Flashcards in 6.2 - Patterns of Inheritance Deck (57):
How does sexual reproduction lead to variation in genotypes in species?
Meiosis makes gametes with a unique assortment of alleles through crossing-over & independent assortment of chromosomes. Also random fusion of gametes during fertilisation also increases variation in off-spring.
Explain the meaning of the term crossing over.
Swapping of alleles between maternal & paternal homologous chromosomes during prophase I of meiosis. Pairs of homologous sister chromatids exchange portions resulting in a shuffling of alleles.
Other than inheriting characteristics, what else may have an effect on the phenotype of an individual?
What is genotype?
The genetic constitution of an organism.
What is phenotype?
Appearance due to alleles. Appearance due to environment.
What is a dominant allele?
A allele that is always shown in the phenotype.
What is a recessive allele?
An allele that is only shown in the phenotype when homozygous.
What is heterozygous?
1 dominate allele, 1 recessive allele.
What is homozygous dominant?
2 dominant alleles.
What is homozygous recessive?
2 recessive alleles.
What is co-dominance?
Where both alleles are dominant, so both are expressed in the phenotype.
What is multiple alleles?
Where there are more than 2 alleles of a particular gene.
How can you prove that the allele for a genetic characteristic is on the X chromosome?
Male offspring are positive for the phenotype even when the father is negative and the mother is negative (heterozygous) because faulty X can only be inherited from mother.
How is a genetic diagram laid out? – to explain flower colour being controlled by a single gene with two codominant alleles.
Parental phenotypes: red flower X white flower
Parental genotypes: CR CR X CW CW
Gametes: CR CW
F1 genotypes: All CRCW
F1 phenotypes: All pink
Gametes: CR, CW, CR, CW
F2 genotypes: CRCR ; CRCW ; CRCW ; CWCW
F2 phenotypes: red ; pink (pink) ; white
Expected F2 phenotypic ratio: 1 :2: 1
Why is a recessive genetic disorder, found on the X chromosome, more likely to shown on male’s phenotypes than female’s?
Males only have one X chromosome so only one allele is needed for it to be expressed. Females need two alleles for it to show in their phenotype.
What is monohybrid inheritance?
The inheritance of a characteristics controlled by a single gene.
What is dihybrid inheritance?
The inheritance of 2 characteristics which are controlled by different genes.
What are the predicted phenotypic ratios for monohybrid, dihybrid and codominant crosses?
Monohybrid cross - 3:1 - dominant : recessive
Dihybrid cross - 9:3:3:1 - dominant both: dominant 1st recessive 2nd: recessive 1st dominant 2nd: recessive both
Codominant - 1:2:1 - homozygous for one allele: heterozygous: homozygous for the other allele
Why are the ratios of offspring genotypes not the same in real life to those calculated in genetic diagrams?
Fertilisation is random.
Why can the allele frequency of a lethal genetic disorder decrease over time?
Affected individuals might struggle to reproduce, therefore the allele will not be passed on to future generations.
What is an autosome?
Any chromosome that is not a sex chromosome (X or Y).
Explain the term linkage.
Two or more genes (loci) on the same chromosome. They do not independently assort during meiosis (metaphase I). Their alleles are passed on to offspring together.
How and when can linked genes be split up?
Crossing over during meiosis (prophase I) can split linked genes.
When are linked genes less likely to be split up?
The closer together two genes are on the autosome, the more closely they are said to be linked and the less likely they are to be split up by crossing over.
Describe and explain the predicted ratio of two autosomally linked alleles.
Predicted ratio is the same as a monohybrid cross = 3:1 as the two autosomally linked alleles are inherited together. A higher proportion of the offspring will have parental genotypes and phenotypes.
What is epistasis?
One gene masks the expression of another gene.
What is recessive epistasis?
Having 2 copies of the recessive epistatic allele blocks the expression of the other gene.
What is the predicted ratio of a dihybrid cross involving a recessive epistatic allele?
What is dominant epistasis?
Having one copy of the dominant epistatic allele blocks the expression of the other gene.
What is the predicted ratio of a dihybrid cross involving a dominant epistatic allele?
12:3:1 (and 13:3)
Suggest how one gene may inhibit the action of another.
One gene produces a protein that blocks transcription of another gene, possibly by binding to the promoter region for that gene. The product of one gene acts as a transcription factor for another gene and represses transcription of that gene.
Suggest how one gene may enhance the action of another.
One gene produces a protein that acts as a transcription factor to stimulate transcription of another gene. (NB epistasis can enhance as well as block)
What is the Chi-squared test used for?
To determine the significance of the difference between observed and expected results.
What must your table of data collate to calculate chi-squared?
Expected result (E)
Observed result (O)
O – E
(O – E )^2
(O – E )^2 / E
How do you calculate degrees of freedom?
Number of classes – 1
What has happened if your X^2 value is larger than the critical value?
There is a significant difference between the observed and expected results. Something other than chance is causing the difference and the null hypothesis can be rejected.
What does the Hardy-Weinberg principle predict?
Allele frequencies will stay the same from generation to generation.
Providing no mutation occurs, there’s a large population, population genetically isolated, mating is random and there is no migration.
Explain how evolution occurs by natural selection.
In a population, some individuals are better adapted to the selection pressures.
Those with an advantageous allele are more likely to survive (than those with different alleles), reproduce and pass on this allele.
A greater proportion of the next generation inherit the advantageous allele. Over time, the frequency of the advantageous allele increases from generation to generation.
What effects which characteristics become more common?
What are the factors that can affect the evolution of a species?
Stabilising selection; directional selection; genetic drift; genetic bottleneck; founder effect.
What is stabilising selection?
Where selection favours average individuals. This preserves the characteristics of a population. Occurs when the environment is stable.
What is directional selection?
Where selection favours individuals that vary in one direction from the mean of the population. This causes a change in the characteristics of the population. Occurs when there is a change in the environment.
Describe and explain genetic drift.
Variation exists in a population but by chance, the allele for one genotype is passed onto the offspring more often than others. The number of individuals with this allele increases. If this happens by chance again and again, it can lead to evolution as the allele becomes more common in the population.
What population size does genetic drift have a greater effect on evolution?
Smaller populations where chance has a greater influence.
What is genetic bottlenecking?
An event that drastically reduces the size of the population of a species (for at least 1 generation). The reduction in population means that there is a reduction in the variety of alleles in the population (smaller gene pool).
What is the Founder effect?
When members of a species move to and colonise a new environment/area. There are only a small number of different alleles in the initial population reducing the genetic diversity.
How can genetic diversity be measured?
By counting the number of different genes in a gene pool.
What is the difference between variation and genetic diversity?
Genetic diversity can be measured, variation is only expected to occur as a result of meiosis.
What is artificial selection?
Humans select individuals in a population to breed together to get desirable traits.
What characteristics have been selected for in plants?
Higher yield; better taste; richer colour; drought resistance/pathogen resistance.
How does selective breeding reduce genetic diversity?
Only organisms with similar traits are bred together. These will all have similar alleles. It results in a type of genetic bottleneck as it reduces the number of alleles in the gene pool.
A reduced gene pool could cause problems in the future – why? What can we do about it?
Potentially useful alleles are lost from the population when other alleles are being selected for. If a new disease appears there is less chance of resistant alleles being present in the population. It is important to maintain resources of genetic material for the future – seed banks.
Describe how a population evolves to be better adapted to an environment.
Variation in original colonisers as mutations took place. Some better adapted to survival in the conditions. Greater reproductive success. Allele frequencies change.
What is speciation?
The development of a new species.
What is the difference between allopatric speciation and sympatric speciation?
Allopatric speciation is due to geographical isolation.
Sympatric speciation is due to reproductive isolation.
What is allopatric speciation?
Geographical isolation leads to separate gene pools. Variation in population due to mutation. Different environmental conditions. Different selection pressures so the best adapted to the conditions survive. Increase in frequency of allele. Occurs over a long period of time. Eventually populations can no longer interbreed as they have a separate gene pool.