Numerical Chromosomal Abnormalities

Question 1

What is haploid?

Answer 1

one set of chromosomes (n=23) as in a normal gamete

Question 2

What is diploid?

Answer 2

cell contains two sets of chromosomes (2n=46, normal in human)

Question 3

What is polyploid?

Answer 3

Multiple of the haploid number

Question 4

What happens in recombination?

Answer 4

• Homologous chromosomes align
• Form a bivalent structure
• Exchange genetic material (recombine)

Question 5

How does meiotic non-disjunction occur?

Answer 5

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.

Question 6

What does an imbalance of chromosomes result in?

Answer 6

Imbalance of chromosomes results in trisomy or monosomy.

Question 7

What happens if non-disjunction occurs in mitosis?

Answer 7

• 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

Question 8

What is the definition of mosaicism?

Answer 8

The presence of two or more genetically different cell lines derived from a single zygote

Question 9

What happens to monosomic and trisomic cells?

Answer 9

• Monosomic cells die out, but all of trisomic cells continue through mitosis as normal.
• All daughter cells continue as 47 chromosomes

Question 10

Define recombination

Answer 10

• 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.

Question 11

What keeps the chromosomes together during recombination?

Answer 11

Connected by Synaptonemal complex – series of proteins keeping chromosomes together so enables them to exchange genetic material.

Question 12

What is crossing over?

Answer 12

• The same as Recombination and results in new recombinant chromosomes and new allele combinations.
• Generates genetic variation between generations.

Question 13

What does meiosis result in?

Answer 13

Daughter cells that are genetically unique

Question 14

Why is meiosis important?

Answer 14

• Meiosis leads to genetic variation – good thing
• When these processes go wrong, it can lead to disease – chromosomal abnormalities.

Question 15

What are autosomal aneuploidies?

Answer 15

Abnormalities that do not involve the sex chromosomes

Question 16

What are some examples of disorders caused by autosomal aneuploidies?

Answer 16

• Patau’s
• Edward’s
• Down’s
• Others happen but do not carry to term - can occur in any chromosome

Question 17

What causes pataus syndrome and how common is it?

Answer 17

• Trisomy 13
• 2 in 10,000 births

Question 18

What causes edwards syndrome and how common is it??

Answer 18

• Trisomy 18
• 3 in 10,000

Question 19

What causes downs and how common is it?

Answer 19

• Trisomy 21
• 15 in 10,000

Question 20

What are sex chromosome aneuploidies?

Answer 20

• 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.

Question 21

What is aneuploid?

Answer 21

Chromosome number which is not an exact multiple of haploid number due to an extra or missing chromosome

Question 22

What are some examples of sex chromosome aneuploidies?

Answer 22

• 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)

Question 23

What causes Turner’s and how common is it?

Answer 23

• Females with only one X chromosome (45 chromosomes instead of 46)
• 1 in 5000 female births

Question 24

What causes Triple X syndrome and how common is it?

Answer 24

• Females with 3 X chromosome instead of 2 (47 chromosomes instead of 46)
• 1 in 1000 female births

Question 25

What causes Klinefelter’s and how common is it?

Answer 25

• Males with extra X chromosome (XXY) (47 chromosomes instead of 46)
• 1 in 1000 female births

Question 26

What is a bivalent structure?

Answer 26

Two chromosomes on top of each other

Question 27

What is the bivalent structure joined by?

Answer 27

Synpatonemal complex (protein structure)

Question 28

What is a non-recombinant chromosome?

Answer 28

The one thats the same as the parent strand

Question 29

What are the most common aneuploidies?

Answer 29

13, 18 and 21

Question 30

When are sex chromosome aneuploidies diagnosed?

Answer 30

At puberty or when they cause fertility problems

Question 31

What is mitotic non-disjunction?

Answer 31

When the majority of cells are 2n but some are 2n+1

Question 32

Why are there no 2n-1 cells when mitotic non-disjunction takes place?

Answer 32

They die out bc they dont have the full set of chromosomes

Question 33

What is mosaicism?

Answer 33

The presence of two or more genetically different cell lines derived from a single zygote

Question 34

What are the two ways you can get mosaicism?

Answer 34

Starting from normal cell that non-disjunction happens in

Or

Start with a trisomic cell → trisomic rescue

Question 35

What happens in trisomic rescue?

Answer 35

Cell recognises that it has the wrong number of chromosomes and throws one out at random

Question 36

What is partial trisomy and monosomy compared to full?

Answer 36

• Partial monosomy/trisomy (microdeletion/duplication syndromes) far more common – mechanism different.

Question 37

How does full monosomy arise?

Answer 37

Nondisjunction