6.Chromosome mutation- numerical and structural.Marker chromosomes.Chromosomal fragile sites

Chromosome Mutation

Numerical Chromosome Mutation

• Involves changes in the number of chromosomes in a cell (aneuploidy) or the addition of one or more complete haploid complements(polyploidy)

• Can occur due to errors during cell division (meiosis or mitosis).- non-disjunction

  • Trisomy- gain of one extra chromosome - most other than the examples result in pregnancy loss

    • DOWN SYNDROME-TRISOMY 21

    • EDWARDS SYNDROME- TRISOMY 18

    • PATAU SSYNDROME- TRISOMY 13

  • Monosomy- absence of a single chromosome( if it is of an autosome it is almost always incompatible with life

    • can be caused by anaphase lag- loss of a chromosome as it moves to the pole of the cell during anaphase

      • TURNER SYNDROME-45,X

  • Polyploidy- cells contain multiples of the haploid number of chromosomes

    • can be caused by failure of a maturation separation in a gamete or for example polyspermy

Structural Chromosome Mutations

• Involves changes in the structure of chromosomes.

• Can occur due to breakage and rearrangement of chromosome segments.

• Can be balanced and unbalanced

  • balanced- no loss or gain of genetic material( generally harmless)

    • carriers of a balanced abnormality are at risk of producing an offspring with an unbalanced one

  • unbalanced- genetic material if gainer or lost

  • Types of structural chromosome mutations include:

    • Deletion: Loss of a chromosome segment.(results in the monosomy of the segment)

      • Cri du chat- loss of material from chromosomes 4 and 5

    • Duplication: Presence of an extra copy of a chromosome segment.

    • Inversion: Reversal of the orientation of a chromosome segment.(2 break points)

      • segment flips 180 degrees

      • if the segment involves the centromere-PERICENTRIC

      • if it doesn’t involve the centromre-PARACENTRIC

      • balanced inversion carriers have a high chance of passing an unbalanced inversion to their offspring

    • Translocation: Exchange of chromosome segments between non-homologous chromosomes.

      • a break occurs in each of the two chromosomes

      • Robertsonian translocation- the breakpoints are located close to the centromere of acrocentric chromosomes(chromosomes are reduced to 45 but there is no loss or gain so its balances)

      • Reciprocal translocations- breakage of at least two chromosomes with an exchange of segments

    • Insertions-a segment of one chromosome gets inserted into another one

Marker Chromosomes

• Marker chromosomes are structurally abnormal chromosomes that cannot be identified as a specific chromosome they are also small

• They are often small and contain duplicated or rearranged genetic material.

• Marker chromosomes can be derived from normal chromosomes or result from chromosomal rearrangements.

• Their presence can be associated with genetic disorders or have no apparent phenotypic effects.

• Identification of marker chromosomes is important for genetic counseling and diagnosis.

Chromosomal Fragile Sites

• Chromosomal fragile sites are specific regions of chromosomes that are prone to breakage under certain conditions.

• They are characterized by gaps, breaks, or constrictions in the chromosomes.

• Fragile sites can be classified as common or rare, depending on their frequency in the population.

• Fragile sites can be induced by various factors, including chemicals, radiation, or specific DNA sequences.

• Fragile sites are associated with genetic disorders, such as fragile X syndrome, which is caused by a fragile site on the X chromosome.

Rings = a portion of 1 chromosome has broken off and formed a circle/ring

● Isochromosome = formed by the mirror image copy of a chromosome segment including the centromere

● Dicentric chromosome = has 2 centromeres