1. Mistakes in Meiosis
Section 4.4

2. Mistakes in Meiosis
Learning Goal: To learn how mistakes in meiosis can cause genetic disorders and how they are detected.
Success Criteria:
I know I am succeeding when I can…
explain that errors in meiosis, such as non-disjunction, can result in an abnormal number of chromosomes and can cause serious genetic disorders
explain that karyotypes and prenatal testing are tools for diagnosing certain genetic disorders
create and interpret a human karyotype

3. Review: Human Chromosomes
46 chromosomes (23 homologous pairs) in each somatic cell
22 pairs of autosomes
1 pair of sex chromosomes
XX = Female, XY = Male
Karyotype – picture of chromosomes arranged according to shape and size
1. The X and Y are not homologous, but contain enough matching regions to still pair up during prophase I of meiosis to separate
2. so male sperm may possess an X or a Y; while female ova can only have an X

4. Sex Determination Cont’d…
This differs in other types of organisms!

5. Normal Meiosis 46 23 23 23 23 23 23

6. Non-disjunction “not coming apart”
produces gametes with 22 or 24 chromosomes
due to a failure of homologous chromosomes to separate in meiosis I, or the failure of sister chromatids to separate during meiosis II or mitosis
results in a cell with an imbalance of chromosomes
cells that have an extra chromosome or that are missing a chromosome are not able to function normally

7. Abnormal Meiosis I 46 24 22 24 24 22 22
ALL 4 Abnormal

8. Sister chromatids fail to separate
Abnormal Meiosis II 46
Sister chromatids fail to separate 23 23 24 22 23 23

9. Non-Disjunction Cont’d…
trisomy: are three homologous chromosomes in place of a homologous pair
monosomy: a single chromosome in place of a homologous pair
if zygote survives, each cell it produces by mitosis as embryo grows will retain chromosomal abnormality

10. Non-Disjunction Disorders
Down syndrome: a chromosomal abnormality in which an individual has three copies of chromosome number 21; also referred to as trisomy 21
A Female with Down Syndrome
A Normal Female

11. Non-Disjunction Disorders Cont’d…
individuals with Down syndrome experience a range of both physical and mental challenges
are generally able to carry out rich, fulfilling lives
1:800 births
probability of having a baby with Down syndrome increases with age
1. The extra chromosome means that there is excess genetic information and an unpaired chromosome in every cell.
2. common traits among people with Down syndrome may include a round, full face; short height; and a large forehead people with Down syndrome maintain a wide range of abilities
3. It is believed that approximately 1 in 800 live births produces a child with Down syndrome. The probability of having a baby with Down syndrome increases with age.
4. For example, a woman in her forties has a 1 in 40 chance of having a child with Down syndrome, whereas a woman in her twenties has only a 1 in 1000 chance.

12. Non-Disjunction Disorders Cont’d…
some additional human non-disjunction disorders

13. Diagnosing Non-disjunction
non-disjunction disorders are usually confirmed by preparing a karyotype
prenatal testing tests for a genetic disorder that occurs prior to birth
amniocentesis involves the use of a long syringe and an ultrasound machine, the collected cells are then used to prepare a karyotype chart
during the 15th and 20th weeks of pregnancy, women can have a blood test called multiple marker screening, which tests for hormone levels
Technicians usually prepare a karyotype by obtaining and mixing a small sample of white blood cells with a solution that stimulates mitotic division. A different solution is added that stops division at metaphase, when the chromosomes are most condensed and can be photographed and sorted. The completed karyotype is then examined and the disorder is diagnosed

14. Structural Abnormalities
Mutation: any change to the DNA of a cell
Deletions: a portion of chromosome is missing or deleted
e.g. Wolf-Hirschhorn syndrome, and Jacobsen syndrome

16. Structural Abnormalities
Duplications: a portion of chromosome is duplicated, resulting in extra genetic material
e.g. Charcot-Marie-Tooth disease type 1A

18. Structural Abnormalities
Translocations: a portion of one chromosome is transferred to another chromosome

20. Structural Abnormalities
Inversions: a portion of chromosome has broken off, turned upside down and reattached, therefore genetic material is inverted

21. Mistakes in Meiosis
Learning Goal: To learn how mistakes in meiosis can cause genetic disorders and how they are detected.
Success Criteria:
I know I am succeeding when I can…
explain that errors in meiosis, such as non-disjunction, can result in an abnormal number of chromosomes and can cause serious genetic disorders
explain that karyotypes and prenatal testing are tools for diagnosing certain genetic disorders
create and interpret a human karyotype

22. Lab: Interpreting Karyotypes
interpret and model the construction of karyotypes

23. Homework
Read pages in your text
Complete #1-3, 6, 7, 9