C HAPTER 6: G ENETIC DISEASES

I NTRODUCTION Cystic fibrosis:

W HAT ARE GENETIC DISEASES / DISORDERS ? A genetic disorder is an illness caused by abnormalities in genes or chromosomes What are chromosomes?

C HROMOSOMES A normal human being has 46 of these chromosomes in each cell (excepting reproductive cells) Of these 46 chromosomes, 44 are ‘autosomal’ 2 chromosomes are ‘sex chromosomes’ X and Y

G ENETIC DISORDERS Sometimes genetic disorders are caused by a whole chromosome:: Down syndrome Klinefelter syndrome Down syndrome is caused by a trisomy: Instead of a pair of chromosomes, you have 3 In this case 21 Down syndrome is also called ‘trisomy 23’. Other trisomies can occur, but often result in a miscarriage For example: trisomy 16

T RISOMIES The most common types of autosomal trisomy that survive to birth in humans are: Trisomy 21 (Down syndrome) Trisomy 18 (Edwards syndrome) Trisomy 13 (Patau syndrome) Trisomy 9 Trisomy 8 (Warkany syndrome 2) Trisomy 22 Sometimes, there is an extra copy of a sex chromosome present: XXX, XXY or XYY (XYY is not really a disorder) Klinefelter syndrome is an example: XXY

G ENETIC DISORDERS Other genetic disorders are not caused by a whole chromosome, but by a fault in one or more genes A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a polypeptide or for an RNA chain that has a function in the organism.

G ENES Cystic fibrosis is an example of a disorder caused by only 1 faulty gene You can only get it when this faulty gene is present on both chromosomes of chromosome 7 If only one chromosome 7 has the faulty gene you are a carrier You don’t get ill because the normal gene is dominant over the faulty gene

C YSTIC FIBROSIS

H UNTINGTON ’ S DISEASE Cystic fibrosis is caused by a recessive (non- dominant) gene Some genetic disorders are caused by faulty genes that are dominant Huntington’s disease In this case you will get the disease, even if only one copy of your chromosome 4 has the gene Q

W ORK Read pages 76 – 78 Answer questions 1 - 6