Lecture 17: Large Scale Chromosomal Changes (Part 1) Flashcards
(50 cards)
1
Q
two main themes underlying the observations on chromosomal changes
A
- karyotypes generally remain constant within a species
- most genetic imbalances result in a selective disadvantage - related species usually have different karyotypes
- closely-related species differ by only a few rearrangements
- distantly-related species differ by many rearrangements
- correlation between karyotypic rearrangements and speciation
2
Q
x =
A
number of different/unique chromosomes that make up a single complete set
eg 23 in humans
3
Q
n =
A
number of chromosomes in a gamete
eg 23 in humans
4
Q
relation between n and x in diploids
A
n = x
- each gamete contains a single complete set of chromosomes
5
Q
relation between n and x in hexaploids
A
n = 3x
6
Q
euploidy
A
A condition where a cell has a complete set(s) of chromosomes.
7
Q
aneuploidy
A
A condition where a cell has an abnormal number of individual chromosomes, not involving whole sets.
8
Q
give 4 examples of euploid
A
monoploidy (x)
diploidy (2x)
triploidy (3x)
tetraploidy (4x)
9
Q
using diploid species as a basis, explain:
- euploidy
- nullisomy
- monosomy
- trisomy
A
euploidy: 2n
nullisomy: 2n-2
monosomy: 2n-1
trisomy: 2n+1
10
Q
monoploidy
A
- male bees, wasps, and ants
- they undergo parthenogenesis
- usually lethal in other organisms
11
Q
parthenogenesis
A
development of unfertilised egg into an embryo (with no fertilisation):
- single set of chromosomes
- produce gametes by mitosis
12
Q
why is monoploidy lethal in most organisms?
A
- unmasks recessive lethal alleles (eg X-linked diseases)
- if an individual survives to adulthood, this most likely leads to sterility
13
Q
uses of monoploid plants
A
- visualize recessive traits directly
- introduction of mutations
14
Q
how can we create a monoploid plant?
A
- take a diploid plant
- haploid pollen grains are treated and plated onto agar
- growth of haploid embryoids
- embryoids treated with plant hormones
- monoploid plant (usually sterile)
15
Q
what is the issue with having a monoploid plant?
A
- in meiosis, chromosomes are supposed to pair up and separate.
- in monoploids, there’s nothing to pair with, so meiosis is disrupted, leading to infertile gametes.
16
Q
how is colchicine useful to scientists?
A
- colchicine inhibits the formation of the mitotic spindle
- thus the plant cells become diploid and meiosis is able to occur normally
17
Q
in what organisms is polyploidy particularly common?
A
in plants
18
Q
tetraploidy in plants
A
alfalfa, coffee, peanuts, large apples, pears, grapes
19
Q
octoploidy in plants
A
large strawberries
20
Q
results of polyploidy in plants
A
- associated with origin of new species
- may positively correlate with size and vigor
21
Q
two types of polyploids
A
- autopolyploids
- allopolyploids
22
Q
autopolyploids
A
- originate within a species
- all polyploids with an odd number of chromosome sets are sterile because they cannot produce balanced gametes, producing aneuploid gametes
23
Q
when can autopolyploids with an odd number of chromosome sets produce balanced gametes?
A
- if x is small
- balanced gametes are only produced if two copies of each chromosome always segregate to the same daughter cell and the third to the other
24
Q
generation of autotetraploids
A
- when the 2x genome of a diploid is doubled to 4x, with all four sets coming from the same species
- could be spontaneous or induced by a drug such as colchicine
- often the source of a new species
25
how are balanced gametes generated in autotetraploids?
- each chromosome has three homologs to choose from
- in order to form balanced gametes, the four copies of each group of homologues must form two bivalents
- successful tetraploids produce balanced 2x gametes and are fertile
26
allopolyploids
- hybrid of two or more closely-related species
- partially homologous chromosomes
27
amphidiploid
doubled diploid: contains two different diploid genomes
28
amphipolyploids in agriculture
F1 hybrid of wheat and rye (triticale) is sterile because there are no pairing partners for the rye chromosomes
29
describe the different triticale hybrids that have been generated
- some combine high yield of wheat with ability of rye to grow in unfavourable environments
- some combine high level of protein from what with high level of lysine from rye
30
4 examples of aneuploidy in sex chromosomes
XXY, XXX, XO, XYY
31
draw diagrams for nondisjunction during the first and second meiotic division, as well as after fertilisation by a normal gamete
32
what are the potential consequences of mitotic nondisjunction?
can result in a mosaic - am individual has two or more populations of genetically different cells in their body.
33
gynandromorph
an organism that has both male and female physical characteristics — often with distinct regions of male and female tissues
34
two types of mitotic nondisjunction
1. mitotic nondisjunction
2. mitotic chromosome loss
35
why do fertile aneuploids generate aneuploid progeny?
offspring of fertile aneuploids have an extremely high chance of aneuploidy because of production of unbalanced gametes
36
aneuploidy in the human population
- incidence of abnormal phenotypes caused by aberrant chromosome organisation or number is 0.4%
- half of spontaneously aborted foetuses have chromosome abnormalities
- incidence of abnormal phenotypes caused by single-gene mutations is 0.01%
37
monosomy in humans
2n-1; usually lethal in utero in humans, but there are a few exceptions:
- Monosomy 21: born with severe multiple abnormalities but die shortly after birth
- Turner Syndrome (XO): 99% of affected foetuses are not born. Those who are born have developmental abnormalities
38
defining characteristics of Turner Syndrome
short stature, sterile due to rudimentary ovaries and lack of menstruation
39
X inactivation occurs in XX individuals, so why are there abnormalities in XO individuals?
- embryogenesis: X inactivation occurs after several rounds of cell division, so it is thought that the 2nd X chromosome has important function within the first hundred divisions
- some of the genes on the 'inactivated' X chromosome are expressed
- germ-line: X chromosome reactivation
40
trisomy in humans
2n+1: often lethal in animals owing to chromosome imbalance
41
trisomy 21
Down syndrome
- females can be fertile
- males infertile
- average life expectancy is 40-60 years due to congenital heart disease
42
trisomy 18
Edward syndrome
- severe physical and mental abnormalities
- heart defects, growth retardation, small jaw, kidney abnormalities, narrow pelvis, rocker bottom feet, clenched fists
- average life expectancy of a few weeks
- 1/6000 to 1/10000 live births
43
trisomy 13
patau syndrome
- severe physical and mental abnormalities
- major abnormalities of heart kidneys, brain, face and limbs, small or absent eyes, harelip, small malformed head
- average life expectancy of 130 days
- 1/12500 to 1/21700 live births
44
three types of sex chromosome trisomies
XYY
XXX
XXY (Klinefelter syndrome)
45
Klinefelter syndrome
1/1000 male births
- humans tolerate X chromosome aneuploidy because of X inactivation
- sterile
46
why can aneuploidy for X have phenotypic consequences?
some X-linked genes escape inactivation — especially those in the pseudoautosomal regions (PARs). One key gene, SHOX, is essential for bone growth. Loss (like in Turner syndrome) or gain (like in XXX or XXY) of SHOX copy number leads to short or tall stature, respectively.
47
prenatal diagnostic testing
- look for abnormal karyotypes
- possible to screen for biochemical and genetic disorders
- tests are done in combination with blood tests for certain maternal and fetal proteins, and with ultrasound tests
48
prenatal testing - screening tests
first trimester screening test (11 to 13 weeks) ultrasound (nuchal translucency) and maternal blood test [pregnancy associated plasma protein-A (PAPP-A) and β-human chorionic gonadotrophin (β-hCG)]
49
prenatal testing - diagnostic tests
- chorionic Villi Sampling, CVS (10 to 13 weeks)
- amniocentesis (16+ weeks), less invasive
50
which comes first - screening tests or diagnostic tests
screening tests - if there are abnormalities, we then conduct diagnostic tests.
