Consequences of Abnormal Meiosis

Question 1

Non-Disjunction of Chromosomes

Answer 1

During Anaphase 1, one or more homologous pairs of chromosomes may not separate
During Anaphase 2, the sister chromatids of one or more chromosomes may not separate
This is referred to non-disjunction
As a result of non-disjunction, gametes forming from meiosis may have one extra chromosome, one less chromosome, no chromosomes or an extra set of chromosomes
An abnormal gamete may fuse with a normal gamete or another abnormal gamete, leading to different genetic disorders
Non-disjunction may lead to two general conditions; Aneuploidy or Polyploidy

Question 2

Aneuploidy

Answer 2

In aneuploidy, one gamete receives two copies of the same chromosome, while the other gamete receives no copy
If fertilization occurs with either of these gametes by a normal gamete, the zygote will have an abnormal number of chromosomes
If the zygote has one extra chromosome, we say it is trisomic
If the zygote has one chromosome short, we say it is monosomic
Down Syndrome is an example of aneuploidy

Question 3

Down Syndrome

Answer 3

Result of an extra chromosome number 21. The cells are trisomic for chromosome number 21, Down Syndrome is also called trisomy 21
In meiosis 1, the chromosome pair 21 may not separate or,
In meiosis 2, the chromatids of chromosome 21 may not separate
Gametes will therefore have an extra copy (2 copies) of chromosome number 21
Gamete with 2 copies of chromosome 21 fuses with a normal gamete with 1 copy of chromosome 21
The resulting zygote will have 3 copies of chromosome number 21 (47 chromosomes instead of 46 chromosomes) resulting in Down Syndrome
Characterized by mental retardation, hearing loss, hearing defects, decreased muscle tone, upwardly slanting eyes, a small mouth and nose, abnormal ear shape and a depressed nasal bridge
There is no cure for Down Syndrome. Only the symptoms are treated
The karyotype resulting from such an analysis is then examined to determine if there is an abnormal number of chromosomes

Question 4

Polyploidy

Answer 4

Most animals and plants contain cells that are diploid
When there is non-disjunction of all the chromosomes during meiosis, (a gamete will be diploid instead of haploid)
If an abnormal, diploid gamete fuses with a normal haploid gamete, the zygote will be triploid (3n)
If an abnormal diploid gamete fuses with another abnormal diploid gamete, the zygote will be tetraploid (4n)
If a plant or animal has more than two complete sets of chromosomes, we say it is polyploid
Polyploid is very common in plants , especially in angiosperms

Question 5

Advantages of Polyploidy in Agriculture

Answer 5

Polyploidy plants not only have larger cells but the plants are often larger. This has led to the deliberate creation of polyploidy varieties of such plants as watermelons, marigolds and snapdragons
Polyploid individuals can also be created in plant breeding laboratories by treating dividing cells with colchicine. This drug prevents the formation of spindle during mitosis. As a result the duplicated chromosome fail to separate in mitosis. In this way, scientists have produced plants with larger fruits, flowers and storage organs