Chromosomal Basis of Inherited Disorders
Biology: IB Diploma Higher Level, topic 3.1.2
Chromosomal Basis of Inherited Disorders
Inherited disorders can arise when chromosomes behave abnormally during meiosis. Chromosome disorders can be due to abnormalities in chromosome number or due to chromosomal structural rearrangements.
Identification of chromosomes
- The isolation and microscopic observation of chromosomes forms the basis of cytogenetics and is the primary method by which clinicians detect chromosomal abnormalities in humans.
- A karyotype is the number and appearance of chromosomes, and includes their length, banding pattern, and centromere position.
- To obtain a view of an individual’s karyotype, cytologists photograph the chromosomes and then cut and paste each chromosome into a chart, or karyogram
Disorders in chromosome number
- Disorders of chromosome number include the duplication or loss of entire chromosomes, as well as changes in the number of complete sets of chromosomes.
- They are caused by nondisjunction, which occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis.
- Misaligned or incomplete synapsis, or a dysfunction of the spindle apparatus, can cause nondisjunction.
- The risk of nondisjunction occurring increases with the age of the parents.
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Nondisjunction
- Nondisjunction can occur during either meiosis I or II, with differing results.
- If homologous chromosomes fail to separate during meiosis I:
- The result is two gametes that lack that particular chromosome and two gametes with two copies of the chromosome.
- If sister chromatids fail to separate during meiosis II:
- The result is one gamete that lacks that chromosome, two normal gametes with one copy of the chromosome, and one gamete with two copies of the chromosome.
Nondisjunction
Aneuploidy
- An individual with the appropriate number of chromosomes for their species is called euploid.
- In humans, euploidy corresponds to 22 pairs of autosomes and one pair of sex chromosomes.
- An individual with an error in chromosome number is described as aneuploid, a term that includes:
- Monosomy, a loss of one chromosome.
- Trisomy, a gain of an extraneous chromosome.
Trisomy 21
- The most common trisomy among viable births is that of chromosome 21, which corresponds to Down Syndrome.
- Individuals with an extra chromosome 21 may synthesize an abundance of the gene products encoded by that chromosome.
- This extra dose (150 percent) of specific genes can lead to a number of functional challenges.
- Individuals with this inherited disorder are characterized by short stature and stunted digits, facial distinctions, and developmental delays.
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