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I'm Dr. Simon Holden.
I'm a consultant in clinical genetics in
the Department of Clinical Genetics at Addenbrooke's Hospital in Cambridge,
and I'm also an associate lecturer at the University of Cambridge.
This presentation is the second part of the talk on chromosome disorders.
The aims are: that students should be
familiar with the different types of chromosome disorders,
understand how they relate to other genomic disorders,
be familiar with the nomenclature used to
describe the more common chromosome rearrangements,
understand how the behavior of chromosome rearrangements at
meiosis dictates their inheritance and their recurrence risks,
and be able to describe a chromosome rearrangement
and its clinical implications to a patient.
So, this is just a revision slide showing how common
the different types of chromosome rearrangements and imbalances are.
This slide, again, is to orientate us; it shows a G banded male karyotype.
We will now look at numerical chromosome abnormalities that can take several forms.
First, let's consider polyploidy.
This is the presence of one or more additional haploid chromosome sets.
The most common form of polyploidy is triploidy.
Here, we have three sets of the normal haploid chromosome number,
which is represented as the letter n. So, this can give rise to karyotypes such as 69, XYY.
Triploidy is the most likely numerical chromosome abnormalities
to be encountered in the first trimester of pregnancy.
It occurs when a double chromosome content,
so two haploid cells or 2n,
comes from one parent.
Triploidy affects one to three percent of recognized conceptions and
most triploid pregnancies are lost as first-trimester miscarriage or very occasionally,
the second-trimester fetal death in the womb.