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A-Level Biology > Unit 7 - Genetics, Populations, Evolution & Ecosystems > Flashcards

Unit 7 - Genetics, Populations, Evolution & Ecosystems Flashcards

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1
Q

define dominant allele

A

allele that has the same effect on the phenotype whether it is present in homozygous or heterozygous state

allele that is always expressed in the phenotype

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2
Q

define gene

A

a section of DNA that codes for the production of a particular polypeptide –> determines one characteristic in an organism

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3
Q

define allele

A

one of a number of different forms of a gene that result in different polypeptides being made

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4
Q

define genotype

A

the genetic make up of an organism/what alleles the organism has

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5
Q

define phenotype

A

the physical characteristics of an organism - a result of the expression of the genotype & its interaction with the environment

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6
Q

define diploid

A

having 2 copies of each chromosome

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7
Q

define haploid

A

having only 1 copy of each chromosome

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8
Q

define homologous

A

a pair of chromosomes from an organism with identical genes
(but not necessarily identical alleles)

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9
Q

define locus

A

the specific position on homologous chromosomes of a gene

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10
Q

define recessive

A

an allele that only affects the phenotype when present in the homozygous state/when both recessive alleles are present

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11
Q

define codominant

A

pairs of alleles that both affect the phenotype when present in a heterozygote

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12
Q

define homozygous & heterozygous

A

homozygous: having 2 identical alleles of a gene
heterozygous: having 2 different alleles of a gene

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13
Q

define carrier

A

an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele

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14
Q

define mutation

A

a permanent change in DNA base sequence in a gene
often leads to a new allele being formed –> genetic variation

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15
Q

define monohybrid cross

A

inheritance of 1 gene

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16
Q

define dihybrid cross

A

inheritance of 2 genes

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17
Q

lots of traits are controlled by many genes

A

polygenic

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18
Q

for a characteristic controlled by a single gene, what will the individual’s gametes contain?

A

one allele of each gene
there is an = probability that it will contain the maternal or paternal allele

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19
Q

what are the 4 methods for genetic crossing & the types of alleles inherited?

A

dominance - dominant, recessive
co-dominance - 2 codominant alleles (heterozygous is a blend)
sex linkage - sex-linked dominant or recessive, sex-linked co-dominant
multiple alleles - ‘multiple’ bc more than 2 alleles of the gene exist in the population

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20
Q

describe dominance

A
  1. dominant allele impacts the phenotype when only one is present
  2. recessive allele only affects the phenotype in the absence of the dominant allele

2 possible phenotypes

expected phenotype ratio from heterozygous cross 3:1

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21
Q

describe co-dominance

A

both alleles are codominant
both alleles are expressed in the phenotype of a heterozygous individual, creating an intermediate/blend phenotype b/w the homozygous phenotypes

expected phenotype ratio from heterozygous cross 1:2:1

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22
Q

describe inheritance of sex

A

determined by which 2 sex chromosomes the individual has
females have XX - all eggs contain X
males have XY - sperm have either X or Y
–> sex of offspring is determined by the sex chromosome carried by the sperm

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23
Q

describe the inheritance of sex-linked genes

A

females have 2 alleles/copies of a sex-linked gene
males have 1 allele/copy of a sex-linked gene = can’t be heterozygous
males are more likely to have sex-linked phenotypes than females bc males only need one allele for it to be expressed
a male can only inherit a sex-linked allele (only on X chromosome) from his mother

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24
Q

do genetic cross for sex-linked genes

A

see booklet

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25
Q

describe the inheritance of multiple alleles

A

definition: inheritance of a gene which exists as more than 2 alleles within the population’s gene pool

sometimes multiple alleles show dominance hierarchy with each allele being dominant to the alleles after it

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26
Q

do genetic cross for multiple alleles

A

see booklet

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27
Q

describe test cross

A

performed to distinguish b/w homozygous dominant & heterozygous individuals
(same phenotype different genotype)
each unknown individual is crossed with a homozygous recessive
if any of the offspring show recessive phenotype, the unknown parent must be heterozygous

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28
Q

do genetic cross for test cross

A

see booklet

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29
Q

why might a test cross not be conclusive?

A

is the sample is small - element of chance

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30
Q

how do you prove traits are caused by dominant alleles?

A

parents have the trait with unaffected offspring –> the parents must be heterozygous carriers of the recessive allele

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31
Q

how do you prove traits are caused by recessive alleles?

A

unaffected parents have affected offspring –> parents must be heterozygous carriers of the recessive allele

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32
Q

how do you prove traits are caused by sex-linked genes?

A

different inheritance patterns b/w sexes
unaffected mother & affected offspring - males can’t be carriers of sex-linked genes
(don’t assume male = XY & female = XX bc depends on species)

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33
Q

describe dihybrid cross (inheritance of 2 genes simultaneously)

A

there are 4 possible combinations of 2 characteristics
for each characteristic there are 2

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34
Q

what is the expected phenotype ratio for dihybrid cross of 2 heterozygous parents?

A

1:1:1:1

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35
Q

do a dihybrid genetic cross for one heterozygous & one homozygous recessive parent

A

see booklet

36
Q

describe the use of Chi Squared tests in genetics

A

determines whether there is a statistically significant difference b/w observed & expected results in phenotype ratios

autosomal linkage is a big factor for why observed phenotype ratios often don’t match up with expected ratios

37
Q

what is the formula for the Chi Squared test?

A

Σ (O-E)^2/E

38
Q

explain how meiosis results in cells that have a haploid number of chromosomes & show genetic variation

A
  1. homologous chromosomes pair up on equator of cell
  2. there is crossing over b/w homologous chromosomes
  3. this prouces new combinations of alleles
  4. independent segregation of homologous chromosomes
  5. homologous chromosomes separate in meiosis 1
  6. produces varying combinations of chromosomes/genes/alleles
  7. sister chromatids are separated in meiosis 2
39
Q

how to do Chi Squared test Qs with genetics

A

of categories - 1 = degrees of freedom

x^2 value = less than critical value –> P value greater than 0.05 –> no dif.
vice versa

explanation:
1. yes/no significant difference b/w observed & expected ratios
2. any differences are due to chance
3. accept/reject null hypothesis

40
Q

describe epistasis

A

the allele of one gene has an effect on the expression of another gene

41
Q

do epistasis genetic cross & what are the expected ratios?

A

see booklet

9:3:4
12:3:1
13:3
9:7

42
Q

when does autosomal linkage occur?

A

when genes are located on the same chromosome
so alleles do not separate during meiosis when gametes are formed
alleles are inherited together as they do not sort independently

43
Q

what is the degree of autosomal linkage determined by?

A

how close the genes are physically
the closer the genes, the less likely crossing over takes place
crossing over during meiosis –> formation of recombinants

44
Q

define linkage group

A

a set of genes at different loci on the same chromosome that, except for crossing over, tend to be inherited together

45
Q

what are the letters for linked genes with/without crossing over & what do most gametes contain?

A

without: AB or ab = same as parent genotype = parental gametes
with: Ab or aB = new combination of alleles = recombinant gametes

most gametes formed are the same as the parent genotype

46
Q

describe what happens when crossing over does occur b/w genes

A

crossing over of homologous chromosomes happens at a point b/w genes as they are not linked so new combinations of gametes are formed
this will affect phenotype ratios

47
Q

define allele frequency

A

number of times an allele occurs within the gene pool

48
Q

define genotype frequency

A

the # of individuals with a particular genotype in a population

49
Q

calculating allele frequency by multiplying genotype by 2…

A

see booklet

50
Q

what is the Hardy-Weinberg equation?

A

p^2 + 2pq + q^2 = 1
dominant phenotype is p^2 + 2pq
recessive phenotype is q^2

51
Q

what does the H-W principle predict?

A

that the proportion of dominant & recessive alleles of a gene in a population remains the same from one generation to the next if certain conditions are met

52
Q

what conditions must be met for H-W principle to be valid/what are the H-W assumptions?

A
  1. the population is large
  2. the organisms are diploid & reproduce sexually
  3. mating is random
  4. no new mutations occur
  5. no natural selection occurs (all alleles are equally likely to be passed to next generation)
  6. the population is isolated (so no flow of alleles into or out of population)

only 1 & 2 can be met

53
Q

H-W exam Q tips

A

q^2 is the key

q^2 allows you to calculate q which unlocks allele frequency equation p+q=1

54
Q

what can the trend in population growth be if uncontrolled by selection pressures?

A

exponential

55
Q

what are the 2 types & e.gs of selection pressures?

A

biotic: predation, competition, disease

abiotic: temp., sunlight, pH

56
Q

what are the causes of variation?

A

genetic:
mutations create new alleles - gene mutations (new alleles), chromosome mutations
sexual reproduction creates new combinations of existing alleles - independent segregation, crossing over - in meiosis & random fertilisation

environmental - not passed on to offspring

57
Q

what are the different types of variation?

A

discontinuous variation - not influenced by the environment - controlled by one or a few genes, categoric

continuous variation - influenced by the environment - often controlled by many genes & alleles = polygenic

58
Q

what is reproductive success?

A

passing on alleles to the next generation

59
Q

what is a selection pressure?

A

a factor that limits a population

60
Q

what are the different types of selection?

A

directional, stabilising & disruptive

61
Q

describe directional selection

A

definition: a type of natural selection in which a single phenotype is favoured, causing the allele frequency to continuously shift in one direction

there is a selection pressure to move the mean phenotype in one direction/way

the advantageous allele will increase in frequency & will become fixed/the only allele

directional selection will change the characteristics/phenotype of the population over time

be specific to Q

62
Q

what does the graph for directional selection look like?

A

see booklet

63
Q

describe how antibiotic resistance evolves as an example of directional selection

A
  1. variation in population of bacteria due to random mutation of DNA, leading to resistance allele
  2. when antibiotic is used, bacteria with resistance allele survive & reproduce, passing on beneficial resistance allele
  3. frequency of resistance allele increases in population over time
64
Q

how do antibiotics work?

A

stop binary fission by:
- preventing cell wall synthesis
- inhibiting metabolism

65
Q

why do we not say bacteria are immune to antibiotic?

A

immune to pathogen
resistant to drug

66
Q

describe stabilising selection

A

extremes from the mean are selected against so the characteristics of a population are maintained
extreme characteristics at both ends are affected by selection pressures

67
Q

what does the graph for stabilising selection look like

A

see booklet

68
Q

describe how human birth weight evolves as an example of stabilising selection

A

lighter & heavier infants have higher infant mortality rate

69
Q

define disruptive selection

A

individuals with values close to the mean are selected against –> often leads to speciation

70
Q

what does the graph for disruptive selection look like?

A

see booklet

71
Q

define gene pool

A

all of the alleles of all the genes in all individuals in a population at a given time

72
Q

how does natural selection affect H-W principle?

A

allele/genotype frequencies are altered

e.g. dominant phenotype favoured so decrease q & q^2
or
heterozygous genotype is favoured so decrease q^2 & p^2

73
Q

define species

A

a group of organisms that can potentially interbreed to produce fertile offspring

74
Q

define population

A

all organisms of the same species present in a particular area at a particular time

75
Q

define speciation

A

formation of more than one species from an original species

76
Q

describe speciation overview

A

gene pools must be isolated so there is no flow of alleles b/w the 2 populations (this allows the genes pools to diverge until they become sufficiently different)

divergence can be due to natural selection if 2 populations have different selection pressure, mutations or genetic drift

77
Q

describe the different types of speciation

A

1- allopatric speciation - creating a new species when the 2 populations are separated by a physical/geographical isolation

2- sympatric speciation - creating a new species when the 2 populations are in the same area but have reproductive isolation

78
Q

describe allopatric speciation

A
  1. single population of a species with individuals that can interbreed to produce fertile offspring
  2. physical/geographical barrier separates 2 different areas & individuals in them forming 2 separate populations
  3. different conditions in each area = different selection pressures
  4. so alleles & phenotypes & gene pool of the 2 populations become increasingly different
  5. the differences are so great that individuals can’t/won’t interbreed with each other to produce fertile offspring so a new species has been formed

apply this framework to Q

79
Q

what types of isolation are involved in sympatric speciation

A

pre-zygotic isolation
temporal isolation - breeding seasons do not coincide

behavioural isolation - related to courtship behaviour

mechanical isolation - anatomical differences prevent mating

leads to reproductive isolation - gene pools are isolated & no gene flow

80
Q

describe temporal isolation

A

2 different species produce gametes at different times of year –> very unlikely to fertilise each other
random mutation caused different timing of gamete production
–> disruptive selection

81
Q

hybrid infertility

A

e.g. 2 parents w different chromosome # - mule
= post-zygotic mechanism

82
Q

define evolution

A

a change in the allele frequencies in a population over time

83
Q

what are the major contributing factors to evolution?

A

mutations & selection

84
Q

define genetic drift

A

in a small population, the chance differences in which individuals reproduce & which die impact alleles passed on to offspring even if the alleles are neutral
this impacts allele frequency & so evolution

85
Q

what can genetic drift cause?

A

alleles being lost from a population randomly (not due to selection pressure or natural selection)

86
Q

as a population gets larger,

A

the chance differences in the frequency of alleles passed on gets smaller & the impact of selection is larger

A-Level Biology (10 decks)

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