Ch. 7 skip 7.1 (Fig. 7.1 ranks as perhaps least useful figure in book)
Exam will be online after class today at 8pm. Questions in office hours. Turned in only online. Etc.
Euploid -- multiples of a full set of chromosomes
- haploid ?
- diploid ?
- monoploid (males in bees, wasps, ants.) why bad ? deleterious recessives
autopolyploid - from single parent, doubles. can be made with colchicine injection which arrests mitosis. how do chromosomes pair? sometimes as sets of bivalents or often as quadrivalents
allopolyploid - from two different parents, usually F1 sterile, then doubles. homeologous chromosomes
end result of most polyploids is fractionation -- reduction down to previous numbers. corn is an ancient tetraploid 10 -> 20 -> 10 but still has 2 copies of many genes in its genome.
What about odd multiples of ploidy? triploidy? pentaploidy?
with odd multiples problem is ? draw triploid. explain bivalents and segregation of univalents in meiosis resulting in aneuploid gametes (can be one tri (bi + uni) or normal bi + uni
Incomplete sets, too many or few of one or more chromosomes
- disomic, monosomic, trisomic etc.
- e.g. ? Down's syndrome or trisomy 21.
- the process that creates this is nondisjunction usually at meiosis
- turns out an important function of crossing over is to hold chromosomes together. mutants that are achiasmatic or have other strange biases in meiosis sometimes also have increase rates of nondisjunction
- aneuploidy causes
In human females ovules are arrested in prophase of meiosis at birth, and only finish meiosis during ovulation
as women age, their meiotic products have increasing risk of nondisjunction, thought to be because of failing or loss of cohesion of chiasmata
relative expression levels of genes matters.
- imagine gene that controls edge of leaf and one that controls center of leaf. could end up with some strange looking leaves if genes are out of balance
- double every gene in the genome, what's the effect on the relative ratio? (none)
- double copies of a gene, what effect on transcription? (double)
- examples of Datura in book -- extra chromosome copy disrupts gene balance alters morphology
- but most organisms not so tolerant
however, can you think of a "normal" case of aneuploidy ?
sex chromosomes! In drosophila fixed by 2x expressing everything on X in males. In humans one X is deactivated in each cell so expresses same amount. this is called dosage compensation
deletion and insertion we've discussed to some degree — but can happen to large regions
- why would a very large (100's of kb) deletion very likely be deleterious? not having lots of genes is really bad in homozygote, but why bad in het ? recessive deleterious expressed. (hemizygous)
DSBs common. if multiple breaks and wrong ends get stuck together -> rearrangement
Also faulty recombination. Draw genes and TEs on 1 chromosomes, but could happen even between chromosomes in rare occasions.
Figure 17-19 in book does a great job showing these.
17-26, 17-28, 17-29 great figures showing effects of inversions and how meiosis works in inversion heterozygote. simply a piece of DNA that flips. has any effect on gene number ?
may have phenotypic effects because it rearranges genes
prevents recombination because of loop structures that form in meiosis for pairing to occur. recombination results end with deletions (see above figures)
are being shown to be important in adaptation! (mimulus example)
two chromosomes switch arms (reciprocal). leads to some very strange pairing in meiosis (hard to draw, check out 17-30 in the book). depending on how meiosis finishes, can produce invialbe or viable gametes
nonreciprocal translocation (one of the translocation copies or one arm piece lost). leads to gene imbalance. once version of this is Robertsonian translocation. involves acrocentric chromosomes (compare to metacentric) moving long arms together, losing small arms often not necessary