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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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  • 30 oktober 2021
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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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