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66 Cards in this Set
- Front
- Back
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What is continuous variation? |
When individuals in a population vary within a range - no distinct categories. Can be shown by quantitative data (values that can be measured by numbers) |
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What is discontinuous variation? |
When there are two or more distinct categories - each individual fall into only one of these blood groups, no intermediates. Shown by qualitative data (data that doesn't contain any numbers). |
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What is phenotypic variation? |
The variation in an organism's phenotype. |
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Describe the genes and alleles of individuals of the same species. |
Individuals of the same organism have the same genes, but different alleles. |
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Explain how sexual reproduction leads to variation in genotypes within a species. |
• Meiosis produces gametes with a unique assortment of alleles through crossing over and independent assortment of chromosomes. • Random fusion of gametes during fertilisation increases genetic variation of offspring • Differences in genotype result in phenotypic variation. |
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Example of genotype leading to phenotypic variation. |
Human blood group - three different blood group alleles, resulting in four different blood groups. |
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Describe polygenic characteristics. |
• Characteristics influenced by many genes • Characteristics show continuous variation e.g. skin colour |
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Define monogenic characteristics. |
• Characteristic controlled by one gene • Characteristic shows discontinuous variation. e.g. flower colour (white or violet) |
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Describe how the environment can cause phenotypic variation. |
• Differences in the environment (climate, food, lifestyle) affect characteristics • Characteristics controlled by environmental factors change over an organism's life |
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2 examples of phenotypic variation as a result of the environment. |
ETIOLATION ↳ plants grow abnormally long and spindly as not receiving enough light CHLOROSIS ↳ plants don't produce enough chlorophyll and turn yellow ↳ caused by lack of magnesium in soil. |
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Describe how phenotypic variation can arise due to both genotype and the environment. |
• Genotype influences the characteristics and organism is born with, but environmental factors can influence how the characteristics develop. • Most phenotypic variation caused by the combination of both genotype and environmental factors • Usually show continuous variation |
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Example of phenotypic variation in animals caused by both the genotype and environmental factors. |
BODY MASS ↳ partly genetic, strongly influenced by environmental factors such as diet ↳ if diet doesn't contain enough of right nutrients, body mass is likely lower than that determined by genes ↳ mass varies in a range, so continuous variation |
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Example of phenotypic variation in plants caused by both the genotype and environmental factors. |
HEIGHT OF PEA PLANTS ↳ there are tall and dwarf forms (discontinuous) determined by genotype ↳ exact height of tall and dwarf plant varies (continuous) due to environmental factors (light intensity, water availability) |
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Explain why most plants and animals have two alleles of each gene. |
• One allele from each parent • Inherit one copy of each chromosome of a pair from parents |
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Define homozygous. |
Organism carries two copies of the same allele. |
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Define heterozygous. |
Organism carries two different alleles. |
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Define dominant allele. How are they represented? |
An allele whose characteristic appears in the phenotype even when there's only one copy. Represented by a capital letter. |
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Define recessive allele. How are they represented? |
An allele whose characteristic appears in the phenotype only when there are two copies present. Shown by lower case letters. |
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Define codominant allele. |
An allele whose characteristic appears together with another allele in the phenotype because neither allele is recessive. |
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2 examples of codominant alleles. |
• Horses → alleles for white hair or coloured hair are codominant, so a horse with one copy of each allele has a roan coat (mixture of white and coloured hairs) • Alleles for sickle-cell anaemia, genetic disorder caused by mutation in haemoglobin gene → cause red blood cells to be sickle shaped. |
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Define carrier. |
A person carrying an allele which is not expressed in the phenotype but that can be passed on to offspring. |
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Example of a carrier. |
CYSTIC FIBROSIS • inherited disease caused by mutation in CFTR gene • recessive disease, so both CFTR alleles need to be mutated in order for someone to get the disease • if someone has one mutated CFTR allele and one normal CFTR allele, they won't have cystic fibrosis but will be a carrier. |
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Describe how the genotype of an organism is produced. |
• Gametes contain one allele for each gene • Gametes from two parents fuse together, the allele from each parent forms the genotype of the offspring produced. |
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What is monogenic inheritance? |
The inheritance of a characteristic controlled by a single gene. |
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What do monogenic crosses show? |
The likelihood that different alleles of a gene with a monogenic characteristic are inherited by offspring of particular parents. |
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What is the F₁ generation? |
The crossing of two homozygous parents. |
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Describe the result of a monogenic cross with two homozygous parents. |
Always produce all heterozygous offspring in the F₁ generation. |
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What is the F₂ generation? |
The crossing between the parents from the F₁ generation i.e. two heterozygous parents. |
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Define phenotypic ratios. |
The ratio of different phenotypes in the offspring. |
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State the predicted results of a monogenic cross with two heterozygous parents. |
3 : 1 ratio of dominant : recessive characteristics. |
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Why will you not always get the expected phenotypic ratios? |
Due to linkage and epistasis. |
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Hᴺ → normal haemoglobin Hⁿ → sickle haemoglobin Describe the different phenotypes that can arise and their corresponding genotypes. |
• Homozygous for normal haemoglobin → don't have disease → HᴺHᴺ • Homozygous for sickle haemoglobin → have sickle-cell anaemia → HⁿHⁿ • Heterozygous → have sickle-cell trait → some normal and some sickle haemoglobin → HᴺHⁿ |
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State the phenotypic ratio for the monogenic cross of two heterozygous parents with codominant alleles. |
1 : 2 : 1 ratio of homozygous for one allele : heterozygous : homozygous for other allele |
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In ABO blood group system, there are three alleles for blood type: • Iº is allele for blood group O • Iᴬ is allele for blood group A • Iᴮ is allele for blood group B State the different phenotypes that can arise and their corresponding genotypes. |
Allele Iº is recessive, and alleles Iᴬ and Iᴮ are codominant. • Group A → IᴬIᴬ, IᴬIº • Group B → IᴮIᴮ, IᴮIº • Group AB → IᴬIᴮ • Group O → IºIº |
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What is dihybrid inheritance? |
The inheritance of two characteristics, which are controlled by different genes. Each of the two genes will have different alleles. |
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What do dihybrid crosses show? |
The likelihood of offspring inheriting certain combinations of the two characteristics from particular parents. |
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What is the phenotypic ratio when you do a dihybrid cross with two heterozygous parents? |
9 : 3 : 3 : 1 of dominant both : dominant first, recessive second : recessive first, dominant second : recessive both |
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State what sex chromosomes males and females have. |
• Females have two X chromosomes (XX) ↳ homogametic, only one kind of sex chromosome • Males have one X chromosome and one Y chromosome (XY) ↳ heterogametic, two kinds of sex chromosomes |
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What does it mean if a characteristic is sex-linked? |
The alleles that code for those characteristics are located on a sex chromosome. |
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State the names for chromosomes that are carried only on the X chromosome and Y chromosome. |
• Only carried on X chromosome → X-linked genes • Only carried on Y chromosome → Y-linked genes |
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Explain why most genes on sex chromosomes are only carried on the X chromosome (X-lined genes). |
The Y chromosome is smaller than the X chromosome so carries fewer genes. |
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Explain why males are more likely than females to show recessive phenotypes for sex-linked genes. |
• Males only have one X chromosome, so often only have one allele for sex-linked alleles. • As they only have one copy, they express the characteristic of this allele even if it is recessive. |
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What are X-linked and Y-linked disorders? |
• X-linked disorders are genetic disorders carried on the X chromosome • Y-linked disorders are genetic disorders carried on the Y chromosome |
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Examples of genetic disorders. |
Colour blindness and haemophilia ↳ both X-linked disorders |
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How do you express sex-linked alleles in genetic diagrams. |
As they are sex-linked you need to represent both the chromosome and the allele. ↳ e.g. Xⁿ → X represents the X chromosome and the n represents the recessive allele. |
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If the allele for colour blindness is recessive and X-linked, what genotype is required for males and females? Is colour blindness more common in males or females? |
• Females require two copies of recessive allele to be colour blind • Males only need one copy of the recessive allele to be colour blind • Colour blindness more common in males. |
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• X - female • Y - male • N - normal colour vision allele • n - faulty colour vision allele (X-linked) Draw the genetic cross diagram for the offspring between a carrier female and an unaffected male. State the phenotypic ratios. |
• 3 : 1 ratio of offspring without colourblindness : offspring with colour-blindness • 2 : 1 : 1 ratio of female offspring without colour blindness : male offspring without colour blindness : male offspring with colour blindness |
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• X - female • Y - male • N - normal colour vision allele • n - faulty colour vision allele (X-linked) Draw the genetic cross diagram for the offspring between a carrier female and a male with colour blindness. State the phenotypic ratio. |
1 : 1 ratio of offspring with colour-blindness : offspring without colour-blindness ↳ ratio will be the same for each gender |
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Define autosome. |
Any chromosome that isn't a sex chromosome. |
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Define autosomal genes. |
Genes that are located on the autosomes. |
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What does it mean that autosomes are linked? |
• They are on the same autosome • They will stay together during the independent assortment of chromosomes in meiosis I, so their alleles will be passed on to the offspring together |
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When are two genes on an autosome more closely linked? Explain. |
• Genes that are closer together on the autosome are more closely linked. • Less likely to be split up during crossing over. |
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If two genes are autosomally linked, explain the effect on the phenotypic ratio in the offspring of a dihybrid cross between heterozygous parents. |
• Expect 9 : 3 : 3 : 1 ratio in the offspring • However, phenotypic ratio more similar to that of a monogenic cross between two parents (3 : 1) as two autosomally-linked genes will be inherited together ↳ higher proportion of offspring have parents' genotype (heterozygous) and phenotype |
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Explain the difference in the expected phenotypic ratio and the actual phenotypic ratio. |
For the NnRr and nnrr genotype to be so common, the NR and nr alleles in the NnRr parents must have been linked. ↳ mostly produced NR and nr gametes ↳ some Nr and nR gametes produced due to crossing over, but fewer Nnrr and nnRr offspring overall As a result, a higher proportion of the offspring have their parents' phenotypes. |
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What is epistasis? |
• Different genes can control the same characteristic → by interacting together, they form the phenotype • Epistasis is when the allele of one gene masks the expression of the alleles of other genes. |
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What does it mean if gene 1 is epistatic to gene 2? |
Gene 1 can mask the expression of gene 2. |
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What does it mean if the epistatic allele is recessive? |
Two copies of the epistatic allele are required to mask the expression of the other gene. |
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If you have a recessive epistatic allele, what is the expected phenotypic ratio from two heterozygous parents. |
9 : 3 : 4 phenotypic ratio of dominant both : dominant epistatic, recessive other : recessive epistatic. |
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What does it mean if the epistatic allele is dominant? |
Having at least one copy of the epistatic allele will mask the expression of the other gene. |
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If you have a dominant epistatic allele, what is the expected phenotypic ratio from two heterozygous parents. |
12 : 3 : 1 phenotypic ratio of dominant epistatic : recessive epistatic, dominant other : recessive both. |
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What is the chi-squared test, χ²? |
Statistical test used to see if the results of an experiment support a theory. |
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How to carry out the chi-squared test to see if results of an experiment support a theory. |
1. Make a null hypothesis 2. Use theory to predict the results = expected results 3. Carry out experiment and record actual result = observed results 4. Carry out χ² test to see whether outcome supports or rejects the null hypothesis |
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How to work out the number of offspring expected for each phenotype for a chi squared test. |
E = [total number of offspring / ratio total] x predicted ratio. |
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What is the critical value? |
The value of χ² that corresponds to a 0.05 (5%) level of probability that the difference between the observed and expected results is due to chance. |
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What does it mean if your χ² value is greater than the critical value? |
- There is a significant difference between the observed and expected results (so something other than chance is causing the difference) - The null hypothesis is rejected |
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What does it mean if your χ² value is less than the critical value? |
- There is no significant difference between the observed and expected results - The null hypothesis is accepted. |
