1. VARIATION IN CHROMOSOME STRUCTURE AND NUMBER
CHAPTER 8 part 2
VARIATION IN CHROMOSOME STRUCTURE AND NUMBER

2. Duplications
Like deletions, the phenotypic consequences of duplications tend to be correlated to size
Duplications are more likely to have phenotypic effects if they involve a large piece of the chromosome
However, duplications tend to have less harmful effects than deletions of comparable size
In humans, relatively few well-defined syndromes are caused by small chromosomal duplications
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3. Bridges’ Experiment Investigating the Bar-Eye Phenotype in Drosophila
Bar eyes is a trait in which flies have a reduced number of facets
Ultra-bar (or double-bar) is a trait in which flies have even fewer facets than the bar homozygote
Both traits are X-linked and show incomplete dominance
Figure 8.6
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4. Bridges’ Experiment Investigating the Bar-Eye Phenotype in Drosophila
Calvin Bridges in the 1930s investigated the bar/ultra-bar phenomenon at the cytological level
The cells of the salivary gland of Drosophila have gigantic chromosomes, termed polytene chromosomes
The banding patterns on these chromosomes is easily seen
It is thus possible to detect the duplication or deletion of single genes
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5. Testing the Hypothesis
Information concerning the nature of the bar and ultra-bar phenotypes may be revealed by a cytological examination of polytene chromosomes
Testing the Hypothesis
Refer to Figure 8.7
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6. Figure 8.7
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7. The Data
This is a drawing of a short segment of a polytene chromosome that corresponds to the region of the X chromosome where the bar allele is located. This bar allele is found within the region designated 16A
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8. Interpreting the Data 8-24
The 16A region duplication returned to the wild-type banding pattern
Interpreting the Data
Bar phenotype is caused by a duplication in region 16A of the X chromosome
Ultra-bar phenotype is caused by three copies in the 16A region
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9. Interpreting the Data
The mechanism of formation of the bar allele can be explained by a misaligned crossover
Likewise for the formation of ultra-bar and bar-revertant alleles
Figure 8.8
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10. Interpreting the Data
The bar and ultra-bar alleles are also associated with the phenomenon of position effect
A female that is homozygous for the bar allele has four copies of region 16A
And 70 facets
A female that is heterozygous for the ultra-bar and normal alleles also has four copies of region 16A
But only 45 facets
Figure 8.6
The positioning of three copies next to each other on the X chromosome increases the severity of the defect
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11. Duplications and Gene Families
The majority of small chromosomal duplications have no phenotypic effect
However, they are vital because they provide raw material for additional genes
This can ultimately lead to the formation of gene families
A gene family consists of two or more genes that are similar to each other
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12. 8-28 Figure 8.9 Genes derived from a single ancestral gene
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13. A well-studied example is the globin gene family
The genes encode polypeptides which function in proteins that bind oxygen
Hemoglobin
The globin gene family is composed of 14 homologous genes on three different chromosomes
All 14 genes are derived from a single ancestral gene
Accumulation of different mutations in the members of the gene family created
1. Globin genes that are expressed during different stages of human development
2. Globin proteins that are more specialized in their function
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14. 8-30 Figure 8.10 Expressed very early in embryonic life
Expressed maximally during the second and third trimesters
Expressed after birth
Duplication
Better at binding and storing oxygen in muscle cells
Better at binding and transporting oxygen via red blood cells
Figure 8.10
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15. Inversions
A chromosomal inversion is a segment that has been flipped to the opposite orientation
Centromere lies within inverted region
Centromere lies outside inverted region
Figure 8.11
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16. In rare cases, inversions can alter the phenotype of an individual
In an inversion, the total amount of genetic information stays the same
Therefore, the great majority of inversions have no phenotypic consequences
In rare cases, inversions can alter the phenotype of an individual
Break point effect
The breaks leading to the inversion occur in a vital gene
Position effect
A gene is repositioned in a way that alters its gene expression
About 2% of the human population carries inversions that are detectable with a light microscope
Most of these individuals are phenotypically normal
However, a few an produce offspring with genetic abnormalities
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17. Inversion Heterozygotes
Individuals with one copy of a normal chromosome and one copy of an inverted chromosome
Such individuals may be phenotypically normal
They also may have a high probability of producing gametes that are abnormal in their genetic content
The abnormality is due to crossing-over in the inverted segment
During meiosis I, homologous chromosomes synapse with each other
For the normal and inversion chromosome to synapse properly, an inversion loop must form
If a cross-over occurs within the inversion loop, highly abnormal chromosomes are produced
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18. Figure 8.12
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19. Translocations
A chromosomal translocation occurs when a segment of one chromosome becomes attached to another
In reciprocal translocations two non-homologous chromosomes exchange genetic material
Reciprocal translocations arise from two different mechanisms
1. Chromosomal breakage and DNA repair
2. Abnormal crossovers
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20. Telomeres prevent chromosomal DNA from sticking to each other
Figure 8.13
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21. Translocations
Reciprocal translocations lead to a rearrangement of the genetic material, not a change in the total amount
Thus, they are also called balanced translocations
Reciprocal translocations, like inversions, are usually without phenotypic consequences
In a few cases, they can result in position effect
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22. Example: Familial Down Syndrome
In simple translocations the transfer of genetic material occurs in only one direction
These are also called unbalanced translocations
Unbalanced translocations are associated with phenotypic abnormalities or even lethality
Example: Familial Down Syndrome
In this condition, the majority of chromosome 21 is attached to chromosome 14 (Figure 8.14a)
The individual would have three copies of genes found on a large segment of chromosome 21
Therefore, they exhibit the characteristics of Down syndrome
Refer to Figure 8.14b
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23. Familial Down Syndrome is an example of Robertsonian translocation
This translocation occurs as such
Breaks occur at the extreme ends of the short arms of two non-homologous acrocentric chromosomes
The small acentric fragments are lost
The larger fragments fuse at their centromeic regions to form a single chromosome
This type of translocation is the most common type of chromosomal rearrangement in humans
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24. 8.2 VARIATION IN CHROMOSOME NUMBER
Chromosome numbers can vary in two main ways
Euploidy
Variation in the number of complete sets of chromosome
Aneuploidy
Variation in the number of particular chromosomes within a set
Euploid variations occur occasionally in animals and frequently in plants
Aneuploid variations, on the other hand, are regarded as abnormal conditions
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25. Figure 8.16 Polyploid organisms have three or more sets of chromosomes
Individual is said to be trisomic
Individual is said to be monosomic
Figure 8.16
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26. Aneuploidy
The phenotype of every eukaryotic species is influenced by thousands of different genes
The expression of these genes has to be intricately coordinated to produce a phenotypically normal individual
Aneuploidy commonly causes an abnormal phenotype
It leads to an imbalance in the amount of gene products
Refer to Figure 8.17
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27. In most cases, these effects are detrimental
They produce individuals that are less likely to survive than a euploid individual
Figure 8.17
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28. Aneuploidy
Alterations in chromosome number occur frequently during gamete formation
About 5-10% of embryos have an abnormal chromosome number
Indeed, ~ 50% of spontaneous abortions are due to such abnormalities
In some cases, an abnormality in chromosome number produces an offspring that can survive
Refer to Table 8.1
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30. The autosomal aneuploidies compatible with survival are trisomies 13, 18 and 21
These involve chromosomes that are relatively small
Aneuploidies involving sex chromosomes generally have less severe effects than those of autosomes
This is explained by X inactivation
All additional X chromosomes are converted into Barr bodies
The phenotypic effects listed in Table 8.1 may be due to
1. The expression of X-linked genes prior to embryonic X-inactivation
2. An imbalance in the expression of pseudoautosomal genes
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31. Some human aneuploidies are influenced by the age of the parents
Older parents more likely to produce abnormal offspring
Example: Down syndrome (Trisomy 21)
Incidence rises with the age of either parent, especially mothers
Figure 8.19
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32. Down syndrome is caused by the failure of chromosome 21 to segregate properly
This nondisjunction most commonly occurs during meiosis I in the oocyte
The correlation between maternal age and Down symdrome could be due to the age of oocytes
Human primary oocytes are produced in the ovary of the female fetus prior to birth
They are however arrested in prophase I until the time of ovulation
As a woman ages, her primary oocytes have been arrested in prophase I for a progressively longer period of time
This added length of time may contribute to an increased frequency of nondisjunction
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33. Euploidy Most species of animals are diploid
In many cases, changes in euploidy are not tolerated
Polyploidy in animals is generally a lethal condition
Some euploidy variations are naturally occurring
Female bees are diploid
Male bees (drones) are monoploid
Contain a single set of chromosomes
A few examples of vertebrate polyploid animals have been discovered
Rat - Argentinean
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34. Euploidy
In many animals, certain body tissues display normal variations in the number of sets of chromosomes
Diploid animals sometimes produce tissues that are polyploid
This phenomenon is termed endopolyploidy
Liver cells, for example, can be triploid, tetraploid or even octaploid (8n)
Polytene chromosomes of insects provide an unusual example of natural variation in ploidy
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35. Polytene Chromosomes
Occur mainly in the salivary glands of Drosophila and a few other insects
Chromosomes undergo repeated rounds of chromosome replication without cellular division
In Drosophila, pairs of chromosomes double approximately nine times (29 = 512)
These doublings produce a bundle of chromosomes that lie together in a parallel fashion
This bundle is termed a polytene chromosome
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36. Each chromosome attaches to the chromoventer near its centromere
Central point where chromosomes aggregate
Figure 8.21
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37. Because of their size, polytene chromosomes lend themselves to an easy microscopic examination
They are so large, they can be even seen in interphase
Polytene chromosomes exhibit a characteristic banding pattern (Figure 8.21b)
Each dark band is known as a chromomere
The DNA within the dark band is more compact than that in the interband region
Cytogeneticists have identified about 5,000 bands
Polytene chromosomes have facilitated the study of the organization and functioning of interphase chromosomes
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38. Euploidy In contrast to animals, plants commonly exhibit polyploidy
30-35% of ferns and flowering plants are polyploid
Many of the fruits and grain we eat come from polyploid plants
Refer to Figure 8.22a
In many instances, polyploid strains of plants display outstanding agricultural characteristics
They are often larger in size and more robust
Refer to Figure 8.22b
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39. and two copies of other chromosomes one copy of some chromosomes
Polyploids having an odd number of chromosome sets are usually sterile
These plants produce highly aneuploid gametes
Example: In a triploid organism there is an unequal separation of homologous chromosomes (three each) during anaphase I
and two copies of other chromosomes
Each cell receives
one copy of some chromosomes
Figure 8.23
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40. Sterility is generally a detrimental trait
However, it can be agriculturally desirable because it may result in
1. Seedless fruit
Seedless watermelons and bananas
Triploid varieties
Asexually propagated by human via cuttings
2. Seedless flowers
Marigold flowering plants
Developed by Burpee (Seed producers)
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42. UGA UAG UAA