Cytogenetics

Translocations, inversions and deletions

What is a reciprocal translocation?

• one part of a chromosome is exchanged with another

• There’s no net loss of genetic material

Breakpoint

point of breakage event

Derivative chromosomes / der

rearranged chromosomes

centric segment

part that included the centromere

translocated segment

part that comes from another chromosome

What are the 2 types of effects of a translocation?

1. Balanced

2. Unbalanced

What is the effect of a balanced translocation?

• (Usually) No effect

• Unless a critical gene is affected

  • Efect can be on reproduction/ fertility due to meiosis

What is the effect of a balanced translocation?

• some loss / gain

Quadrivalent

a structure that forms during meiosis when two normal chromosomes and two derivative chromosomes come together

What are the 3 modes of quadrivalent segregation?

1. 2:2

2. 3:1

3. 4:0

What are the 3 types of 2:2 quadrivalent segregation?

1. alternate

2. adjacent-1

3. adjacent-2

What is the only form of quadrivalent segregation that results in balanced segregation?

alternate 2:2

What are the 2 possible gamete results of 4:0 segregation?

1. Doubly disomic

2. Nullisomic

What is the translocation pattern for 2:2 adjacent-1 translocation?

What is the translocation pattern for 2:2 adjacent-2 translocation?

What is the translocation pattern for 2:2 alternate translocation?

What is the translocation pattern for 3:0 translocation?

Robertsonian Translocations

a chromosomal abnormality where the long arms of two different chromosomes fuse together

What gametes can be formed as a result of Robertsonian translocations?

1. alternate

2. adjacent

What are the 3 possible effects of Robertsonian translocations?

1. Balanced

2. Trisomy

3. Monosomy

What is uniparental disomy?

2 copies of chromosomes from the same parent

Are monosomies of autosomes viable?

NO

Are monosomies of X chromosomes viable?

Yes

What is uniparental heterodisomy?

the two different chromosomes are inherited from one parent

How does uniparental heterodisomy occur?

Trisomy rescue

What is uniparental isodisomy?

two of the same chromosomes come from the same parent

How does uniparental isodisomy occur?

• trisomy or monosomy resure

On what chromosomes can robertsonian translocations occur?

13,14,15,21,22

Why cam Robertsonian translocations only occur on certain chromosomes?

• Chromosomes that are acrocentric

• Acrocentric = ends of short arms have similar DNA seqs

  • Increased likelihood to fuse

What is the effect of robertsonian translocations in carriers?

45 chromosomes

• fusion of 2 q arms

• loss of two joint short arms

What are the 2 types of inversions?

1. Pericentric

2. Paracentric

What determines the type of inversion?

Position of the centromere

Where is the centromere in paracentric inversions?

• Outside inverted regions

• AWAY from centromere

Where is the centromere in pericentric inversions?

• Centromere is within inverted regions

• IN centromere

Which type of inversion leads to more likely imbalanced offspring?

PERICENTRIC

How is the ISCN written for inversions?

• Number of Chromosomes

• Sex Chromosomes

• inv(x) - where x is chromosome number

• (breakpoints) - with no ; between the breakpoint

e.g. 46,XY,inv(1)(p11q22)