Genetic disorder presentation

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… {5C22544A-7EE6-4342-B048-85BDC9FD1C3A}Slide Number 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!