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Errors in recombination

Structural abnormalities

This is another complicated step in this course where we have used a lot of new terminology. These are all defined in the glossary in Week 1 but we thought it might help to just mention a few terms we use prior to reading the text below: translocation balanced unbalanced homologous

Prior to segregation in meiosis, homologous chromosomes pair and genetic material is swapped. This is known as recombination and is key to ensuring offspring have different genetic traits to their parents. However, if homologous chromosomes (chromosomes of the same pair) misalign or non-homologous chromosomes pair, a number of different structural chromosome abnormalities may result including chromosome deletions / duplications; translocations and chromosome inversions.

Recombination Example of Recombination during Meiosis

© St George’s, University of London

Deletions or Duplications

This refers to the loss (deletion) or gain (duplication) of genetic material. A deletion or duplication is called interstitial when it occurs in the middle of the chromosome and terminal when it occurs at the tip of the chromosome.

Deletion and Duplication Example of interstitial deletion and duplication

© St George’s, University of London

The phenotypic (clinical) effect of a deletion or duplication depends on the genes involved. Deletions are generally more likely to have a phenotypic effect than duplication. Large deletions are likely to be lethal. One of the commonest microdeletion syndromes is Di George syndrome, involving deletion of a region of chromosome 22 called 22q11. Di George syndrome can cause a variety of problems, including heart abnormalities, defects of the palate, problems of immunity and calcium control. Children and adults often have a characteristic facial appearance.

Child with Di George syndrome Child with Di George syndrome

©Image: “Facial appearance of patient with Del22 syndrome” by Images in Paediatric Cardiology. Licensed under CC BY-NC-SA 3.0

With the advent of array testing, an increasing number of recurrent microdeletion and microduplication syndromes are being recognised. This is discussed further in week 3.

Translocations

A translocation describes when a portion of one chromosome is transferred to another chromosome. Translocations can be balanced or unbalanced depending upon whether there is a net gain or loss of genetic material. They are broadly classified into reciprocal or Robertsonian translocations.

a. Reciprocal translocations arise when any two chromosomes swap non-homologous segments. A carrier of a balanced reciprocal translocation may have offspring with an unbalanced translocation i.e. trisomy of one of the translocated segments and monosomy of the other.

Reciprocal translocations Example of Reciprocal translocation

© St George’s, University of London

b. Roberstonian translocations describe when two acrocentric chromosomes are “stuck together”. Acrocentric chromosomes are chromosomes where the centromere is very close to the end and include chromosomes 13, 14, 15, 21, 22 and Y.

Roberstonian translocations Example of Roberstonian translocation

© St George’s, University of London

Inversions

An inversion is when a section of the chromosome has broken away, twisted around 180° (i.e. inverted end to end) and re-inserted into the chromosome. if this section spans the centromere, it is called a pericentric. If the inversion does not include the centromere, it is called a paracentric inversion.

Inversion Example of Inversion

© St George’s, University of London

Usually inversions are not associated with any loss or gain of genetic material and so a carrier is asymptomatic (unless a critical gene is disrupted at the breakpoints which is rare). They may only become of aware of their carrier status if they have a child with an unbalanced arrangement or they have chromosome investigations for infertility or recurrent miscarriages.

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This article is from the free online course:

The Genomics Era: the Future of Genetics in Medicine

St George's, University of London

Course highlights Get a taste of this course before you join:

  • Welcome to Week 1
    Welcome to Week 1
    video

    In this video, Lead Educator, Dr Kate Tatton-Brown welcomes learners to the course and explains the course aims and outcomes.

  • Did you know?
    Did you know?
    video

    Our resident scientist tells you his favourite genomics facts.

  • Responsibility in the genomic era
    Responsibility in the genomic era
    video

    In this tutorial, you will hear from Dr Carwyn Rhys Hooper on the concept of responsibility for health.