Variations in Chromosomal Aberrations PDF

VARIATIONS IN CHROMOSOMAL ABERRATIONS [email protected]  Chromosomal Aberration or Chromosomal abnormalities ✓ Occur as a result of a defect in the number of chromosomes in a cell of an organism or in the arrangement of genetic material (Genes) on the chromosome. ✓ Chromosomal abno...

VARIATIONS IN CHROMOSOMAL ABERRATIONS [email protected]  Chromosomal Aberration or Chromosomal abnormalities ✓ Occur as a result of a defect in the number of chromosomes in a cell of an organism or in the arrangement of genetic material (Genes) on the chromosome. ✓ Chromosomal abnormalities give rise to specific physical symptoms, however, the severity of these depends on the type of aberration. ✓ Abnormalities may be in the form of additional material attached to a chromosome, a whole chromosome is missing, or even a faulty chromosome formation. ✓ Any increase (eg., duplication) or decrease (eg., deletion) in the chromosomal material interferes with normal development and function. ✓ There are 46 sets of chromosomes in humans. Any change in the chromosome number or structure may result in anomalies. ✓ The term “karyotype” is used to refer to the full set of chromosomes from an individual. Abnormalities in chromosomes are detected or confirmed by karyotype comparison of a given genome to a “normal” karyotype for the species through genetic testing. ✓ Chromosome abnormalities usually occur when there is an error in cell division following meiosis or mitosis. ✓ Numerical Aberrations ✓ Structural aberrations Numerical Aberrations ✓ They are caused by a failure in chromosome division during meiosis that results in gametic cells with an extra chromosome or a deficiency in the number of chromosomes. Aneuploidy ✓ Occurs when an organism gains or loses one or more chromosomes, but not a complete set. ✓ It leads to the variation in the number of chromosomes but does not involve the whole set of chromosomes. ✓ The nuclei of aneuploids contain chromosomes whose number is not the true multiple of the basic number (n). Examples include Down syndrome (47 chromosomes instead of 46), and Turner syndrome (45 chromosomes instead of 46). Type of aneuploidy Monosomy- individuals lose one chromosome. Typical is Turner syndrome (45 chromosomes in humans) Trisomy- The gain of one extra chromosome. Examples ✓ Down syndrome (2n+1= 47 chromosomes in humans). ✓ Trisomy 18 (Edwards syndrome) has an additional copy of chromosome 18 ✓ Trisomy 13 (Patau syndrome) has an additional copy of chromosome 13 ✓ Trisomy 8 (Warkany syndrome 2) has an additional copy of chromosome 8 Tetrasomy- individuals gain two (2) extra chromosomes Eg., ✓ XXXY syndrome (Klinefelter’s syndrome) ✓ XXXX syndrome ( 48 chromosomes) ✓ XXYY syndrome ( 48 chromosomes) Pentasomy- individuals gain three (3) extra chromosomes Example: ✓ Penta X Syndrome: (49, XXXXX), the female has five X chromosomes instead of the normal two. ✓ Signs are intellectual disability, short height, less muscle tone, and delay in development. Nullisomy- a pair of homologous chromosomes is completely lost. ✓ Humans with this disorder will not survive. Euploidy ✓ When one or more complete haploid sets of chromosomes are involved in the aberration, the resulting abnormality is called Euploidy. ✓ It is more tolerated in plants rather than animals. ✓ For example, if a human cell has an extra set of 23 chromosomes, it will have Euploidy. Types of Euploidy ✓ Ploidy refers to the number of homologous sets of chromosomes in the genome of a cell or an organism. Each set is designated by n. Monoploidy - The state of having a single set of chromosomes and is represented by 1n. ✓ The cell or organism with a single set of chromosomes is called a monoploid. ✓ Monoploidy is lethal in animals but in the case of plant species, this can be more tolerated. ✓ In most animal species this could mean death but there are few animal species where monoploidy is a normal part of the life cycle, such as male wasps, ants, and bees. ✓ The offspring that have arisen from monoploidy are those that have developed from unfertilized eggs. Polyploidy - a normally diploid cell or organism acquires one or more additional sets of chromosomes. ✓ the polyploid cell or organism has three or more times the number of haploid chromosomes. ✓ Polyploidy arises as a result of the total nondisjunction of the chromosomes during mitosis or meiosis. Structural aberrations ✓ These occur due to a loss of genetic material, or a reorganization in the location of the genetic material. ✓ They include deletions, duplications, inversions, ring formations, and translocations. ✓ Unbalanced rearrangements include deletions, duplications, or insertions of a chromosomal segment. ✓ Balanced rearrangements include inverted or translocated chromosomal regions. ✓ Since the full complement of DNA material is still present, balanced chromosomal rearrangements can go unnoticed because they may not cause disease. The disease can arise as a result of a balanced rearrangement if breaks in the chromosomes occur in one gene, resulting in a missing or non-functional protein, or if the fusion of chromosomal segments results in a hybrid of two genes, producing a new protein product whose function is detrimental to the cell Types of structural abberations Deletions-A part of the chromosome is missing or removed. - Eg., Wolf-Hirschhorn syndrome (caused by partial removal of the short arm from chromosome) - Jacobsen syndrome, also called 11q terminal deletion disorder (caused by the terminal removal of the q arm of chromosome 11). Duplications-A part of the chromosome is duplicated, resulting in additional genetic material. -Eg., Charcot-Marie-Tooth disease type 1A (caused by duplication of the gene encoding peripheral myelin protein 22 (PMP22) on chromosome 17). Translocations- When a part of a chromosome is transferred to another chromosome. ✓ There are two main types of translocations. - reciprocal translocation- segments of two different chromosomes are exchanged. - Robertsonian translocation- one complete chromosome joins another in the centromere Robertsonian translocation only occur with chromosomes 13, 14, 15, 21, and 22. Inversions- a part of the chromosome is broken, upside down, and reattached, therefore, the genetic material is inverted. Insertions- A portion of one chromosome is deleted from its normal place and inserted into another chromosome. Rings- Ring chromosomes can result when one chromosome undergoes two breaks and the broken ends are fused into a circular chromosome. This can happen with or without loss of genetic material. Isochromosome- An isochromosome can form when one arm of the chromosome is missing and the remaining arm is duplicated. How chromosomal aberrations or abnormalities occur ✓ By accident when the egg or sperm forms or during the early stages of fetus development. ✓ The mother’s age ✓ certain environmental factors ✓ Most chromosomal abnormalities occur as an accident in the egg or sperm and are therefore not inherited. ✓ The abnormality is present in all cells of the body; however, some abnormalities can occur after conception, resulting in mosaicism (where some cells have the abnormality and others do not). Prenatal exams and tests can be done to examine the fetus’s chromosomes and detect some, but not all, types of chromosomal abnormalities. THANK YOU