSION
In inversion, a gene segment momentarily becomes free from its chromosome and then reinserts in the opposite order. This can completely alter the gene’s activities.

26. TRANSLOCATION
In translocation, part of a chromosome changes place with another part of either the same or a non-homologous chromosome.
Translocations can result in some cancers, Down Syndrome, and leukemia.

27. GENETIC TESTING
Examining genes can allow for the diagnosis, treatment, and prevention of genetic illnesses.
Several types of testing:
Karyotype analysis - discovers chromosome abnormalities
BiologySource
Carrier testing - tests parental genes before conception
Cystic fibrosis (CF) and Tay-Sachs caused by gene mutations passed on to offspring by both parents.
Presymptomatic (predictive) testing - searches for specific genetic diseases that run in families.
Usually done for disorders that appear later in life
Diagnostic genetic testing - confirms a diagnosis
This type of test can be done at any point in a person’s life.

28. GENETIC TESTING
Prenatal testing: detects chromosomal problems in a fetus.
Embryo becomes a fetus about 7 weeks.
Down syndrome and spina bifida common tests
Amniocentesis - genetically testing of fetus between weeks 14 to 20 of pregnancy.
Chorionic villus sampling is also used.
Tissue surrounding the fetus is removed and tested.
These tests have risks and are used only if concerns arise
Newborn screening - tests for some genetic disorders shortly after birth.
Phenylketonuria (PKU) testing is done this way.

29. Genetic Testing In-utero