Genetic Diversity Flashcards
(29 cards)
gene mutation
change in the base sequence of chromosomes
when do gene mutations arise?
spontaneously during DNA replication
what increases the rate of gene mutation?
mutagenic agents
2 types of gene mutation
substitution, deletion
why may a base substitution mutation not affect the sequence of encoded amino acids?
as the genetic code is degenerate, so some amino acids are codes for by multiple DNA base triplets. The altered triplet can therefore code for the same amino acid as the original, and the encoded amino acid sequence remains unchanged.
base deletion mutation effects
Decrease in number of bases present, causes a frameshift to the left. All the following triplets are read differently and code for different amino acids.
how do mutations in the number of chromosomes arise?
spontaneously by chromosome non-disjunction during meiosis. This is when chromosomes fail to separate properly.
Process of meiosis
- before meiosis, DNA replicates and condenses.
meiosis 1 - chromosomes line up n their homologous pairs and one from each homologous pair goes into each daughter cell.
meiosis 2 - pairs of sister chromatids that form each chromosome are separated as the centromere divides and drawn into different daughter cells.
How does independent segregation of chromosomes during meiosis lead to genetic variation in offspring?
Homologous pairs of chromosomes line up randomly in M1 and are then separated. Which chromosome - maternal or paternal - enters each daughter cell here is determined by chance - so daughter cells are genetically unique with different combinations of alleles.
How does crossing over of chromosomes in meiosis lead to genetic variation in offspring?
- M1: homologous chromosomes pair up
- chromatids twist around each other and equivalent portions break off and are exchanged.
- chromatids contain different combinations of alleles.
- 4 daughter cells and hence potential offspring have different combinations of alleles.
What happens during crossing over of chromosomes - meiosis?
Homologous chromosomes line up in their pairs and chromatids twist around each other. equivalent portions are exchanged.
Expression to calculate the number of possible different chromosome combinations following meiosis (without crossing over)
2^n
what does meiosis produce?
4 genetically different haploid gametes.
How does random fertilisation of gametes lead to increased genetic variation?
2 genetically unique gametes fuse and form a zygote from which the new organism develops, which has a unique combination of alleles from both parents.
what is a diploid cell?
a cell that has 2 complete sets of chromosomes - 1 from each parent
What is a haploid cell?
a cell that only contains 1 complete set of chromosomes, produced by meiosis
when does meiosis occur in a lifecycle?
when the chromosome number halves (to n), production of gametes
genetic diversity
the number of different alleles of genes in a population
what does genetic diversity enable to occur?
natural selection
What are the principles of natural selection?
1) random mutations occur, resulting in new alleles of genes
2) many mutations are harmful, but in some environments the new allele may be beneficial, meaning its possessor is more likely to survive and reproduce.
3) the advantageous allele is inherited by members of the next generation.
4) over many generations, allele frequency of the advantageous allele increases in the population.
3 types of selection
directional, stabilising, disruptive
directional selection
individuals with alleles for a phenotype at 1 of the extremes are more likely to survive and reproduce. This could be due to environmental change
stabilising selection
individuals with alleles for phenotypes around the mean are more likely to survive, reproduce and pass their advantageous alleles on to the next generation.
This occurs when the environment isn’t changing
the range of phenotypes decreases
example for stabilising selection
human birth weights
small babies are less likely to survive - as they have difficulty in keeping body temperature constant.
large babies are difficult to give birth to so are also less likely to survive.
medium-sized babies are most likely to survive reproduce, passing their advantageous alleles for size on to offspring - so frequency of medium-sized babied increases in the next generation
