Ch. 6 Chromosome Variation Flashcards
What is the “ploidy” status of cultivated bananas?
Polyploidy (3n, 4n, or higher)
The Cavendish strain of bananas is threatened by what?
Soil Fungus more recently but also pests and diseases
The banana of choice before the Cavendish was what?
Gros Michel (until disease in 1950’s - 1960’s wiped it out)
Why can’t we just replant the Gros Michel?
Because they would all be genetically identical cultivated and vulnerable to the same pathogen and pests as before.
How are we helping the banana issue?
Using genome sequence scientists have already identified several genes that play a role in resistance to fungal diseases and are exploring ways to breed and genetically engineer bananas
Chromosome mutations
Variations in the number and structure of chromosomes
Karyotype
Complete set of chromosomes possessed by an organism
What types of cells are karyotypes prepared from?
actively dividing cells such as wbc, bonemarrow cells, treated with chemical (colchicine) to prevent them from entering anaphase.
special staining techniques within karyotyping help to reveal what?
“bands of DNA (G band, C band, Q band, R band)
G bands - DNA rich in A-T base pairs
Q bands - differences of c-g and a-t base pairs
C bands - regions of DNA occupied by centromeric heterochromatin
R bands - DNA rich in C-G base pairs
three basic categories of chromosome mutations
- 6.2 chromosome rearrangements (alter structure)
- 6.3 aneuploidy (add number)
- 6.4 polyploidy (add sets)
*** 6.2 Chromosome rearrangements
alter the STRUCTURE of chromosomes (duplicated, deleted or inverted)
*** 6.3 aneuploidy
NUMBER of individual chromosomes is altered (addition or deletion)
*** 6.4 polyploidy
SETS of chromosomes are added (3n, 4n, or more etc. . ..)
6.2 Chromosome duplication
mutation in which part of the chromosome has been doubled. Example: instead of ABCDEFG the duplication might be ABCDEF-EF-G (tandem duplication)
6.2 Tandem duplication
the type of duplication in which the duplicated segment is immediately adjacent to the original segment
ABCDEFG
ABCDEFEFG
6.2 Displaced duplication
if the duplicated segment is located some distance from the original segment either on the same chromosome or on a different one
ABCDEFG
ABCDEFGEF
6.2 reverse duplication
duplication is inverted
ABCDEFG
ABCDEFFEG
6.2 Deletion
a segment of the chromosome is deleted
ABCDEFG
ABCDG
6.2 Inversion
segment of the chromosome is turned 180 degrees.
ABCDEFG
ABCFEDG
6.2 Translocation
segment of a chromosome moves from one chromosome to a nonhomologous chromosome or to another place on the same chromosome.
Loop segment in meiosis
a way to detect duplication or deletion in a heterozygous for duplication. within meiosis the duplicated region must loop out to allow the homologous sequences of the chromosomes to align.
how do duplications and deletions often arise?
Duplications and deletions often arise from unequal crossing over, in which duplicated segments of chromosomes misalign during the process.
What’s the most frequent cause of red-green color blindness in humans?
unequal crossing over
Segmental duplication
duplications greater than 1000 base pairs in length.
pseudodominance
expression of a normally recessive mutation. it is an indication that one of the homologous chromosomes has a deletion.
paracentric inversions
inversions that DO NOT include the centromere
pericentric inversions
inversions that DO include the centromere.
position effect
many genes are regulated in a position-depentednt manner; if their positions are altered by an inversion, their expression may be altered i.e. position effect.
dicentric chromatid
acentric chromatid
- two centromeres
- no centromere
How is recombination reduced within pericentric inversions?
no dicentric bridges or acentric fragments are produced, but the recombinant chromosomes have too many copies of some genes and no copies of others, so gametes that receive the recombinant chromosomes cannot produce viable progeny
nonreciprocal translocation
genetic material moves from one chromosome to another without any reciprocal exchange.
Reciprocal translocation
two way exchange of segments between the chromosomes
Robertsonian Translocation
the long arms of two acrocentric chromosomes become joined to a common centromere through a translocation generating a metacentric chromosome with two long arms and another chromosome with two very short arms.
fragile sites
chromosome regions susceptible to breakage under certain conditions.
fragile-X syndrome
one of the most intensively studied fragile sites on the human x chromosome. a disorder that includes intellectual disability. x linked inheritance 1 in 5000 male births. results due to the increase in the number of repeats of a CGG trinucleotide.
What is the outcome of a Robertsonian translocation?
One large chromosome and one very small chromosome with two very short arms.
copy number variations (CNVs)
include duplications and deletions that range in length from thousands of base pairs to several million base pairs. Many of these variants encompass at least one gene and may encompass several genes.
3 main ways aneuploidy can arise?
- chromosome may be lost in the course of mitosis or meiosis.
- the small chromosome generated by a robertsonian translocation may be lost in mitosis or meiosis.
- nondisjunction. failure of homologous chromosomes or sister chromatids to separate in meiosis or mitosis. some gametic cells will gain a single chromosome some will lose a single chromosome.
Aneuploidy - nullisomy
loss of both members of a homologous pair of chromosomes. 2n-2
Aneuploidy - Monosomy
Loss of a single chromosome 2n-1
Aneuploidy - Trisomy
gain of a single chromosome 2n+1
Aneuploiody - Tetrasomy
gain of two homologous chromosomes 2n+2
Down syndrome
trisomy 21. most common autosomal aneuploidy in humans.
primary down syndrome
three full copies of chromosome 21. arises from spontaneous nondisjunction in egg formation: about 75% of the nondisjunction events that cause down syndrome are maternal in origin most arising in meiosis 1.
familial down syndrome
4% of people with down syndrome have this type. NOT TRISOMIC. extra copy of part of chromosome 21 is attached to another chromoosome through translocation. so still 46 chromosomes but the second part of 21 tacs on to another chromosome so karyotype might look like 45. .
translocation carrier
increased chance of producing children with down syndrome although they themselves don’t have down syndrome.
Trisomy 18 (Edward Syndrome)
severe intellectual disability low set ears, short neck, deformed feet, clenched fingers heart problems. life expectency <1 year.
Trisomy 13 (Patau syndrome)
severe intellectual disability, small head, sloping forehead, small eyes, cleft lip and palate, extra fingers and toes and numerous other problems. life expectency < 3 years.
Trisomy 8
Most rare of trisomies- intelectual disability, contracted fingers and toes, low set malformed ears, prominent forehead. normal life expectancy.
Does the frequency of Aneuploidy increase or decrease with maternal age?
it increases with maternal age. due to being associated with nondisjunction.
Autopolyploidy
all chromosome sets are from a single species
Allopolyploidy
chromosome sets are from two or more species
two types of polyploidy
- autopolyploidy
- allopolyploidy
unbalanced gametes
various number of chromosomes in gametes due to random segregation when multiple are involved.
amphidiploid
type of allopolyploid consisting of two combined diploid genomes.
