Numerical Chromosomal Abnormalities Flashcards
What is haploid?
one set of chromosomes (n=23) as in a normal gamete
What is diploid?
cell contains two sets of chromosomes (2n=46, normal in human)
What is polyploid?
Multiple of the haploid number
What happens in recombination?
- Homologous chromosomes align
- Form a bivalent structure
- Exchange genetic material (recombine)
How does meiotic non-disjunction occur?
When there is an error:
- In the segregation of the pairs of homologous chromosomes in meiosis 1 – both go into daughter cell
- Meiosis 2 where one pair separates correctly but 2 chromatids go into the same cell after meiosis 2.
What does an imbalance of chromosomes result in?
Imbalance of chromosomes results in trisomy or monosomy.
What happens if non-disjunction occurs in mitosis?
- If non-disjunction occurs in mitosis, then only a proportion of cells are affected and is called Mosaicism
- The individual has a mixture of cell types with respect to chromosomal complement.
- Majority of cells 2n, some 2n+1 = mosaic
What is the definition of mosaicism?
The presence of two or more genetically different cell lines derived from a single zygote
What happens to monosomic and trisomic cells?
- Monosomic cells die out, but all of trisomic cells continue through mitosis as normal.
- All daughter cells continue as 47 chromosomes
Define recombination
- The physical exchange of genetic material between homologous chromosomes and is the reason for why they align.
- Key difference between mitosis and meiosis.
- Occurs in metaphase 1.
What keeps the chromosomes together during recombination?
Connected by Synaptonemal complex – series of proteins keeping chromosomes together so enables them to exchange genetic material.
What is crossing over?
- The same as Recombination and results in new recombinant chromosomes and new allele combinations.
- Generates genetic variation between generations.
What does meiosis result in?
Daughter cells that are genetically unique
Why is meiosis important?
- Meiosis leads to genetic variation – good thing
- When these processes go wrong, it can lead to disease – chromosomal abnormalities.
What are autosomal aneuploidies?
Abnormalities that do not involve the sex chromosomes
What are some examples of disorders caused by autosomal aneuploidies?
- Patau’s
- Edward’s
- Down’s
- Others happen but do not carry to term - can occur in any chromosome
What causes pataus syndrome and how common is it?
- Trisomy 13
- 2 in 10,000 births
What causes edwards syndrome and how common is it??
- Trisomy 18
- 3 in 10,000
What causes downs and how common is it?
- Trisomy 21
- 15 in 10,000
What are sex chromosome aneuploidies?
- A group of conditions in which individuals have an abnormal number of sex chromosomes
- Sex chromosomal aneuploidies are not as severe as autosomal aneuploidies in terms of clinical impact.
What is aneuploid?
Chromosome number which is not an exact multiple of haploid number due to an extra or missing chromosome
What are some examples of sex chromosome aneuploidies?
- Turner’s (45, X) (1 in 5000 female births)
- Triple X syndrome (47, XXX) (1 in 1000 female births
- Klinefelter’s (47, XXY) (1 In 1000 male births)
What causes Turner’s and how common is it?
- Females with only one X chromosome (45 chromosomes instead of 46)
- 1 in 5000 female births
What causes Triple X syndrome and how common is it?
- Females with 3 X chromosome instead of 2 (47 chromosomes instead of 46)
- 1 in 1000 female births
What causes Klinefelter’s and how common is it?
- Males with extra X chromosome (XXY) (47 chromosomes instead of 46)
- 1 in 1000 female births
What is a bivalent structure?
Two chromosomes on top of each other
What is the bivalent structure joined by?
Synpatonemal complex (protein structure)
What is a non-recombinant chromosome?
The one thats the same as the parent strand
What are the most common aneuploidies?
13, 18 and 21
When are sex chromosome aneuploidies diagnosed?
At puberty or when they cause fertility problems
What is mitotic non-disjunction?
When the majority of cells are 2n but some are 2n+1
Why are there no 2n-1 cells when mitotic non-disjunction takes place?
They die out bc they dont have the full set of chromosomes
What is mosaicism?
The presence of two or more genetically different cell lines derived from a single zygote
What are the two ways you can get mosaicism?
Starting from normal cell that non-disjunction happens in
Or
Start with a trisomic cell → trisomic rescue
What happens in trisomic rescue?
Cell recognises that it has the wrong number of chromosomes and throws one out at random
What is partial trisomy and monosomy compared to full?
- Partial monosomy/trisomy (microdeletion/duplication syndromes) far more common – mechanism different.
How does full monosomy arise?
Nondisjunction
