Genetics

Question 0

Define MOSAIC

Answer 0

An individual with two or more genetically different cell lines derived from a single zygote

Question 1

Define HEMIZYGOUS

Answer 1

Having only one allele of a particular gene

Question 2

Define CHIMERA

Answer 2

An individual with two or more genetically different cell lines derived from more than one zygote (for example, an individual who has received a bone marrow transplant)

Question 3

Define PENETRANCE

Answer 3

The proportion of people with a genetic mutation who exhibit the associated signs and symptoms

Question 4

Define EXPRESSIVITY

Answer 4

The extent to which the phenotype is expressed

Question 5

What is the difference between PENETRANCE and EXPRESSIVITY

Answer 5

Penetrance is looking at the probability that the phenotype will be expressed at all whereas expressivity is about the extent to which the phenotype is expressed. I.e. With expressivity, we’re saying “yes, the phenotype is definitely being expressed but how much of it is actually being expressed”

Question 6

Difference between PERICENTRIC INVERSION and PARACENTRIC INVERSION

Answer 6

Pericentric inversion is when each break is on a different arm

Paracentric inversion is when both breaks are on the same arm

Question 7

Give 3 trisomy examples

Answer 7

Trisomy 13 - Patau syndrome

Trisomy 18 - Edwards syndrome

Trisomy 21 - Down syndrome

Question 8

Give 2 examples of sex chromosome numerical abnormalities

Answer 8

Klinefelter syndrome - 47, XXY

Turner syndrome - 45, X

Both cause mild intellectual impairment and infertility

Question 9

Define HAPLOINSUFFICIENCY

Answer 9

Haploinsufficiency is when a loss-of-function mutation results in a dominant condition*

*Loss-of-function mutations usually cause recessive conditions while gain-of-function mutations usually cause dominant conditions

Question 10

What are POLYMORPHISMS

Answer 10

Changes to the coding sequences within a gene which doesn’t affect the gene product

Question 11

Give an example of a mitochondreal disease

Answer 11

1) MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis and Stroke-like episodes)
2) MERRF (Myocolonic Epilepsy with Ragged Red Fibres)

Question 12

Explain the terms MONO ALLELIC EXPRESSION and GENE IMPRINTING?

Answer 12

Mono allelic expression is when you have two alleles of a gene but only one of them is expressed (unlike hemizygous genes whereby you only have one copy of the gene)

Gene imprinting is a special case of mono allelic expression whereby the expressed allele is always either the maternal copy or the paternal copy

Question 13

Give examples of gene imprinting syndromes

Answer 13

1) PRADER-WILLI SYNDROME

2) ANGELMAN SYNDROME

Question 14

Give an example of a disease causes by tandem trinucleotide repeat

Answer 14

Fragile-X syndrome (a tandem trinucleotide repeat within the 5’ UTR)

Question 15

Define ANTICIPATION

Answer 15

When the age of onset of a disease is reduced or the severity is increased with each subsequent generation

Usually seen in conditions associated with tandem repeats. This is because with each successive generation, the number of repeats increases and hence, the gene becomes even more unstable!