Sex Linkage + Chromosome Theory

Question 1

Proof of the chromosome theory

Answer 1

Reciprocal crosses in drosophila
Gave different results if male or female

P1: Red female x White male
F1: All red
F2: Females - all red, Males - 1/2 red 1/2 white

P1: White female x Red male
F1: Females - all red, Males - all white
F2: Both - 1/2 red 1/2 white

Locus for eye colour carried on X chromosome
Evidence that genes are borne on chromosomes

Question 2

Unusual eye colour cross in drosophila

Answer 2

P1: White female x Red male
F1: Females - all white, Males - all red
Due to female anueploids, error in cell division → 2 or 0 X chromosomes
XXX and Y0 die
X0 - red male
XXY - white female

Question 3

Sex-linked inheritance

Answer 3

Gene carried on sex chromosome
Generally recessive - carried in females, shown in males
Males only have one X so recessive alleles show

Question 4

Examples of sex-linked inheritance

Answer 4

Drosophila eye colour
Colour blindness
Lesch-Nyan syndrome - eats own flesh
Haemophilia - factor VIII deficiency, no blood clotting, fatal in women from menstruation

Question 5

X chromosome inactivation

Answer 5

Lyonisation - Mary Lyon
Equal dose of X-linked alleles in male and female
Females randomly inactivate one X in each cell
Inactivated chromosome can be viewed under microscope - Barr Body

Question 6

Examples of X chromosome inactivation

Answer 6

Tortoiseshell cat - only female, cells randomly have black or orange colour gene turned on
Humans - females who are het. for colour blindness can have patches of normal and colour blind cells in retina
Duchenne Muscular Dystrophy - male condition, muscles waste away, case of 2 identical twins girls, one had it and one didn’t due to X chromosome inactivation in early development

Question 7

Chromosome arrangements

Answer 7

Patau syndrome - associated with trisomy 13
Downsyndrome - extra chromosome 21
Chromosome 1 in humans due to 2 fused primate chromosomes

Question 8

Centromeres

Answer 8

Where chromosome attaches to spindle in cell division
Separates chromosomes into short P arm and long Q arm
Metacentric chromosomes - centromere at centre
Acrocentric chromosomes - centromere close to one end
Telocentric chromosomes - centromere very near one end

Question 9

Telomeres

Answer 9

Repeated sequence at ends of each chromosome

Question 10

Polytene chromosomes

Answer 10

Oversized chromosomes found in drosophila salivary glands

Question 11

Chromosome mapping methods

Answer 11

Somatic Cell Hybridisation

Fluorescent in situ hybridisation (FISH)

Question 12

Somatic Cell Hybridisation

Answer 12

Fuse human and mouse cells using sendai virus, virus can stick to both cells and when it tries to enter they fuse + hybridise
Sort hybrid cells using poison that needs both human and mouse enzymes to break it down

Question 13

Fluorescent in situ hybridisation (FISH)

Answer 13

Localise specific DNA sequences on chromosomes, fluorescent probes only binds to the parts of the chromosome where they show high complementarity
Use fluorescence microscopy to find where the probe has bound

Question 14

Deletion

Answer 14

Section of chromosome removed

Question 15

Examples of deletion

Answer 15

Notch wing in Drosophila
Cri-du-chat in humans - tip of chromosome 5 lost
Deletion mapping used to map Cystic Fibrosis

Question 16

Duplication

Answer 16

Section of chromosome is doubled up

Can cause mispairings when chiasma forms

Question 17

Examples of duplication

Answer 17

Globin chromosomes 11, 16, 22 = myoglobin higher affinity for oxygen
Huntington’s disease
Cross bar eyed drosophila → mix of normal and ‘double bar’
Visible bands in polytene chromosomes show bar has duplicated segment, double bar has extra copy of duplication, normal has lost duplication and returned to wild type

Question 18

Inversion

Answer 18

Section of chromosome rotated and replaced
Must make a loop
Gives dicentric and acentric fragments
Dicentric breaks and acentric lost at cell division
Inversions are crossover suppressors

Question 19

Translocation

Answer 19

Two non-homologous chromosomes exchange parts
Can be lethal
Can have similar effects as having an extra chromosome

Question 20

Examples of translocation

Answer 20

Used to control tsetse and mosquitos in Africa, zapped with x-rays so offspring are sterile
Associated with cancers eg. Chronic Myeloid Leukaemia (9-22 translocation) and Burkitt’s lymphoma (8-14)

Question 21

Anueploidy

Answer 21

Individuals with chromosome number different from normal

Question 22

Nondisjunction

Answer 22

Error in cell division, homologous chromosomes fail to separate → trisomy + monosomy

Question 23

Examples of anueploidy/nondisjunction

Answer 23

Klinefelter’s syndrome: XXY - sterile male with female characteristics; XYY - male; XXX - female
Turner’s syndrome: X0 - female

Question 24

Somatic anueploidy

Answer 24

Drosophila gyandromorphs - red eyed side female, white eyed side male, due to XX nondisjunction in early development

Question 25

Polyploidisation

Answer 25

Changes in no. of chromosome sets, usually lethal in humans, odd no. chromosomes associated with infertility - can’t pair in meiosis
Haploidy sex determination in bees + wasps - unfertilised (n) = male drone, fertilised (2n) = female worker sterile

Question 26

Allopolyploid

Answer 26

Union of 2+ distinct chromosome sets, doubling of chromosome number

Question 27

Autopolyploid

Answer 27

duplication of one diploid chromosome set

Question 28

Triticale

Answer 28

allopolyploid of emmer wheat and rye