Bentzen 10 - Chromosome Variations Flashcards
What are chromosome variations (mutations)?
Permanent chromosmal changes that can be passed on to offspring if they occur in cells that will become gametes.
What are two general types of chromosome variations (mutations)?
- Chromosomal rearrangement - changes in the structure of individual chromosomes
- Variation in chromosome numbers - changes in the number of chromosomes. One or more individual chromosomes are added or deleted (includes aneuploids and polyploids)
How many autosomes do humans have?
22
How many sex chromosomes do humans have?
1
What does a metacentric chromosome look like?
Equal length of the chromosome arms extending from the centromere
What does submetacentric chromosome look like?
There is a short arm and a long arm
What does an acrocentric chromosome look like?
Stubby arms and long arms
What does a telocentric chromosome look like?
Two arms extending from the centromere
What are chromosome rearrangements?
When a length of the chromosome is duplicated or something else happens to it. There is no change in ploidy level
What is aneuploidy?
Also known as trisomy. Where there is ONE extra chromosome added
What is polyploidy?
Also known as autotriploid, where there are multiple numbers of chromosomes made, eg. 3n=9
What is the genetic reason for the Cri du Chat syndrome?
A deleted region of chromosome 5
What is the genetic reason for Down Syndrome?
Trisomy (aneuploidy) of chromosome 21
What are four types of chromosome rearrangements?
Duplication (length in chromosome duplicated)
Inversion (Where a sequence in chromosome inverts)
Deletion (Where a sequence in a chromosome is deleted)
Reciprocal Translocation (where a sequence in a chromosome is switched with another chromosome)
How are chromosome rearrangements made?
They are a consequence of repair mechanisms gone wrong after double-stranded DNA breaks
At what part of a cell’s life cycle can deletion loops be detected?
During meiosis
How can you detect chromosome deletions?
There is a formation of a deletion loop during pairing of homologs in prophase I, in prophase I, the normal chromosome must loop out for the homologous sequences of the chromosomes to align.
What is a consequence of chromosome deletions?
The genes that are deleted will not be deleted on the homologous chromosome so that the other genes will be expressed no matter what (in terms of recession or dominance).
Will lead to more mutant traits expressed (rather than the wild type)
What is a tandem chromosomal duplication?
It is the simplest type of duplication where the two segments are adjacent to one another.
What are the effects of duplication of a segment of a chromosome?
There is no loss of genetic information.
Usually viable, but bad effects from abnormal gene dosage can occur.
Important in evolution
What generates chromosomal duplications and deletions?
Unequal crossing over of misaligned chromosomes generates duplications and deletions
How can you detect tandem duplications in chromosomes?
During alignment of homologous chromosomes in prophase I of meiosis a duplicated chromosome forms a loop
What does more copies of Bar gene region in Drosophilia lead to?
Fewer eye facets. A consequence of duplication
What is it called in duplication where both copies of a gene remain the same after duplication?
redundancy
What is it called where only one duplicated gene becomes inactive?
Pseudogene
What is it called where one duplicated gene acquires a new function?
Neofunctionalization (important in evolution, often tandem to the original duplicated gene)
What causes inversions in chromosomes?
2 breaks on a chromosome
What is the consequence of inversion in chromosomes?
Gametes are usually viable because genetic information is not lost or gained, but viability changes if crossing over occurs!
What is pericentric inversion?
Where inversion of the chromosome involves the centromere
What is paracentric inversion?
Where inversion in the chromosome does not involve the centromere (just somewhere in the arms)
What is the position effect from chromosome inversions?
It is where new location of genes on the chromosome alters gene expression in some cells (gene is silenced in some but not in others, leading to ‘patchy’ phenotype)
What is the consequence of crossing over after inversion in a chromosome?
Crossing over outside of inverted region: viable gametes
Crossing over inside inverted region: some nonviable gametes
Inverted chromosomes forms an inversion loop
What is a dicentric chromatid?
Where there are two centromeres attached by chromatid from crossing over in paracentric inverted chromosomes (no centromere involved). The link between the chromatids is called the dicentric bridge.
Dicentric = two centromeres
What is a acentric chromatid?
A chromatid with no centromere, can happen when dicentric bridge is formed as a consequence of crossing over in paracentric inverted chromosomes.
What is the consequence of recombination in paracentric inversion chromosomes?
It will produce non-viable gametes because they are missing some genes
What is the consequence of recombination and pericentric inversion in chromosomes?
Recombinant gametes are nonviable because genes are missing or present in too many copies
What is reciprocal translocation?
Echange of genetic material between nonhomologous chromosomes
also in prophase I of meiosis there is a pairing configuration between the two heterozygous chromosomes (they make a + shape)
What is a consequence of reciprocal translocation?
Creation of a gene fusion or hybrid gene. Can lead to protein produced with improper functions, leading to cancer
How can reciprocal translocation be detected?
In a karyotype using chromosome painting to show different coloured chromosome pieces in one chromosome (which should all be one colour)
What are three different ways that reciprocal translocated chromosomes may segregate in anaphase? Which one produces viable gametes?
Alternate segregation (Where the two normal copies separate from the translocated copies)
Adjacent-1 segregation (where the T1 and N2 separate from T2 and N1)
Adjacent-2 Segregation (where the T1 and N1 segregate form T2 and N2 - rare)
Only alternate segregation produces viable gametes
What is Robertsonian translocation?
The fusion of two chromosomes, leading to the loss of genetic information.
Two acrocentric chromosomes form break point which subsequently fuse leading to a metacentric chromosome and a chromosome fragment (which is lost due to a failure to segregate).
What are three ways that chromosomes can separate in Robersonian Translocation?
Adjacent-1 segregation (common and unbalanced, two fused with fragment, and two fused no fragment)
Adjacent-2 Segregation (Rare, unbalanced, two fused with long fragment, two non fused)
Alternate segregation (common and balanced, two fused and two normal)
How does Robsertsonian translocation provide evidence for evolution?
Humans have 2n=46 chromosomes and great apes hae 2n=48 chromosomes. Logically there must be two fused chromosomes in humans and there is, it is human chromosome 2, which is seperated in two for the great apes.
Are copy number variations (CNV) common in the human genome? How big are they?
Yes. Thousands of base pairs. The average person has 1000 and most have no observable effects, but some are associated with deseases such as osteoporosis, autism and schizophrenia
