Topic 5

Question 1

What’s the differences between gene mutations and chromosome mutations?

Answer 1

Gene mutations are changes that take place within a gene
Chromosome mutations are changes in a chromosome region encompassing multiple genes

Chromosome mutations divide into 2 groups:
Changes in chromosome number (# of DNA molecules changed)
Changes in chromosomes structure (result in novel sequence arrangements within one or more DNA double helicies

Slide 3

Question 2

What are the 2 types of changes in chromosomes number?

Answer 2

Changes in whole chromosome sets (results in condition called aberrant euploidy

Changes in parts of chromosome sets (resulting in condition called aneuploidy

Question 3

What is cytogenetics?

Answer 3

Geneticists use stains to identify specific chromosomes and to analyze their structures

Slides 4-6

Question 4

What do each of these terms mean:
Euploid
Polyploid
Aneuploid
Monosomic 
Trisomic

Answer 4

Euploid- having a chromosome number that is an exact multiple of the monoploid number
(Monoploid=1n, diploid=2n, triploid=3n)

Polyploid- extra copy/copies of each chromosome of species that normally are diploid (polyploid >2n)

Aneuploid- abnormal number of one or more of the chromosomes, but not all

Monosomic- one missing chromosome compared to the 2n wild type
Chromosome #= 2n-1

Trisomic- one extra chromosome compared to the 2n wild type chromosome number
Chromosome #= 2n+1

Slide 7-10

Question 5

What is autopolyploid?

What is allopolyploid?

Answer 5

Autopolyploid- all the chromosomes originated from the same organism
Increasing ploidy levels can generate traits that are more valued by the consumer

Allopolyploid- one set of chromosomes originated from one organism and the other from another (original hybrid)
The parent organisms need to be related to produce viable progeny
Mixing 2 genomes can lead to new phenotypes in the offspring

Slide 11-13

Question 6

What are the 2 ways autopolyploid are generated?

Answer 6

Naturally formed autopolyploids- mistakes in meiosis that lead to failure in executing one of the meiotic divisions produces a gamete with twice as many chromosomes as wild type gamete

Induced autopolyploids in plants-
Treat meiotic or mitotic cells with microtubules inhibitor to arrest cells in metaphase, identify variable polyploid individuals by karyotyping

Slide 14-16

Question 7

What are polyploid tissues?

Answer 7

In some tissues the cells replicate the DNA in S phase but not divide, leading to the formation of a polyploid tissue in an otherwise non polyploid organism
To become polyploid cells replicate their DNA (S), but do not divide
Polyploid can be a mechanism of increasing cell volume and/or call metabolism

Slide 17-19

Question 8

What are the agricultural advantages of monoploids?

Answer 8

Used in research for:
Combining recessive mutations in one genotype
Screening for resistance to chemicals, easier to do as a haploid, the the resistance is recessive

Slide 20

Question 9

What are monosomies?

What are trisomies?

Answer 9

Aneuploidy is gain or loss of 1 chromosome due to non-disjunction errors in meiosis

Monosomies- one missing chromosome, in humans autosomic monosomies die in utero 
Turner syndrome (XO)

Trisomies- one extra chromosome, in humans only autosomic monosomies for large chromosomes die in utero
Down syndrome
Klinefelter syndrome

Slide 21-26

Question 10

What are trisomies/monosomies so deleterious?

Answer 10

The product of genes typically do not work in their own pathway, but interact with other to contribute to a process/pathway
Gene imbalance is often not sufficient to perturb a pathway, however the combo of many pathways slightly perturbed may lead to an embryo that can not develop
The amount of gene product (transcription) is in part dependant on how many template (copies) of the gene there are for the RNA polymerase to act upon
Too much transcription (trisomies) and too little transcription (monosomies)

Slides 27-31

Question 11

Why are trisomies for sex chromosomes less deleterious compared to trisomies for autosomes?

Answer 11

Imbalance of the Y chromosomes will have a effect in fertility since this chromosome encodes mostly for genes involved in sex determination and function
Imbalance of X chromosomes will have a minor effect since the X chromosome has naturally evolved to be dosage compensated between the 2 sexes

Slide 32-34

Question 12

How do abnormalities in chromosome number appear?

Answer 12

Lack of crossovers causes non disjunction in the first meiotic division (homologs don’t separate)
Defect in chromosomes cohesion cause non disjunction in the second meiotic division (sister chromatids don’t separate)
Trisomies increase in frequency with increased maternal age

Slides 35-40

Question 13

Study the 5 types of chromosomal changes slide 41-42

Answer 13

Translocation 
Inversion 
Duplication
Deletion 
Gain of genetic material 

Slide 41-42

Question 14

What is unbalanced vs balanced rearrangements?

Answer 14

Unbalanced rearrangements- lead to change gene number, these can be caused by deletions and duplications, when they involve atleast one gene

Balanced rearrangements- do not lead to change gene number, will be rearrangements that change gene order, like inversions and reciprocal translocations

Slide 43

Question 15

What is the effect of a deletion?

Answer 15

Unmasking of recessive mutation
Pseudodominance- recessive alleles uncovered by the deletion will appear as if they are dominant
Can lead to defects if deletion involves haplinisufficient genes (genes that need to be expressed in both homologs to provide enough protein)

Slide 44
Slide 46-47

Question 16

What is the effect of duplications?

Answer 16

Genes in the duplicated region have a potential to be expressed twice as much
Depending on which and how many genes effected this can cause gene imbalance

Slide 45

Question 17

What is cri du chat syndrome?

Answer 17

Named for the cat like cry of baby affected with disease
Includes mental retardation and micro encaphaly
Deletion is small arm of chromosome 5 during meiosis
CTNND2 gene important for cell adhesion and neuronal migration in the developing brain

Slide 47

Question 18

What is Williams syndrome?

Answer 18

Unequal crossover causing human genetic disorder

Unusual brain development associated with developmental delays and learning disabilities
Deletion that has a dominant phenotype likely caused by haploinsufficiency

Slide 48

Question 19

What is the effects of inversions?

Answer 19

Inversions may cause a change that affects gene function
Potential for disrupting genes, modifying gene expression and creating new genes is possible at any breakpoint and type of rearmament
If no gene is effected inversions can be homozygous viable and undetected unless an heterozygous for the inversion is formed

Slide 50-51

Question 20

What are the effects of translocations?

Answer 20

If no gene is affected by the breakpoint and a centromere is not part of the translocation, it can be homozygous viable and undetected
Translocation cause human genetic disorders (Down syndrome)
Translocation cause new phenotypes (variegation in drosophila)
Translocations cause cancer (lymphoma)- proto-oncogene when mutated squires novel activity to promote cancer

Slides 52-59