6.1.2 Patterns of inheritance Flashcards
Types of variation
- continuous= phenotype in population varies within a range i.e height (polygenic and influenced by environment)
- discontinuous= when there are 2 or more distinct categories the phenotype can fall into (monogenic and not influenced by environment)
Environmental factors on phenotype variation
-able to influence gene expression through controlling characteristics i.e chlorosis + animal body mass
-caused by differences in environment
Chlorosis
-when plants don’t produce enough chlorophyll + leaves turn yellow/pale
-most plants do show normal genes coding for chlorophyll production but environmental factors affect their phenotype and stop production of proteins i.e
- lack of light(plant stops production to conserve resources)
- mineral deficiencies i.e lack of iron + magnesium means a plant cannot produce chlorophyll
- viral infections(affect metabolism of cells)
Animal body mass
-determined by a combination of environmental + genetic factors
-amount + quality of food, quantity of exercise, presence of disease= environmental
-genetic mutations could change the pattern of fat deposition in the body + cause obesity
Genetic variation affecting phenotype variation
-created through the individual mix of alleles inherited from both parents–} combination determined by meiosis(crossing over + independent assortment), random fusion of gametes + any mutations in DNA replication
-i.e human blood group
Genotypes and phenotypes
- genotype= combination of allele an organism inherits for a characteristic
-for most genes there are 2 alleles inherited, one from each parent - phenotype= observable characteristics of an organism
-any changes the environment makes to an organism’s phenotype are not inherited, they are modifications not mutations(changes to DNA)
Definition of gene and allele
-a gene is a sequence of DNA that codes for a protein(resulting in a characteristic)
-alleles are different versions of the same gene –} order of bases in each is slightly different
Definition of recessive and dominant alleles
-dominant= the version of the gene that will always be expressed if present in an organism
-recessive= will only be expressed if 2 copies of this allele are present in an organism
Definition of homozygous and heterozygous
-homozygous= 2 identical alleles for a characteristic –} could be dominant(2 alleles for dominant phenotype) or recessive(2 alleles for recessive phenotype)
-heterozygous= 2 different alleles for a characteristic
–} dominant allele will be expressed
What are genetic cross diagrams?
-show the possible genotypes of an offspring so can be used to predict the genotypes + phenotypes of offspring produced when two parents are bred
What is monogenic inheritance?
-inheritance of a characteristic controlled by a single gene
-show the likelihood of different alleles of that gene being inherited by offspring
Steps to performing a a genetic cross
-state the phenotype of both parents i.e Green
-state the genotype of both parents i.e GG
-state the gametes of each parent(contain only one allele for each gene)
-use a Punnett square to find potential alleles for offspring
-state the proportion of each genotype + state corresponding phenotype
3 rules for monogenic inheritance
-true bred cross (homozygous) will always produce heterozygous offspring
-F1 cross is a cross of offspring from a true bred
-F1 crosses for monogenic always produce a 3:1 phenotypic ratio
What is codominance?
-when 2 different alleles occur for a gene and both are equally dominant
-as a result, both alleles of the gene are expressed in the phenotype of the organism if present
Snapdragon example of codominance
-snapdragons’ petal colour is controlled by the C gene and its allele can either be red or white:
- Cr Cr (red flowers, homozygous)
- Cw Cw (white flowers, homozygous)
- Cr Cw (pink flowers, heterozygous)
What is the expected F1 ratio for codominance?
1:2:1
homozygous: heterozygous: homozygous
What are multiple allele crosses?
-some genes have more than 2 versions or multiple alleles–} however as an organism only carries 2 versions of the gene, only 2 alleles will be present in the individual
How are human blood groups determined?
-determined by 3 alleles(a, o, b) on gene I
-the gene encodes a specific antigen that will be present on the surface of the RBC
-Ia and Ib are codominant and when both are present, they will both be expressed in the phenotype
What is the expected phenotypic ratio for the different blood groups?
-because any offspring could have any one of the 4 blood groups, the ratio would be 1:1:1:1
(A:B:O:AB)
What is a dihybrid cross?
-the inheritance of 2 different characteristics, caused by 2 genes that may be located on different pairs of homologous chromosomes
-each of the 2 genes can have two or more alleles
-the crosses can be used to show the likelihood of offspring inheriting certain combinations of the 2 characteristics
How many alleles in a genotype and in a gamete?
-4 alleles in a genotype(2 for each characteristic)
-2 alleles per gamete
What is the rule for a true bred cross in dihybrid crosses?
-all the offspring produced in the F1 generation will have a heterozygous genotype i.e YyRr
What is the expected phenotypic ratio for an F2 dihybrid generation?
9:3:3:1
both dominant: dominant first recessive second: recessive first, dominant second: both recessive
What factors can affect the phenotypic ratio?
-crossing over
-random fertilisation of gametes
What is sex linkage?
-when the gene coding for a certain characteristic is located on a sex chromosome
-the Y chromosome is smaller than the X chromosome and carries less genes–} males only have 1 X chromosome so often only have 1 allele for sex-linked genes
-therefore they are more likely to express that characteristic in their phenotype even if it is recessive
-if there us a faulty gene on the X chromosome of females, they are more likely to become carriers for the disease + only display it if the faulty gene is on both x chromosomes
Haemophilia
-recessive blood disorder carried on the X chromosome
-if a male inherits the recessive allele that codes for haemophilia, they develop the condition
-females who are heterozygous for the haemophilia gene are carriers
-the alleles for the condition are shown alongside the chromosome they are found on i.e Xh
What is autosomal linkage?
-occurs on any chromosome that is not a sex chromosome
-genes that are linked when they are on the same chromosome–} they will stay together during independent assortment in meiosis 1 and their alleles will be passed on to the offspring together unless crossing over changes the combination of genes on the chromosome
-linked genes will be inherited together
How does crossing over affect autosomal linkage?
-the closer together two genes are on the chromosome, the less likely that crossing over will split them up and vice versa
-offspring affected by crossing over are known as recombinant offspring
What is recombination frequency and how is it calculated?
-a measure of the amount of crossing over that has happened in meiosis
- recombinant frequency: no. of recombinant offspring/total no. of offspring
-recombination of 50%= no linkage and genes are on separate chromosomes
-recombination of less than 50%= gene linkage and the random process of independent assortment is hindered
What is the expected phenotypic ratio for F1 offspring?
1:1:1:1
What is epistasis?
-when the allele of one gene masks the expression of the alleles of another gene
-many genes control the same characteristic + they interact to form the phenotype
Examples of epistasis
-widow’s peak is controlled by 1 gene and baldness is controlled by another–} the bald gene would mask the widow’s peak gene
-flower pigment is controlled by 2 genes: yellow pigment + a gene that codes for an enzyme that turns the pigment orange–} yellow pigment is epistatic to colour changing gene(doesn’t matter if you have gene 2 when you do not have the yellow pigment gene)
What is the expected phenotypic ratio for recessive epistatic gene in the F2 generation?
-crossing a homozygous recessive parent with homozygous dominant parent results in 9:3:4
-dominant epistatic: recessive other: recessive epistatic
What is the expected phenotypic ratio for dominant epistatic gene in the F2 generation?
-crossing a homozygous recessive parent with a homozygous dominant parent would produce a 12:3:1 phenotypic ratio
How many alleles in each genotype and gamete for epistasis?
-4 alleles per genotype and 2 alleles per gamete
What is chi-squared and why is it used?
-stat test that measures the size of the difference between observed and expected results and whether expected results fit the observed pattern
-determines whether differences in the expected and observed results are statistically significant or due to chance (accept/reject null hypothesis)
what is the chi-squared formula?
X^{2}=total of (observed value-expected value)^{2}/expected value
steps to using chi-squared?
-start with an experimental theory that is used to make a prediction of the results-expected results
-experiment is carried out and the actual results are recorded-observed result
-state the null hypothesis (no statistically significant different between the observed and expected result)
-calculate chi-squared and compare to the critical value
-the outcome will either accept the null hypothesis(due to chance) or reject it
comparing chi-squared to critical value
-if the calculated value is greater than/equal to the critical value, there is a significant difference, less than 5% probability that the results are due to chance
-if the calculated value is less than the critical value, there is no significant difference, more than 5% probability that the results are due to chance
What is a gene pool?
-the range of alleles present in a population
what is allele frequency?
-the relative frequency of an allele in a population
what affects allele frequency and how does this link to evolution?
-allele frequency changes over time dependent on environmental influence
-evolution is a long term change in the allele frequency of a population over time
Factors affecting evolution: mutation
-new alleles are usually generated by mutations in genes–} leads to genetic variation
Factors affecting evolution: sexual selection
-leads to an increase in frequency of alleles which code for characteristics that improve mating successes
Factors affecting evolution: gene flow
-the movement of alleles between populations i.e emigration results in changes of allele frequency within a population
Factors affecting evolution: natural selection
-evolution in larger populations is predominantly driven by natural selection(any changes by chance in allele frequency tend to even out in a larger population)
-leads to an increase in the no. of individuals with beneficial characteristics
-more likely to reproduce and pass on alleles–} high frequency of beneficial alleles
Factors affecting evolution: genetic drift
-usually has a bigger impact on evolution in small populations*
-change in allele frequency due to the random nature of mutations
*less alleles in the gene pool of a smaller population
How does genetic drift work?
-individuals within a population show variation in their genotypes i.e A and B
-by chance, the allele for genotype B is passed on to the offspring more often than others–} no. of individuals with that allele increases
-if this continues, it leads to evolution as the allele becomes more common in a population
How does population size affect evolution?
-genetic drift and natural selection work alongside each other to drive evolution but the one with the greatest impact is determined by population size
-small populations with limited genetic diversity cannot adapt to change as easily and are more likely to become extinct than large populations with a larger gene pool(easy to adapt to change over time)
Genetic bottlenecks
-an event that causes a big reduction in population size i.e natural disasters/diseases/habitat disruptions from humans
-the resulting population would have a small gene pool(may lead to the extinction of a particular allele) and limited genetic diversity
What is a positive aspect of genetic bottleneck?
-a beneficial mutation will have a much greater impact and lead to the quicker development pf a new species
Founder effect
-when a new population is created with a few individuals from a larger population due to migration and geographical isolation or through cultural choice
-these small populations have much smaller gene pools and less genetic variation
-alleles that were rare in the original population will be much higher and have bigger impact on natural selection in the new population if carried
What does the stability an ecosystem affect?
-which characteristics are selected for by natural selection
Describe stabilising selection
-the environment of the organism remains mostly unchanged
-the phenotype of the organism favours the average and not the extremes
Describe directional selection
-occurs when there is a selection pressure against one of the phenotypic extremes
-distribution of the phenotypes move more towards the extreme
Describe disruptive selection
-favours the extreme of the phenotype + the intermediate phenotype is selected against
What is the Hardy-Weinberg principle and what are its assumptions?
-can be used to predict allele frequencies
-predicts that the frequencies of alleles in a stable population will remain constant from one generation to the next and there will be no evolution
-assumes that:
- the population is large enough to make a sampling error negligible
- there is no selective advantage for any genotype–} no selection
- there is no mutation, migration or genetic drift
- random mating(all possible genotypes can breed with another)
(conditions virtually never occur in a natural environment)
Hardy-Weinberg equation for allele frequency
p + q = 1
-p= frequency of dominant allele
-q= frequency of recessive allele
(total frequency always equals 1)
Hardy-Weinberg equation for genotype frequency
p² + 2pq + q² = 1
-p²= frequency of homozygous dominant genotype
-2pq= frequency of heterozygous genotype
-q²= frequency of homozygous recessive genotype
What is speciation?
-the formation of a new species through evolution
-the organisms belonging to the new species will no longer be able to interbreed to produce fertile offspring with the original species
What are the events leading up to speciation?
-members of the population become isolated and no longer interbreed with the rest of the population–} in no gene flow between the groups
-alleles in both groups undergo random mutations + environment of each group may change/be different–} different selected characteristics
-accumulation of mutations + changes in allele frequencies over generations lead to large changes in the phenotype–} reproductively isolated and become a new species
Describe allopatric speciation
-formation of 2 different species from 1 due to geographical isolation–} physical barrier i.e sea
-populations adapt to different conditions in their environment–} different characteristics will be more common via natural selection(different selection pressures)
-mutations change allele frequencies different and lead to different changes in their phenotype
Describe sympatric speciation
-formation of 2 different species from 1 due to reproductive isolation within the same habitat
-random mutations could occur within a population, preventing member of that population breeding with members of the same species
-hybrid fertile offspring(formed from 2 different species interbreeding) may not be able to interbreed with the members of either parent population–} stops gene flow
What act as reproductive barriers?
-seasonal changes
-mechanical changes
-behavioural changes
What is artificial selection?
-artificial change in the environment leading to survival of the fittest–} selection for breeding of plants/animals with desirable characteristics by farmers
-populations are usually polymorphic for most characteristics
-alleles coding for the most common characteristic is called the wild type allele and other forms are called the mutant
Examples of artificial selection
-dairy cows: farmer breeds female with high milk yield and male whose mother had a high milk yield together, select the best offspring and breed them together, continued over many gens
(milk should be high quality + cows should have a long lactation period, large udders, be resistant to diseases, a calm temperament
-bread wheat: wheat plants with a high wheat yield are bred together, offspring with best yield are bred together, continued over many gens
(wheat should have a high tolerance of cold, short stalks(don’t collapse), uniform stalk height(easier harvesting)
What is inbreeding and what is the problem with it?
-the breeding of closely related individuals
-leads to a limited gene pool–} decreases genetic diversity and limits chances of a population of inbred organisms evolving
-being too genetically similar increases chances of having the same recessive alleles which reduces ability to survive and reproduce(more susceptible to genetic disorders)
What are gene/seed banks and how are they useful?
-seed banks keep samples of seeds from both wild + domesticated varieties
-gene banks store biological samples i.e eggs
-alleles from banks can be used to increase genetic diversity via outbreeding(breeding unrelated/distantly related varieties)–} reduces occurrence of homozygous recessives + increases potential to adapt to environmental changes
