Chromosome mutations

Question 1

How do mutations in the number of chromosomes arise?

Answer 1

• Mutations in number of chromosomes can arise spontaneously by chromosome non- disjunction during meiosis.

Question 2

What is non-disjunction?

Answer 2

• Non disjunction is when chromosomes/ chromatids do not split/ separate equally during anaphase (so, cells produced won’t have the correct no of chromosomes.)

() - extra info for clarification.

Question 3

True or False

Chromosome non-disjunction can only occur in meiosis I.

Answer 3

• False.
• Chromosome non-disjunction can occur in either meiosis I or meiosis II.
• Meiosis I: either one/ all chromosomes don’t separate equally.
• Meiosis II: either either one/ all chromatids don’t separate equally.

Question 4

What are the 2 different effects that chromosome non-disjunction can have?

Answer 4

• Changes in whole sets of chromosomes - polyploidy.
• Changes in the number of individual chromosomes - aneuploidy.

Question 5

When does polyploidy occur?

Answer 5

• Polyploidy occurs when you have changes in whole sets of chromosomes.
• Changes in whole sets of chromsomes occur when organisms have three/ more sets of chromosomes rather than the usual two.

Question 6

Describe process that would lead to polyploidy (non-disjunction in meiosis I.)

Answer 6

• Each homologous pair is doubled (due to DNA replication in interphase.)
• Non disjunction in meiosis I (all chromosomes fail to separate equally.)
• Normal division in meiosis 2, chromatids separate equally.
• 4 gametes produced. 2, 2n, diploid gametes and 2 gametes with no chromosomes.

Question 7

What would happen if a 2n gamete (produced by non-disjunction) fuses with a normal n gamete?

Answer 7

• A triploid (3n) cell would be produced - rather than a diploid cell.

Question 8

How does aneuploidy actually occur?

Answer 8

• Changes in number of individual chromosomes.
• Individual homolgous pairs of chromosomes fail to separate in meiosis.
• Results in gamete hacing one more/ one fewer chromosome.
• When zygote with one more/ less chromosome fuses with normal gamete, the zygote will have more/ fewer chromosomes than in body cells.

Question 9

Describe process that would lead to aneuploidy (non-disjunction occuring in meiosis I.)

Answer 9

• Each homologous pair is doubled (due to DNA replication in interphase.)
• Non disjunction in meiosis I (one chromosome does not separate equally.)
• Normal division in meiosis 2, chromatids separate equally.
• 4 gametes produced: two n+1 haploid gametes with one additional chromosome AND two n-1 haploid gametes with one missing chromosome.

Question 10

If chromosome non- disjunction occurs in meiosis I - leading to polyploidy, what issue happens during meiosis?

Answer 10

• All chromosomes fail to separate equally (ie. one cell ends up with all the chromosomes/ other cell doesn’t contain any chromosomes after meiosis I.)

() - extra info for clarification.

Question 11

If chromosome non- disjunction occurs in meiosis I - leading to aneuploidy, what issue happens during meiosis?

Answer 11

• One chromosome doesn’t separate equally (one cell has two of same homologous pair and other cell doesn’t contain that specific homologous pair.)

Question 12

If chromosome non- disjunction occurs in meiosis II - leading to polyploidy, what issue happens during meiosis?

Answer 12

• All the chromatids don’t separate equally (4 gametes –> 2 have no chromosomes/ 2 have 2 copies of each chromosome.)

Question 13

If chromosome non- disjunction occurs in meiosis II - leading to aneuploidy, what issue happens during meiosis?

Answer 13

• One chromatid doesn’t separate equally.
• 4 gametes –> two have 1 missing chromosome, 2 have normal number of chromosomes.)

Question 14

What will happen when n+1 gamete fuses with n (normal gamete?)

Answer 14

• Forms 2n+1 zygote (diploid.)
• Resultant zygote will have 1 extra chromosome.