Chapter 6: Chromosome Variation Flashcards
What are the four major types of eukaryotic centromere positions (from lowest to highest position)?
- Metacentric (lowest)
- Submetacentric
- Acrocentric
- Telocentric (highest)
What does staining allow overall?
Allows you to pair the homologous chromosomes together, and to count the total number of chromosomes
What is C-banding?
Centromeric heterochromatin
What is Q-banding?
A-T rich regions
What is G-banding?
Differences in the amount of C-G
What is R-banding?
C-G rich regions
What do banding patterns determine?
Determines whether there is an abnormality within the karyotype
What leads to developmental abnormalities?
Unbalanced gene dosage
Why do unbalanced gene dosages lead to developmental abnormalities?
- Expression must occur in a specific way
- Timing, amount of gene product, and environment must be appropriate
Define chromosome rearrangement.
Change in chromosome structure
Define chromosome duplication.
Duplication of a chromosome segment
Define chromosome deletion.
Deletion of a chromosome segment
Define inversion.
Chromosome segment is inverted 180 degrees
Define paracentric inversion.
Inversion that does NOT include the centromere in the inverted region
Define pericentric inversion.
Inversion that includes the centromere in the inverted region
Define translocation.
Movement of a chromosome segment to a non-homologous chromosome or to another region of the same chromosome
Define aneuploidy.
- Change in the number of individual chromosomes
- One or more individual chromosomes are added or deleted
Define polyploidy.
Addition of entire chromosome sets (3n, 4n, 5n, or more)
Define nullisomy.
Loss of both members of a homologous pair
Define monosomy.
Loss of one member of a homologous pair
Define trisomy.
Gain of one chromosome, resulting in three homologous chromosomes
Define tetrasomy.
Gain of two homologous chromosomes, resulting in four homologous chromosomes
Define autopolyploidy.
Polyploidy in which extra chromosome sets are derived from the same species
Define allopolyploidy.
Polyploidy in which extra chromosome sets are derived from two or more species
What are the three types of chromosome mutations?
- Rearrangements
- Aneuploids
- Polyploids
What are the four basic types of chromosome rearrangement?
- Duplication
- Deletion
- Inversion
- Translocation
What happens to duplicated regions during pairing in prophase I?
The duplicated region must loop out to allow the homologous sequences of the chromosomes to align
What does the Bar phenotype in Drosophila result from?
An X-linked duplication
What are the possible consequences of unequal crossing-over?
One chromosome has a duplication, and the other a deletion
What is the consequence of unequal crossing-over between chromosomes both containing a duplication?
- Chromosome with three copies of the gene
- Wild-type chromosome (one copy)
What happens during meiosis in an individual heterozygous for a deletion?
The normal chromosome loops out during chromosome pairing in prophase I
What is the Notch phenotype in Drosophila? How is it produced?
- Produced by a chromosome deletion that includes the Notch gene
- Produces wing veination
Give an example of a phenotypic change in humans caused by a deletion.
- Cri-du-chat
- Smaller head, mental retardation, cries that resemble a cat
What occurs in an individual heterozygous for paracentric or pericentric inversion?
Chromosomes form an inversion loop during pairing in prophase I
Why does looping-out occur in prophase I?
To allow for the homologous sequences to align
In a heterozygous individual, a single cross-over within a paracentric or pericentric inversion leads to waht?
Leads to ABNORMAL gametes
How does chromosome 4 differ in humans and chimpanzees? What does that explain?
- Differs in a pericentric inversion
- Explains why we are related to chimpanzee, but have a different genotype
What is a Robertsonian translocation?
The short arm of one acrocentric chromosome is exchanged with the long arm of the other
What does a Robsertsonian translocation create?
- Large metacentric chromosome
- Fragment that often fails to segregate and is lost
In an individual heterozygous for a reciprocal translocation, what forms during homologous pairing? When?
- Cross-like structures
- During prophase I of meiosis
Chromosome 2 in humans contains a ___________________________ that is NOT present in chimpanzees, gorillas, or orangutans?
Robertsonian translocation
How does aneuploidy differ from rearrangement?
Rearrangement only concerns a small segment of a single chromosome
What are the consequences of aneuploids produced through non-disjunction in meiosis I? What are the zygotes formed?
- 2 trisomic
- 2 monosomic
What are the consequences of aneuploids produced through non-disjunction in meiosis II? What are the zygotes formed?
- 1 trisomic
- 1 monosomic
- 2 normal diploid
What are the consequences of aneuploids at the mitosis level?
Cancer and tumor cells
Where do the most common aneuploidy seen in living humans occur? Give examples.
- Sex-chromosomes
- Turner and Klinefelter syndrome
What is the cause of Down syndrome?
Three copies of one or more genes located on chromosome 21
What are the two types of Down syndrome? How do they differ?
- Primary
- Familial
- Differ in the source of the third chromosome
What is the cause of primary Down syndrome?
- Non-disjunction in one parent, which results in three copies of chromosome 21 of the progeny
- Happens randomly in the population
When does primary Down syndrome usually occur?
The incidence increases with maternal age (past 40 years old)
Why does the incidence of aneuploidy increase with maternal age?
- Because the ovules are produced early in development; they age with us
- Ovules that are 50 years old have a much higher chance of creating aneuploidy than 20 year olds
Familial Down syndrome arises in offspring whose parents are what?
- Carriers of chromosomes that have undergone a Robertsonian translocation
- Most commonly between chromosome 21 and 14
How many chromosomes do carriers of Down syndrome have? Do they exhibit symptoms?
- 45 chromosomes
- No symptoms
Why is our organism capable of tolerating multiple copies of chromosome 21?
Because chromosome 21 is the smallest chromosome in the karyotype (affects little biological processes)
Vegetables and fruits are examples of what?
Polyploids
What can autopolyploidy arise from?
Non-disjunction in mitosis
What can autoploidy arise from?
Non-disjunction in meiosis
Define chromosome mutations.
Variations in chromosome structure AND number
What is a karyotype?
The complete set of chromosomes possessed by an organism
What is the cause of many chromosome rearrangements?
- Originate when double-strand breaks occur in the DNA molecules found within a chromosome. Broken ends are either rejoined correctly, or wrong ends are connected (chromosome rearrangement)
- Or, errors in crossing-over, or when crossing-over occurs between repeated DNA strand
Differentiate tandem, displaced, and reverse duplication.
- Tandem: duplicated segment is immediately adjacent to the original
- Displaced: duplicated segment is located some distance from the original
- Reverse: when the duplication is inverted
How does a chromosome duplication alter the phenotype?
- Abnormal gene dosage
- If the amount of one protein increases while the amounts of others remain constant, problems can result
What are segmental duplications?
Duplications that are greater than 1000 bp in length
Chromosome duplications often result in abnormal phenotypes because:
A) developmental processes depend on the relative amounts of proteins encoded by different genes
B) extra copies of the genes within the duplicated region do not pair in meiosis
C) the chromosome is more likely to break when it loops in meiosis
D) Extra DNA must be replicated, which slows cell division
A) developmental processes depend on the relative amounts of proteins encoded by different genes
What occurs when a deletion includes the centromere?
The chromosome will not segregate in meiosis or mitosis and will usually be lost
Why are deletions lethal in the homozygous state?
Because all copies of any essential genes located in the deleted region are missing
How can deletions affect normally recessive mutations?
If the wild-type allele has been deleted, it will no longer be able to mask the recessive allele’s expression
What is pseudodominance? What does it indicate?
- The expression of a normally recessive mutation
- Indicates that one of the homologous chromosomes has a deletion
What is a haplo-insufficient gene?
- When a single copy of a gene is not sufficient to produce a wild-type phenotype
- Ex: Notch in Drosophila
Which chromosomal mutation leads to a gene being broken in two parts, and one part potentially moving to a new location?
Inversion
What are the consequences of an individual that is homozygous for a particular inversion during meiosis?
No special problems arise
What are the results for crossing-over within a paracentric inversion?
- Dicentric chromosome
- The chromosome lacking a centromere is lost in anaphase I
- Non-recombinant gametes: one viable (wild-type), one with paracentric inversion
- Recombinant gametes: missing some genes (non-viable)
Can recombinant progeny result from crossing-over with a paracentric inversion?
The recombinant ones are non-viable
Why are the recombinant gametes resulting from pericentric inversion non-viable?
Because genes are either missing or present in too many copies
A dicentric chromosome is produced when crossing-over takes place in an individual heterozygous for which type of chromosome rearrangement? A) Duplication B) Deletion C) Paracentric inversion D) Pericentric inversion
C) Paracentric inversion
Differentiate translocation and crossing-over.
- Translocation: movement of genetic material between non-homologous chromosomes
- Crossing-over: movement of genetic material between homologous chromosomes
Differentiate a non-reciprocal and a reciprocal translocation.
Reciprocal implies a two-way exchange of segments
Why do individuals heterozygous for a reciprocal translocation exhibit reduced fertility?
Because only about half of the gametes from them are expected to be functional
What are the three possible causes of aneuploidy?
1) Chromosome lost in mitosis or meiosis if its centromere is deleted
2) Small chromosome generated through the Robertsonian translocation may be lost in mitosis or meiosis
3) Non-disjunction
How many chromosomes does a nullisomic zygotes possess?
- Loss of both members of a homologous pair of chromosomes
- 44 chromosomes
How many chromosomes does a monosomic zygotes possess?
- Loss of a single chromosome
- 45 chromosomes
How many chromosomes does a trisomic zygotes possess?
- Gain of a single chromosome
- 47 chromosomes
How many chromosomes does a tetrasomic zygotes possess?
- Gain of two HOMOLOGOUS chromosomes (four homologous copies of a particular chromosome)
- 48 chromosomes
A diploid organism has 2n = 36 chromosomes. How many chromosomes will be found in a trisomic member of this species?
37 chromosomes
How many chromosomes do individuals with familial Down syndrome have?
- They have 46 chromosomes (NOT 47)
- An extra copy of part of chromosome 21 is attached to ANOTHER chromosome through translocation (usually 14)
Maternal age is associated with _________.
non-disjunction
Why is there no age effect in males for incidences of non-disjunction?
Because sperm are produced continuously after puberty
Why are sex-chromosome aneuploidies more common than autosomal aneuploidies in humans and other mammals?
- Dosage compensation prevents the expression of additional copies of X-linked genes in mammals
- Little information in the Y chromosome
- Little gene dosage effects in both cases
- Autosomal aneuploidies are often lethal
What arises when a non-disjunction in meiosis produces a diploid gamete that then fuses with a normal haploid gamete?
Produces an autotriploid (3n)
What is autopolyploidy caused by?
Accidents of mitosis or meiosis that produce extra sets of chromosomes, all derived from a SINGLE species
Why is polyploidy wanted in agriculture?
More chromosomes = more DNA = bigger size
Why don’t triploids usually produce viable offspring?
- Unbalanced gamete fuses with a normal gamete
- The difference in number creates unbalanced gene dosage in the offspring (lethal)
What are the two causes of autopolyploidy?
- Non-disjunction in mitosis
- Non-disjunction in meiosis
What does allopolyploidy arise from?
- The hybridization between two species
- The resulting polyploid carries chromosome sets derived from two or more species
What is hybridization?
The “forced” process of breeding two species that would not normally fertilize
What F1 generation does a hybridization between two diploid species produce? What do they result in during meiosis?
- A hybrid with NON-HOMOLOGOUS chromosomes
- They do NOT pair and segregate properly in meiosis
- Unbalanced, non-viable gametes
How may a hybrid produce normal, viable gametes? Why?
- By forcing non-disjunction at an early mitotic cell division
- Non-disjunction leads to the doubling of all chromosomes, producing an allopolyploid
What is an amphidiploid?
Allopolyploid, consisting of two combined diploid genomes (produced through non-disjunction)
Species A has 2n = 16 chromosomes and species B has 2n = 14. How many chromosomes would be found in an allotriploid of these two species? A) 21 or 24 B) 42 or 48 C) 22 or 23 D) 45
C) 22 or 23
