Patterns of Inheritance and Variation

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  • Created by: LBCW0502
  • Created on: 11-03-17 13:39
What is a gene?
A section of DNA which codes for a protein
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What are the three types of environmental variation?
Etiolation, chlorosis and body mass
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What is etiolation?
The process in flowering plants grown in partial or complete absence of light
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What is chlorosis?
Leaves look pale or yellow because cells are not producing the normal amount of chlorophyll (reduces ability of plant to photosynthesise)
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What are the three environmental factors which cause chlorosis?
Lack of light, mineral deficiencies and viral infections
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Which factors determine an organism's body mass?
Genetic and environmental factors
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What are alleles?
Different versions of the same gene
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What is a locus?
Position of alleles on chromosome
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What is a genotype?
The combination of alleles an organism inherits for a characteristic
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What is a phenotype?
The observable characteristics of an organism
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What are modifications?
Any changes the environment makes to a person's phenotype are not inherited
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Can mutations in gametes be passed onto the offspring?
Yes
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What is a dominant allele?
The version of the gene that will always be expressed if present in an organism
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What is a recessive allele?
Only expressed if two copies of this allele are present in an organism
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What does homozygous mean?
Two identical alleles for a characteristic
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What does homozygous dominant mean?
Contains two alleles for the dominant phenotype
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What does homozygous recessive mean?
Contains two alleles for the recessive phenotype
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What does heterozygous mean?
Two different alleles for a characteristic. The allele for the dominant phenotype will be expressed
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The variation of a characteristic displayed within a species can be divided into which two groups?
Continuous and discontinuous variation
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What is continuous variation?
A characteristic that can take any value within a range
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What is discontinuous variation?
A characteristic that can only appear in specific (discrete) values
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Describe the features of discontinuous variation?
Individuals fall into distinct groups and one gene is involved with little effect from the environment
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Give examples of discontinuous variation
Blood group, albinism, round/wrinkled pea shape
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Describe the features of continuous variation
There are two extremes with every degree of variation possible in between
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Give examples of continuous variation
Height, weight, leaf surface area, animal mass, skin colour
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Which factors cause continuous variation?
Genetic and environmental
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Which factors cause discontinuous variation?
Genetic
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How many genes affect continuous variation?
A number of genes - polygenic (large effect from environment)
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How many genes affect discontinuous variation?
One or two genes - usually monogenic
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How is continuous variation represented as?
Line graph (bell shaped, normal distribution)
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How is discontinuous variation represented as?
Bar chart
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What is monogenic inheritance?
Inheritance of a single gene
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What is step 1 of showing a genetic cross?
State the phenotype of both the parents
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What is step 2 of showing a genetic cross?
State the genotype of both parents. Assign a letter code to present alleles for dominant (capital) and recessive (lowercase)
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What is step 3 of showing a genetic cross?
State the gametes of each parent. Circle the letters
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What is step 4 of showing a genetic cross?
Use a Punnett Square to show the results of random fusion of gametes during fertilisation. Label the gametes on the edges of the square
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What is step 5 of showing a genetic cross?
State the proportion of each genotype which are produced among the offspring (% or ratio)
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What is step 6 of showing a genetic cross?
State the corresponding phenotype for each of the possible genotypes. It must be clear that you know which phenotype results from each genotype
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Describe the offspring (in the first generation) of two true breeding parents?
All the offspring are heterozygous
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Describe the offspring (in the second generation) when you cross two of the heterozygous offspring from the first generation?
Two are heterozygous and two are homozygous (phenotypic ratio of 3:1)
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What is codominance?
When two alleles occur for a gene - both of which are equally dominant. As a result both alleles of the gene are expressed in the phenotype of the organism if present
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Give examples of codominance
Snapdragons (produce red/white/pink flowers), roan horses, AB blood group, cystic fibrosis
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What are multiple alleles?
A gene with more than two possible alleles
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Give examples of factors determined by multiple alleles
Blood group and the immunoglobulin gene
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How is the sex of the offspring determined?
It is determined by whether the sperm fertilising the egg contains a Y or X chromosomes
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How are some genes sex-linked?
Some characteristics are determined by genes carried on the sex chromosomes
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Give examples of sex-linked disorders
Red-green colour blindness, haemophilia, duchenne muscular dystrophy
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Why are males more likely to develop haemophilia?
If a male has a recessive allele on their X chromosome, they cannot have the dominant allele on their Y chromosome to mask the recessive allele
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Why are females carriers of haemophilia?
Females are heterozygous for the haemophilia coding genes. They do not suffer from this disorder but may pass on the allele to their children (result in children suffering from haemophilia)
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What is dihybrid inheritance?
Inheritance of two genes
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How many alleles are analysed in a dihybrid cross?
Four alleles (two for each characteristic)
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Describe the offspring (in the first generation) when a true breeding homozygous pea plant with yellow round seeds is crossed with a true breeding homozygous pea plant with green wrinkled seeds
All the offspring are heterozygous (all yellow round seeds)
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Describe phenotypic ratio in the offspring (in the second generation) when two seeds from the first generation are crossed
9: 3: 3: 1
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Why might the actual ratio differ from the expected ratio?
The fertilisation of gametes is a random process so in a small sample a few chance events can lead to a skewed ratio. Genes studies are on the same chromosome (linked genes) - if no crossing over occurs alleles for two features are inherited together
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What is autosomal linkage?
When the genes that are linked are found on one of the other pairs of chromosome. Linked genes are inherited as one unit (no independent assortment unless separated by chiasmata)
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Give examples of linked characteristics in fruit flies
Body colour and wing length
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What is recombinant frequency?
A measure of the amount of crossing over that has happened in meiosis
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How do you calculate recombination frequency?
Number of recombinant offspring / total number of offspring
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What does a recombination frequency of 50% indicate?
There is no linkage and the genes are on separate chromosomes
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What does a recombination frequency of less than 50% indicate?
There is gene linkage and the random process of independent assortment has been hindered
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What is the degree of crossing over determined by?
How close the genes are on a chromosome. As the degree of crossing over reduces, the recombination frequency also gets smaller. The closer the genes the less likely they will be separated during crossing over
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What does a recombination frequency of 1% relate to?
A distance of one map unit on a chromosome (The recombination frequencies for a number of characteristics coded for by genes on the same chromosome can be used to map the genes on the chromosome)
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What is the phenotypic ratio from a dihybrid cross in the F1 generation from heterozygous parents?
9: 3: 3: 1
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What is the phenotypic ratio form a dihybrid cross in the F2 generation from pure breeding parents?
9: 3: 3: 1 (F1 generation are all heterozygous)
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What is the phenotypic ratio for sex linked disorders usually?
1: 2: 1 (Carrier: Affected: Not affected) or 2:1 (Affected: Not affected)
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What is an autosome?
Chromosome that is not a sex chromosome (body cell)
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What is epistasis?
When one gene masks or suppresses the expression of another
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Name the three types of epistasis
Recessive, dominant and complementary
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How are alleles antagonistic?
Alleles on one locus (epistatic) can present the expression of alleles on another locus (hypostatic)
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What is the chi-squared test?
A statistical test that measures the size of the difference between the results you actually get (observe) and those you expected to get
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What are the three types of environmental variation?

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Etiolation, chlorosis and body mass

Card 3

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Card 4

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What is chlorosis?

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Card 5

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What are the three environmental factors which cause chlorosis?

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