Cytogenetics

Question 1

Frequency of down’s

Answer 1

1/700

Question 2

What causes Down’s syndrome

Answer 2

Trisomy 21

Maternal non-disjunction of chromosomes during meiosis.

Question 3

What causes Patau’s syndrome

Answer 3

Trisomy 13

Robertsonian tranlocation - non-reciprocal trnalocation involving two homologous (paired chromosomes) which have a long and a short arm. Long arms fuse to form single chormosome

Question 4

What are features of Patau’s

Answer 4

Dysmorphic facial features
Scalp skin defects
Incomplete cleavage of embryonic forebrain
Congential heart disease

Question 5

What causes Edward’s Syndrome

Answer 5

Trisomy 18

Question 6

What is translocation?

Answer 6

Exchange of chromosome segments that generally involve dissimilar chormosomes

Question 7

What are reciprocal translocation?

Answer 7

Balanced - no loss of genetic materia
Individual is phenotypically normal

Gametes however may not contain a complete copy of hte genome

Question 8

What is Robertsonian translocation

Answer 8

Long arms of two chormosomes fuse at a centromere and short arms are lost

Individual will have 45 chromosomes but appear phenotypically normal

Question 9

What is inversion

Answer 9

Two breaks in a chromosome and the DNA between the breaks rotates by 180 degrees

Usually balanced however increased chance of duplication and deletion of chromosomal region and unbalanced chromosomal arrangmenet in offspring

Question 10

What are isochromosomes?

Answer 10

Duplication of one arm but lack of the other

Result of breakage of a chromatid in the transverse direction Usually lethal

Question 11

What is duplication

Answer 11

Extra copy of a chromosome region

Can be inherited, or de novo from unequal crossing over, translocation, inversion

Question 12

What are frame shifts?

Answer 12

Deletion and insertion of one/two nucleotide bases resulting in incorrect reading during translation and hence an incorrect amino acid sequence and premature termination of the polypeptide chain

Question 13

Examples of trinucleotide repeat disorders

Answer 13

Myotonic dystrophy
Fragile X sydnrome
Huntington’s disease

Question 14

What is a missense mutation

Answer 14

Single nucleotide change leading to different amino acid substitution in polypeptide chain

Question 15

What is a nonsense mutations

Answer 15

Single nucleotide change resulting in a STOP codon (UAA, UAG, UGA) polypeptide cahin is shortened

Question 16

What is a splice site mutation

Answer 16

Single nucleotide base change alters a critical splice junction - abnormal RNA processing or reduction in the normal gene product or prevents the addition of poly-A tail for effective transcription