Sometimes, there can be breakage of a chromosome. It can produce variety of arrangements. These arrangements affect the genes of that chromosome. There are following types of chromosomal structural changes:
Fig: Different aberrations in structure of chromosome
DELETION OR DEFICIENCY
The loss of part of chromosome is called deletion. Fragments without centromeres are usually lost during cell division. Therefore, the chromosome will miss certain genes. This causes several serious genetic diseases.
Deletion may occurs due to
(a) Losses from translocation
(b) Crossovers within an inversion
(c) Unequal crossing over
(d) Breaking without rejoining
Types of deletions
‘There are two types of deletions
(a)Interstitial deletions: In this case, internal part of a chromosome is missing.
(b)Terminal deletion: In this case, there is only one break and the homologous chromosome fail to rejoin. Thus tip of the chromosome is lost in cell division.
Effects of deletions
Small deletions are not fatal. But large deletions are usually fatal. Some medium-sized deletions lead to human disorders.
1. Homozygous deletions are lethal. They cause abnormal developments.
- In some deletions. number of base pairs is not equally divisible by three. It causes a frame shift mutation. Thus all of the codons corning after the deletion are read incorrectly during translation. It produces a severely, altered and nonfunctional protein.
- Deletions cause male infertility and two thirds of cases of Duchenne muscular dystrophy. A deletion of part of the short arm of chromosome 5 results in Cri du chat. It is also known as “cry of the cat” syndrome. It is found in I in 50,000 live births. The surviving infants have a distinctive cry, severe mental retardation, and shortened life span.
- Heterozygous deletions shows abnormal pairing in meiosis. The normal chromosome does not have a homologous region for pairing. Thus a loop is formed . Its example is polytene chromosomes of Drosophila.5. Heterozygous deletion may cause pseudo-dominance. In this case, dominant allele is missing. Therefore, recessive allele expresses itself
In this case, the fragment joins to the homologous chromosome with a duplication of genes. It adds one or more gene in the chromosome. Therefore, the organism carries same gene repeated in its haploid chromosome. Gene duplication occurs due to following reasons:
I. Error in homologous recombination.
- A retro transposition event occurs.
- Duplication of an entire chromosome leads to the duplication of a region of DNA containing a gene.There are two types of duplications(a)Tandem duplication: In this case, the duplication is adjacent to the normal chromosome. It has same order as the original order. ft is believed that the homologous-chromosomes ‘overlap each other. Therefore, the segments break simultaneously at different points. If different homologous chromosomes reunite, one chromosome has tandem duplication and other has deletion of duplicated region. Thus the duplication and deletion are reciprocal to each other.
(b) Reverse duplication: In this case, duplication is also adjacent normal chromosome. But the order is reverse as compared to original order.
(c) Displaced duplication: In this case, the duplication is not adjacent to the normal segment.
(d) Heterokaryotypic duplication: In some cases, part of chromosomes bend back and join with the duplicated sequence on the same chromosome. Such individuals with two chromosomal types are called heterokaryotypic. This duplication is called heterokaryotypic duplication.
Effects and significance of duplication
- The homozygous or heterozygous individuals for small duplicated segments are viable. Further evolutionary changes can occur in this individual.
- Duplication has great significance for evolutionary biology. The effected individuals have two genes. It has a single functional gene. This gene has a slow mutation rate. This individual also has one copy of a duplicate set of genes. This gene is free from selective pressure. It can freely mutate. As two copies of genes are present. Therefore. mutations in just one copy of the gene has no deleterious effect on the organism. Thus the second copy is free to mutate randomly.
- Susumu Ohno in his classical book “Evolution by gene duplication” (1970) postulate that gene duplication has a major role in evolution. Duplication is now widely accepted as a major evolutionary force. According to some researchers gene duplication is the most important evolutionary force since the emergence of the universal common ancestor..
- Major genome duplication events also occur. It is predicted that the entire yeast genome underwent duplication about 100 million years ago. Plants are the most important genome duplicators. For example, st.heat is hexaploid. It has six duplicate copies of its genome.
- The to genes that exist after a gene duplication event are called paralogs. Paralogs code for proteins with a different function and structure. This is different from orthologous genes. Ortholgous genes code for proteins with similar functions but exist in different species.
In this case, the fragment reattach to the original chromosome but in reverse orientation. An inversion occurs when a single chromosome undergoes breakage and rearrangement withinitself. Special cytogenetics techniques are needed to detect inversions. Inversion may be single or double. the example of single into inversion is Arrowhead Standard and the example of multiple inversion is Chiricahua Standard. Inversions arc of two types: paracentric and pericentric.
(a) Paracentric inversions: These inversions do not include the centromere. Both breaks occur in one arm of the chromosome.
(h) Pericentric inversion!:: It includes the centromere and there is a break point in each any.
Fig: Two types of inversion
Effects of inversions
- In some cases, the rearrangement is balanced after inversion and there is no extra or missing genetic information. Such inversions usually. do not cause any abnormalities in carriers. However, there is an increased chance for the production of unbalanced chromosome rearrangements in the offspring of carriers. Families that may he carriers of inversions must undergo genetic counseling and genetic testing.
- –1–he most common in inversion seen in humans is on chromosome 9. This inversion is generally has no deleterious or harmful effects. But it may leads to an increased risk for miscarriage for about 30% of affected couples.
- heterozygous individuals or an ins inversion can be recognized by the presence of inverted loop. Such loops are present in polytene chrormosomes.