Lecture 9 :Chromosomal Mutations

Question 1

1. Define chromosomal mutations.

Answer 1

Chromosomal mutations refer to heritable changes that involve chromosome level deviations in shape or quantity.

Question 2

Define nondisjunction. How do the consequences of nondisjunction in meiosis I and II differ? How
are these consequences similar?

Answer 2

failure of chromosomes to separate during meiosis or mitosis which results in cells with excess chromosomes and cells lacking a chromosome.

In meiosis 1: one set of gametes receive an entire tetrad the other gametes will receive no chromosomes–> results in all offspring being aneuplodic (one set of gametes is trisomic and the other being monosomic)

In meiosis ii: only half the gametes are aneuploid

Question 3

Monosomies other than Turner syndrome are never seen in humans. Why are monosomies often
lethal in animals, and why might chromosome IV in Drosophila be an exception?

Answer 3

Monosomies are often lethal in animals because genes are dose dependendent so lack of a chromosome can greatly affect gene expression/ development.

• in diploid organisms monosomies harbor lethal recessive mutations as there are no dominant wild type alleles expressed that can out express the recessive mutations

Chromosome IV is a rather small chromosome with less genese encoded on it so loss of said chromosome will not affect gene expression as much as a large chromosome.

Question 4

Plants tolerate monosomy better than animals, but lacking any particular chromosome in pollen
grains is usually not well tolerated. Why?

Answer 4

pollen grains are typically haploid and go through extensive development before fertilization so a nondisjunction in meiosis will greatly affect gene dosage; sensitive to meiotic nondisjunction

Question 5

Explain how some instances of trisomy can be adaptive, whereas most others deleterious

Answer 5

In some yeast species, there are genes on chromosomes that can be adaptative. In S. cerevisiae an extra copy of chromosome 4= better tolerance of heat.

Increased gene dosage of a beneficial/adaptive gene can be good

Question 6

Which trisomies survive to term in humans? Which survive to adulthood? What are some
explanations for why these trisomies in particular survive to adulthood?

Answer 6

survive to term: 18,21, 13, x, y
survive to adulthood–> Trisomy 21; downsyndome, Triplo x, Jacobs syndrome (xxy) typically survive to term in humans

For triplo xxx, jacobs syndrome –> x inactivation
trisomy 21 is a small chromsosme

Question 7

What are some of the common phenotypes associated with Down syndrome?

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

Answer 7

flattened face
eye close together
Shorter

Question 8

How is it known that the extra copy of chromosome 21 is usually maternally derived?
9. Aneuploidy is commonly found in studies of spontaneously aborted embryos/fetuses, w

Answer 8

considering trisome 21 is due to meiotic nondisjunction the older age of the mother there is a longer time arrested in meiosis 1

Question 9

Aneuploidy is commonly found in studies of spontaneously aborted embryos/fetuses, why is it odd
that monosomy almost never seen in these studies, and what explains this phenomenon?

Answer 9

because monosomies are poorly tolerated in animals these monosomic embryos die early on

this is odd because you would expect an equal chance of trisomy and monosomy because when a nondisjunction happens in meiosis I one set of gametes has trisomy and the other has monosomy

Question 10

Define autopolyploidy. What are some of the key consequences of autotetraploidy on meiosis and
reproduction?

Answer 10

Autoploidy refers to adding full haploid sets within a species

tetraploids produced due to the duplication of genetic material without dividing into 2 zygotes

consequences:

instantaneous speciation:
A tetraploid being crosses with a diploid will result in a material triploid and therefore a speciation event as genetic material cannot be further passed down to offspring

Meiosis consequence:
homologs do not separate evenly between cells because there is three copies and one cell will be deficient in that copy

Question 11

Why are polyploid plants often propagated asexually

Answer 11

Considering they are triploid and thus sterile they cannot go through meiosis / sexual fertilization do to not inability to evenly separate homolgs into cells

Question 12

What is the usual effect of polyploidy on cell size? What explains this in yeast?

Answer 12

polyploids typically have larger cells

In yeast this is due to the g1 cyclins being downregulated to cell cycle arrest is altered allowing cells to grow large in size before dividing

Question 13

Explain how allopolyploidy occurs and how it is distinct from autopolyploidy

Answer 13

alloploidy involveds haploid sets from two different species

Question 14

What two conditions are required for a chromosome mutation to be heritable?

Answer 14

when the chromosome break is repaired the composition of the genes is not repaired

mutations must be within the germline

Question 15

Contrast terminal and intercalary deletions. What is a deletion loop?

Answer 15

terminal deletions have only one breaks and everything distal to the break deletes

intercalary deletions have two breaks in which a deletion loop is formed deleting part of the chromosome between the two breaks. The centrometer allows the two ends of the chromosome to join

Question 16

What are two reasons deletions can be deleterious or lethal?

Answer 16

if a deletion removes one copy of a gene then complementation cannot occur to

Question 17

What causes Cri du chat syndrome in humans?

Answer 17

due to a small terminal deletion on chromosome 5

Question 18

What are the two main causes for duplications? Why are rDNA genes normally duplicated?

Answer 18

duplications are typically due to unequally cross over during meiosis

OR
replication error in meiosis

rDNA typically has many duplications of genes because they need to express a lot of the protein

Question 18

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

Answer 18

CUP1 sequesters copper ions to prevent damage to the cell –> multiple copies = greater copper resistance

dependent on the environment because if the plants the yeast are surviving on are being sprayed with cooper they can resist the copper

Question 19

Explain the meiotic consequences of paracentric and pericentric inversions. Why is it technically
incorrect to say that inversions prevent recombination? Why is this effectively true?

Question 20

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?

Question 21

What are the three main types of translocations? Define each.

Answer 21

reciprocal : exchange of segment from non-homogous. chromosomes
nonreciprocal: a segement from one chromosome moved to another
robersonian: the two long arms of acrocentric chromosome fuse together and the short arms are lost

Question 22

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

Question 23

Can translocations have phenotypic consequences? Explain in the context of the SSU1 gene in
yeast.