Introduction to Genetics for beginners

An Introduction to basic genetic concepts and links to health for individuals who

have had minimal experience of genetics

DNA, genes & chromosomes

The objectives of this presentation are to:• Understand the role and structure of DNA, genes

and chromosomes.• Understand that proteins are encoded by genes• Be aware that alterations in genetic material can

cause disease

The structure of DNA, genes & chromosomes

Chromosomes

• Chromosomes are made of DNA. • Each contains genes in a linear

order.• Human body cells contain 46

chromosomes in 23 pairs – one of each pair inherited from each parent

• Chromosome pairs 1 – 22 are called autosomes.

• The 23rd pair are called sex chromosomes: XX is female, XY is male.

Gene for sickle cell disease (chromosome 11)

Gene for cystic fibrosis (chromosome 7)

Chromosomes

pCentromere

q

Chromosome 5

The Karyotype

A normal male chromosome pattern would be described as:

46,XY.

46 = total number of chromosomesXY = sex chromosome constitution

(XY = male, XX = female).

Any further description would refer to any abnormalities orvariants found

Total Genes On Chromosome: 723373 genes in region marked red, 20 are shown

FZD2AKAP10ITGB4KRTHA8WD1SOSTMPP3

MLLT6

STAT3BRCA1 breast cancer 1, early onsetGFAPNRXN4NSFNGFRCACNB1HOXB9HTLVRABCA5CDC6ITGB3

Chromosome 17source: Human Genome Project

Genes are arranged in linear order on chromosomes

Chromosome anomalies• Cause their effects by altering the amounts of products of the

genes involved.

– Three copies of genes (trisomies) = 1.5 times normal amount.

– One copy of genes (deletions)= 0.5 times normal amount.

– Altered amounts may cause anomalies directly or may alter the balance of genes acting in a pathway.

Classification of chromosomal anomalies• Numerical (usually due to de novo error in cell division)

- monosomy- trisomy

• Structural (may be due to new error in cell division or inherited) - dosage changes (e.g. deletions) and rearrangements

Most frequent numerical anomalies in liveborn

AutosomesDown syndrome (trisomy 21: 47,XX,+21)

Edwards syndrome (trisomy 18: 47,XX,+18) Patau syndrome (trisomy 13: 47,XX+13)

Sex chromosomesTurner syndrome 45,X

Klinefelter syndrome 47,XXY

All chromosomesTriploidy (69 chromosomes)

Summary of Chromosome Anomalies

• Change in numbere.g. trisomy 21 Down syndrome;

Edwards’ syndrome; Turner syndrome.Usually an isolated occurrence.

• Change in structuree.g. deletionsMay be inherited.

Trisomy 21

The DNA Double Helix

Disease-Associated Mutations Alter Protein Function

What is a mutation?• Mutation – an alteration or change in the genetic

material• In clinical use usually = “harmful”• Inherited• From exposure to mutagenic agents but more

arise spontaneously through errors in DNA replication / repair

• More likely to be recognised if effects are detrimental

Summary• Human body cells contain 46 chromosomes in

23 pairs – one of each pair is inherited from each parent

• Chromosomes are made of DNA• Each chromosome contains genes in a linear

order• Genes are codes for cells to make proteins• Alterations in genes or chromosomes alter the

protein produced and can hence cause disease