Human Chromosomes and chromosomal abnormalties

Question 1

What are somatic chromosomes?

Answer 1

Contain two copies of each chromosomes (homologous pairs), except the sex chromosomes.

Question 2

How many chromosomes are there in human cells?

How many in a gamete?

Answer 2

46

23

Question 3

What is the karyotype?

Answer 3

A full set of chromosomes in a cell.

Question 4

How many chromosomes are there on each chromosome type?

Answer 4

2 (one maternal and one paternal)

Same genes in the same order on the homologous genes in each pair.

Question 5

How is the order of chromosomes determined?

Except for what?

Answer 5

Size of the chromosome

Except for chromosome 21 which is smaller than 22.

Question 6

Chromosome anatomy:
G band?
Telomeres?
rDNA?
Centromere?

Answer 6

G band = region that takes up Giema stain, giving characteristic pattern (dark band takes up lots of stain).

Telomeres = specialised repeated DNA sequence, protect ends of chromosomes.

rDNA = DNA encoding rRNA.

Centromere = where the spindle attaches to the chromosome.

Question 7

Which chromosome arm size is smaller and which is larger?

Answer 7

p is smaller and q is larger.

Question 8

What are telomeres and why are they kept this length?

Answer 8

Every time we replicate our chromosome, it gets a bit shorter so we have repeated sequences of DNA telomeres.

This means we can lose this DNA and it doesnt matter.

Question 9

What is non-centromeric heterochromatin?

Answer 9

Non-coding genes present here.

Question 10

Give information on fluorescence in situ hyrbidisation

What can chromosome painting detect?

Answer 10

• Specific cloned DNA sequences tagged with fluorescent chemicals are hybridisation (base-pairee) to the chromosome DNA sequence
• Chromosomes viewed under fluorescence microscope: image computer enhanced

Can detect small structural abnormalities in the chromosomes. This technique depends on FISH.

Question 11

What is an inversion on a chromosome?

Answer 11

Inversion abnormality - piece of the chromosome has come out and flipped over and attached back into the chromosome the wrong way around.

Question 12

Which sex chromosome carries fewer genes?

Answer 12

Y chromosome

Question 13

Which gene encodes for testis determining factor?

Answer 13

SRY gene

Question 14

What do Leydig cells of testis secrete?

Answer 14

They secrete testosterone, an androgenic steroid hormone, which activates male-specific gene expression.

Question 15

Explain chromosome X inactivation

Explain how this persists

Explain what a Barr Body is

Answer 15

One X chromosome means that it will get permanently unactivated.
This inactivation choice is random.
All descendants of these cells will have the same X chromosome inactivated.

X inactivation persists through cell division (so the adult female is a genetic mosaic).

Barr body - the inactive X chromosome once it has condensed.

Question 16

Explain polyploidy, triploidy, tetraploidy?

Answer 16

Polyploidy - multiple sets of chromosomes in humans

Triploidy - 3n

Tetraploidy - 4n

Question 17

What is aneuploidy?

What does this arise due to? What is this?

Answer 17

Having one extra or one less number of chromosomes.

Failure of homologous chromosomes to separate at anaphase. This occurs by chance. Cells have double the number of chromosomes.
One cell ends up with two copies and one ends up with no copies when disjunction happens and at second division.

Question 18

Give the 4 subtopics under aneuploidy

Answer 18

• Gene dosage: important for normal development, so aneuploidy causes abnormalities.
• Nullisomy (2n-2). Loss of one homologous pair of chromosomes. No viable human examples
• Monosomy (2n-1). Loss of a single chromosome. One viable human example (Turner syndrome)
• Trisomy (2n+1). One extra chromosome. Several viable human syndromes.

Question 19

What is Klinefelter syndrome?

Answer 19

Extra sex chromosome (2 X and 1 Y).
Male.
Common cause of male fertility.
Tall stature, some learning difficulties, underdeveloped testes, breast development.
Testosterone can improve secondary sexual characteristics.

Question 20

What is Turner syndrome?

Answer 20

X and no other sex chromosome.
Female.
Few noticeable defects until puberty: do not develop secondary sexual characteristics.
Mainly infertile.

Question 21

Explain down syndrome

Answer 21

All autosomal monosomies and most autosomal trisomies can survive birth.

Trisomy chromosome 21
Many developmental abnormalities, low IQ
Only autosomal trisomy to survive to adulthood, but life expectancy low.

Question 22

Why is trisomy 21 the only human autosomal trisomy to survive to adulthood?

Answer 22

This is the shortest autosome.

Extra copy of 21 - not as many genes as if you had an extra number of chromosome 1 meaning that survival is more likely.

Question 23

Why are numerical sex chromosome abnormalities better tolerated than numerical autosomal abnormalities?

Answer 23

Few genes on Y chromosome. All but one X chromosome inactivated. Not as many genes on Y so not missing as many chromosomes if it is not there.

Question 24

So why do Turner syndrome and Klinefelter syndrome individuals have any abnormalities?

Answer 24

Pseudoautosomal region on X chromosome is not included in X inactivation: gene dosage of genes in this region is abnormal.

Question 25

What do structural chromosomal abnormalities arise from?

What are they induced by?

Answer 25

Arises from breaks in chromosomes.

Induced by radiation, viruses, chemicals, transposable elements, errors in crossing over.

Question 26

Inversions:
What is pericentric?
What is paracentric?

Why may it not be a problem?

Answer 26

Pericentric - inversion includes centromere

Paracentric - one chromosome arm

No loss of material, but breaks may occur within genes or regulatory regions.

Question 27

In heterozygous, duplication and deletions can arise when —- takes place within an inversion during —-.

What does this mean for individuals with inversions?

Answer 27

Recombination

Meiosis

Means that the individual may be normal but may have children with severe chromosomal abnormalities.

Question 28

What is a translocation and what are the three types?

Answer 28

Translocation - change in position of chromosome segment.

Reciprocal and non-reciprocal, intrachromosomal.
Reciprocal is swapping bits between chromosomes, non-reciprocal is moving bits from one chromosome into another and intrachromosomal is a translocation in one chromosome.

Question 29

In translocation, heterozygous segregation at meiosis can lead to what …?

Answer 29

Can lead to genetically unbalanced gametes.