Human Chromosomes & Abnormalities

Question 1

What is the chromosome number in a human somatic cell?

Answer 1

46

Question 2

How are the chromosomes ordered? What is the exception?

Answer 2

Ordered by size/length

21 is actually smaller than 22

Question 3

What are homologous chromosomes?

Answer 3

Chromosomes with the same genes in the same order

Question 4

What are the G bands when looking at a karyotype?

Answer 4

Regions of chromosomes which take up Giemsa stain

Question 5

What are telomeres?

Answer 5

Tandem repeats of short, G-rich sequences

Question 6

What the the p and q arms?

Answer 6

Centromere divides chromosome into two parts

Shorter segment is p arm

Longer segment is q arm

Question 7

Describe rDNA.

Answer 7

DNA encoding rRNA on 5 chromosome pairs only

Does not take up Giemsa stain

Found on P arm

Question 8

In what type of DNA is the rDNA found?

Answer 8

Non-centromeric heterochromatin

Question 9

What does 9q34.1 represent?

Answer 9

Chromosome 9, q arm

Region 3, band 4, sub band 1

ABL1 oncogene associated with chronic myeloid leukaemia

Question 10

What gene is found at Xp21?

Answer 10

Duchenne muscular dystrophy gene

Question 11

What gene is found from 7q22.3 - 7q23.1?

Answer 11

Cystic fibrosis gene

Question 12

What is FISH?

Answer 12

Fluorescence in situ hybridisation

Specific cloned DNA sequences tagged with fluorescent chemicals are hybridised to chromosome’s DNA sequence

Question 13

What can chromosome painting be used for?

Answer 13

Detect structural abnormalities in chromosomes

Question 14

What determines the sex of an organism?

Answer 14

Presence/absence of Y chromosome

Question 15

How does the Y chromosome cause an organism to be male?

Answer 15

SRY gene encodes testes determining factor which initiates testes formation

Question 16

What is testosterone?

Answer 16

Androgenic steroid hormone

Question 17

What do Leydig cells of testis do?

Answer 17

Secrete testosterone in the presence of LH which activates male-specific gene expression

Question 18

Why are adult females called genetic mosaics?

Answer 18

One randomly-chosen X chromosome is inactivated in each cell of the blastocyst

Persists through cell division

Question 19

How does the inactivated X chromosome appear histologically?

Answer 19

As a ‘Barr body’

Dark circle at the periphery of the nucleus

Question 20

In what percentage of spontaneous abortions have chromosomal abnormalities?

Answer 20

50%

Question 21

What is polyploidy?

Answer 21

Multiple full sets of chromosomes

Question 22

What is triploidy?

Answer 22

Three full sets of chromosomes (3n)

Question 23

Is triploidy survivable?

Answer 23

No, all die within 3 months

Question 24

What is tetraploidy?

Answer 24

Four full sets of chromosomes (4n)

Question 25

Is tetraploidy survivable?

Answer 25

No, very rare live births and short-lived

Question 26

What is aneuploidy?

Answer 26

Abnormal number of chromosomes

Question 27

Why does aneuploidy occur?

Answer 27

Non-disjunction during meiosis anaphase

Question 28

Why do chromosomal abnormalities result in developmental issues?

Answer 28

Abnormal gene dosage

Question 29

What is nullisomy?

Answer 29

Loss of one homologous pair

Question 30

What is monosomy?

Answer 30

Loss of a single chromosome

Question 31

Give an example of monosomy.

Answer 31

Turner syndrome

Question 32

What is trisomy?

Answer 32

One extra chromosome

Question 33

Give two examples of sex aneuploidy.

Answer 33

Turner syndrome Klinefelter syndrome

Question 34

What is the genotype which results in Turner syndrome?

Answer 34

XO

Question 35

Describe Klinefelter syndrome.

Answer 35

Males with extra X chromosome Common cause of male infertility Tall stature, learning difficulties, underdeveloped testes May have breast development

Question 36

Describe Turner syndrome.

Answer 36

Females with one X chromosome >90% of cases spontaneously aborted Do not develop secondary sexual characteristics Mainly infertile

Question 37

Which three trisomies can survive birth?

Answer 37

Down syndrome 21 Edwards syndrome 18 Patau syndrome 13

Question 38

Describe Down syndrome.

Answer 38

Trisomy of chromosome 21 Many developmental abnormalities, low IQ Premature ageing As maternal age increases, incidence increases

Question 39

Why do those with Down syndrome live longer than other trisomies?

Answer 39

Chromosome 21 is the shortest so less abnormal gene dosage

Question 40

Why are numerical sex chromosome abnormalities not always fatal?

Answer 40

Lyonisation Few genes on Y chromosome

Question 41

Why are there abnormalities in sex aneuploidies if lyonisation occurs?

Answer 41

Pseudoautosomal region on X chromosome is not included in lyonisation Gives abnormal gene dosage

Question 42

What causes chromosome structural abnormalities?

Answer 42

Breaks in chromosomes

Question 43

What can induce breaks in chromosomes? (5)

Answer 43

Radiation Viruses Chemicals Transposable elements Errors in crossing over

Question 44

Give an example of a deletion abnormality.

Answer 44

Cri-du-chat syndrome

Question 45

What is Cri-du-chat syndrome?

Answer 45

5p- (part of p arm deleted on chromosome 5) Mental retardation Physical abnormalities result in larynx defects

Question 46

What are the two types of inversions?

Answer 46

Pericentric Paracentric

Question 47

What is a pericentric inversion?

Answer 47

Inversion of part of chromosome containing centromere

Question 48

What is a paracentric inversion?

Answer 48

Inversion on one chromosome arm

Question 49

When do inversions cause problems?

Answer 49

Duplication and deletion can occur during meiosis Children will have severe chromosomal abnormalities

Question 50

Describe looping in meiosis.

Answer 50

Regions of chromosomes must line up with homologous region in pair Inversions cause looping of chromosome and chromosomes join together instead of swapping material Spindle pulls and breaks chromosomes - deletion of acentric fragments

Question 51

What is a translocation?

Answer 51

Change in position of chromosome segments

Question 52

What are the two types of interchromosomal translocations?

Answer 52

Reciprocal Non-reciprocal

Question 53

What is the difference between inter- and intra- chromosomal translocations?

Answer 53

Inter = between different homologous chromosomes Intra = within a chromosome

Question 54

Why are interchromosomal translocations bad (meiosis)?

Answer 54

Heterozygote with interchromosomal translocation Segregation leads to genetically imbalanced gametes Children have severe abnormalities