Structural Chromosomal Aberrations: Types, Detection, and Significance, Slides of Plant physiology

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Lecture - 8

Chromosomal Aberration: Variation in

chromosome structure – deletion,

duplication, inversion and translocation –

genetic and cytological implications.

1

Chromosomal Aberration Structural change Numerical change Deletion Duplication Inversion Translocation Variation in chromosome structure or number is called Chromosomal Aberration Chromosomal mutations or Structural Chromosomal aberrations

Type of structural chromosomal aberration

1. Deletion
2. Duplication
3. Inversion
4. Translocation Alter gene number in the chromosome Alter sequence of genes in the chromosome Intrachromosomal / homosomal aberrations Interchromosomal aberrations

5 Structural chromosomal changes

1. DELETION / DEFICIENCY

Term coined by Bridges Drosophila, maize, tomato, wheat etc., Two types : 1a) Terminal deletion 1b) Intercalary / Interstitial deletion Deletion / Deficiency : Loss of a portion of segment from a chromosome.

1a. Terminal deletion

• Loss of any one of the terminal segment of a chromosome - Terminal deletion
• If break occurs near the end of a chromosome, a small piece of the terminal end is lost. (The injured end later heals) Two types
• Heterozygous - in one chromosome of a homologous pair
• Homozygous - in both the chromosomes (rare and usually lethal) Normal chromosome before deletion After deletion Only one break occurs a b c d e f g h a b c d e f

1 b. Intercalary / Interstitial deletion ✓ Loss of a portion of segment from a chromosome from the intermediate portion or between telomere and centromere. ✓ Generally does not involve centromere. ✓ Intercalary deletions are more common than terminal deletion. ✓ The deleted portion may have one / two / several genes a b c d e^ f^ g^ h a b c d g h Break occurs at two places Normal chromosome before deletion After deletion

Intercalary deletion – formation of loop

Detection of Deletion

1. Cytological method Meiotic pairing and chromosome length.
2. Genetic method Deletion of dominant gene, results in the expression of recessive gene - change in the phenotype – Psudodominance.

Genetic effects due to Deletion / Deficiency

1. Fertility: Reduced pollen fertility.
2. Viability / Lethal effect: Organisms with homozygous deficiency usually do not survive to an adult stage because a complete set of genes is lacking.
3. Crossing over: is suppressed in the region of deficiency.
4. Phenotype: absence of a dominant gene due to deletion results in the expression of recessive genes, resulting in change in phenotype. Eg: Cat Cry (cri du chat) in human - deletion in the short arm of chromosome.
5. Change in karyotype: Gene number and karyotype of an individual gets changed.

14 Structural chromosomal changes

2 a). Tandem

✓ Sequence of genes in the duplicated segment is similar to that of the sequence of genes in the original segment

2 b). Reverse tandem

✓ Sequence of genes in the duplicated segment is reverse to that of the sequence of genes in the original segment a b c d e f g h a b c (c b) d e f g h a b c d e f g h I j k a b c (b c) d e f g h I j k Normal chromosome before duplication After duplication before duplication After duplication Tandem and reverse tandem are called adjacent duplication Types of duplication

2 c). Displaced

✓ When duplication is found away from the original segment but on the same arm of the chromosome a b c d e f g h a (d e) b c d e f g h Normal chromosome Displaced

2d). Reverse displaced

✓ When duplication is found away from the original segment but on the other arm of the chromosome a b c d e f g h (^) i j k (d e) Reverse Displaced Displaced and Reverse displaced also called as Non - adjacent duplication

Detection of Duplication chromosome

Cytology methods ✓ Duplication loop can be observed during pachytene state when homologous chromosomes pair. ✓ Chromosomes having duplication segment are longer than normal chromosomes. Genetic method ✓ Duplications can also be detected by suppression of recessive characters. ✓ A single dominant gene in the duplicate region is enough to suppress the expression of two recessive alleles.

Genetic effect of Duplications

✓ Duplication are less harmful than deletions. ✓ They do not reduce the viability of an individual. ✓ Phenotype : duplication of certain genetic regions produces specific phenotype. Eg: Gene for bar eye ( 16 A) in drosophila Normal oval shaped – single dose Bar shaped – two dose of 16 A Ultra bar shaped – three dose of 16 A segment ✓ Crossing over: supressed in duplicated region ✓ Gene number is increased ✓ Reduction in pollen fertility in plants