Errors in meiosis and X-inactivation

• Nondisjunction is the failure of chromosomes when separating properly during cell division, resulting in an abnormal distribution of chromosomes.
• There are 2 consequences of nondisjunction:

  1. trisomy
  2. monsomy
• Trisomy occurs when there is one extra copy of a chromosome.
• Monosomy occurs when there is only one copy of a chromosome instead of two copies.
• Down syndrome (trisomy 21) is caused by an error in meiosis two where the daughter cells receive either 3 or 4 copies of chromosome 21
• Aneuploidy is the abnormal number of chromosomes due to gain or loss of one or more chromosomes.
• The consequence of aneuploidy is developmental abnormalities, intellectual activities and severe health issues such as Kinefelter, down, turner syndrome.
• Kinefelter syndrome is male at birth, results to low fertility and has a feminized physique. It is usually XY chromosomes, but has an extra X, so -> XXY.
• Turner syndrome affects females. It has XO, single chromosome. They have menstrual impairment, which depends whether the X chromosome comes from the mother or father.
• If a pair of chromosomes fails to disjoin at anaphase of meiosis 1, the likely chromosome numbers of the 4 resulting gametes is: n+1, n+1, n-1, n-1.
• Deletion occurs when a portion of chromosome is missing or deleted. The consequence is the loss of genetic material.
• Duplication involves the addition of a new sequence of nucleotides to the 3' end of a gene. The consequence is an increase of certain genes.
• Inversion is when a portion of a chromosome is flipped or reversed, so it would not line up. The consequence is that it can disrupt gene expression and chromosomal stability.
• Translocation is the transfer of a portion of a chromosome to another chromosome. The consequence is that it can disrupt the gene function.
• Nondisjunction can occur in meiosis when homologous chromosomes or sister chromatids fail to seperate properly. This can contribute to birth defects and can lead to abnormal number in gametes.
• Behaviour in meiosis

  Aneuploidy contributes to birth defects - trisomy 21 and monsomy X.
• Behaviour in meiosis


  Deletion can occur when a chromosome breaks and the segment is lost. it contributes to birth defects because the loss of critical genetic material.
• Behaviour in meiosis


  Duplication can occur when the chromosome breaks and segment is duplicated. it contributes to birth defects as it can lead to an increase of dosage of certain genes.
• Inversion can occur when a chromosome breaks and rejoins in an inverted orientation. it contributes to birth defects as it disrupts gene expression and chromosomal activity.
• Translocation can occur in meiosis when homologous chromosomes fail to separate properly, leading to exchange in genetic material. it contributes to birth defects as it disrupts the gene function.
• Various forms of polyploidy

  Alloploidy, Autoploidy, Tetraploidy, Triploidy
• Alloploidy occurs when multiple sets of chromosomes are derived from the species. Their formation arises from interspecific hybridization, followed by chromosomes doubling.
• The consequence of alloploidy in meiosis is pairing them between homologous chromosomes from different parental species can be disrupted during meiosis, leading to irregular chromosome segregation.
• Autoploidy is having multiple sets of chromosomes derived by a single species. Their formation comes from errors in meiosis.
• The consequence of Autoploidy in meiosis is that homologous chromosomes may not pair up correctly, leading to unbalanced gametes.
• Tetraploidy is an individual and has complete 4 sets of chromosomes. Their formation can come from errors, resulting in duplication of chromosome sets.
• The consequence of tetraploidy in meiosis is that they may exhibit meiotic irregularities, leading to reduced fertility.
• Odd numbers of chromosomes in segregation can disrupt normal chromosome pairing, synapsis and segregation, leading to formation of unbalanced gametes with aneuloid crhomosome numbers.
• X-inactivation occurs in female mammals to balance gene dosage between males and females, as males only have 4 X chromosomes and females have 2 X chromosomes.
• The process of X-inactivation involved silencing X chromosome in each somatic cell of females, resulting in Barr bodies (inactive X chromosomes).
• The consequence to X-inactivation leads to mosiac patterning, X-linked disorders and formation of barr bodies.