Genetics Flashcards
What did Mendel do and why?
Mendel worked with Pisum Sativa (peas) because they had many characteristics to study and flowers that were manipulatable.
He crossed tall (dominant) and short (recessive) plants. In the first generation there were no short plants, but in the second generation short plants came back in pure form with a 3:1 ratio.
State and describe Mendel’s first principle.
The principle of dominance:
a. Alleles come in pairs, one from a female parent and one from a male parent.
b. One of the two alleles is dominant and the other recessive; dominant masks recessive.
What is a genotype?
The two alleles for a characteristic, one from female parent and other from male parent.
What is a phenotype?
The trait for a characteristic that is expressed.
What is the dominant allele?
The allele that is always expressed.
What is the recessive allele?
The allele that is only expressed in pure form.
What is the locus?
The position of an allele on a chromosome.
Describe the positioning of the same characteristic on a chromosome.
The alleles for a characteristic must be at the same position on the chromosome.
What did Thomas Hunt Morgan do and how?
He conducted the first genesis experiment. He used fruit flies (drosophila melangaster) because they produced many generations in a short period of time and had lots of study-able traits. He studied the red-eyed ad white-eyed flies.
What is the law of segregation and who founded it?
Two alleles of a gene would separate into different haploid gametes during meiosis. Founded by Gregor Mendel.
What is Mendel’s law of assortment?
The 9:3:3:1 ratio shows that the four types of gametes are equally common and the inheritance of two genes is independent (the presence of an allele of one of the genes in a gamete has no influence over which allele of the other gene is present in the gamete)
What are possible reasons for ratio’s other than the 9:3:3:1 ratio to result from a dihybrid cross?
- Co-dominant alleles
- A homozygous parent
- Sex linkage (not autosomal)
- Interaction between genes
What is an example of an organism with interaction between it’s genes?
Mice: two genes affect coat colour of mice; one controls if it has a colour or not and the other controls what the colour would be.
Describe unlinked genes.
The genes are located on different chromosomes called homologous chromosomes (bivalent). When pairing homologous chromosomes during prophase of meiosis, the alleles of the homologous genes are on different pairs of chromosomes.
Describe and explain what happens to bivalents during metaphase and anaphase.
During metaphase, bivalents are assorted randomly on the equator. This means the pole to which an allele on one bivalent goes to when the homologous chromosomes separate in anaphase don’t affect where the other alleles go. So if AaBb produces the gametes AB, Ab, aB, ab, all are equally probable because A and B are on different chromosomes.
What is gene linkage?
It is when pairs of genes are on the same chromosome; they are inherited together.
Describe the loci of linked genes.
Linked genes have loci on the same chromosome.
How are new combinations produced for linked genes?
By recombination (crossing over) to produce recombinants by swapping the DNA between chromatids.
What did Thomas Hunt Morgan develop?
The linked genes idea to account for the anomalies of Mendel’s law of independent assortment theory.
How are anomalies of Mendel and Morgan’s theories explained?
If the pattern of inheritance is the same on males and females but they don’t follow mendelian ratios, then the two genes are on the same non-sex chromosomes (autosomes).
What identifies recombinants?
Recombinants have different phenotypes from their parent’s.
How can you identify gene linkage from ratios?
The expected ratio for unlinked genes is 9:3:3:1; if the observed ratio is far off from this ratio then gene linkage is occurring.
What are non-sister chromatids?
Chromatids of two different chromosomes in a pair.
Define crossing over.
The exchange of DNA material between non-sister homologous chromatids.
Where does crossing over occur?
Prophase I of meiosis.
Describe crossing over.
Homologous chromosomes, each consisting of two sister chromatids, pair up in prophase I of meiosis. When the chromosomes are paired, sections of the chromatids are exchanged creating chromosomes with non-sister chromatids.
What does recombination rely on and why?
Crossing over, because otherwise it would be impossible to produce new combinations and the offspring would all have the same combinations as their parents.
Name and describe the steps for the chi-squared test.
- Draw a table of observed results: number of individuals for each phenotype resulting from the cross.
- Calculate the expected frequencies: mendelian ratio value divided by 16 multiplied by total.
- Determine number of degrees of freedom: total number of phenotypes minus 1.
- Find the critical region of chi-squared from the table: use calculated degree of freedom and the significance level of 0.05 (5%).
- Calculate chi squared with formula.
- Compare: if calc. value is in critical region - results do not fit mendelian ratio. If value is outside critical region - the difference between expected and observed results are not statistically significant (fit M. ratio), suggesting that the genes are unlinked.
Describe discrete variation.
Every individual fits into a number of non-overlapping classes.
Describe continuous variation.
Any variable is possible between the two extremities.
How does continuous variation occur?
Genetically determined. It is due to the combined effects of two or more genes, which is called polygenic inheritance.
What other factor effect polygenic traits? Give an example.
Environmental factors: height is continuous variation as any height is possible between the two extremities (shortest and tallest), however it is also effected by nutrition.
What is a gene pool?
A pool which consists of all the genes and their different alleles in an interbreeding population.
Describe the theory behind gene pools.
If there is random mating (interbreeding) in a population, any two individuals could be the parent of an offspring individual. So the individual could inherit any of the genes in the gene pool .
How does evolution contribute to gene pools?
Evolution involves a change overtime in allele frequency in a gene pool.
How do you calculate the frequency of an allele?
The number of that allele in a population divided by the total number of alleles of the gene (ranges from 0 to 1).
Why would geographically isolated populations have different allele frequencies?
It may be due to the difference sin natural selection or to random drift.
Name the three types of natural selection.
- Directional
- Stabilising
- Disruptive
Describe the directional type of natural selection and give an example.
One extreme is selected for and the other extreme is selected against.
In the bird species Parus major, breeding success has been greater earlier than later due to the availability of prey being earlier.
Describe the stabilising type of natural selection and give an example.
Intermediates are selected for and extremes are selected against.
In the bird species Parus major, breeding success has been most successful with intermediate clutch sizes (number of eggs). Large clutch sizes have low survival rates and small clutch sizes mean fewer offspring with no greater chance of survival.
Describe the disruptive type of natural selection and give an example.
Extremes are selected for and intermediates are selected against.
In the bird species Passerina amoena, year old males with the dullest and brightest plumage are most successful at obtaining higher qualities of territories, pairing with females and creating offspring than males with intermediate plumage.
Describe speciation.
Speciation is the formation of a new species which forms when a pre-existing species which is reproductively isolated (not interbreeding with other populations). Eventually the isolated population won’t be able to interbreed because a new species has been created.
Name and describe the two speeds at which speciation can occur.
- Gradualism - over a long period of time.
2. Punctuated equilibrium - abruptly.
What does the speed at which speciation occurs depend on?
The type of reproductive isolation.
Name and describe the three main types of reproductive isolation.
- Temporal - when populations of a species breed at different times.
- Behavioural - when populations of a species have behaviour that prevent it from breeding.
- Geographical - when populations of a species can’t breed because they live in different areas.
What is polyploidy?
Instant speciation. An error which results in an individual having more than two sets of chromosomes.
Describe polyploidy using the species allium.
Allium is a species which has 16 chromosomes. In that species, an individual with 32 chromosomes is a tetraploid. If this tetraploid crosses with a diploid individual, the offspring would be a triploid and, therefore, reproductively isolated.
