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topic 8 Flashcards

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Biology 101 flashcards
1
Q

8.1 [origins of genetic variation]

What’s genetic variation?

What causes genetic variation?

A
  • The difference in the genetic code (DNA) + the number of alleles of each gene present in the gene pool of a population
Genetic variation arises through...
1. Mutations
2. Meiosis  (crossing over + independent assortment)
3. Random fertilisation
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2
Q

Meiosis introduces genetic variation through..

  1. Crossing over

(prophase 1)

A
  • A large enzyme ‘cuts + joins’ bits of maternal + paternal chromatids together from homologous chromosomes
  • The chiasmata (points where chromatids break) are important in 2 ways:
  • Exchange of genetic material between homologous chromosomes = genetic variation = re-combination of alleles arise
  • Errors in the process lead to mutations - introducing new combinations into the genetic make-up of a species
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3
Q

Meiosis introduces genetic variation through..

  1. Independent assortment

(metaphase 1)

A
  • chromosomes from parents are distributed to gametes completely randomly
  • New gamete can have any amount of chromosomes from either parent
  • pairs of homologous chromosomes line up at equator randomly
  • This guarantees great variety in gametes = new combination of alleles in gametes
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4
Q

Genetic variation through mutations

What’s a mutation?
Different types?

A

Mutation - permanent change in DNA of organism
Single codon changes → different amino acid produced
→effects proteins produced

  1. Gene (point) mutation
  2. Chromosomal mutation
  3. Chromose mutation
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5
Q
  1. Gene (point) mutation
A

A change in one or a small number of nucleotides affecting a single gene

  • Substitution: 1 base substituted for another
  • Insertion: extra base added
  • Deletion: a base completely lost / removed in the sequence

EG
- Sickle-cell anaemia
- cystic fibrosis (CF)

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6
Q
  1. Chromosomal mutation
A
  • Changes in the position of entire genes within a chromosome

EG
- Turner’s Syndrome
- Down’s Syndrome

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7
Q
  1. Chromosome mutation
A
  • An entire chromosome is gained or lost (during meiosis)
  • usualy have major impact on organism
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8
Q

Genetic variation through random fertilisation

A
  • male + female gametes from 2 unrelated indivduals fuse to form new genetic individual
  • introduces variation because the combination of gametes that fuse to form the zygote is random
  • Random selection of 1 egg to be released + only 1 of the 20-150 million sperm released in ejacution will be able to fertilise the egg
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9
Q

(not in spec)

  1. Explain why germ line mutations can be inherited whilst somatic mutations (mitosis) cannot
  2. Explain why germ line mutations are more important than somatic mutations in the evolution of species
A

1.
- Germ line mutations are mutations that occur in cells involved in gamete production
- Therefore, these mutations can be passed on / inherited
- This would affect all the cells in the offspring

2.
- Germ line mutations affect cells that produce gametes
- These will affect offspring + natural selection acts on populations
- Somatic mutations will not introduce new alleles into the population

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10
Q

8.2 [transfer of genetic material]

  1. Genotype?
  2. Phenotype?
    Allele?
A
  1. Genotype:
  • Combination of alleles of a particular gene / genes present in a haploid gamete / diploid organism
  • The genetic makeup of an organism
  1. Phenotype:
  • (Measurable) physical + chemical characteristic that make up the appearance of an organism
  • The expression of an organism’s genetic makeup combined with its interactions with the enviornment

Allele = alternative version/form of a gene

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11
Q
  1. Homozygote?
  2. Heterozygote?
A
  1. Homozygote:
  • An individual where both of the alleles coding for a particular characteristic are identical
  1. Heterozygote:
  • An individual where the two alleles coding for a particular characteristic are different
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12
Q
  1. Dominance?
  2. Recessive?
A
  1. Dominance:
  • When a phenotype is expressed whether the individual is homozygous for the characteristic or not
  1. Recessive:
  • When a phenotype is only expressed when both alleles code for the feature
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13
Q
  1. Codominance?
  2. Multiple alleles?
A
  1. Codominance:
  • Both alleles are expressed and the proteins they code for act together without mixing to produce a given
    phenotype
  • 2 dominant alleles that both contribute to the phenotype, either by showing a blend of both characteristics, or the characteristics appearing together
  • eg A + B blood group alleles are codominant = both alleles expressed + produce their proteins, which act together without mixing
  1. Multiple alleles:
  • A gene with more than two alleles
  • There are more than two possible variants
  • (however, diploid individual will only inherit 2 of them, regardles of how many possible alleles there are)
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14
Q

Mendel’s first law - Law of segregation

  • Monohybrid / monogenic crosses?
  • What is the expected ratio of phenotypes for monogenic crosses?
A

Mendal’s 1st law - Law of segregation:

  • an individual can pass only 1allele for a characteristic into a gamete
  • one allele for each trait is inherited from each parent to give a total of 2 alleles for each trait

Monohybrid crosses

  • look at characteristics determined by 1 single gene
  • can predict potential outcomes

3:1 ratio of dominant to recessive phenotypes

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15
Q

Polygenic?

A

Phenotypic traits that are determined by several interacting genes, not just 1

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16
Q

(Digenic) Dihybrid Inheritance of 2 unrelated unlinked genes

Dihybrid crosses?

A
  • Will be inherited separately
  • Inheritance of 2 pairs of contrasting characteristics at the same time

Dihybrid crosses

  • Looks at the pattern of inheritance when 2 genes are considered at the same time
  • breeding experiments involving the Inheritance of 2 pairs of contrasting characteristics at the same time
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17
Q

Mendel’s second law - the law of independent assortment

A
  • That different traits are inherited independently of each other
  • States that the inheritance of 1 characteristc will have no effect on the inheritance of another
  • only when genes are not linked on the chromosome
  • = The inheritance of allele for one phenotype for 1 characteristic (such as grey/ebony bodies of Drosophila) has nothing to do with the inheritance of alleles for other characteristics (such as wing length / eye colour)
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18
Q

What ratio will the crossing of 2 non-linked heterozygotes form in a dihybrid cross?

A

The ratio of the crossing of 2 non-linked heterozygotes in a dihybrid cross will always result in a 9:3:3:1 ratio of phenotypes

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19
Q

Reasons why sometimes ratio is not 9:3:3:1

A
  • small smaple size
  • unexpected ratios can mean the genes being examined are both of the same chromosome (they are linked)
  • experimental errors
20
Q

Difference between independent assortment + autosomal linkage

A

Independent assortment occurs when allels located on different chromosomes

Autosomal linkage occurs when alleles are located on same chromosome

21
Q

What’s an autosome?

A
  • autosome = any chromosome not involved in inheritance of sex
  • any genes located on same autosome will show autosomal linkage
  • linked genes will not show independent assortment as the combinations of alleles passed down from parents tend to stay together (meiosis)
  • = most gametes have the combination of alleles found in parents’ chromosomes (meiosis)
  • only when DNA is swapped between chromosmes in crossing over can recombinant gametes be produced
22
Q

Example of autosomal linkage?

A

Drosophila (fruit flies)
- gene for body colour (black/grey) + gene for wing length (long/vestigial) are autosomally linked = meaning they are inherited in pairs
- gene for body colour: allele for grey body is dominant to allele for black body
- gene for wing length: alllele for normal wings is dominant over the allele for vestigial wings (tiny wings that don’t work)

23
Q

Explain why autosomally linked genes are inherited in pairs, referring to meiosis

A
  • Genes on the same chromosome, particularly those closest together, are unlikely to undergo recombination during meiosis
  • Therefore they are inherited as if they were the same gene
24
Q

offspring results of autosomal linkage, dihybrid cross of 2 heterozygotes

A

In autosomal linkage, dihybrid cross of 2 heterozygotes result in…

  • 3/4 of the offspring having both dominant characteristics
  • 1/4 of the offspring having both recessive characteristics

3:1

25
Q

[not in spec]

Chromosome mapping

A
  • studying gene linkage on autosome gives us a way of working out how close together various genes are on a chromosomes
  • Using linked genes we can build up a genetic map of a chromosome
  • to make genetic map - crossover value (cov) needed:
  • COV = N of recombinant offspring / total N of offspring x100
26
Q

Sex linkage on X chromosome

A
  • sex-linked inheritance occurs when genes located on X chromosome
  • involves just 1 gene
  • An allele is located on 1 of the sex chromosomes, meaning its expression depends on the sex of the individual (eg haemophilia)
  • sex chromosomes are the final pair of chromosome (23rd pair)
27
Q

Difference between male + female interms of sex chromosomes

A
  • Female = 2 X chromosomes (XX) = all her eggs contain X chromosome = female referred to as homogametic
  • Male = has 1 X chromosome + 1 Y chromosome (XY) = half sperm will contain X + half contain Y chromosome
    = male referred to as heterogametic
28
Q

EG of a sex linked inherited disorder

Haemophillia

A
  • disorder caused by the recessive allele of a gene on the X chromosome
  • 1 protein needed for clotting of blood is missing
  • blood clotting casade coded for by multiple genes
  • many of these genes are caried on the X chromosome
  • = problems with blood clotting are often sex-linked diseases
29
Q

Why are males more likely to express a recessive sex-linked allele?

A
  • Most sex-linked alleles are located on the X chromosome
  • Therefore males only get one copy of the allele, so will express this characteristic even if it’s recessive
  • males can’t be heterozygous = more likely to be affected
  • Since females get two alleles, this is less likely
  • heterozygoes females are carriers
30
Q

Chi squared test?

A
  • A statistical test to find out whether the difference between observed + expected data is due to chance or a real effect
  • Test of significance to see if its statistically significant or not

[check google for formula]

  • H0 (null) hypothesis = there is no differences between observed and expected
  • H1 Hypothesis = There is difference between observed + expected
  • Critical value always 0.05
31
Q

Chi squared test

degree of freedom?
[df]

A

Degree of freedom = n - 1
( N = number of traits / phenotype )

  • read the critical value using degree of freedom + column 0.05
  • to find probability that a difference is due to chance or not
32
Q

Criteria for the chi squared test

A
  • Data placed in discrete categories
  • Large sample size
  • Only raw count data allowed (i.e. not percentages)
  • No data values equal zero
33
Q

How chi squared test is used to check for significance:

A
  • The formula results in a number, which is then compared to a critical value (for the corresponding degrees of freedom)
  • If the number is greater than or equal to the critical value = there is no significant difference = the results occured due to chance
34
Q

8.3 [Gene pools]

Population ?

Gene pools ?

A

Population

  • The number of organisms of a particular species occupying a habitat

Gene pools

  • sum of all the alleles in a population at a given time
35
Q

Allele frequency ?

What does it show us ?

A

Allele frequency = How often a particular specific allele appears in population

  • Allele frequency Tells us if population is changing over time
  • If there’s a permanent change in allele frequency it would show evolution
36
Q

selection pressures act on a population, changing allele frequencies and altering the gene pool

  • 3 types of selection?
A
  1. Stabilising selection maintaining continuity in a population
  2. Disruptive selection leading to changes or speciation
  3. Directional selection from environmental changes
37
Q
  1. Stabilising selection
A
  • Occurs when environmental conditions stay the same
  • Individuals closest to the mean are favoured + any new characteristics are selected against
  • Results in low diversity + variation
  • Preferring the average of the population to survive

–> Acts agaist change by making favourable characteristics more common in an unchanged environment, reducing variety

38
Q
  1. Disruptive selection

[variation of directional selection]

A
  • Extreme phenotype are favoured over the mean
  • least common type of selection but very important = brings evolutionary change
  • Over time, the population becomes phenotypically divided + new species may develop
  • leads to change by making extreme values of a characteristic more common, leading to 2 clear forms in population (speciation)
  • The opposite of stablising selection
39
Q
  1. Directional selection
A
  • Enviornmental pressure applied to population moves the mean value for a characteristic in 1 direction
  • changing the whole population
  • ‘classic’ natural selecon = shows a change from one phenotype to another, which is more advantageous to the environment
40
Q

sometimes changes in allele frequencies can be the result of
chance + not selection, including genetic drift

  • What’s genetic drift?
A
  • Random changes in gene pool in population occuring by chance, not because they cause adv / disadv to offspring + not from selection
  • chances can be due to random sexual reproduction / accidents preventing reproduction
  • alleles are not passed from a population in a way that reflects their frequency in the population
  • effects will be greater in small populations + may lead to a complete loss of an allele if by chance it’s not passed on
  • genetic drift is NOT directional
41
Q

Extreme cases of what genetic drift can cause in small population?

A
  1. Founder effect
  2. Population bottleneck (or genetic bottleneck)
42
Q
  1. Founder effect
A
  • loss of genec variation that occurs when
  • a small number of individuals become isolated, forming a new population with limited gene pool + allele frequencies not representative of the original population
  • new population is established by a few individuals, where the frequency of alleles is different
  • also referred to as a voluntary bottleneck
43
Q
  1. Population bottleneck
A
  • The effect of a catastrophic event/s that dramatically reduces the size of a population (by at least 50%) = only few survive
  • +causes a severe decrease in the gene pool of the population
  • resulting in large changes in allele frequencies + a reduction in genetic diversity
  • natural catastrophe/ a new disease/ hunting by humans/predators
  • Involuntary bottleneck
44
Q

Hardy-Weinberg formula?
What’s it used for?

A
  • Can be used to monitor changes in the allele frequencies in a population

EQUATION:

P2 + 2pq + q2 = 1

  • q = Frequency of Recessive allele
  • p = Frequecny of Dominant allele
  • Allele frequency will always be a decimal as alleles added together = 1 (p + q = 1)
45
Q
  • Hardy - Weinberg equilibrium ?
  • Assumptions made by the Hardy - Weinberg equilibtrium?
A
  • In a Hardy - Weinberg equilibrium, allele frequencies should be constant through generations
  • This assumes nothing is acting to make changes

Assumptions / conditions:

  • There is no mutation (to create new alleles)
  • There is no random mating
  • The population is very large
  • There is no migration (in/out of poulation = population is isolated)
  • There is no selection pressure

= These very unlikely = usefulness of equation is in monitoring changes so causes can be found

bio (alevel) (10 decks)

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