topic 6 chromosomes

Question 1

how can they be altered

Answer 1

• Description of completement of chromosomes
• Alterations -
Loss of whole chromosome or a major part =
reduced viability of a gamete

Question 2

what is aneuploidy

Answer 2

• Aneuploidy
○ Unbalanced chromosome complement
○ Fertilization of gametes in which nondisjunction occurred
§ Offspring with abnormal number of a chromosome
○ A trisomic zygote has 3 copies
A monosomic zygote only has one copy of a particular chromosome

Question 3

downsyndrome ex

Answer 3

• Down syndrome 
		○ Trisomy of chromosome 21 
		○ Most common human chromosome abnormality - 1 in approx 800 births 
		○ Frequency of having a down syndrome child increases w age of mother - correlation not explained
		○ Above 35 it increases 
		○ Paper in 2009 
		○ Mean maturnal age vs birth year 
		○ oogenesis

Question 4

describe oogenesis

Answer 4

• Begins in embryo
• Primordial germ cells become oogonia (both diploid)
• Oogonium:
○ Multiples by mitosis
○ Begins meiosis
Stops at prophase I - primary oocyte - @ puberty influence of FSH initiates division - secondary oocyte - arrested @ metaphase - ovulation completes maturation

Question 5

oogenesis down syndrome possible connection?

Answer 5

• As a result, an egg produced by a 15 yr old is 15, therefore, one by 45 is 45 years old - bc meiosis stops at birth for eggs
  
  • Theory for down syndrome - multiple stops - halting at prophase 1 and metaphase 2 might be connected to disjunction at chromosome 21 - older age accelerates this chance
  • Up to 37 - mostly MM1 error
  • Past - mostly MMII

Question 6

maternal age connection to trisomy 21? is it confirmed?

Answer 6

• Likelihood of a chromosomal disorder caused by
an error in cell division increases with maternal
age
Allen et. al. (2009). Human Genetics Vol. 125
Significant association between advanced maternal age
and chromosome 21 nondisjunction was restricted to
meiotic (meiosis I & Il) errors in the ovum
Increased proportion of MMI cases among women < 15
& those 40-45 years of age
• Prevalence also known to vary by race/ethnicity
○ African: 7.3, White: 9.2, Hispanic: 11.8
○ Could be other mechanisms involved!
• Conclusion: jury is still out

Question 7

describe turner syndrome

Answer 7

• Monosomy of X chromosomes (XO)
  
  • Sterile female
  • 1 in 5,000 female births
  • Also has common characteristics

Question 8

describe klinefelter syndrome

Answer 8

• Ex. 3 klinefelter syndrome
  
  • Sex chromosomes - 2 X and one Y
  • Extra X in a male
  • Trisomy of sex chromosomes

Question 9

For alterations in the sex chromosomes in 
e 
which parent did nondisjunction occur? 
XO
XXX
XXY
XYY

Answer 9

• 1st either
• 2nd either
• 3rd either
4th father only

Question 10

what is polyploidy? how does it happen

Answer 10

• More than 2 sets of homologous chromosomes 
	• 
	• Many species of plants 
Fewer species of animals Failure of reduction 
division in meiosis 
produces unreduced 2N 
gametes => 3N zygote
	• 
	• Results in aneuploid gametes

Question 11

what is autopolyploids

Answer 11

c Autopolyploids
Multiple chromosome
sets originating from
within one species

Question 12

what is allopolyploid

Answer 12

Allopolyploids 
Multiple chromosome 
sets originating from 
two or more species 
Between closely 
related species
	• 
	• Allopolyploid - usually results in hybrids

Question 13

polyploidy in plants result

Answer 13

• Polyploids are often larger 
	& have larger component 
	arts than their relatives 
	• Epidermal leaf cells of tobacco plants 
Increase of cell size (evident 
in stoma size) with increase 
of ploidy 
x 2. grapes 
	• Tetraploid grapes are larger than diploid

Question 14

how can polyploidy occur in animals? ex?

Answer 14

• Via parthenogenesis
○ Flatworms, leeches, brine shrimps
○ Female doesn’t need sperm to multiply
• Salmonids
•
• Hybridization - are polyploid, all female for the minnows
Polyploid frogs & toads may participate in sexual
reproduction but salamanders & lizards are
parthenogenetic
Sterility of triploids has been commercially exploited
2N oysters become unpalatable during breeding season
3N’s are sterile; hence palatable year-round

Question 15

4 types of alternations to chromosome structure? what leads to them

Answer 15

Breakage of a 
chromosome leads to 
four types of changes in 
chromosome structure 
Deletion 
Removal of a segment 
Duplication 
Repetition of a segment 
Inversion 
Reversal of segment 
orientation within a 
chromosome 
Translocation 
Segment moving from one 
chromosome to another

Question 16

ex of chromosome alteration

Answer 16

• Certain cancers, including 
	chronic myelogenous 
	leukemia (CML), are 
	caused by translocations 
	of chromosomes 
Abnormally short 
chromosome 22 — the so- 
called "Philadelphia 
chromosome" & 
abnormally long 
chromosome 9 
Reciprocal translocation in 
a single blood cell 
precursor leading to all 
abnormal descendent cells 
	• Chromosome 9 and 22 leading to the formation of a short chromosome 22

Question 17

Human chromosome mutations: overall incidence?

Answer 17

• Surprisingly frequent in human sexual reproduction
○ Relevant cellular processes prone to high level of error
• Approx. 15% of conceptions abort spontaneously
○ 50% show chromosomal abnormalities
• 0.6% of live births also show abnormalities
○ From aneuploidy and rearrangements
• Study by griffiths
○
○ Suggests there is a strong incidence of abnormailities

Question 18

ex kayotype difference in deer but still look similar

Answer 18

2n=46 vs 2n=8 
i666Äapid and 
number reduction in Munoac Deer Inferred from 
.Mitochondria1 DNA Phylogeny. Evole. 
Karyotypic evolution in Muntjac proceeded 
via reduction in 2N number 
Rate of change in chromosome number in 
e 
Muntjac is one of the fastest among 
vertebrates 
e Involves many fusion

Question 19

summary

Answer 19

e Mutations are random; do not depend on their role in
advantage or disadvantage
Rate of mutation, by itself, quite low; but total input of
e
genetic variation by mutation has significant contribution
in evolutionary process
o Polyploidy, translocations, fusions, or fissions of
chromosomes are extremely important for evolution of
chromosome number
e Mutations of karyotype include polyploidy & structural
rearrangements
Many of them may have reduced fertility in heterozygous condition
Related species may differ strikingly in karyotype but not
in phenotype