9. GENETIC DIVERSITY Flashcards
Define variation
Differences between individuals
State what interspecific variation is
Variation between species
State what intraspecific variation is
Variation within a species
Which two factors contribute to phenotypic variation?
Genetic variation and environmental factors
Define discontinuous variation
Variation that falls into distinct categories
Give an example of discontinuous variation in humans
Blood group, sex
Define continuous variation
Variation that shows gradations from one extreme to the other
Give an example of continuous variation in humans
Height, weight
Why might individuals in the same family / species show phenotypic similarities?
Because they share genetic similarities
Why might individuals in different families / species show phenotypic differences?
Because they have genetic differences
Phenotypic characteristics can be under monogenic control. What does this mean?
The phenotype is controlled by one gene
Phenotypic characteristics can be under polygenic control. What does this mean?
The phenotype is controlled by many genes
Are phenotypic characteristics that are under monogenic control more likely to be continuous or discontinuous?
Discontinuous
Are phenotypic characteristics that are under polygenic control more likely to be continuous or discontinuous?
Continuous
Describe the structure of a chromosome
A long strand of wound up chromatin
How many chromosomes do human somatic cells have?
23 pairs / 46
How many chromosomes do human gametes have?
23
Which type of cells have a diploid number of chromosomes?
Somatic cells (body cells)
Define what homologous chromosomes are
Chromosomes with the same genes at the same loci
In terms of n, write a haploid number of chromosomes
n
In terms of n, write a diploid number of chromosomes
2n
Which process is this showing: n + n = 2n ?
Fertilisation (gametes fusing)
Define what sister chromatids are
Identical copies of the same chromosome
When are sister chromatids formed?
DNA replication
How are sister chromatids held together?
By centromeres
Meiosis is a type of cell division that produces which type of cells?
Gametes
How many nuclear divisions occur in meiosis?
Two - meiosis 1 and meiosis 2
When is the only time DNA replication occur in meiosis?
In interphase before meiosis 1
Describe what happens in metaphase 1 in meiosis
Homologous chromosomes randomly assort when they line up on the equator of the cell. Crossing over occurs between adjacent non-sister chromatids.
Describe what random assortment is
The random positioning of the maternal and paternal chromosome in a homologous pair either side of the equator of the cell
Define crossing over
Equal lengths of alleles are exchanged between the non-sister chromatids in meiosis 1
Describe how crossing over occurs
Chiasmata form between homologous chromosomes to form a bivalent. Equal lengths of alleles break and rejoin onto different chromatids.
Describe what a bivalent is
Four chromatids held together by a chiasmata
Describe what chiasmata is
The point of contact where two non-sister chromatids have twisted around each other in a bivalent
Describe what happens in anaphase 1 in meiosis
Homologous chromosomes are pulled to the poles of the cell
Describe the chromosome number in the cells produced from meiosis 1
Diploid / 2n
Describe what happens in metaphase 2 in meiosis
Sister chromatids line up on the equator of the cell
Describe what happens in anaphase 2 in meiosis
Sister chromatids are pulled to the poles of the cell
Describe the daughter cells are formed from meiosis
Four genetically non-identical, haploid cells (gametes)
State the number of chromosomes present in each daughter cell produced from meiosis in humans
23 single chromosomes
State the formula to calculate the number of different possible combinations of chromosomes of each daughter cell produced from meiosis
2^n where n = haploid number of chromosomes
Calculate the number of possible chromosome combinations in gametes produced from meiosis in an organism with a diploid chromosome number of 18
2^(18/2) - 2^9 = 512
State the formula to calculate the the possible number of different combinations of chromosomes in zygotes following random fertilisation of two gametes
(2^n)^2 where n = haploid number of chromosomes
Calculate the number of possible combinations of chromosomes in zygotes following random fertilisation of two gametes whose cells have a diploid number of 66
(2^(66/2))^2 = (2^33)^2 = 8589934592^2 = 7.38 x 10^19
Compare the number of nuclear divisions that occur in mitosis and meiosis
Mitosis has one nuclear division, whereas meiosis has two
Compare the daughter cells produced from mitosis and meiosis
Mitosis produces two genetically identical diploid daughter cells, whereas meiosis produces four genetically non-identical haploid daughter cells
Describe three things that occur in meiosis that don’t occur in mitosis
Homologous chromosomes separate, crossing over, and random assortment
Describe what chromosome mutation is
Changes in the structure or number of whole chromosomes
Describe what chromosome non-disjunction is
When individual homologous pairs of chromosomes or chromatids fail to separate during meiosis
Describe non-disjunction if it occurred in meiosis 1
Homologous chromosomes fail to separate (during anaphase 1)
Describe non-disjunction if it occurred in meiosis 2
Sister chromatids fail to separate (during anaphase 2)
What would the result of chromosome non-disjunction be?
The gametes produced would either have missing or extra chromosome(s) compared to normal.
Which gametes produced from chromosome non-disjunction will not be able to fuse with another gamete during fertilisation?
Gametes that have fewer than the haploid number of chromosomes
Which gametes produced from chromosome non-disjunction will be able to fuse with another gamete during fertilisation?
Gametes that have more than the haploid number of chromosomes
Name the two types of ploidy that describes the chromosome number in the gametes produced from chromosome non-disjunction
Polyploid and aneuploidy
Define polyploidy
A change in the number of whole sets of chromosomes
An example of polyploidy is cells that are triploid. Using n, write the shorthand for a triploid number of chromosomes
3n
Using n, write the formula to show the gametes that would have fertilised to produce a triploid zygote
n + 2n = 3n
An example of polyploidy is cells that are tetraploid. Using n, write the shorthand for a tetraploid number of chromosomes
4n
Using n, write the formula to show the gametes that would have fertilised to produce a tetraploid zygote
2n + 2n = 4n
In which organisms does polyploidy most commonly occur?
Plants
Define aneuploidy
A change in the number of individual chromosomes
Using n, write the shorthand for the aneuploidy gametes produced from a chromosome non-disjunction event if one pair of sister chromatids failed to separate during meiosis 2
n+1, and n-1
Give an example of a human condition caused by an aneuploidy gamete being created from a chromosome non-disjunction event in meiosis
Down’s Syndrome
Down Syndrome is produced when cells contain an extra copy of chromosome 21 (three copies instead of two). How many chromosomes in total would someone’s somatic cells have if they have Down’s Syndrome?
47
When can organisms not produce all fertile gametes?
When their diploid cells have an odd number of chromosomes
Why can organisms with an odd number of chromosomes in their diploid cells not produce all fertile gametes?
Because half of their gametes would not be haploid, and could therefore not pair with another gamete during fertilisation
Define genetic variation between individuals
Differences in the base sequence of DNA
Define genetic variation of a population
The total number of different alleles in a population
Recall the five factors that cause genetic variation
Mutation
Random fertilisation
Random assortment
Crossing over
Chromosome non-disjunction
Which of the five factors that cause genetic variation will always occur in meiosis?
Random assortment and crossing over
Describe how random assortment in meiosis 1 produces genetic variation in the gametes
Each gamete has a different combination of maternal and paternal chromosomes
Describe how crossing over in meiosis 1 produces genetic variation in the gametes
Each chromatid will contain a different combination of alleles
What effect does selection have on genetic variation in populations?
Selection reduces genetic variation in populations
Name the two types of reproduction
Sexual and asexual
Name the type of asexual reproduction that occurs in bacteria
Binary fission
In which type of reproduction are genetically identical offspring produced?
Asexual
In which type of reproduction are genetically non-identical offspring produced?
Sexual
In populations that reproduce asexually, what is the only way genetic variation is produced?
Mutation
Apart from genetic variation, how else can phenotypic variation be produced in populations that reproduce asexually?
Environmental factors
Why do organisms that reproduce sexually have an even number of chromosomes?
So during meiosis, the chromosome can be split into two haploid cells of an equal number. This means the diploid number of chromosomes can be restored during fertilisation.
Describe the advantages of reproducing asexually
There are fewer stages so reproduction is quicker.
A higher number of offspring can be produced.
There is only one parent so a new environment can be colonised easily.
Clones are produced so phenotypes are always maintained.
Describe the disadvantages of reproducing asexually
It decreases genetic diversity which decreases the chances of survival.
Describe the advantages of reproducing sexually
It increases genetic diversity which increases the chances of survival.
Describe the disadvantages of reproducing sexually
There are more stages so reproduction is slower.
A lower number of offspring can be produced.
Two parents are required so it’s more difficult to colonise new areas.
Genetically different offspring are produced so phenotypes are not always maintained.
