3.4.3 Genetic diversity can arise as a result of mutation or during meiosis

Question 1

Explain what is meant by the terms haploid and diploid.

Answer 1

Haploid (n) - there’s only one copy of each chromosome (in gametes).
Diploid (2n) - each body cell contains two of each chromosome.

Question 2

What happens to the chromosome number at fertilisation?

Answer 2

At fertilisation, the chromosome number halves, resulting in the production of haploid daughter cells.

Question 3

What is a chromatid?

Answer 3

A chromatid is a single separate strand of a chromosome.

Question 4

Give three ways in which the outcome of meiosis differs from the outcome of mitosis.

Answer 4

Meiosis:

• Produces cells with half the number of chromosomes as the parent cell.
• Daughter cells are genetically different from one another and the parent cell.
• Produces four daughter cells.

Mitosis:

• Produces cells with the same number of chromosomes as the parent cell.
• Daughter cells are genetically identical from one another and the parent cell.
• Produces two daughter cells.

Question 5

Drosophila (fruit flies) only have four chromosomes.
Crossing over does not occur very frequently in male Drosophila. Explain what crossing over is and how it leads to genetic variation.
[4 marks]

Answer 5

• During meiosis homologous pairs of chromosomes come together.
• The chromatids twist around each other and bits swap over.
• The chromatids now contain different combinations of alleles.
• This means each of the four daughter cells will contain chromatids with different combinations of alleles.

Question 6

Drosophila (fruit flies) only have four chromosomes.
Explain how independent segregation leads to genetic variation.
[2 marks]

Answer 6

• Independent segregation means the homologous chromosome pairs can split up in any way.
• So, the daughter cells produced can contain any combination of maternal and paternal chromosomes with different alleles.

Question 7

Turner syndrome is a genetic condition affecting females. It is caused by non-disjunction of the sex chromosomes. Females usually have two X chromosomes. Some females with Turner syndrome have only one X chromosome.
Suggest and explain how chromosome non-disjunction could cause Turner syndrome.
[3 marks]

Answer 7

• Chromosome non-disjunction may mean that the sex chromosomes fail to separate during meiosis.
• This could mean that one of the daughter cells/gametes ends up without a copy of the X chromosome, whilst another daughter cell/gamete gets two X chromosomes.
• If the gamete without an X chromosome is fertilised, the resulting zygote will be missing one X chromosome, resulting in Turner syndrome.

Question 8

What are mutagenic agents?

Answer 8

Any substance or radiation that increases the rate of mutation.

Question 9

What are mutations?

Answer 9

Gene mutations involve a change in the DNA base sequence of chromosomes.

Question 10

What are some of the types of errors that can occur during DNA replication?

Answer 10

• Substitution
• Deletion
• Addition

Question 11

Not all base substitutions cause a change in the sequence of encoded amino acids.
Explain why.

Answer 11

• The degenerate nature of the genetic code means that some amino acids are coded for by more than one DNA triplet.
• e.g. Tyrosine can be code for by TAT or TAC in DNA.
• This means that not all substitution mutations will result in a change to the amino acid sequence of the protein.

Question 12

How does the deletion of a base affect the base sequence?

Answer 12

The deletion of a base will change the number of bases present, which will cause a shift in all the base triplets after it.

Question 13

Explain how the chromosome number is halved during meiosis.

[2 marks]

Answer 13

• Homologous chromosomes (pair).

- One of each (pair) goes to each (daughter) cell/to opposite poles.

Question 14

Crossing over greatly increases diversity in this species of moss.
Describe the process of crossing over and explain how it increases genetic diversity.
[4 marks]

Answer 14

• Homologous pairs of chromosomes associate.
• Chiasma(ta) form.
• (Equal) lengths of (non-sister) chromatids/alleles are exchanged.
• Producing new combinations of alleles.

Question 15

Patau syndrome is a condition caused by a mutation affecting chromosome number.
Describe how the change in the chromosome number in Patau syndrome was produced.
[2 marks]

Answer 15

• In meiosis…

- …homologous chromosomes/sister chromatids do not separate.

Question 16

Patau syndrome is a condition caused by a mutation affecting chromosome number. All the cells of the body will have this mutation.
Explain why all the cells of the body will have this mutation.
[2 marks]

Answer 16

• All cells derived from a single cell by mitosis.

- Mitosis produces genetically identical cells.