6.2.3 Variation: Sexual Reproduction Flashcards
1
Q
What is genetic variation
A
Variation in the genes (or DNA base sequences) between organisms within a species
2
Q
How does genetic variation occur in sexual reproduction?
A
- Crossing over of non-sister chromatids during prophase 1 of meiosis
- Independent assortment of homologous chromosomes during metaphase 1 of meiosis
- Random fusion of gametes during fertilisation
3
Q
What happens during crossing over?
A
- Non-sister chromatids exchange alleles
- During meiosis 1 homologous pairs of chromosomes cross over and get entangled
- Crossing points are called chiasmata
- Entanglement place stress on DNA molecules
- Resulting in section of chromatid breaking and rejoining with a the chromatid from the other chromosome
- Gives rise to new combination of alleles on the 2 chromosomes
4
Q
Independent Assortment
A
- Produces different random combinations of alleles in daughter cells, increasing genetic variation
- Random alignment of homologous pairs of chromosomes along the equator of the cell in metaphase 1
- Orientation of one homologous pair of chromosomes is independent by the orientation of another pair
.
To work out the number of different possible chromosome combinations the formula 2n can be used, where n corresponds to the number of chromosomes in a haploid cell
For humans this is 2^23 which gives 8 324 608 different combinations
5
Q
How does random fusion of gametes work?
A
- Each gamete carries different alleles (due to crossing over and independent assortment)
- Any male gamete (Sperm cell) can fuse with the female gamete (egg cell) to form a zygote (diploid cell from which an organism develops, made by fusion of 2 gametes)
- Random fusion of gametes causes genetic variation between zygotes with unique combinations of alleles
6
Q
How can mutations cause genetic variation?
A
- Random change in DNA base sequence results in generation of new allele
- Advantageous, disadvantageous
- Or no change at all, genetic code is degenerate
