M2 L9: chromosomal mutations

Question 1

chromosomal mutation

Answer 1

heritable changes at the chromosomal level (change in structure or number)

Question 2

euploidy

Answer 2

having an even multiple of N chromosomes (any number of full sets)

Question 3

aneuploidy

Answer 3

having an extra or one less copy of a chromosome (or multiple) - not an exact multiple of N

Question 4

nondisjunction

Answer 4

chromosomes not separating in meiosis I or II –> makes aneuploid gametes

Question 5

monosomy

Answer 5

when a diploid organism is missing one copy of a chromosome

Question 6

trisomy

Answer 6

when a diploid organism has an extra copy of a chromosome

Question 7

polyploidy

Answer 7

karyotypes that are even multiples of N (full sets) but more than 2N

Question 8

how do we get variation in chromosome number (aneuploidy)?

Answer 8

nondisjunction (or mitotic errors –> mosacis)

Question 9

how are the effects of nondisjunction different for meiosis I and II

Answer 9

nondisjunction in meiosis I –> 2 gametes have an extra chrom, 2 gametes missing one

in meiosis II –> 1 gamete has an extra chrom, 1 gamete missing one, 2 are normal

Question 10

most common monosomy karyotype

Answer 10

2N-1

Question 11

viable monosomy in humans and drosophila and why

Answer 11

turner syndrome (X): would only have one X active anyway (all others = barr bodies)

chromosome IV: very small, only 5% of genes, still smaller than normal

Question 12

why is monosomy usually lethal in animals

Answer 12

expression of recessive lethals that would normally be masked by dominant alleles

improper gene dosage and no way of compensation

Question 13

effect of monosomy in plants? exceptions?

Answer 13

tolerate in autosomes better than in animals but still develop smaller/less viable

pollen very sensitive to meiotic nondisjunction because normally haploid and require development before fertilization; if missing a chromosome –> can’t develop enough to fertilize egg

Question 14

typical trisomy karyotype

Answer 14

2N+1

Question 15

how is trisomy tolerated in plants and animals/yeast/humans

Answer 15

better than monosomy as long as the chromosome is small

still poorly tolerated in humans

plants tolerate it well

can be advantageous in yeast

Question 16

examples of advantageous trisomies in yeast? Why are they advantageous?

Answer 16

S cerevisiae: trisomy 3 –> better heat tolerance

candida albicans (trisomy 5) and cryptococcus (trisomy 1): better tolerance of azole drugs

all caused by increase in gene dosage for relevant genes

Question 17

only viable autosomal trisomy in humans? who discovered the syndrome? how common is it? phenotypes?

Answer 17

trisomy 21

down syndrome

langon down

1/800 live births

short stature, intellectual disability, heart malformations, decreased F fertility, M usually sterile

Question 18

Which parent contributes the extra chromosome for down syndrome/how do we know

Answer 18

The mother

Rate of trisomy 21 associated with maternal age but not paternal age

Question 19

what trisomies have been observed in humans? Which survive to term? Which are compatible with life?

Answer 19

All have been observed

5 survive to term: X, Y, 13, 18, 21

3 compitable with life: XXX, XYY, 21 (triplo X, jacobs, down)

Question 20

what trisomies are compatible with human life for usually only a few weeks?

Answer 20

13: patau syndrome
18: edwards syndrome

Question 21

Why are monosomes less common among miscarried fetuses compared to trisomies? What’s expected?

Answer 21

Should be obs in equal ratios bc for every gamete with an extra chromosome, there is a gamete with a missing chromosome

monosomy fetuses don’t even develop enough to be recovered, but trisomy fetuses do

Question 22

what’s autopolyploidy? is it more common in plants or animals?

Answer 22

adding full sets of N to organisms within a species

more common in plants, has happened in lizards, fish, amphibians

Question 23

what are the three ways to get a triploid (autopolyploidy, 2N –> 3N)

Answer 23

1) nondisjunction of all chrom in meiosis –> diploid gamete that unites with haploid gamete

2) two sperm and one egg

3) diploid mates with a tetraploid (diploid makes N gametes, tetraploid makes 2N gametes)

Question 24

how to make a tetraploid

Answer 24

one main way

duplication without division (zygote duplicates DNA but then doesn’t divide)

Question 25

impact of autotetraploidy? (making a tetraploid via autopolyploidy) why?

Answer 25

tetraploid mates w/ normal diploid and makes a triploid

meiosis in triploids fails

triploid is reproductively isolated –> instantaneous speciation

Question 26

why does meiosis fail in triploids

Answer 26

3rd pair of homologs separate in meiosis 1, but in order to make balanced gametes, one homolog (for every chromosome) must go to each daughter cell in meiosis I –> very unlikely so gametes usually aneuploid

Question 27

probability of making a haploid or diploid gamete? probability of making a haploid gamete? probability of making a diploid gamete? where does this come from?

Answer 27

(1/2)^(N-1)

1/2 x (1/2)^(N-1)

1/2 x (1/2)^(N-1)

extra homologs assort independently from each other (no way to make 3rd maternal and paternal homologs go to opposite daughters)

Question 28

how are polyploid plants produced and why? what are some examples

Answer 28

propagated asexually/cloning bc meiosis usually fails (same reason as in triploids)

also larger than diploids

bananas and winesap apples

Question 29

why are polyploid yeast cells larger than diploids

Answer 29

more than 2 copies of each chrom (being polyploid) decreases production of cyclins in G1 phase of cell cycle –> cell grows larger before division is triggered

Question 30

what’s allopolyploidy

Answer 30

combining full sets (N) from two different species

Question 31

are allopolyploidy hybrid offspring sterile? Why/why not?

Answer 31

Would expect them to be but can go through whole genome duplication –> all chrom from both species’ haploid sets have a homolog –> can do meiosis

Question 32

is allopolyploidy more common in plants or animals and why

Answer 32

more common in plants

hybridization between distantly related animal species is rare

Question 33

what’s the phenotype of allopolyploid hybrid offspring? common allopolyploid plants?

Answer 33

can have some traits from each parent or a completely new phenotype

gossypium hisutum (hybrid of old/new world cotton)

sporobolus anglicus (hybrid of 2 grasses, new phenotype)

Question 34

Types of mutations within individual chromosomes? What causes them?

Answer 34

duplications, deletions (terminal and intercalary), inversions (peri and paracentric), translocations (reciprocal, nonreciprocal, robertsonian)

all are from chromosomes repairing breaks

Question 35

what’s a duplication

Answer 35

doubling genetic material on a chrom

Question 36

what’s a deletion

Answer 36

losing genetic material on a chrom

terminal is from the end, intercalary is from the middle

Question 37

what are translocations

Answer 37

reciprocal: genetic info exchanged between non homologs (like crossing over with nonhomologous autosomes)

non reciprocal: genetic info transferred from one chrom to another

Question 38

what are inversions

Answer 38

gene order reversed for a segment of a chromosome

paracentric does not involve centromere, pericentric does

Question 39

what 2 things must be true for a chrom mutation to be heritable

Answer 39

must be in the germ line

must change the original composition or gene content of the chromosome

Question 40

how many breaks do deletions require? what part of the chromosome is kept? how do they synapse? what’s the effect on phenotype?

Answer 40

terminal = 1 break
intercalary = 2 breaks

keep part with centromere

homolog w/o deletion forms a deletion loop (compensation loop)

can lead to expression of recessive lethals or deleterious alleles

effect can change based on haplosufficiency/insuficiency

Question 41

human disorder caused by a deletion? where is the deletion? what is the phenotype? how rare is it?

Answer 41

crit du chat

terminal deletion on chromosome 5

size of deletion determines severity, usually intellectual disability, malformation of glottis and larynx –> cry sounds like a cat

1/25,000-50,000 live births

Question 42

how do chrom with duplications synapse? what are the 2 main ways to get duplications

Answer 42

chrom w/ duplications forms compensation loop

1) unequal crossing over in meiosis (loci not lined up)

2) replication error in mitosis

Question 43

what types of genes are normally present as duplications/many copies

Answer 43

rRNA and rDNA clusters (for making ribosomes bc we need lots of them)

Question 44

phenotypic consequence of duplications in yeast?

Answer 44

duplications in CUP1 gene –> make more proteins (sequester copper ions in the cell/prevent oxidative damage)

Question 45

how many breaks do inversions require? how do they synapse? how many viable chrom are produced after recombination?

Answer 45

2 breaks

inverted chrom forms inversion loop

paracentric doesn’t involve centromere –> 1 normal non recomb, 1 dicentric recomb with dup and del, 1 acentric recomb with dup and del, 1 inverted non recomb

pericentric does involve centromere –> 1 normal non recomb, 1 inverted nonrecomb, 1 recomb with dup and del, 1 recomb with dup and del

Question 46

how do inversions affect recombination?

Answer 46

don’t prevent recombination but make it so recombinants at those loci are not expressed in hets (they’re inviable)

Question 47

what are robertsonian translocations

Answer 47

small arms of two nonhomologous chromosomes are lost, long arms fuse and form one chromosome

Question 48

effects of robertsonian translocations (phenotypes, synapsis, and gametes/meiosis)

Answer 48

familial down syndrome (inheriting a 21 from each parent and a translocated 21 –> down syndrome)

cause abnormal synapsis

chance of semi sterility from making unbalanced gametes (have duplications and missing genes) bc chromosomes w/ translocations still assort independently

Question 49

how can inversion and translocation change gene expression? example?

Answer 49

put genes under dif promoters/closer to promoter –> change amount of expression

2 translocations and an inversion in SSU1 s cerevisae gene –> more SSU1 expression = more membrane bound sulfite pumps = more sulfite resistance

Question 50

Explain how CUP1 duplications can be adaptive for yeast. Is the benefit of these mutations dependent on the environment? Why or why not?

Answer 50

CUP1 gene produces proteins that sequester copper ions in the cells and prevent them from causing oxidative damage. Duplication = more of these proteins = more protection from copper ions. The benefit depends on the environment because if the yeast were not in an environment frequently sprayed with copper, it would not be advantageous to them.

Question 51

Suppose the offspring of two inversion heterozygotes (who harbor the same inversion) receives an inverted copy of a chromosome from both its mother and father. Will meiosis be affected?

Answer 51

No, since both of the offspring’s chromosomes have the same inversion, they will be able to synapse and recombine with each other.

Question 52

Explain the meiotic consequences of a translocation in the context of independent assortment.

Answer 52

A translocation het will be semisterile. Following independent assortment, 50% of their gametes will not be balanced and will lack a full haploid set of genes.

Question 53

What frequency would you expect a Down syndrome affected parent to produce Down syndrome afflicted offspring?

Answer 53

50% because the extra copy of 21 has to go to one of 2 daughter cells in meiosis I