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Summary Genetics Chapter 8
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A summary of Genetics Chapter 8 of the book 'Genetics Analysis& Principles'.
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8 Variation in chromosome structure+number
Genetic variation= genetic differences among members of the same species or among different specie
Allelic variation= genetic variation in a population that involves the occurrence of 2/ more different alleles
for a particular gene.
8.1 Microscopic examination of eukaryotic chromosomes
Cytogeneticists= studies chromosomes under the microscope.
Determine common chromosomal composition and identify individuals with variation in chromosome
number/ structure+ provides a way to distinguish between species.
3 main features to classify+ identify chromosomes: location of the centromere, size+ banding patterns (revealed
when chromosomes are treated with stains)-> classified regarding centromere location-> centromere never exactly
in the center; each chromosome has a short arm (p)+ long arm (q):
Metacentric= centromere near the middle.
Submetacentric= centromere slightly off center.
Acrocentric= centromere significantly off center, but not at the end.
Telocentric= centromere is at one end (short arm may be nonexistent).
Karyotype= photographic representation in which all the chromosomes within a single cell have been arranged in a
standard fashion-> short arms on top, long arms on the bottom-> largest chromosome smallest number (except X+Y)
For detailed identification: treated with stains; produces characteristic banding patterns-> chromosomes
treated with mild heat/ proteolytic enzymes that partially digest chromosomal proteins beforehand.
G bands= chromosomal banding pattern; when the chromosomes have been treated with Giemsa stain-
> some regions bind the dye heavily (dark bands); some don’t bind the stain well (light bands).
300 G bands in metaphase; more G bands in prometaphase (chromosomes are less compacted).
Banding pattern useful for:
Distinguishing chromosomes.
Detecting changes in chromosome structure.
Assessing evolutionary relationships between species.
Similar chromosome banding pattern; genetic relatedness.
8.2 Changes in chromosome structure (overview)-> fig
8.2
CHANGES IN THE TOTAL AMOUNT OF GENETIC INFORMATION IN A SINGLE CHROMOSOME
Deletion/ deficiency= a segment of DNA is missing.
Duplication= repetition of a DNA segment more than 1x within a chromosome/ genome.
CHROMOSOMAL REARRANGEMENTS
Inversion= change in the orientation of genetic material along a chromosome.
Segment is flipped/ reversed from the normal order.
Translocation= 1 chromosome segment becomes attached to a different (part of the same) chromosome:
Simple translocation= 1 piece of a chromosome becomes attached to a different chromosome.
Reciprocal translocation= 2 different chromosomes exchange pieces.
, 8.3 Deletions and duplications
The loss of genetic material in a deletion tends to be detrimental to an
organism (fig 8.3)
Chromosomal deletion: occurs when a chromosome breaks in 1/ more places+ fragment of the chromosome is lost.
Broken into 2 fragments: terminal deletion= segment is lost from the end of a linear chromosome.
Broken into 3 fragments: interstitial deletion= internal segment is lost from a linear chromosome.
Phenotypic consequences of a chromosomal deletion depend on:
Size of the deletion+ whether it includes (portions of) genes are vital to the development of the organism.
When they have a phenotypic effect; usually harmful-> larger: more harmful; more genes are missing.
Human genetic disease cri-du-chat syndrome= deletion in a segment of the short arm of human
chromosome 5-> mental deficiencies, unique facial anomalies+ unusual catlike cry in infancy.
Duplications tend to be less harmful than deletions
Duplications may be caused by abnormal crossover events:
During meiosis: crossing over after homologous chromosomes have properly aligned with each other.
Rare: crossover at misaligned sites on homologs (fig 8.5).
Misaligned: chromosome carries 2/ more homologous DNA segments that have identical/ similar
sequences; repetitive sequences= short DNA sequences; occur many times within a species’ genome.
Nonallelic homologous recombination= recombination that occurs at homologous sites within chromosomes, where
the sites aren’t alleles of the same gene-> often due to the occurrence of repetitive sequences.
Result: 1 chromatid with duplication+ 1 with deletion
Chromosome with extra genetic material; gene duplication= increase in the copy number of a gene; can
lead to the evolution of gene families.
Phenotypic consequences of duplications tend to be correlated with size:
More likely to have phenotypic effects if they involve a large piece of a chromosome.
Small duplications are less likely to have harmful effects than deletions of comparable size.
Having only 1 copy of a gene is more harmful than having 3 copies.
In humans, relatively few well-defined syndromes are caused by small chromosomal duplications.
Duplications provide additional material for gene evolution, sometimes leading
to the formation of gene families (fig 8.6)
Gene family= 2/ more genes within a single species that are like each other; derived from the same ancestral gene.
2 copies of an ancestral gene can accumulate different mutations-> after many generations: 2 genes similar
but not identical-> can occur several times during evolution; creating a family of many similar genes.
Homologous= 2/ more genes derived from the same ancestral gene; similar DNA sequences.
Paralogs= homologous genes within a single species.
Globin genes: alle encode protein subunits that bind oxygen-> 14 paralogs on 3 different chromosomes->
advantageous to have a gene family (more mutations):
Similar function but some have more specialized functions.
Differences in the gene expression during different stages of life.
Copy number variation is relatively common among members of the same
species (fig 8.8)
Copy number variation (CNV)= DNA segment (1000 bp or more in
length) commonly exhibits copy number differences among
members of the same species; occurs at the population level.
Some species members may carry a chromosome that is
missing a (part of a) gene.
May be due to a duplication (fig 8.8)-> some undergo a
segmental duplication= duplication in which a small
chromosome segment has more than 1 copy of the
same gene.
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