Chapter 8 - Chromosome Variation Flashcards
What is monoploidy?
- One set of chromosomes
- Males are usually infertile
- No pairing in Meiosis I so cannot make gametes
- Ex. ants, bees, wasps
What is polyploidy?
- More than normal # of chromosomes
- Ex. triploid/tetraploid
- Bigger and can out-compete
- So evolution favours these plants
- 30-35% of flowering plants
- Rare in animals
What are autopolyploids?
- Sets are identical
- Chromosomes derived from same species
- More vegetative growth and less seed production
- Larger cells
How/when does autopolyploidy occur through mitosis?
- Cytokinesis stops
- No cell division
- 2x genetic info in autotetraploid
How/when does autopolyploidy occur through meiosis?
- Non-disjunction occurs in Meiosis I (no segregation)
- Produces a 2n gamete
- Produces an autotriploid zygote
What are some examples of polyploid plants?
- Seedless varieties of citrus are triploids or pentaploids
- Odd # makes plants sterile
- Not as much genetic diversity
- Ex. watermelon, bananas, grapes
- Strawberries = 8N
What is a problem with polyploidy?
- Many are sterile due to problems with pairing/separation of homologous chromosomes in meiosis
- Ex. Bananas 3N = 33
- > 11 univalents and 11 bivalents
- > Number of chromosomes can vary
- > need stoichiometrically equal # of genes to be viable
- > could produce gametes with 2 copies of one chromosome and 1 copy of another
What are the 3 ways that triploid cells can segregate/pair in meiosis?
- 2 homologous chromosomes pair, whereas the other segregates randomly
- All 3 chromosomes pair and segregate randomly
- None of the chromosomes pair and all 3 move to the same cell
What are allopolyploids?
- Some polyploids are the result of crosses b/n 2 or more species (usually related)
What is the process of creating allopolyploids?
- Gametes from 2 diploid plants form hybrid
- The hybrid is sterile b/c meiosis is irregular
- The chromosomes are doubled, creating a tetraploid
- Meiosis in the tetraploid is regular
- Euploid gametes produced by the tetraploid can combine to propagate the organism sexually
What is an example of a allohexaploid?
- Bread wheat
2N=14 x 2N=14 Sterile 2N=14 - Meiosis will not work bc there are no homologous chromosomes Doubled to 4N=28 - Can now undergo meiosis I and II Crossed with 2N=14 Sterile 3N=21 Doubled to 6N=42 (allohexaploid) or can be called 3N=42 (amphidiploid)
What is an example of an allotetraploid?
- Upland cotton
- 2N = 52
Cross b/n American and Old World cotton (both 2N=26)
2N=26 Sterile
Doubled to 4N=52 Fertile
What was crossed in the development of triticale?
Rye and wheat
4N=28 x 2N=14
Sterile 3N=21
Doubled to 6N=42 Triticale (A functional diploid or amphidiploid)
What is special about triticale?
- High protein of wheat
- High lysine of rye
- Hardy even in poor soil conditions
What occurs in animal polyploidy? What is an example?
- Interspecies crosses can result in sterile animal
- Chromosome doubling is not well tolerated
Ex. Sterile Mule
(Hardy and strong) - Liger is fertile bc it has homologous chromosomes
When can polyploids be fertile?
Polyploids produced by chromosome doubling in interspecific hybrids may be fertile if their constituent genomes segregate independently
What is aneuploidy? What can cause it?
- Diploid genome which lacks a chromosome or has an extra chromosome
- Ex. 2N+1 trisomies
- Ex. 2N-1 monosomies
- Meiotic nondisjunction
What is the karyotype of Down syndrome?
47, +21
What is the karyotype of Patau syndrome?
47, +13
What is the karyotype of Edward syndrome?
47, +18
What is the karyotype of Turner syndrome?
45, X
What is the karyotype of Klinefelter syndrome?
47, XXY
What is the karyotype of Triplo-X syndrome?
47, XXX
A diploid organism has 2n = 36 chromosomes. How many chromosomes will be found in a trisomic member of this species?
2n + 1 = 36 + 1 = 37 chromosomes
How does frequency of trisomies compare with age?
Frequency of trisomies increases with age
- Eggs are stalled in Prophase I
- Recombination between the homologous pair
How does formation of eggs in humans work?
- Meiosis begins in fetus then arrests
- Germ cells remain arrested in prophase I until ovulation
- This is a prophase arrest (DNA exchange can occur at this point in time)
Why is there a correlation b/n the incidence of trisomies and maternal age?
- Bivalents with crossovers near ends are fragile and increasingly unstable over time
- If bivalent dissociates, nondisjunction of homologues occurs
Why might association b/n sister chromatids (cohesion) breakdown in older women?
- Unknown Possibilities: - Length of prophase arrest - Hormonal changes - Environmental factors
What chemicals can cause nondisjunction?
- Colchicine (used to treat plants to promote chromosome doubling)
- Bisphenol A (BPA; used in plastic bottles)
How does Down syndrome occur?
- 96% due to meiotic nondisjunction events
- About 80% in oogenesis and 20% in spermatogenesis
What are some treatments for Turner syndrome (XO)?
- Growth hormone therapy to increase stature
- Estrogen for breast development
What are some treatments for Klinefelter syndrome (XXY)?
- Testosterone therapy coupled with breast tissue removal
- More typical male phenotype
Why do Triple X females have a milder phenotype than XO?
- X-inactivation
What is a deletion/deficiency?
- Missing chromosome segment
What is a duplication?
- Extra chromosome segment
What is hypoploidy? What is an example?
- Occurs as a result of deletion
- Less genetic material
- Ex. Cri-du-chat syndrome
46, XX (5p-)
What is hyperploidy?
- More genetic material
- Occurs as a result of duplication of a chromosome segment or gain of entire chromosomes
How can a chromosome be rearranged?
- A chromosome may become rearranged internally
OR - It may become joined to another chromosome
What is pericentric inversion?
- Portion of 2 chromosomes is swapped
What is paracentric inversion?
- Heterozygote has one normal chromosome and one with inverted segment
- In Prophase I, chromosomes form inversion loop (allows homologous sequences to align)
What is a translocation?
- Segment from one chromosome is detached and reattached to a different (nonhomologous) chromosome
What is reciprocal translocation?
- Pieces of 2 nonhomologous chromosomes are exchanged without net loss of genetic material
What configuration forms between chromosomes of a heterozygous individual with translocation?
- Crosslike configuration forms during prophase I of meiosis
- Because each chromosome has sections that are homologous to 2 other chromosomes
What are the 3 ways chromosomes can separate in anaphase I after translocation?
- Alternate
- Adjacent-1
- Adjacent-2
What does alternate segregation look like and what is the result?
- 2 translocated chromosomes together and 2 normal together
- Joined diagonally
- Viable gametes are formed
- 2 gametes have both normal chromosomes, and other 2 have both translocated chromosomes
What does adjacent-1 segregation look like and what is the result?
- Split horizontally (N1 with T2 and T1 with N2)
- Nonviable gametes are formed
- Each gamete has 1 normal and one translocated chromosome
- Some genes are present in 2 copies and others are missing
What does adjacent-2 segregation look like and what is the result?
- Rare
- Split vertically (N1 with T1 and N2 with T2)
- Nonviable gametes are formed
- Each gamete has 1 normal and one translocated chromosome
- Some genes are present in 2 copies and others are missing in each gamete
What are compound chromosomes and how are they formed?
- Formed by fusion of homologous chromosomes, sister chromatids or homologous chromosome segments
What are robertsonian translocations and how are they formed?
- Fusion of 2 nonhomologous chromosomes at their centromeres
- Short arm of acrocentric chromosome is exchanged with long arm of another
- Creates a large metacentric chromosome
- A fragment that often fails to segregate is lost
What is special about human chromosome 2?
- It is metacentric
- Has arms that correspond to 2 different acrocentric chromosomes in the genomes of great apes
What are some examples of translocation leading to disease?
- 5% of Down syndrome cases are caused by Robertsonian translocation of chromosome 21 long arm onto long arm of #14.
- Translocations can cause cancer by moving a gene to a chromosome where it will be misregulated
How is translocation seen when a parent who is a translocation carrier is crossed with a normal parent?
- Carrier parent has normal phenotype
- Gametogenesis produces gametes (each have one translocated chromosome)
- 2/3 of offspring will have normal phenotype and 1/3 will show disease (ex. Down syndrome)
- Other chromosomal combinations result in spontaneously aborted embryos
