Module 4 - Genetic diversity can arise as a result of mutation or during meiosis Flashcards
Define homologous pair of chromosomes
2 chromosomes that carry the same genes
6 marks
Describe what happens to chromsomes in meoisis
- Chromosomes condense
- Chromosomes associate in homologous pairs
- Crossing over
- Join to spindle fibres
- at equator of cell via centromere
- Homologous pairs move to opposite poles
- Pairs of chromatids seperated in 2nd division
4 marks
Explain crossing over
- Chromatids can become twisted around each other
- This puts tension on chromatids
- Causing pairs of the chromatid to break
- Broken parts of chromatid recombine with another chromatid
- =New combination of alleles
4 marks
Describe and explain meoisis that result in genetic variation within a species
- In metaphase I of meoisis crossing over between homologous pairs= variation in gametes
- Also in metaphase I independent segration of the homologous pairs leads to variation of different alleles in the gametes
- Random fertlisation of gametes
- New combination of alleles in gametes
Explain how chromosome number is halved
- Homologous chromosomes (pair);
- One of each (pair) goes to each (daughter) cell / to opposite poles;
2 marks
Explain why meosis is important in sexual reproduction, apart from producing gametes that are genetically different
- Produces haploid cells
- Restoring diploid number
- Introduces genetic variation
3 marks
Explain how crossing over can contribute to genetic variation
- Sections of chromatids exchanged
- Sections have different alleles
- New combination of alleles
5 marks
Meiosis results in genetic variation in the gametes which leads to variation in the offspring formed by sexual reproduction.
Describe how meiosis causes this variation and explain the advantage of variation to the species.
- Crossing over
- Independent segregation
- Random fertilisation
- Different adaptations
- Some survive
- To reproduce
- Pass on allele
The offspring produced from farmed trout are sterile. Suggest and explain why.
- Too many copies of chromosomes
- Homologous chromosomes do not pair
OR
Homologous chromosomes do not separate
evenly - So no meiosis
4 marks
Describe how the process of meiosis results in haploid cells.
Do not include descriptions of how genetic variation is produced in meiosis.
- DNA replication
- Two divisions
- Separation of homologous chromosomes in first division
- Separation of sister chromatids in second division
- Produces 4 haploid cells
2 marks
Explain the role of independent segregation in meiosis
- To provide genetic variation
- Allows different combinations of maternal and paternal chromosomes / alleles
Explain the importance of meiosis in the life cycles of organisms which reproduce sexually. (3)
meiosis halves the chromosome number / from diploid to haploid / produces haploid / n cells;
when gametes fuse / at fertilisation, the diploid number is restored / this keeps the chromosome number constant
introduces genetic variation / independent assortment / crossing over;
3 Advantages of asexual reproduction
Fewer stages so quicker
Only one parent involved so can colonise new environment
Produces clone so successful geno / phenotype maintained
Advantage of sexual reproduction
Increase genetic diversity so greater chance of survival
Healthy cells with damaged DNA produce enzyme X. This enzyme slows the
cell cycle by delaying the start of DNA replication. People with the disease
ataxia telangiectasia (AT) do not produce functional enzyme X. Mutations occur at a higher rate in people with AT.
Suggest and explain why ‘mutations occur at a higher rate’ in people with AT (lines 15–16).
- No functional enzyme x produced
- So faster cell cycle
- Faster DNA replication
- Mutations more likely to occur
Scientists have produced a mutated rice variety in which there is no crossing over.
A population of the mutant rice variety produced by sexual reproduction shows genetic variation. Populations of non-mutant rice varieties also show genetic variation.
Suggest and explain the similarities and differences in the causes of genetic variation within these rice populations.
(Similarities)
1. (Both populations) have (variation due to) independent segregation/assortment (of chromosomes/chromatids)2. (Both populations) have (variation due to) random fertilisation (of gametes);
(Difference)
3. Crossing over causes variation in non-mutant only;
2 marks
Patau syndrome is a condition caused by a mutation affecting chromosome number. All the cells of the body will have this mutation. Describe how the change in chromosome number in Patau syndrome was produced
- In meiosis
- Homologous chromosomes / sister chromatids do not separate (non disjunction)
2 marks
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.
- Mutation / extra chromosome in gamete
- All cells derived by mitosis
OR - All cells derived from a single cell / zygote by mitosis
- Mitosis produces genetically identical cells
