Sex Linkage And Chromosome Rearrangements Flashcards Preview

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Flashcards in Sex Linkage And Chromosome Rearrangements Deck (43):
1

Which aneuploid sex chromosome combinations are viable in drosophila and humans?

Drosophila- XXY (female) XO (male)
Humans- XO XXX (female), XXY (male)

2

What are the long and short arms of a chromosome called?

Short P
Long Q

3

What are metacentric, acrocentric and telocentric chromosomes?

Metacentric- centromere at centre
Acrocentric- closer to one end
Telocentric- near the very end

4

Where does proof of the chromosome theory come from?

Thomas Hunt Morgan made reciprocal crosses in Drosophila
-> Gave different result depending on whether Male/female carried the mutant allele
(X^W is recessive for white eyes)
X^WY mutant Male
X^WX^W mutant female

-> Pattern of inheritance follows that of X chromosome (physical object)

5

What was definite proof of the chromosome theory?

Aneuploid cross
-> females that produce only X^WX^W eggs or O eggs
-> Cross with XY

6

Define sex linkage and give examples

Genes carried on X chromosome. When recessive:
- Transmitted mainly through females
- Manifests mainly in males

• Colour blindness
• Lesch-Nyhan syndrome- fatal error in purine metabolism
• Haemophilia/factor VII deficiency

7

Outline Lesch-Nyhan syndrome

• Mutation in HPRT gene on X chromosome- sex linked
• HGPRT deficiency causes Utica acid build up in body fluids
• Fatal error in purine metabolism
• Leads to self mutilation and death
• Affects 1/380,000 live births

8

Outline haemophilia A

• X-linked recessive
• Deficiency in factor VIII, causes increased bleeding as blood can’t clot properly
• Mild haemophilia A can occur in heterozygote females due to XCI
• 1/5000 males

9

Define lyonisation and give examples

X-chromosome inactivation. Females randomly inactivate one X in each cell early in development- Barr body in cell nucleus

• Tortoiseshell cat- patches of orange and black fur
• Colour blindness in human females- patches of normal and colour blind cells in retina
• Duchenne muscular dystrophy- heterozygous female twins, one with, one without.

10

What determines sex in humans, insects and birds and butterflies?

Insects- number of Xs
Humans- presence of Y
Birds and butterflies- males are homogametic WW and females are heterogametic ZW

11

What is cytogenetics?

The study of chromosomes

12

Why do we study whole chromosomes?

1. Many human diseases are associated with whole chromosome changes
• Cancers often associated with chromosome arrangements
• Patau syndrome- Trisomy 13

2. Evolutionary genetics
• Chromosome 2 in humans is result of fusion of two primate chromosomes

13

Define centromere

Section where the chromosome attaches to the spindle during cell division

14

Define telomere

Repeated sequence at the end of each chromosome.
Protect ends of chromosomes- prevent from binding to others.

15

Define deletion and give an example

Section of chromosome is removed (deletion loop is formed)

• Notch wing in Drosophila- dominant X chromosome mutation. Homozygotes lethal
• Cri-du-chat in humans- tip of chromosome 5 is lost- causes pinched facial features, mental retardation, cat-like cry

16

Define duplication and give examples

Common. A section of chromosome is doubled up. Important force in evolution.
Tandem ABCBCD, reverse ABCCBD

• Globin gene families- alpha and beta groups on different chromosomes
• Odorant gene family

17

What is replication slippage? Give an example

• tandem duplication
• chiasma forms
• results in further duplication
Important force in mutation

Drosophila bar eye:
• Results from tandem duplication
• Cross two bar eye flies together and can result in normal and double bar eyed flies
• Due to chiasma forming and crossing over

Huntington’s disease:
• increase in number of CAG repeats

18

Define inversion

Rare. Section of chromosome rotated and reinserted. Often viable as no change in amount of material.
Prevents crossing over of inverted chromosomes with unconverted chromosomes as a dicentric and acentric (centromeres) fragment are formed
Dicentric breaks
Acentric lost at cell division

In heterozygotes for the inversion, abnormal gametes will be formed. Excess of some genes, deficient of others. Small-> viable gametes, abnormal child, large-> lethal

19

Give an example of inversion

Some drosophila species have many overlapping inversions all over their chromosomes- all multiple inversion heterozygotes.
-> No recombination at all. One fixed set of genes

20

Define translocation and give an example

Two non-homologous chromosomes exchange parts

• Associated with cancers- chronic myeloid leukaemia- 9-22 translocation, Burkitt’s lymphoma- 8-14 translocation

21

Outline Burkitt’s lymphoma

• Disease of lymphatic system
• extended lymph glands in the neck
• Translocation from c8 to c14
• Translocation of oncogene next to an active immune system gene- position effect variegation
• Lymphoma gene is more active than it should be

22

What has translocation been used for?

1. Releasing insects with translocations means it will become more and more common-> most of offspring will be heterozygous and will die
• Was used to wipe out tsetse flies in parts of Africa, responsible for transmitting sleeping sickness.

2. Used in mapping genes such as Testis-determining Factor (TDF/SDY). XX males have crucial part of Y translocated onto X so search narrowed down.

3. Suggested position effect variegation

23

Define position effect variegation and give an example

A gene’s effect can be modified by it’s position on a chromosome

Red eye/white eye Drosophila
Translocation of recessive allele to somewhere else= mixture of red and white eye phenotype

Translocation of oncogenes to somewhere on the genome near a more active promoter- spreads faster
• Burkitt’s lymphoma

24

What is a Giemsa stain?

G-banding- staining of condensed chromosomes to produce a visible karyotype

25

Outline how cancer can involve chromosomal changes

1. Deletions- loss of tumour suppressor genes
2. Duplications- doubling up of Porto-oncogenes- promote cell division
3. Translocations- activity of genes can be increased if they’re moved next to different genes. Eg, Burkitt’s lymphoma- proto-oncogene in chromosome 8 moved close to a gene that controls expression of antibodies in B-lymphocytes on c14-> increase expression

26

What is nondisjunction?

Error in cell division resulting in trisomy and monosomy

27

Outline Down’s syndrome

• Trisomy 21 causes 95%
• 88% maternal gamete nondisjunction, 8% paternal
• Can be caused by mosaicism- mitotic nondisjunction after conception
• Translocation of part of c21 onto c14- Down’s with only 46 c’s
• 1/800 births- increase with mothers age
• Low IQ(20-50), happy disposition, flattened face, short hands and feet, breathing difficulties, single palmar crease

29

Outline Turner’s syndrome

XO female
• Sterile- suggests 2X needed for normal development despite lyonisation
• High risk for heart disease
• Majority of cases spontaneously abort- rather than Klinefelters

30

What are XYY and XXX?

Supermale
• effects on impulse control

Superfemale
• no symptoms- X inactivated by lyonisation
• May lead to sterility and sometimes low IQ
• Up to 8 Xs have been found

31

Outline Edwards syndrome

Trisomy 18
• Organ abnormalities
• Die within a few weeks of birth

Trisomies of larger chromosomes never seen, die early in development. In spontaneous abortions incidence of trisomy is high 1/2.

32

What causes gynandromorphs? Give an example

Somatic aneuploidy
XX nondisjunction in early development
Eg, Drosophila heterozygous for sex-linked recessive white eye: Some have one red eye (XX female) one white (XO male)

33

What is haplodiploidy sex determination?

Ants, bees, wasps
Males are haploid n
Females are diploid 2n

34

Outline Klinefelter syndrome

XXY male
• Some female characteristics
• Low steroid levels
• 1/500-1000
• Up to 6X and 1Y found- still male

34

What is polyploidisation?

Changes in number of chromosome sets
Odd numbers > infertility

34

What is autopolyploidy?

Multiplication of the number of sets of chromosomes within a particular species

36

What is allopolyploidy?

Hybridisation of species and mixing of their chromosome sets

37

What ploidy is today’s wheat?

Hexaploidy 6n

38

Why are bananas sterile?

They’re triploid so have an odd number of chromosomes

39

What is triticale?

A recent allopolyploidy of emmer wheat and rye.
Grows in dry cold conditions, used as animal feed

40

How are seedless fruits made?

Adding colchicine to cells in culture medium increases frequency of polyploidisation

Triploids formed and are sterile

41

Describe polyploidy in humans

Invariably lethal
Occasionally babies born with mix of polyploid and diploid cells- chimaeras, but die very young

42

Give an example of polyploidy in cancer

Adenocarcinoma- nearly all cells have 4n
• Generate large and competitive cells which can simultaneously evolve resistance to several anti cancer drugs as they have extra copies of their genome
• Can see particularly in methotrexate resistance
• May be multiple copies of resistant chromosomes- why cancers return after “cure”

43

What is synteny?

Shared sections of chromosomes in different groups

Shows importance of chromosome reshuffling- structure and number. Eg, globin families

• over 90% of mouse and human chromosome constituents are in syntentic blocks. Eg, part of mouse 2 makes up whole of human 20. X very similar in both species.
• Last common ancestor 75mya