4.3 Genetic diversity can arise as a result of mutation or during meiosis Flashcards
What are gene mutations?
Gene mutations involve a change in the base sequence of
chromosomes resulting in the formation of a new allele. Gene mutations can arise spontaneously during DNA replication.
Define the term mutagenic agent.
A factor that increases the rate of mutations.
How might a gene mutation have no effect on a phenotype?
Due to the degenerate nature of the genetic code, not all base
substitutions cause a change in the sequence of encoded amino
acids.
Mutation could occur in an intron, so amino acid sequence is not changed.
Mutation may change an amino acid but not effect the tertiary structure of the protein (chaining one hydrophobic amino acid for another, for example).
The new allele, assuming it produces a less functional protein, will be recessive so will not influence phenotype (as that allele is passed to offspring down the generations eventually a descendent individual might be homozygous recessive - it is at this point the allele may influence phenotype).
How might a gene mutation have a positive effect on a phenotype?
If the mutation results in change in a polypeptide that positively changes the properties of the protein in a way that increases the reproductive success, or probability of survival, of the possessor.
What is a non-disjunction mutation?
A non-disjunction mutation is a change in the number of chromosomes. It is an example of a chromosomal mutation (as opposed to a gene mutation).
Non-disjunction mutations occur during meiosis, and there are two causes: either a failure to separate the homologous chromosomes at anaphase 1, or a failure to separate the sister chromatids at anaphase 2. They happen spontaneously, usually happen because something went wrong the spindle fibres.
Following a non-disjunction mutation, one of the resulting gametes will have too many chromosomes, while another has too few. Given that gametes are haploid, missing a whole chromosome means missing certain genes entirely, so these gametes are usually non-viable. However, having an extra chromosome can be viable, and these ‘polyploidy’ gametes can fuse with normal gametes in fertilization.
There are 4 ways in which genetic variation can be introduced.
Mutation - new alleles
Independent segregation - new combination of alleles
Crossing over - new combination of alleles
Random fusion of gametes - new combination of chromosomes
Describe the process of crossing over and explain how it increases genetic diversity.
Homologous pairs of chromosomes associate to form a bivalent.
Chiasmata form as non-sister chromatids cross over each other.
Equal lengths of non-sister chromatids are exchanged
So that the chromatids now contain new combinations of alleles
