Biology Chapter 9 Flashcards
How many daughter cells does mitosis produce and what are the number of chromosomes of each daughter cell in relation to the parent cell?
2 daughter cells
Each daughter cell has the same number of chromosomes as the parent cell
How many daughter cells does meiosis produce and what are the number of chromosomes of each daughter cell in relation to the parent cell?
4 daughter cell
Each daughter cell has half the number of chromosomes as the parent cell
Why does the number of chromosomes halve during meiosis?
To keep the number of chromosomes constant through the generations/ so that when gametes fuse they have one complete set of chromosomes
What is the haploid number of chromosomes in a human?
23
How many nuclear division are involved in meiosis?
2
Besides halving the number of chromosomes, what is the function of meiosis?
To introduce genetic variation/diversity
What is the advantage of genetic diversity to an organism?
To adapt to changing environmental conditions
Give 2 ways in which meiosis brings about genetic variation
- Independent segregation (homologous chromosomes separated into different cells)
- Crossing over (recombination)
Explain how crossing over occurs
Homologous chromosomes pair up. Their chromatids wrap round one another and equivalent sections may be exchanged
When does crossing over happen?
During stage 1 of meiosis (first division)
What is meant by the ‘locus’ of a gene?
It’s position on a chromosome or DNA molecule
Explain how independent segregation of chromosomes occurs
In meiosis 1 the homologous pairs line up at random so the combinations of chromosomes that enters the 2 daughter cells is also random
How many homologous pairs are there in a human cell at the start of meiosis 1?
23
How many cells are formed by the end of meiosis 2?
4
Do the characteristics determined by the genes on the chromosome have to be the same in each of the chromosomes in a homologous pair?
Yes
Do the alleles have to be the same in each of the chromosomes in a homologous pair?
No
What happens when chromosomes twist around each other?
Force is generated at the point of twisting and a part of the chromatid breaks off
How does this result in recombination?
The broken parts of the chromatids re-join with their homologous partner chromatids
how does this increase variety?
It means that each of the cells produced is genetically different
Imagine a cell contained 21 chromosomes. Is it likely to be haploid or diploid?
Haploid- diploid cells usually have an even number of chromosomes as they occur in homologous pairs
Variation between individuals in a species is due to:
Variation between alleles
What is the advantage to a species of greater genetic diversity?
The species if more likely to survive a change in the environment
Why is this?
They have a wider range of characteristics and so some individuals will be able to adapt to change
Summarise the effect of both mutations on inheritance and allele frequency
The beneficial allele is inherited so will increase in frequency in the population, whereas the harmful allele is not inherited and will decrease in frequency in the population
What affects the allele frequency in populations?
Differences between the reproductive success of individuals
Why is there competition between members of a species to be the ones that survive?
More offspring are produced that can be supported by the supply of food, light, space etc.
What makes certain individuals better able to survive in competition with others?
Those with combinations of alleles that make them fitter
What is selection?
The process by which organisms that are better adapted to their environment survive and breed, while those that are less well adapted fail to do so
What type of selection favours individuals that vary in one direction from the mean of the population?
Directional selection
What type of selection favours average individuals?
Stabilising selection
What type of curve describes the variation about the mean for a characteristic?
Normal distribution curve
