Variation in chromosome structure and arrangement
The chromosomes of each species has a characteristic morphology (structure) and number. But, sometimes due to certain accidents or irregularities at the time of cell division, crossing over or fertilization some alterations in the morphology and number of chromosomes take place. The slightest variation in the organization of chromosomes is manifested phenotypically and is of great genetical interest. The changes in the genome involving chromosome parts, whole chromosomes, or whole chromosome sets are called chromosome aberrations or chromosome mutations
The chromosomal abberations due to structure change involve following types
1:Deletion
2:Duplication
3:Inversion
4:Translocation
The structural abnormalities can occur in both homologous chromosomes of a pair or in only one of them. When both homologous chromosomes are involved, these are called structural homozygotes, e.g., deletion homozygote, duplication homozygote, etc. When only one homologous chromosome is involved, it is called structural heterozygote.
Deletion or deficiencies
Deletion or deficiency as name shows is the lost of a segment of a chromosome or of the part of a genes.It was first discovered by Bridges in 1917 when he found that a part of gene concerned with eye color is lost.
The portion of chromosome with centromere will survive and the portion without the centromere will be lost after mitosis or meiosis in the resultant cells.
The chromosomes with deletions can never revert to a normal condition. If gametes arise from the cells having a deleted chromosome, this deletion is transmitted to the next generation.
The deletion or deficiency may be
Terminal Deletion
In terminal deletion a terminal section of a chromosome is absent and it is resulted by only one break.
Intercalary Deletion
While in the intercalary deletion, an intermediate section or portion of chromosome is lost and it is caused by two breaks —one on either end of the deleted region.Thus, in the latter case, the chromosome is broken into three pieces, the middle one of which is lost and the remaining two pieces get joined again.
Compensation loop
During synapsis between structural heterozygotes, the unpaired region of the normal homologue buckles out. Such a configuration is called a deficiency loop or compensation loop.Such loops can be observed in salivary gland chromosomes of Drosophila.
Genetic effect of deletion
If too much genetic information is lost as a result of deletion,The abberation may be lethal and it will cause some genetical effects in organisms.For effective study of genetical effect of deletion first we should understand the term pseudodominance.
Pseudodominance
In the presence of a deletion , a recessive allele of the normal homologous chromosome will behave like a dominant allele,mean it will be phenotypically expressed, this phenomenon is called pseudodominance.
Examples of pseudodominance (deletion)
1:Human babies missing a portion of the short arm of chromosome 5 (autosome) have a distinctive cat-like cry; derived from the French name “cri du chat” (cry of the cat) syndrome.They are also mentally retarded,have malformation in the larynx, moon faces, saddle noses, small mandibles,malformed low-set ears and microcephally (small head).
2:. Deletion of part of the short arm of one X chromosome produces a typical Turner syndrome.
The chromosomes of each species has a characteristic morphology (structure) and number. But, sometimes due to certain accidents or irregularities at the time of cell division, crossing over or fertilization some alterations in the morphology and number of chromosomes take place. The slightest variation in the organization of chromosomes is manifested phenotypically and is of great genetical interest. The changes in the genome involving chromosome parts, whole chromosomes, or whole chromosome sets are called chromosome aberrations or chromosome mutations
The chromosomal abberations due to structure change involve following types
1:Deletion
2:Duplication
3:Inversion
4:Translocation
The structural abnormalities can occur in both homologous chromosomes of a pair or in only one of them. When both homologous chromosomes are involved, these are called structural homozygotes, e.g., deletion homozygote, duplication homozygote, etc. When only one homologous chromosome is involved, it is called structural heterozygote.
Deletion or deficiencies
Deletion or deficiency as name shows is the lost of a segment of a chromosome or of the part of a genes.It was first discovered by Bridges in 1917 when he found that a part of gene concerned with eye color is lost.
The portion of chromosome with centromere will survive and the portion without the centromere will be lost after mitosis or meiosis in the resultant cells.
The chromosomes with deletions can never revert to a normal condition. If gametes arise from the cells having a deleted chromosome, this deletion is transmitted to the next generation.
The deletion or deficiency may be
Terminal Deletion
In terminal deletion a terminal section of a chromosome is absent and it is resulted by only one break.
Intercalary Deletion
While in the intercalary deletion, an intermediate section or portion of chromosome is lost and it is caused by two breaks —one on either end of the deleted region.Thus, in the latter case, the chromosome is broken into three pieces, the middle one of which is lost and the remaining two pieces get joined again.
Compensation loop
During synapsis between structural heterozygotes, the unpaired region of the normal homologue buckles out. Such a configuration is called a deficiency loop or compensation loop.Such loops can be observed in salivary gland chromosomes of Drosophila.
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| compesation loop |
Genetic effect of deletion
If too much genetic information is lost as a result of deletion,The abberation may be lethal and it will cause some genetical effects in organisms.For effective study of genetical effect of deletion first we should understand the term pseudodominance.
Pseudodominance
In the presence of a deletion , a recessive allele of the normal homologous chromosome will behave like a dominant allele,mean it will be phenotypically expressed, this phenomenon is called pseudodominance.
Examples of pseudodominance (deletion)
1:Human babies missing a portion of the short arm of chromosome 5 (autosome) have a distinctive cat-like cry; derived from the French name “cri du chat” (cry of the cat) syndrome.They are also mentally retarded,have malformation in the larynx, moon faces, saddle noses, small mandibles,malformed low-set ears and microcephally (small head).
2:. Deletion of part of the short arm of one X chromosome produces a typical Turner syndrome.
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| Chromosomal abberaion |
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