Ch. 6 Variation in # and Structure of Chromosomes Flashcards
Aneuploidy
missing or additional single chromosomes
Polyploidy
additional sets of chromosomes
Chromosome mutations - different types
Modifications at the level of the chromosomes.
Changes in the total number of chromosomes.
Deletion or duplication of segments of chromosomes.
Rearrangements within or among chromosomes.
Chromosomes mutations have an impact on?
Phenotypes. Bc genetic component of organisms is a very delicate balance.
Changes in location or content of the genetic variation?
Changes the phenotype
Changes in location or content of the genetic variation in animals and plants?
In animals, most such changes are lethal.
Plants are more tolerant - can use this to create plants that are bigger and more viable
Cytological analysis steps
How to diagnose chromosome mutations.
- Take blood sample
- Separate cells from serum by centrifugation
- Remove white cells and culture in vitro
- Stimulate cells to divide
- Disable mitotic spindle (cells arrested in metaphase)
- Add hypotonic solution to swell cells.
- Squash cells on slide, fix, and stain.
- Examine chromosomes (karyotype)
Cytological analysis - different staining techniques
Giemsa, quinacrine mustard, reveal characteristic bands; FISH and fluorescent microscopy can reveal locations of genes etc.
Human kayrotype
p= petite, small arm of chromosome
q= long arm
Use staining techniques to reveal banding pattern. Experienced lab technicians can see if there is a piece of chromosome missing.
ploidy
“fold” as in “twofold”
Euploid
“good fold”
Having complete sets of chromosomes (diploid=2n; triploid=3n; tetraploid=4n; polyploid= multiples of n)
Aneuploid
“not good fold”
Having particular chromosomes of parts under or over represented.
Aneuploidy vs polyploidy
Aneuploidy implies a genetic imbalance; polyploidy does not.
Monosomy
2n-1
Loss of a single chromosome in an otherwise diploid organism.
Trisomy
2n+1
Gain of a single chromosome in an otherwise diploid organism.
Autopolyploidy
Multiples of the same genome
Allopolyploidy (amphidiploidy)
Multiples of closely related genomes
Aneuploidy
variation in chromosome number.
Organisms gains or loses one or more single chromosome.
Hypoploid
an organism in which a chromosome or chromosome segment is underrepresented
Hyperploid
an organism in which a chromosome or chromosome segment is overrepresented
Nondisjunction causes?
monosomy and trisomy
Nondisjunction
random errors during the formation of gametes.
Paired homologs fail to disjoin during segregation.
Results: normal distribution of chromosomes into gametes is disrupted.
Nondisjunction can happen during meiosis 1 or 2. - outcome is different depending on where.
(homologous chromosomes pair up in meiosis, random error happens and pair fails to disjoin during segregation.
Nondisjunction in meiosis 1
In metaphase 1, homologous chromosomes don’t separate.
1 daughter has more chromosomes, the other is missing some.
2 gametes are n+1 (extra), 2 gametes are n-1 (missing). No normal gametes.
Nondisjunction in meiosis 2
In metaphase 2, sister chromatids don’t separate.
1 gamete is n+1 (extra), 1 gamete is n-1 (missing), 2 gametes are n (normal)
Fertilized gametes- first division nondisjunction
Can result in either trisomic or monosomic zygotes
Fertilized gametes- second division nondisjunction
Can result in disomic (normal), trisomic, or monosomic zygotes
Monosomic and trisomic zygotes
Most fail to develop, so frequency is not as high
Only known monosomy in humans (in X)
Turner syndrome
Monosomy overview
loss of one chromosome (2n-1).
Although one copy remains, if an allele is lethal, organisms is not viable (Monosomy unmasks lethal alleles).
Haploinsufficiency: when one copy of a gene is not sufficient for an organism to survive.
Monosomy of sex chromosomes: Turner syndrome (45,X).
Autosomal monosomy not tolerated in humans or other animals. (causes organism to lose too much genetic material to survive).
In Drosophila, flies that are monosomic for chromosome IV (less than 5% of the organism’s genes) develop more slowly, exhibit reduced body size, and have impaired viability.
More common in plants (tabacco, maize) but plants are usually less viable.
Trisomy overview
Addition of a chromosome (2n+1).
Trisomies for autosomes are often lethal during development.
Addition of chromosomes produces more viable organisms than monospmy (as long as the chromosomes are small)- depends on the amount of genetic material.
Plant tirsomies are viable. Ex: Datura stramonium with altered phenotype (Jimson weed)
Jimson weed
The capsule of the fruit of the jimson weed, the phenotype of which is uniquely altered by each of the possible 12 trisomic conditions
Monosomies and Trisomies: impact
depends on the size/information content of the chromosome in question.
Only known Trisomies in humans where baby survives the chromosomes are small and contain less genetic material than large ones.
Human aneuploidy
About 20% of all conceptions end in spontaneous miscarriages during early pregnancy (first trimester) very often before she is aware of pregnancy.
About 30% of these show some form of chromosomal imbalance.
In humans, only 3 autosomic trisomies are known to survive to term:
Patau Syndrome (47, 13+)
Edwards syndrome (47, 18+)
Down syndrome (47, 21+) - only one where affected person survives into adulthood.
Trisomy 21 (Down Syndrome) s/s
Affected individuals experience cognitive delays, but the intellectual disability is usually mild to moderate.
Characteristic facial appearance.
Weak muscle tone (hypotonia) in infancy.
Variety of birth defects: about half of all affected children are born with a heart defect.
20 times more likely to develop leukemia.
Alzheimer’s disease in older individuals.
DSCR: Down Syndrome critical region on chromosome 21
Genes are dosage sensitive.
Responsible for many phenotypic-associated syndromes.
Since 2004: mouse model trisomic for DSCR
Cognitive deficiencies are most likely caused by the presence of 3 copies of DSCR, but other unknown factors many contribute.
Decreased risk of developing cancers with solid tumors, like lung cancer and melanoma.
Origin of Extra 21st chromosome
Nondisjunction of chromosome 21 during meiosis.
Homologs do not disjoin in anaphase 1 or 2.
Leads to n+1 gametes.
Ovum is the source of 95% of trisomy cases (means maternally caused)
Increased incidence with increasing maternal age.
Newer research suggest that the paternal age plays role too.
Incidence of Down Syndrome syndrome births related to maternal age
Trisomies are caused through random nondisjunction for various unknown reasons, but there is a correlation with maternal age (exception: Familial Down syndrome) 35 - 3/1000 40 - 10/1000 45 - 1/30 50 - 1/15
Down syndrome and age men vs women
In spermatogenesis, gametes are continuously produced from puberty into adulthood.
IN oogenesis, primary oocytes are produced in the female embryo.
Meiosis is arrested in the primary oocytes in prophase 1, ovaries are inactive during infancy and childhood.
Starting with puberty, every month one primary oocyte completes meiosis and becomes an egg.
An egg released by a 40 yr old woman, originates from a 40 yr old oocyte.
Chromosome nondisjunction is more likely to happen in an older egg.