Chromosomal alterations Flashcards
Chromosomal territories
contain a single chromosome within an arbitrary region that is not bound by any membranes
karyotype
We can visualise our chromosomes by means of
autosomes
Chromosomes 1-22
sex chromosome
Chromosome 23 (X and Y, identified separately but together make the homologous 23rd )
How are chromosomes divided?
Chromosomes are divided at the centromere into chromosome arms
Usually unequal lengths
Short arm (p arm)
Long arm (q arm)
nondisjunction
When chromosomes and sister chromatids fail to separate properly
What are the effects of non disjunction?
Affects the numbers of chromosomes within cells
Adds or removes large amounts of genetic material
Usually alters the phenotype, affects development and/or fertility
Bc incorrect dosage compensation
euploid
Chromosome numbers that are a multiple of the haploid number (ie 2n, 3n, 4n, etc)
aneuploid
If we add or remove a chromosome, it alters the euploid number and generates an
aneuploidy
Caused by Nondisjunction in germ-line cells
Describe what happens if non disjuniction occurs in meiosis 1
In meiosis I, results in the failure of homologous chromosome separation
Gametes end up with one extra or one missing chromosome
Trisomic (2n+1): three of one chromosome instead of a homologous pair
Monosomic (2n-1): single copy of one of the chromosomes instead of a homologous pair
Describe what happens if non disjuniction occurs in meiosis 2
In meiosis II, results in the failure of sister chromatid separation
Typically follows normal meiosis I
Two resulting gametes produce normally
Two resulting gametes become (n+1) and (n-1
After fertilisation, we get: Trisomic (2n+1): three of one chromosome instead of a homologous pair
Monosomic (2n-1): single copy of one of the chromosomes instead of a homologous pair
What does aneuploid alter?
changes the gene dosage of all of the genes on the affected chromosome- if we have an extra cope of chromosome 1 we will have extra expression of it
In diploid organisms, gene dosage is 100%
Monosomic individuals, gene dosage is 50%
Trisomic individuals, gene dosage is 150%!
Leads to imbalance of gene products
How are plants and animals affected by aneuploidy?
Animals are usually more affected by gene dosage
Developmental delays and/or nervous system underdevelopment due to aneuploidy
Often lethal
Plants are usually a little bit more tolerant of changes in gene dosage
Different developmental programming than animals
How does aneuploid affect humans?
We have potentially 24 different kinds of trisomy (one for each autosome, one for X, and one for Y)
Only really see trisomy 13, 18, and 21 and rarely any monosomies in autosomes
Often see trisomies in sex chromosomes
Other trisomies/monosomies usually result in death of the embryo
Developmental abnormalities are so severe, the zygote cannot survive and aborts
Trisomy 21 (Down Syndrome)
The most common autosomal aneuploidy in humans
Linked to age of the mother
Meiosis begins in the fetal ovaries, where it reaches synapsis in prophase I and then stops
At puberty, monthly cycles reinitiate meiosis in a few follicles and the egg is released into the fallopian tubes after meiosis I
Meiosis II begins after fertilisation with a sperm
Meisos in female sex cells is very slow
90% of trisomy 21 cases were a result of meiosis I nondisjunction
Progeny have 2 copies of maternal C21, and one copy of paternal C21
Turner Syndrome
(X0 females)- monosomy missing an x
Although one X chromosome is normally inactivated to compensate for gene dosage, 2 X chromosomes are still needed for normal development in females
The genes on the single X chromosome in Turner syndrome are not enough for normal development
Results in no secondary sexual characteristics, infertility, short stature, webbed neck
Triple X Syndrome
(XXX females)
Tall stature, possible reduction in fertility, menstrual irregularity
May have speech delays
Mosaicism
X-inactivation mosaicism
One X chromosome in female somatic cells is randomly inactivated
- condition in which an individual is composed of 2 or more cell types having different genetic or chromosomal makeup
How can Mosaicism develop
Mitotic nondisjunction
Occurs during embryogenesis early in development
25-30% of cases of Turner syndrome can result in 45, X0 monosomy and others 46, XX.
Other individuals can also of mosaicism carrying 47, XXX cells
Derived from a 46, XX zygote undergoing nondisjunction
Uniparental disomy
both copies of the homologous chromosome pair are derived from the same parent- normally we have one from each parent