INTRODUCTION TO GENETIC DISEASES

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The human genome

diploid, 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes thus 46 pairs of chromosomes. Of a pair of chromosomes , 1 is maternal the other paternal. Sex chromosomes determine is individual is male XY or female XX

Karyotype

Chromosome makeup of an individual

Each chromosome of the karyotype is sorted and numbered according to size

What is lyonisation and why is it necessary

Lyonisation: when one X chromosome in each cell is inactivated during early development of the embryo.

Necessary to stop female from having double the expression of all X chromosome proteins in her cells where such overexpression would be harmful and very likely fatal.

Barr body

inactivated chromosome that is excluded from the nucleus

Chromosomal mutations

esult in visible changes in the chromosomes.

Aneuploidy cause is non-disjunction( non-seperation) of chromatids or parts of chromatids during meisosis.

Chromosomal disorders

Occur when entire chromosome is affected. Whole chromosome may be deleted or duplicated or there may be large breakage, deletions or duplications within chromosome. A euploid person has 23 pairs of chromosomes

An aneuploid does not have 23 pairs e.g Down syndrome

List the chromosomal disorders

E.g of chromosomal disorders:

Down syndrome (trisomy 21) extra chromosome

Kleinfelter syndrome (47 XXY) extra X chromoxome

Turner syndrome (45 XO) second sex chromosome is lacking

Autosomal disorders

Due to mutation in a single gene on an autosome. May be dominant ( one damaged allele)or recessive (both alleles damaged)

Dominant autosomal disorder

tends to affect structural proteins

disease affects heterozygotes

non-uniform expression

onset of disease may be delayed

neither parent may have the disease

When inherited, generations may be skipped

Marfan syndrome

Half building blocks are defective, cell cannot assemble structure properly. Is an autosomal dominant disorder due to mutation in one of the fibrillin genes.

Recessive autosomal disorder

affects enzymes

disease occurs in homozygotes

more uniform expression

onset of disease is early in life

Generally both parents have disease or are carriers

more common than autosomal dominant

when inherited generations may be skipped

Cystic fibrosis

autosomal recessive disorder

X-linked disorders

inherited on the X chromosome

Most are recessive, disease expressed when X chromosome is abnormal.

Female affected if the X chromosomes are abnormal, has Turner syndrome or as a result of lyonisation. Only males are affected and daughters inherit the disease from father, sons don't inherit

Females are carriers

All daughters of affected male are carriers

Sex-linked disorders

Haemophilia A

red-green colour- blindness

Duchenne muscular dystrophy

Mitochondrial disorders

All mitochondria inherited through the female line

Typically progressive and not obvious at birth and affect cells non-uniformly

Mitochondria are important in neurological tissues and skeletal muscles

Cannot be inherited from father

e.g lebers hereditary optic atrophy