Cytogenetics

Question 1

Metacentric chromosome

Answer 1

Centromere is in the middle p=q

Question 2

Submetacentric chromosome

Answer 2

Off-center centromere

Question 3

Acrocentric chromosome

Answer 3

Centromere near the end (ends are called “satellite DNA” on p arm

Question 4

Short arm

Answer 4

p

Question 5

Long arm

Answer 5

q

Question 6

What is euploidy?

Answer 6

abnormal number of copies of all chromosomes

Question 7

What is triploidy?

Answer 7

3n
- usually SAB in 1st trimester
- 17% of SABs
- Usually 2 sperm cells fertilize 1 egg

Question 8

What is tetraploidy?

Answer 8

4n
few cases, nonviable
1-2% of SABs
Possible due to failure of cells to cleave during division

Question 9

What is the incidence of Down Syndrome?

Answer 9

1 in 185 births

Question 10

What causes Down Syndrome?

Answer 10

95% T21 (90% maternal meiosis I, 10% paternal meiosis II)
4% Robertsomian translocation
Usually with 14 or 22
Rare: 21q21q translocation - isochromosome
Rare: partial T21 (part of q arm)
Rare: mosaic T21

Question 11

What are the psuedoautosomal regions on the X and Y chromosomes?

Answer 11

Regions on X and Y that exhibit autosomal inheritance
Site of recombination - failure can lead to nondisjunction

Question 12

What is uniparental disomy (UPD), and what types are there?

Answer 12

Both chromosomes from the same parent

Isodisomy: Chromosomes are identical and inherited from sister chromatids
Heterodisomy: inheritance of both homologs from one parent

This often happens from trisomy rescue

Question 13

What is a marker chromosome? What are the most common types?

Answer 13

small chromosomes in addition to the normal amount

Extra chromosome 15
Ring chromosomes - marker chromosome with no telomeres (usually mosaic)

Question 14

What is an isochromosome?

Answer 14

one arm is duplicated → mirrored chromosome
Opposite arm may be deleted
Trisomy of duplicated arm, monosomy of deleted arm

Question 15

What is a Robertsonian translocation?

Answer 15

Most common rearrangement in humans
Fusion of two acrocentric chromosomes near the centromere
Loss of short arms is fine - little genetic material

Question 16

What is an inversion, and what types are there?

Answer 16

two breaks in a single chromosomes and the segment flips

Paracentric - includes only one arm
Pericentric - includes both arms (and centromere)
Balanced - usually no phenotype in carriers

Question 17

What is allelic heterogeneity?

Answer 17

different mutations in a gene → same phenotype

Question 18

What is locus heterogeneity?

Answer 18

mutations in different genes → same phenotype