Chapter 8 Part 2 Flashcards
Aneuploidy
Change in the number of individual chromosomes; most often an increase or a decrease of one or two chromosomes
Ex. Trisomy 21… instead of having two pairs of homologous pair…we have three chromosomes that make a homologous pair…attributes to down syndrome disorder
Polyploid
Possession of more than two sets of chromosomes
Ex… 3n or 4n or 5n…sets of chromosomes than maybe 2n
What can Aneuploidy?
- A chromosome may be lost in the course of mitosis or meiosis…for example, if its centromere is deleted….Loss of the centromere prevents the spindle microtubules from attaching, so the chromosome fails to move to the spindle pole and does not become incorporates into a nucleus
- The small chromosome generated in Robertsonian Translocation may be lost in mitosis or meiosis
- Aneuploidy may arise through nondisjunction, the failure of homologous chromosomes or sister chromatic to separate in meiosis or mitosis
Non-disfunction
This process leads to some gametes or cells that contain an extra chromosome and other gametes or cells that are missing a chromosome
Monosomy
Is the loss of a single chromosome, represented as 2n -1
A human monosomic zygote has 45 chromosomes
Nullisomy
Is the loss of both members of a homologous pair of chromosomes
It is represented as 2n-2, where n refers to the haploid number of chromosomes
Thus, among humans who normally possess 2n=46 chromosome, a nullisomic zygote has 44 chromosomes
Trisomic
Is the gain of a single chromosome, represented as 2n + 1
The gain of a chromosome means that there are three homologous chromosome
Most cases of Down syndrome results from trisomy of chromosome 21
Tetrasomy
Is the gain of two homologous chromosomes, represented as 2n+2
A human tetratomic zygote has 48 chromosomes
Tetrasomy is not the gain of any two extra chromosomes but rather the gain of two homologous chromosomes, so there are four homologous copies of a particular chromosome
What happens when more than one aneuploid mutation may occur in the same individual?
An individual that has an extra copy of each two different – non homologous – chromosomes is referred to as being double trisomic; this condition is represented as 2n + 1 + 1
Similarly, a double monosomic individual has two fewer non homologous chromosomes than normal 2n - 1 -1
A double tetratomic individual has two extra pairs of homologous chromosomes
2n + 2 + 2
What effects does Aneuploidy cause in animals and plants?
It affects the number of gene copies but not their nucleotide sequences, the effects are mostly likely due to abnormal gene dosage
Extra Stuff –> Just for fun/flip the card!
Aneuploidy in humans usually produces such serious development problems that spontaneous abortion results
Only about 2% of all fetuses with a chromosome mutation survive to birth
Causes have been arise to a genetic variant known as PLK4 that increases this outcome by interfering with centrioles
Down syndrome
Trisomy 21 –> see definition in one of the other cards
Down syndrome
Trisomy 21 –> see definition in one of the other cards
Familial Down syndrome
Arises in offspring whose parents are carriers of chromosomes that have undergone a Robertsonian translocation, most commonly between chromosome 21 an chromosome 14: the long arm of 21 and the short arm of 14 exchange places
The long arms of 21 and 14 remain homologous to normal 14
The short arm of 21 and 14 leaves the cytoplasm resulting in monosomy 21
Translocation carriers
Individual organisms heterozygous for a chromosome translocation…they possess only 45 chromosomes…but are more likely to produce children with Down Syndrome
Explain why human and other mammals, sex chromosomes have more mutations commonly than autosomal chromosomes
Dosage compensation prevents the expression of additional copies of X-linked genes in mammals, and there is little information in the Y-chromosome, so extra copies of the X and Y chromosomes do not have major effects on development
Uniparental Disomy
Normally, the two chromosomes of a homologous pair are inherited from two different parents – one from the father and one from the mother
“Both homologous chromosomes inherited from the same parents”
Example…PW syndrome
Genetic Mosaic
Condition in which regions of tissue within a single individual have different chromosome constitutions
Gynandromorphs
Individual organism that is a genetic mosaic for the sex chromosomes, possessing tissues with different sex-chromosome constitutions
Example:
XX - fruit fly female
XO - fruit fly Male
Autopolyploidy
In which all chromosome sets are from a single species
Allopolyploidy
In which chromosome sets are from two or more species
Colchicine
A chemical that induces artificial nondisjunction by disrupting the formation of the spindle apparatus
–> used in agriculturally and ornamentally important plants
Unbalanced Gametes
A gamete produced by meiosis in such an autotriploid might receive, say, two copies of chromosome 1, one copy of chromosome 2, and three copies of chromosome 3, and no copies of chromosome 4
