NATS 1675 Lecture Notes - Lecture 13: Sister Chromatids, Robert Crippen, Nondisjunction
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NATS 1675 – LECTURE #13 – CHROMOSOMAL ABNORMALITIES
Professor: Robert Crippen
Many gene reside on the same chromosome so in meiosis, they will travel together
These traits that the particular genes produced link or both occur on the same
Following Mandel’s 3rd principle [Principle of Independent Assortment or Recombination],
the assortment of the genes on the same chromosome is inter-dependent (so they travel
together) not independent
Genes on a different chromosome travel independent of each other
What is Chromosomal Abnormalities?
It includes a variety of alterations either in the configuration of chromosomes [deletion or
translocation], number of chromosomes or mosaicism
These abnormalities are more extensive than mutations & many are lethal (deadly)
Remember: Mutations is dealing with one gene and Chromosomal Abnormalities are dealing
with many genes
Is a person normal or abnormal?
Answer: Chromosomes are abnormal, people are normal
If we are talking about deletion, we are talking about the loss of a part of a chromosome
Chromosomes can be fragmented (breaking off) by 3 things: Chemicals, Viral Infections
Terminal Deletion – this type of deletion occurs towards the end of the chromosome. This type
of deletion may cause abnormal facial or skull structure, mental retardation and growth delay
This is when a whole chromosome, or a segment of one chromosome breaks away and
attaches to another non-homologous chromosomes
2-3% of Down’s individuals result from the translocation of 21/14 or 14/21
Changes in the Chromosome #
It can occur by 1) non-disjunction, 2) a process called “lagging” or *3) the penetration of
an egg in 2 sperm cells
*How would you tell if the 3rd one occurs? If you are seeing 3 sets of chromosomes.
Why are chromosomal abnormalities lethal?
Because chromosomal abnormalities deals with many genes and so, for many children they
could have part of their chromosomes missing
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1) Non- Disjunction
It is a failure of a pair of homologous chromosomes to separate during Anaphase I or a
failure of sister chromatids to separate during Anaphase II
In Anaphase I results in no normal gametes so ½ gametes are missing a chromosome
and ½ gametes will have ½ of the chromosome
Failure of sister chromatids that separate in Anaphase II results in 1 gamete with an extra
chromosome, 1 gamete missing a chromosome & 2 normal gametes
Pair of homologous
Egg will have one extra chromosome
Egg will have one less chromosome
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