Chapter 8 - Chromsome Variation Flashcards
What do the following chromosomal morphologies indicate?
- Metacentric
- Submetacentric
- Acrocentric
- Telocentric
Meta - middle, so middle of chromosome is where centromere is located
Submeta - below middle, so centromere is just below (or above depending on orientation) the middle of chromosome. Creating p (petit, short arm) and q (long arm) arms.
Acro - top/high point, so centromere is near the top/bottom of the chromosome (depending on orientation. the short region is known as a “satellite”
Telo - end, so centromere is very near to the end of the chromosome.
What are the different types of chromosome abnormalities?
Aneuploidy: addition/removal of one or more individual chromosome is added (2n+x)
Polyploidy: one or more complete set(s) of chromosomes are added (Xn. i.e. 3n)
Chromosomal rearrangement: pieces of chromosomes may be duplicated/deleted/inverted/translocated (2n but rearranged)
What is a chromosomal duplication? What are the different types of duplications?
The repetition of segments in a chromosome. The duplicated segment can either be positioned adjacently (tandem duplication) or in a different segment (displaced duplication). The orientation of the duplicated segment can either be forward or reversed.
Example:
Normal segment: ABC - DEFG
Tandem duplication: ABC - DEEEFG
Displaced duplication ABC - DEFDG
Reverse duplication (can follow either positional pattern): ABC - DEFFEDG
What are the large duplicated regions in humans called?
Segmental duplications, which are duplications that are over 1000bp long. These are typically intrachromosomal (on same chromosome) but can also be interchromosomal (on different chromsomes).
What is the effect of chromosomal duplication?
Chromosome duplication can lead to unbalance gene dosage which can lead to higher transcriptions of some genes. This can cause developmental issues. An example of this is the Bar gene in Drosophilia melanogaster. Bar gene is an X-linked gene and if it has duplications of the bar gene on one or more of the chromosomes (in females) then there will be a smaller eye size in the organism.
What is the main reason for duplications and deletions to occur in chromosomes?
Unequal crossing over. During prophase I alignment of homologous chromosomes occur and this can sometimes be inexact and incorrect. For example, the red and green opsin genes (which code for production of colour cones in eyes) are on the X chromosomes in humans. It is often that the X chromosomes are aligned improperly which causes the crossing over of two different regions, this causes an imbalance of the opsin genes as one x chromosome has an extra opsin gene and one had their opsin gene deleted. So this can cause colour blindness in the offspring that lacks the gene. More common in men as there is only 1 X chromosome.
Why are segmental duplications beneficial?
Segmental duplications, duplications >1000bp are beneficial as it can frontier evolution. An example of this are the specialised types of globin proteins that carry oxygen for specific reasons. These genes were duplicated at some point and allowed for beneficial mutations to occur, increasing individual fitness over time.
What can chromosomal deletions and heterozygous duplications cause in chromosomes?
can cause unequal pairing and lead to loops forming as the chromosomes will chemically want to form homologous pairing.
What are the phenotypic effects of chromosomal deletion?
Can lead to three things:
- Abnormal development due to genetic imbalances.
- Can cause pseudodominance, whereby a normally recessive mutation is expressed due to the deletion of the dominant wild-type allele
- Can lead to haploinsufficiency which is the lack of key genes for physiological function. So, if this occurs during fertilisation the off-spring will not be fully functional.
What is a chromosome inversion? and what are the types of chromosomal inversion?
A chromosome segment is inverted in its order.
i.e. ABC - DEFGH -> ABC - DGFEH.
Inversions require a chromosome to break in two places.
Paracentric inversions (para – next to): inversions that do not include the centromere
Pericentric inversions (peri – around): inversions that include the centromere
How is it known that chromosomal inversions play an important role in human evolution?
Due to G-banding patterns its been discovered that some chromosomes differ between from chimpanzees and humans solely by a pericentric inversion.
What are the phenotypic effects of chromosomal inversions?
Whilst the genetic data isn’t lost on a chromosome, the order is imperative in its expression. So having an incorrect order can make genes inactive/overtly active which has issues. An example of this is in Dosophila where if the white locus is inverted to an area where the chromosomes is highly condensed and inactive. It won’t be expressed properly and lead to an eye with white and red spots.
What is the effect of chromosomal paracentric inversion on meiosis?
As it is a paracentric inversion (para - next to) the centromeres are still positioned equally between the chromosomes. In prophase 1 an inversion loop will form as the chromosomes will try to interact normally (homologous). This loop will then have crossing over take place between the chromosomes which will cause one chromatid to now have 2 centromeres and one chromatid to have no centromere. During anaphase I the chromosome lacking a centromere is lost as it cannot bind to spindle microtubules. The dicentric chromatid forms a dicentric bridge as it is pulled to both poles of the cell, which eventually breaks into two pieces.
At the end, after anaphase II. there are:
– 1 non-recombinant viable gamete (wild-type)
– 1 non-recombinant gamete with paracentric inversion which can cause issues.
– 2 nonviable recombinant gametes (as it was the dicentric chromosome that was split apart)
What is the effect of chromosomal pericentric inversion on meiosis
As it is pericentric (peri - around) inversion the centromeres are located in different areas when chromosomes are lining up in prophase I. This causes a inversion loop and if crossing over occurs once this leaves two chromatids having too many of the same gene and some have none (deleted) after meiosis.
This leaves:
1 non-recombinant viable gamete
2 recombinant non-viable gametes
1 non-recombinant gamete with pericentric inversion, which can cause problems,
What occurs if two events of crossing over occurs on the same 2 chromosomes during pericentric and paracentric inversion in meiosis?
Nothing, both of them cancel each other out allowing there to be 4 gametes, 2 wild type and 2 inverted.
Why are inversions important in evolution?
it represses the recombinant chromosomes from appearing in progeny and allows for genes that are better adapted for the individual/species to be proliferated more often.
What are dicentric and acentric chromosomes?
Dicentric = has 2 centromeres on a single chromosome
Acentric = has 0 centromeres on a single chromosome
What is a chromosomal translocation? What are the types of chromosomal translocation?
A chromosomal translocation is the movement of genetic code between non-homologous chromosomes.
Reciprocal translocation - genetic material is transferred from one chromosome to another, with the receiving chromosome returning their genetic material to the original sending chromosome.
i.e. AB-CDEF, GHI-JK => AB-CDJK, GHI-EF
Non-reciprocal translocation - genetic material is transferred from one chromosome to another without anything being given to the original sender.
i.e. AB-CDEF, GHI-JK => AB-CD, GHIEFJK
What are the phenotypic effects of chromosomal translocations?
- Can cause genes to be physically linked to one another, which can effect its gene regulation as its under new regulatory sequences.
- Can cause gene breaks (as some of the original genetic material is replaced/lost) which can result in proteins being synthesised incorrectly and can cause severe issues like neurofibromatosis.
What is a Robertsonian translocation?
A Robertsonian translocation is a translocation that occurs between two non-homologous acrocentric chromosomes. This causes the two q arms to associate and the p arms to associate. This forms a long metacentric chromosome and a fragment, which isn’t large enough to split apart during mitosis/meiosis so it is often lost.
This results in the loss of a chromosome and can lead to aneuploidy. Like downsyndrome (trisomy 21 or 14 in most cases)
How does a translocation affect meiosis?
Translocations affect meiosis as during prophase I they form a ‘cross-like’ configuration, as the non-homologous chromosomes associate with one each other as crossing over would usually occur. This can cause three different types of division:
Alternate segregation: as the chromosomes split in a way that, whilst chromosomes are mixed in genetic code. They still pertain the information that existed in both chromosomes equally, just ordered differently. No genes are missing nor have multiple duplicates. these gametes are VIABLE
Adjacent-1-segregation: the chromosomes that split apart have unequal amounts of genetic data and some is lost.
Adjacent-2-segregation: similar as adj-1 but different.
Both adj-1 and 2 both produce UNVIABLE gametes.
Check diagram for visualisation.
What is a ‘fragile site’ on a chromosome
A fragile site is a region of a chromosome that is prone to breakage under certain conditions. Fragile-x-syndrome is a neurological disorder that occurs due to a fragile site in the X chromosome as there is typically a small ‘gap’ in the area on x chromosome that can be filled with CGG repeats hundreds of times.
