Genetics Flashcards
What is a gene?
A gene is a sequence of bases on a DNA molecule that codes for a protein which results in a characteristic.
What is an allele?
Different versions of genes- order of bases is different.
Where is the allele of each gene found?
At a fixed position, called the locus.
What is a phenotype?
The characteristics an organism has as a result of both its genes and the effect the environment has on its genes.
What’s the difference between homozygous and heterozygous?
If an organism carries two copies of the same allele it’s said to be homozygous.
If an organism carries two different alleles then it’s heterozygous.
Define genotype.
The genetic composition of an organism
Define Homologous Chromosomes
A pair of chromosomes that have the same gene loci and determine the same features.
What is a dominant allele?
An allele that is always expressed in the phenotype.
What is a recessive allele?
An allele that is only expressed in the phenotype when there is another identical allele.
Define Sex Linkage
Any gene that is carried on the X or Y chromosome
Define Co Dominance
Both alleles are equally dominant and are both expressed in a phenotype
What are multiple alleles?
More than 2 possible alleles for a particular gene.
What is a gene pool?
All the alleles of all the genes of all the individuals in a population at any one time
Define allelic frequency
The number of times the allele occurs within a gene pool
Define Stabilising Selection
Individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce
Occurs when environment isn’t changing
Reduces range of possible phenotypes
Define Directional Selection
Individuals with alleles for extreme characteristics are more likely to survive and reproduce
In response to an environmental change
How do multiple alleles of a gene arise?
Mutations which are at different positions in the gene.
Why are males more likely to show a phenotype produced from a recessive allele?
Males only have one X chromosome so are more likely to show a phenotype produced from a recessive allele because they only need to inherit it from the mother, and only have one for it to be expressed in the phenotype. A female would have to inherit the recessive allele on both X chromosomes, from the mother and the father, for it to appear in the phenotype, which is less likely.
What does the hardy weinberg do?
Predicts frequencies of alleles in a population
Conditions of the hardy weinberg principle.
Large population with no emigration, immigration, mutations or natural selection
There is random mating
What is natural selection?
- Selective pressure favouring individuals with a certain characteristic
- More likely to survive and reproduce
- Pass on their alleles to offspring
- Allele frequency increases in the population
Explain the process of speciation.
- Populations of the same species become geographically isolated due to a physical barrier, such as a floor or earthquake
- The two populations experience slightly different conditions
- The populations experience different selective pressures so different changes in allele frequencies, due to natural selection and mutations
- Changes in allele frequency leads to differences in the gene pools of the two populations, causing changes in phenotype
- Eventually the individuals from the two populations would have changed so much that they can no longer breed to produce fertile offspring so have become separate species
