Genetics 275 Notes - Week 3
Sex Chromosome Aneuploidy con't
1.
2.
3.
4.
5.
XXY - sterile female
Y0 - lethal
X0 - sterile female
XXX - fertile female
XYY - double Y karyotype
-fertile male, tall, normal IQ
6. Variety of mosaic karyotypes that result from nondisjunction after fertilization
-Secondary Nondisjunction - nondisjunction that occurs in a karyotype that is already
abnormal
example: in XXX & XXY individuals you would expect a high incidence of abnormal
progeny but actually doesn't occur (abnormal gametes are selected against?)
-Down's Syndrome patients (21,21,21) individuals do produce aneuploid (21,21)
gametes
example: Drosophila do produce aneuploid gametes (but not as often as would be
expected)
XXY fertile female → 8% aneuploid game tes (secondary nondisjunction)
XY or XX → 1/1500 aneuploid gametes (primary nondisjunction)
XXY
x
↓
XY
X
XX
Y
X
XY
Y
XXX (lethal)
XXY (fertile female)
XY (fertile male)
YY (lethal)
XX (fertile female) XY (fe rtile male)
XXY (fertile female) XYY (fertile male)
Human Autosomal Aneuploids
-mostly lethal - spontaneous abortions/miscarriages
-3 aneuploids will survive to term (to birth)
Trisomy 13
Patau syndrome
1/5000
only survive a few mo nths
Trisomy 18
Edwards syndrome
1/6500
survive less than a year
Trisomy 21
Downs syndrome
1/750
mental retardation,
short stature,
organ problems
-97% of Down's syndrome are due to trisomy 21 (primary nondisjunction
with no history of Down's syndrome)
-3% are familial (due to a translocation)
[T26] - characteristics of X0, XXY, 47 +21 syndromes
[T27] Frequency of Chromosomal Abnormalities in Spontaneous Abortions & Live
Births
-50% of miscarriages have chromosomal abnormalities
-large chromosomes aren't detected as trisomies in miscarriages - likely aborted
before pregnancy is even detected as there is no reason to think that
nondisjunction
doesn't occur with them
[T28] Maternal age vs. Rate of Nondisjunctio n
-before 35 years of age - low rate of nondisjunction
-after 35 years - ~2% of gametes show primary nondisjunction because female
gametes all made early then mature at one per month whereas male gametes are
continuously produced
Polyploidy
-where chromosome number differs by multiple of n
[T29] polyploids in plants and animals
-rare in animals compared to plants
Wheat:
2n = (2x) = 14
2n = (4x) = 28
2n = (6x) = 42
Monoploidy (n)
Triticum monococcum
"
durum
"
restivum
=haploidy
-only one copy of each chromosome
-sterile due to production of unbalanced gametes during meiosis (because no
chromosome pairs to line up at metaphase plate)
[T30] how to make a monoploid plant
[T31] how to make a monoploid into a diploid
Triploidy (3n)
-occurs naturally in some plant species and in some plant tissues
example: bananas (triploid species don't have seeds)
endosperm tissue
How does one obtain triploids?
1. from diploid gamete x haploid gamete
2. from regular meiosis in a tetraploid plant
-there are some triploid amphibians - would expect unbalanced gametes therefore they
reproduce parthenogenetically (population of only females)
-females lay unreduced triploid eggs
-triploidy is more common in plants
-propagate vegetatively (via mitosis, not meiosis)
[T32] Problems with meiosis in triploids
Tetraploidy (4n)
-more common than triploidy as it is easier to get balanced gametes
1. Autopolyploidy (self) - doubling of chromosome number of a diploid
-How? - failure of a diploid cell to enter anaphase
-Features:
1. larger nuclei and cells therefore entire organism may be larger
2. excellent viability due to increased copy number of each gene
3. usually very good fertility
[T33] Possible tetraploid problems in meiosis
example: Maize (corn)
diploid 2n = 20
tetraploid 4n = 40
selfed a diploid → half the progeny have 40 chromosomes (high frequency)
Genetics of Tetraploids
-complex
[T34] Probability of each possible gamete from a tetraploid - changes genotypic ratio of
progeny
2. Allopolyploidy (other) - from crosses between members of distinct taxonomic groups
-F1 is highly sterile because every chromosome is only present in one copy but somatic
cells are okay
-face extinction unless they
1. are continuously produced
2. undergo asexual reproduction (via shoots)
[T36] Production of Triticale from crossing wheat and rye
[T37] Wheat evolution
[T38] Brassica oleracea
-broccoli, brussel sprouts, cabbage, cauliflower and kohlrabi are all this species
but selected for different starch storage organs
Chromosome Abberations
=alterations in chromosome structure that simultaneously delete or duplicate or rearrange
1000’s – 1000000’s of nucleotides (one or many genes)
-not as deleterious as aneuploidy or polyploidy as abberations only involve part of a
chromosome, not entire chromosomes
Six Major Categories
1. Deletions
2. Duplications
3. Inversions
4. Translocations (involves >1 homologous pair)
5. Fusion (2 chromosomes → 1 chromosome)
6. Fission (1 chromosome → 2 chromosomes)
Note: 1,2,3,5, and 6 are intrachromosomal (within 1 chromosome) whereas 4 is
interchromosomal
Deletions
=pieces of chromosome are missing therefore encoded function(s) for a few to many
genes will be missing
Characteristics
1. Stable (cannot revert)
2. Pseudodominance – allows any recessive mutation in the deleted region on the nondeleted chromosome to be expressed and seen
3. Recessive lethality – deletion in same region on both homologues results in lethality
if any vital genes are deleted
4. Semisterility – because deletion in chromosome may result in problems for gamete
that receives it (especially in plants)
5. Cytogenetic – deletion loops may be visible
Human Example
-Cri du chat syndrome – caused by deletion at tip of chromosome 5 short arm
[T39a] 5p15.2 and 5p15.3 (most distal bands deleted)
-homozygous lethal
-causes crying to sound like a cat, mental retardation, and microencephaly (small head)
[T40] Deletion Mapping – to determine where a certain DNA fragment in located on a
chromosome. You can hybridize fragment to a series of deletion chromosomes (with
different regions deleted) and see which ones it hybridizes with. If it hybridizes the
fragment is not located within that deletion region
Duplications
=presence of a section of a chromosome in excess of the normal amount (1/chromsome)
-size can vary – affects how deleterious they are
-results in duplication loops seen cytogenetically
Types:
1. tandem (side by side in same order)
2. reverse (side by side in reverse orientation)
3. displaced (elsewhere on same chromosome)
4. transposition (duplication on a non- homologous chromosome)
5. free duplication (duplication becomes a small minichromosome not attached to any
other chromosome – must contain centromere to be stable)
Origins:
1. spontaneous ie. we don’t know
-but unequal crossing over can cause duplication if there is already a duplication or
repeat in the region
2. Induced with physical agents ex. X-rays
-viability very good with small duplications (↑ duplication → ↓ viability because it
begins to act like a trisomy)
Genetic Consequences – Evolutionary
-will relieve selective pressure on the original copy
-acts like a “back-up copy” so mutation may occur in one copy without deleterious
effects
[T41] Hemoglobin genes arised from duplication and diverged to have different functions
[T42] duplication example in Drosophila
Inversions
-intrachromosomal
-involves 2 or more breaks in a chromosome and then rejoining of broken ends in
opposite orientation