Chromosome Abnormalities

Question 1

Describe the process of GBanding staining and what it shows.

Answer 1

Metaphase chromosomes are exposed to trypsin which digests proteins, and then stained with a romanwaski type dye, the dark bands are known as G+ve bands, which are AT rich and therefore gene poor, the light bands are known as G-ve bands which are GC rich and therefore gene rich

Question 2

Give some reasons for performing cytogenetic analysis.

Answer 2

For prenatal diagnosis, to assess future reproductive risks, to aloow for better clinical management, and for accurate diagnosis of clinical problems (eg accounting for a phenotype)

Question 3

What are the two methods of obtaining chromosomes for a prenatal diagnosis?

Answer 3

Chronic villus sampling, which can be performed at 11-12 weeks with a 1.2% risk of miscarriage, and aminocentris, which can be performed after 15 weeks with a 0.8% miscarriage risk.

Question 4

What is an aneuploidy?

Answer 4

Loss or gain of whole chromosomes

Question 5

What are the possible causes of aneuploidy?

Answer 5

It can originate from non-disjunction at a meiotic division, or a mitotic division (which leads to mosaicism) forming one gamete with a missing chromosome and one with an extra chromosome.

Question 6

What is the most common cause of polyploidy?

Answer 6

Polyspermy (when more than one eggs fertilise a sperm)

Question 7

Give three examples of trisomys

Answer 7

Down Syndrome (trisomy 21) patau syndrome (trisomy 13) and edwards syndrome (trisomy 18)

Question 8

What is mosacism?

Answer 8

Presence of two or more cell lines in the body

Question 9

What is uniparental disomy?

Answer 9

The presence of homologus chromosomes from one parent

Question 10

What is karotyping?

Answer 10

Metaphase chromosomes that are sorted, paired up and grouped together, a systematic sorting of chromosomes that can be used to detect abnormalities.

Question 11

What is the affect of non-disjunction during meiosis I?

Answer 11

Two trisomys and two monosomy cells

Question 12

What is the affect of non-disjunction during meiosis II?

Answer 12

1 trisomy, 1 monosomy and 2 normal cells

Question 13

What is the problem in turner syndrome?

Answer 13

Monosomy X chromosome

Question 14

Why is a monosomy of the X chromosome a problem?

Answer 14

As there are two psuedoautosomal regions in the X and y chromosome, which contain the genes such as the SHOX gene that is associated with short stature.

Question 15

What are the different origins of Uni Parental Disomy?

Answer 15

Trisomy rescue, monosomy rescue, mitotic error, gamete complementarity.

Question 16

What factor determines moscaism?

Answer 16

Non dyjunction in mitosis, if it happens at first mitotic division there will be a non-mosaic karyotype, if it occurs after this there will be a mosaic karyotype.

Question 17

What is non dysjunction and what might cause it?

Answer 17

A different number of chromosomes being left in a cell in either mitotic or meiotic cell division, may be caused by anaphase lag, where chromosomes are left behind in cell division possibility due to defects in spindle function and attachment to chromosomes.

Question 18

What is a reciprocal translocation ?

Answer 18

Two breaks in chromosomes, and the different parts on chromosomes swap places.

Question 19

What are the different segregation types in reciprocal translocations?

Answer 19

3:1 non disjunction, 4:0 non-disjunction, alternate and adjacent 1 and adjacent 2.

Question 20

What is a robertsoian translocation?

Answer 20

Where two acrocentric chromosomes (13,14,15,21,22) fuse together.

Question 21

What are the risks associated with robertsonian translocation?

Answer 21

Aneuploidy risk

Question 22

What are the two types of chromosome deletions?

Answer 22

Terminal, at the ends of chromosomes, and instial within the body of chromosomes