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Chromosomes:
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by chaperone proteins, bind to and condense the DNA molecule to maintain its integrity
Recall we have 46 Chromosomes (2N) Diploid or 23 (1N) Haploid Chromosomes
A Human with 46 intact chromosomes.
Karyotype
Karyotype
A karyotype is a preparation of the complete set of metaphase chromosomes in the cells of a species or in an individual organism, sorted by length, centromere location and other features and for a test that detects this complement or counts the number of chromosomes
Non-disjunction
refers to the chromosomes failing to separate correctly, resulting in gametes with one extra, or one missing, chromosome (aneuploidy)
The failure of chromosomes to separate may occur via:
Failure of homologs to separate in Anaphase I (resulting in four affected daughter cells)
Failure of sister chromatids to separate in Anaphase II (resulting in only two daughter cells being affected)
Chromosomal Abnormalities
If a zygote is formed from a gamete that has experienced a non-disjunction event, the resulting offspring will have extra or missing chromosomes in every cell of their body - We call this Errors of Non-Disjunction
Conditions that arise from non-disjunction events include:
Patau’s Syndrome (trisomy 13)
Edwards Syndrome (trisomy 18)
Down Syndrome (trisomy 21)
Klinefelter Syndrome (XXY)
Turner’s Syndrome (monosomy X)
Down Syndrome
Individuals with Down syndrome have three copies of chromosome 21 (trisomy 21)
One of the parental gametes had two copies of chromosome 21 as a result of non-disjunction
The other parental gamete was normal and had a single copy of chromosome 21
When the two gametes fused during fertilization, the resulting zygote had three copies of chromosome 21
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