6.1.2 Patterns of Inheritance Flashcards
1
Q
what are the two types of variation
A
continuous and discontinuous
2
Q
what is continuous variation
A
when individuals in a population vary within a range, there are no distinct categories
3
Q
what are examples of continuous variation
A
- height
- waist circumference
- fur length
- leaf surface area
4
Q
what is discontinuous variation
A
when there are two or more distinct categories, and each individual falls into only one of these categories, with no intermediates
5
Q
what are examples of discontinuous variation
A
- blood group
- violet flower colour
6
Q
what two things can influence variation
A
genetics
environment
or both
7
Q
what makes up the different genotypes of organisms
A
- different species have different genes
- individuals of the same species have the same genes, but different versions of them (alleles)
- the genes and alleles make up genotype
8
Q
how does sexual reproduction lead to variation in a species
A
- meiosis makes gametes with a unique assortment of alleles
- due to crossing over
- and independent assortment of alleles
- random fusion of gametes during fertilisation
- increases genetic variation in the offspring
9
Q
what is phenotypic variation, and give a human example
A
- the variation in phenotypes of organisms
- caused by the difference in genotypes
- e.g. human blood groups, which have 3 different blood group alleles, and 4 different blood groups
10
Q
how many genes code for inherited characteristics that show continuous variation
A
- many genes
- said to be polygenic
- e.g. human skin colour is said to be polygenic, as comes in loads of different shades
11
Q
how many genes code for inherited characteristics that show discontinuous characteristics
A
- usually influenced by only one, or a small number of genes
- said to be monogenic
- e.g. violet flower colour, either purple or white
12
Q
how does the environment influence variation
A
- caused by differences in the environment
- e.g. climate, food lifestyle
- characteristics controlled by the environment CHANGE over an organism’s life
13
Q
what are examples of environmental factors affecting variation
A
- etiolation: when plants grow abnormally long and spindly because they’re not getting enough light
- chlorosis: when plants don’t produce enough chlorophyll and turn yellow, caused by several environmental factors, e.g. lack of magnesium in the soil
- diet: in animals
14
Q
what are the differences in genetic and environmental factors
A
genetic: determine the genotype and characteristics the organism is BORN with
environmental: influence how some of these characteristics develop
15
Q
what is most phenotypic variation influenced by
A
- combination of BOTH factors
- usually showing continuous variation
16
Q
what are examples of variation being caused by both environmental and genetic factors
A
1) height of pea plants: pea plants come in tall and dwarf forms (discontinuous variation) but the exact height of tall and dwarf varieties varies (continuous variation) due to environmental factors (e.g. light intensity and water availability affecting how tall plant grows)
2) body mass in animals: partly genetic, but strongly influenced by environmental factors, such as diet. e.g. if your diet doesn’t include right nutrients, body mass is likely to be lower than determined by genes. continuous variation
17
Q
what is a gene
A
a sequence of bases on a DNA molecule that codes for a protein, which results in a characteristic
18
Q
what is an allele
A
a different version of a gene
- most plants and animals have 2 versions of each gene, one from mom and one from dad
- represented using letters, e.g. B and b
- order of bases in each allele is slightly different, coding for different versions of the same characteristic
19
Q
what is genotype
A
the alleles an organism has, e.g. Bb
20
Q
what is phenotype
A
the characteristics alleles produce, e.g. brown eyes
21
Q
what is dominant
A
an allele whose characteristic appears in the phenotype even when there is only one copy
- shown via capital letter
22
Q
what is recessive
A
an allele whose characteristic is only expressed in the phenotype if two copies are present
- shown via lowercase letter
23
Q
what is codominant
A
alleles that are both expressed in the phenotype, neither one is recessive
- e.g. haemoglobin
24
Q
what is locus
A
the fixed position of a gene on a chromosome
- alleles of a gene are found at the same locus on each chromosome in a pair
25
what is homozygote
an organism that carries two copies of the same allele
26
what is heterozygote
an organism that carries two different alleles, Bb
27
what is a carrier
a person carrying an allele that is not expressed in the phenotype but that can be passed on to offspring
28
for each gene, how many alleles does a person have
2
29
what contains only one allele for each gene
gammetes
30
what are genetic diagrams used for
predicting the phenotypes and genotypes of offspring if two parents are crossed
31
what is monogenic inheritance
inheritance of a characteristic controlled by a single gene
32
what do monogenic crosses show
the likelihood of different alleles of a gene being inherited by offspring of particular parents
33
what is the first and second set of offspring called
the F₁ and F₂ generations
34
how would you do a punnett square for a normal, monogenic cross
1) list the parental phenotypes
2) list the parental genotypes
3) list the gametes from them
4) cross them in 4x4 grid
5) list the phenotypic ratios of the offspring
35
what does it mean when alleles show codominance
both alleles are expressed in the phenotype, as neither is recessive
36
explain codominance in reference to sickle cell anaemia
- those with HOMOzygous for normal haemoglobin, HᴺHᴺ, don't have the disease
- those HETEROzygous for haemoglobin, HᴺHᶻ have a sickle-cell trait, meaning they have a bit of both sickle-cell and normal haemoglobin
- those HOMOzygous for sickle-cell haemoglobin HᶻHᶻ, have sickle-cell anaemia, so all of their blood cells are sickle-shaped
37
what does multiple alleles mean
when there are more than 2 alleles of the same gene
38
explain multiple alleles in terms of blood groups
Io = recessive O blood group
IA = dominant A blood group
IB = dominant B blood group
- blood groups A and B are codominant, so people with IAIB with have blood group AB
- IoIA= A blood group
- IoIB= B blood group
39
what is dihybrid inheritance
the inheritance of two characteristics which are controlled by different genes
40
what do dihybrid crosses show you
looks at how the two different genes are inherited at the same time
41
how do you carry out a dihybrid cross
- each genotype comes with the 4 alleles, 2 for each gene, together
- split into the 4 gametes (like cross multiplying)
- put into a 4 by 4 dihybrid cross, and work out the ratio
- for 2 heterozygous individuals, ratio of offspring will be 9:3:3:1
42
what is phenotypic ratio
the ratio of the different phenotypes in offspring
- can be predicted via genetic diagrams/punnett squares
43
what are some common phenotypic ratios
MONOGENIC: hetero x hetero = 3:1
- dominant : recessive
DIHYBRID: hetero x hetero = 9:3:3:1
- dominant : dominant recessive : recessive dominant : recessive
44
why may you not get the expected phenotypic ratio
- sex linkage
- autosomal linkage
- epistasis
45
where is the genetic information for biological sex carried
on the 2 sex chromosomes
- females have 2 XX chromosomes
- males have XY chromosomes
46
what is sex linkage
a characteristic is sex linked if the allele that codes for it is carried on the sex chromosome
47
why are so many genes X-linked
- the Y chromosome is smaller than the X chromosome
- so carries fewer genes
- so most genes on the sex chromosomes are only carries on the X chromosome
- called X-linked genes
48
why are males more likely to show recessive phenotypes for sex-linked genes
- males only have one X chromosome
- so they often only have one allele for sex-linked genes
- as they only have one copy
- they will express the characteristic for this allele, even if recessive, as no other one
- whereas females would need 2 recessive copies to show the characteristic, so much more rare
49
when would you ever get a monogenic cross with ratio 1:1
- with a sex linked characteristic
- heterozygous female and recessive male
50
what are genetic disorders caused by a faulty allele on the sex chromosome
colour blindness and haemophilia
- the faulty alleles for both of these disorders are carried on the X chromosome
- so called X-linked disorders
51
how would you draw a punnett square for a sex-linkage
- the female genotype contains XX, and each X has a subscript at the top of dominant or recessive characteristic
- the male has an X with the subscript, but a Y that has none, as does not carry any gene
- then you cross as normal, and identify the phenotypes and genotypes
52
what does autosome mean
any chromosome that isn't a sex chromosome
53
what does autosomal genes mean
genes located on autosomes
54
why are genes on the same autosome linked
- if they are on the same chromosome
- they will stay together during the independent assortment of chromosomes, and their alleles will be passed onto their offspring together
- only reason this won't occur is if crossing over splits them up
55
which genes are more likely to be linked on an autosome
the closer together the two genes, the more likely they are said to be linked
- because crossing over is less likely to split them up
56
why does linkage alter phenotypic ratios
- in a dihybrid cross, you can expect a 9:3:3:1 ratio
- BUT, it is more likely to be like a monohybrid ratio of 3:1
- as the two autosomally-linked alleles are inherited together
- so a higher proportion of offspring will have their parents' (heterozygous) genotypes
57
what is epistasis
where the allele of one gene masks the expression of the alleles of other genes
- means that many different genes control the same characteristic , so interact to form the phenotype
58
how does epistasis occur in humans with widows peak and baldness
- each characteristic controlled by another gene
- if you have alleles that code for baldness, doesn't matter whether you have alleles for widows peak
- as you will be bald
- baldness genes are epistatic to the widows peak gene
- as they mask their expression
59
how does epistasis occur in flower pigments
- controlled by 2 genes
- one determines whether you have any colour or not
- and the other determines what colour it changes to
60
what can you predict would be the phenotypic ratio of recessive epistasis
- where having two recessive genes blocks the expression of others
- yy masks any colour
- R is orange
- r is yellow
9:3:4
61
what can you predict would be the phenotypic ratio of dominant epistasis
- where just one dominant allele blocks the expression of another
- W for no colour
- Y for yellow
- y for green
12:3:1
62
what is chi-squared
63
what is always the null hypothesis in a chi-squared test
there is no significant difference between the observed and expected results
64
what is chi squared formula
65
what is the critical value you use for chi-squared
5% probability that results are due to chance only
- corresponding to degree of freedom (n-1)
66
what does chi-squared tell you compared to critical value
- if value greater than CV, reject Ho
- if the value is smaller than CV, accept Ho
67
what is gene pool
the complete range of alleles present in a population
68
how are new alleles generated
mutations in genes
69
what is gene frequency
how often an allele occurs in a population
- given as percentage or decimal
70
what is evolution in terms of genes and alleles
the frequency of an allele in population changing over time
71
explain how evolution occurs through natural selection
- individuals in a population vary because they have different alleles (new alleles are usually generated through mutation or meiosis)
- selection pressures create a struggle for survival
- some individuals are better adapted to selection pressures than others
- individuals with advantageous alleles, which increase chance of survival, are more likely to survive, reproduce and pass on their advantageous allele than individuals with different alleles
- so greater proportion of the next generation inherit the advantageous allele
- so again, more likely to survive, reproduce and pass on allele
- the frequency of the advantageous allele increases generation to generation
72
what is a selection pressure
anything that affects an organism's chance of survival and reproduction
- NECESSARY for natural selection, as an allele is only advantageous with the right selection pressure, and without it, won't take place
73
what factors affect the evolution of a species
- stabilising selection
- directional selection
- genetic drift
- genetic bottleneck
- founder effect
74
what is stabilising selection
in a stable environment which isn't changing much, those with characteristics in the middle of the range are more likely to survive and reproduce
- reduces the range of possible phenotypes
75
what is an example of stabilising selection
- fur length
- in mammals, have a range of fur length
- in stable climate however, having furs at the extremes of the range reduces the chances of survival
- as harder to maintain right body temperature
- animals with alleles for average fur length more likely to survive, reproduce and pass on their allele, so increases in frequency, so proportion of population with average fur length increases and range of fur decreases
76
how would you show a graph of stabilising selection
- normal graph first, bell shaped
- breeding population is a tighter bell shape inside
- overtime, in next graph, the fur length gets tighter
77
what is directional selection
when there is a change in the environment, individuals with alleles at an extreme are more likely to survive and reproduce
78
what is an example of directional selection
- if an environment becomes very cold
- individuals with alleles for long fur find it easier to maintain the right body temperature than those with short fur
- so more likely to survive, reproduce, pass on the allele, an overtime the frequency of alleles for long fur length increases
79
how would you show graphs for directional selection
- normal bell shaped graph
- those at one extreme form the breeding population
- so in next graph, the bell shape shifts towards this extreme
80
what is another way, other than natural selection, that evolution can occur
genetic drift
81
what is genetic drift
where instead of environmental factors affecting which individuals survive and reproduce, CHANCE dictates which alleles are passed on
82
how does genetic drift work
- individuals in a population show variation in phenotypes
- by chance, the allele for one genotype is passed onto offspring more than others
- the number of individuals with the allele increases
- if by chance the same allele is passed on more often again and again, it can lead to evolution as it becomes more common
83
does natural selection or genetic drift affect the evolution of a population more
- work alongside each other
- but one process can drive evolution more depending on population size
- but in SMALLER POPULATIONS: genetic drift had a greater effect as chance has a greater influence (in larger populations, any variation in allele frequency usually evens out across total population)
84
how is human blood groups good example of genetic drift
- tribes were found in isolated, smaller groups
- and different tribes show different blood group frequencies
- blood groups doesn't affect chances of survival or reproduction, so NOT due to evolution by natural selection
- just by CHANCE, one allele for a blood group was passed on more often, so over time became more common
- same as human populations in the past, which were also much smaller
85
what is a genetic bottleneck
an event, such as a natural disaster, that causes a big reduction in population size, leading to reduction in gene pool
86
what is an example of a genetic bottleneck effecting genetic drift
- mice can be either black or grey, and colour doesn't impact their survival
- natural disaster leaves only one black mouse and rest grey mice
- grey becomes common colour due to genetic drift
87
what is the founder effect
what happens when a few organisms from a population start a new population and there are only a small number of alleles in the initial gene pool
88
explain the founder effect
1) individuals within a population show variation in their genotype
2) some of these individuals start a new population
3) by chance, most of these individuals have one particular genotype
4) without any other gene flow (introduction of new alleles from outside the population) the new population will grow with reduced genetic variation
5) as the population is small, it is more heavily influenced by genetic drift than a larger population
89
why may the founder effect occur
- migration: leading to geographical separation
- other reasons: so that the new colony is separated to the original population, e.g. religion
- e.g. The Amish: all descend from a small number of Swiss who migrated their, and show little genetic diversity, and due to religious beliefs, have remained isolated from surrounding population, so very few alleles have been introduced, and have unusually high incidences of genetic disorders
90
what is the Hardy-Weinberg principle
predicts that frequencies of alleles in a population won't change from one generation to the next
91
under what conditions is the Hardy-Weinberg principle true
- large population
- no immigration
- emigration
- mutations
- natural selection
- random mating (all possible genotypes can breed with all others)
92
what do the hardy weinberg equations predict, and what are they based on
- based on principle that allele frequencies dont change from one generation to the next
- can be used to estimate the frequency of particular alleles and genotypes within populations
- if they do change: immigration, emigration, natural selection or mutations must have happened
93
what are the hardy weinberg equations
TO FIND THE FREQUENCY OF ONE ALLELE FROM ANOTHER:
p+q=1
p = frequency of dominant allele
q = frequency of recessive allele
TO FIND THE FREQUENCY OF ONE GENOTYPE IF YOU KNOW THE FREQUENCY OF THE OTHERS:
p2+2pq+q2=1
p2= frequency of homo dom. genotype
2pq= frequency of hetero genotype
q2= frequency of homo rec. genotype
CAN USE BOTH EQUATIONS
94
what is artificial selection
when humans select individuals in a population to breed together to get desirable traits
- can be done in both animals and plants
95
what is another word for artificial selection
selective breeding
96
how is artificial selection carried out in modern dairy cattle
- modern dairy cows produce many litres of milk a day as a result of artificial selection
1) farmers select a female with very high milk yield and a male whose mother had a very high milk yield to breed together
2) they then select the offspring with the highest milk yield and breed them together
3) continued over several generations until a very high milk-yielding cow is produced
97
how does artificial selection occur in bread wheat
bread wheat is the plant from which flour is produced for bread making, and produced a high yield of wheat due to artificial selection by humans
1) wheat plants with high wheat yield are bred together
2) the offspring with the highest yield are then bred together
3) continued over several generations, to produce a plant with a very high yield
98
why does artificial selection reduce the gene pool
only organisms with similar traits, and therefore similar alleles are bred together
- so reduced the number of alleles in the gene pool
99
why is a reduced gene pool from artificial selection a bad thing
- could cause problems in the future
- if a new disease appears, there's less chance of the alleles that could offer resistance to the disease being present in the population
- also means potentially useful alleles are accidently lost from the population when other alleles are being selected for
100
as gene pool reduces due to artificial selection, what is important to still maintain
- maintain sources of genetic material for use in the future
- e.g. by preserving the original wild type organisms that haven't undergone any artificial selection
101
what problems can artificial selection cause for the organisms involved
it can exaggerate certain traits, leading to health problems
102
what problems has artificial selection caused in pedigree dogs
- all modern pedigree dog breeds are descended from a single, wolf-like ancestor
- each breed has gone through many generations of artificial selection
- e.g. bred for a certain characteristic
- e.g. pugs and bulldogs for a flat, squashed up face
- BUT trait has become so exaggerated, that the dogs have breathing problems as a result
- their reduced gene pool also causes an increased incidence of genetic disease
- e.g. hereditary deafness in certain breeds
103
what are the ethical considerations with artificial selection raised
many people don't think it is fair to keep artificially selecting traits in dogs that cause health problems
104
what is a species
a group of similar organisms that can reproduce to give fertile offspring
105
what is speciation
the development of a new species
106
what is a brief overview of speciation
- populations of the same species become reproductively isolated
- chances in their allele frequency means that
- they can no longer breed together to produce fertile offspring
107
what is allopatric speciation
geographical isolation
- MORE COMMON THAN SYMPATRIC
108
when does geographical isolation happen
when a physical barrier divides a population of a species
- e.g. floods, volcanic eruptions and earthquakes
- means that some individuals are isolated from the main population
109
what is the process of allopatric speciation
1) physical barrier divides a population of species
2) conditions either side of the barrier will be slightly different, e.g. different climate on either side
3) as the environment is different on either side, there are different selection pressures each side
- so different characteristics will be advantageous on each side
- the allele frequency will change in each population
- mutations will also take place independently in each population, also changing the allele frequency
- changes in allele frequency lead to changes in phenotype frequency
- so different characteristics more common on each side, due to natural selection
4) eventually, individuals from the different populations will have changed so much they won't be able to breed together to produce fertile offspring
5) will be reproductively isolated, and split into 2 species
110
why does reproductive isolation occur
the changes in the alleles and phenotypes of the two populations prevent them from successfully breeding together
111
what are the 3 changes leading to reproductive isolation
- SEASONAL: the individuals from the same population develop different flowering and mating seasons, or become sexually active at different times of the year
- MECHANICAL: changes in genitalia prevent successful mating
- BEHAVIOURAL: a group of individuals develop courtship rituals that aren't attractive to the main population
112
what is sympatric speciation
speciation without geographical isolation
- they still become reproductively isolated, but due to random mutations occuring in a population, resulting to changes that prevent members of the population breeding with other members of the species
113
what is an example of sympatric speciation
- most eukaryotic organisms are diploid, so have 2 sets of homologous chromosomes in their cells
- sometimes, mutations can occur that can increase the number of chromosomes, called polyploidy
- these individuals with a different number of chromosomes cannot reproduce sexually to give fertile offspring, so if a polyploid organism emerges in a diploid population, will be reproductively isolated
- however, the polyploid organism could reproduce asexually, and form a new species
114
what is the only time that polyploidy can lead to speciation
if it doesn't prove fatal to the organism and more polyploid organisms can be produces
- more common in plants than animals
