Transcript
SBI3U
Nicole Fisher
Genetic Disorders
Agenda
Extension of meiosis and Mendelian genetics presentations
Nondisjunction
What is it?
Examples
Allelic Disorders
What are they?
What are the types of allelic disorders?
Examples
Nondisjunction
What is nondisjunction?
Error during meiosis or mitosis
2 homologous chromosomes move to the same pole during anaphase
More devastating effects when nondisjunction occurs during meiosis
How does it occur?
Non-disjunction can occur in either meiosis I or meiosis II
In meiosis I, an entire tetrad moves into one of the daughter cells during anaphase I
In meiosis II, one of the chromosomes fails to separate during anaphase II
Nondisjunction Animations
Nondisjunction in meiosis I
http://www.biostudio.com/d_%20Meiotic%20Nondisjunction%20Meiosis%20I.htm
Nondisjunction in meiosis II
http://www.biostudio.com/d_%20Meiotic%20Nondisjunction%20Meiosis%20I.htm
Trisomy
3 homologous chromosomes are present in a cell where there should only be 2
E.g. you have 2 copies of the maternal chromosome 3 and 1 copy of the paternal chromosome 3, where you should only have 1 of each
2n+1 chromosomes in the cell
Example: Down Syndrome (trisomy 21), Klinefelter Syndrome (XXY)
Down Syndrome (Trisomy 21)
1 in 600 babies
Common traits: round & full face, enlarged & creased tongue, large forehead, short height
Exhibit a wide variety of mental abilities
Risk of having a baby with Down Syndrome is 25 times higher for a woman in her 40s compared with a woman in her 20s
Monosomy
A single chromosome in a cell in the place of a homologous pair
E.g. You have the paternal copy of chromosome 1, but not the maternal
2n-1
Example: Turner Syndrome (single X)
Turner Syndrome
Sex chromosomes undergo nondisjunction
Result: female baby with a single X chromosome
Child appears female, but doesn’t ever develop sexually
1 in every 3000 babies
Most are miscarried before week 20
Prenatal Screening Techniques
Amniocentesis – drawing fluid from the amniotic sac (surrounding the fetus)
Can be performed after the 11th week of pregnancy
Cells are used to create a karyotype chart
Multiple Marker Screen (MMS)
Newer technique
Performed after 15-18 weeks
Identifies women at a higher risk of having a baby with a genetic disorder
Uses mother’s blood to measure alphafetoprotein (AFP) levels
Elevated AFP levels may indicate issues with the early nervous system structures
Use amniocentesis to confirm the results
Karyotype Chart
Picture of the chromosomes arranged in homologous pairs
Karyotype Activity
Each table has been given a karyotype chart
Identify where the problem is on your karyotype chart
Do an Internet search for that disorder
Prepare 2-3 points to share with the class
Allelic Disorders
Allelic Disorders
Disorders that are attached to certain allele combinations
Originally discovered by Thomas Hunt Morgan
Did experiments on Drosophila melanogaster
Identified mutations that were autosomal dominant, autosomal recessive and sex-linked
Allele = two or more alternate forms of the same gene
Located at the same position on homologous chromosomes
Autosomal Dominant Disorders
Occur in cells not related to sex determination (body cells)
Only one copy of the allele in question is needed for the disorder to be expressed
Example: Huntington’s chorea
Autosomal Recessive Disorders
Occur in cells not related to sex determination
Both copies of the allele must be recessive in order for this disorder to be expressed
i.e. can be masked by a “normal” dominant allele
Can be lethal or non-lethal
Example: hemophilia or cystic fibrosis
Sex-Linked Disorders
Alleles for sex-linked disorders are located on either the X or Y chromosome
More commonly expressed in males (Why?)
Examples: red-green colour blind, male-pattern baldness
Red Green Colour Blind Test
Red Green Colour Blind Test cont.
Red Green Colour Blind Test cont.
What you should have seen…
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Right
Left
1
6
56
2
25
45
3
8
29
If you have RG colour blind, you would have seen…
{5C22544A-7EE6-4342-B048-85BDC9FD1C3A}Slide Number
Right
Left
1
spots
56
2
25
spots
3
spots
spots
Pedigree Charts
Allelic disorders can be traced through pedigree charts
One way of predicting if a child could have a disorder when they’re born
Gene Therapy
Defective genes are replaced by normal genes to cure genetic disorders
Many disorders have no cure with current medical techniques
Still in the early stages of development
Much debated topic
3 Possible Gene Therapy Strategies
Gene insertion – normal genes are inserted into position on the affected chromosome
Can be accomplished using a virus
Process can be targeted towards a specific cell type
Gene modification – defective genes are modified chemically in an effort to recode the genetic message
More delicate method than gene insertion
Chemical compositions of both the normal and defective genes must be known exactly
Gene surgery – defective gene is actually removed and replaced with the normal gene
Most ambitious method of gene therapy
Ashanti Desilva
http://www.youtube.com/watch?v=EfXK50Bxod8&feature=related
Received gene therapy in 1990 – still alive, and considerably healthier today
Gene therapy’s first success
Why is gene therapy still feared or contested by some?
Thanks!
