1. 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
Why do we resemble our parents? (We resemble our parents because we inherit traits from them.) 2. Why does asexual reproduction result in genetic clones? (During asexual reproduction, the organism makes a copy of itself without combining its genes with those from another organism. The offspring are a genetic copy of the single parent.) 3. Why are the offspring of sexual reproduction not genetically identical to either parent? (During sexual reproduction, genes from two individuals combine to produce offspring with new combinations of genes.)

2. Karyotypes
karyotype: the chromosomes of an individual that have been sorted and arranged according to size, shape and banding patterns
an individual’s chromosome profile can be analysed in this way (eg. sex determination, chromosome abnormalities)
Preparations of chromosomes can be prepared from cells in metaphase of mitosis
The chromosomes are fixed (mitosis stopped), stained, prepared for microscopic examination and photographed
Homologous pairs are then easily matched, classified and numbered

3. Sex Chromosomes and Sex Determination
cell’s 23rd pair of chromosomes are the sex chromosomes, as they determine the sex of the individual (all other chromosomes are referred to as autosomes)
In mammals, there are two types of chromosomes: X and Y
XX  females
XY  males
The X and Y are not homologous, but contain enough matching regions to still pair up during prophase I of meiosis to separate
so male sperm may possess an X or a Y; while female ova can only have an X
Meiosis I, the first division, is often described as a reduction division because the diploid, or 2n, chromosome number is reduced by half to the haploid, or n, chromosome number.
This reduction in chromosome number is a key distinguishing feature of meiosis. In the second division, meiosis II, the number of chromosomes is unchanged, but the total number of cells increases to four

4. Sex Determination Cont’d…
This differs in other types of organisms!
Meiosis I, the first division, is often described as a reduction division because the diploid, or 2n, chromosome number is reduced by half to the haploid, or n, chromosome number.
This reduction in chromosome number is a key distinguishing feature of meiosis. In the second division, meiosis II, the number of chromosomes is unchanged, but the total number of cells increases to four

5. Mistakes in Meiosis
many genetic disorders can be traced back to an error in formation of gametes in meiosis
caused by a difference in the number of chromosomes or the structure of chromosomes
Meiosis I, the first division, is often described as a reduction division because the diploid, or 2n, chromosome number is reduced by half to the haploid, or n, chromosome number.
This reduction in chromosome number is a key distinguishing feature of meiosis. In the second division, meiosis II, the number of chromosomes is unchanged, but the total number of cells increases to four

6. Non-Disjunction
Non-disjunction occurs when homologous chromosomes fail to separate during meiosis
the result is that one of the daughter cells will have an extra chromosome, while the other will be missing a chromosome
cells that have an extra chromosome or that are missing a chromosome are not able to function normally
Meiosis I, the first division, is often described as a reduction division because the diploid, or 2n, chromosome number is reduced by half to the haploid, or n, chromosome number.
This reduction in chromosome number is a key distinguishing feature of meiosis. In the second division, meiosis II, the number of chromosomes is unchanged, but the total number of cells increases to four

7. 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 the zygote survives, each cell that it produces by mitosis as the embryo grows will retain this chromosomal abnormality
Meiosis I, the first division, is often described as a reduction division because the diploid, or 2n, chromosome number is reduced by half to the haploid, or n, chromosome number.
This reduction in chromosome number is a key distinguishing feature of meiosis. In the second division, meiosis II, the number of chromosomes is unchanged, but the total number of cells increases to four

8. 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
Meiosis I, the first division, is often described as a reduction division because the diploid, or 2n, chromosome number is reduced by half to the haploid, or n, chromosome number.
This reduction in chromosome number is a key distinguishing feature of meiosis. In the second division, meiosis II, the number of chromosomes is unchanged, but the total number of cells increases to four
A Female with Down Syndrome
A Normal Female

9. 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
The extra chromosome means that there is excess genetic information and an unpaired chromosome in every cell. 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 and 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. 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.

10. Non-Disjunction Disorders Cont’d…
some additional human non-disjunction disorders
The extra chromosome means that there is excess genetic information and an unpaired chromosome in every cell. 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. 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.

11. 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

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

13. Homework Read pages 161-166 in your text Answer 4.3 questions #1-7 & 9
Centrioles are involved in the formation and organization of spindle fibres, which attach to the chromosomes during cell division. The centromere helps anchor the chromosomes to the spindle fibres