1. Chromosomal Mutations
Entire chromosomes extra/ missing(aneuploidy)
Pieces of chromosome missing or moved onto a different chromosome.
Identified by Karyotypes

2. Non-disjunction…….. how chromosomes go missing
Non = not
Dis = undo
Junction= joining
So non-disjunction is a failure to undo the joining of either:
a) homologous chromosomes or
b) sister chromatids

3. Non-disjunction of homologous chomosomes

4. Non-disjunction of sister chromatids

5. nondisjunction is the leading cause of pregnancy loss and birth defects.
90% of chromosome 21 nondisjunction are due to maternal meiotic errors
homologous chromosomes are arrested in prophase I for 10 to 50 years.

6. Older moms have higher risk

7. Autosomal non-disjunction
Too many or too few autosomes (C 1-22)
Nearly always fatal (miscarriages)
3 Exceptions
1) Trisomy 18 – Edwards syndrome
2) Trisomy 13 - Patau syndrome
3) Trisomy 21 – Down syndrome

8. Trisomy 18 – Edwards syndrome
3 copies of chromosome 18
Half of infants with this condition do not survive beyond the first week of life.
Small head (microcephaly)
Some children have survived to the teenage years, but with serious medical and developmental problems.

9. Trisomy 13 – Patau syndrome
More than 80% of children with trisomy 13 die in the first year.
Small head (microcephaly)

10. Trisomy 21- Down Syndrome
most common cause of human birth defects
Down syndrome symptoms vary from person to person and can range from mild to severe
Most are cognitively impaired
Adults may live semi-independently

11. Sex Chromosome Non-disjuction
At least one X chromosome is needed to live!!
Females who are XXX are fine
Males who are Xyy are fine
y alone will not survive
X alone gives Turner syndrome
Xxy gives Kleinfelter’s syndrome

12. Turner Syndrome XO (monosomy)
Girls with
1) short stature
2) poorly developed ovaries
Treat with
1) growth hormones
2) female hormones like estrogen

13. Kleinfelter’s syndrome
Men with
1) low testosterone levels
2) infertility
Treated with hormone replacement therapy
XXy

14. Why can we survive extra X or y?
Dosage Compensation Theory :
both ♀ & ♂ receive the same dose of X
In ♀ one X Chromosome is inactivated
Inactivated X condenses into a Barr body
75% of genes inactive

15. Mechanism of X inactivation
In each embryonic cell the 2 X C pair up
Then one of them activates its XIST gene
XIST gene products (RNA) wrap up & inactivate that X C
Which X is inactivated is random in each cell
Occurs in morula stage of
Embryo (ball of 2-16 cells)

16. Somatic tissues are a mosaic
Patches of cells
have different
X C inactivated

17. Calico Cats & Sweat glands

18. How boys become boys 2 month old embryos = generic gonads
SRY gene activation : make regulatory proteins
Sex determining Region of the Y chromosome
SRY regulatory proteins activate genes that cause baby to develop into a boy
Absence of these …baby develops into girl

19. Altered Chromosome Structure
1) Deletion: lost section of C
2) Inversion: section of C flipped end for end
3) Translocation: section of C moved to new C
4) Duplication: section of C moves to sister C…one chromatid has 2 copies the other none