patterns of inheritance Flashcards
1
Q
What is chlorosis?
A
When the leaves of a plant look pale or yellow because the cells are not producing the normal amount of chlorophyll. • This lack of chlorophyll reduced the ability of the plant to make nutrients by photosynthesis
2
Q
Describe the plants which
show chlorosis
A
• Most have normal genes coding
for chlorophyll production
• The change in their phenotype is
the result of environmental
3
Q
Give examples of where both genetic and environmental variation play a role in determining an organism’s characteristics
A
- Chlorosis in plants
* Body mass of animals
4
Q
What environmental factors
can cause chlorosis in plants?
A
Lack of light - e.g. when a gardening tool is left on a lawn. In the absence of light, plants turn off chlorophyll production to conserve resources • Mineral deficiencies - e.g lack of iron or magnesium. Iron is needed as a cofactor by some enzymes that make chlorophyll, and magnesium is found at the heart of the chlorophyll molecule • Virus infections - when viruses infect plants, they interfere with the metabolism of cells
5
Q
How can environmental factors
affect animals’ body mass?
A
In the majority of cases, obesity and being severely underweight are a result of environmental factors • Amount and quality of food eaten • Quantity of exercise the organism gets • Presence of disease
6
Q
How can genetic factors affect
animals’ body mass?
A
Genetic mutations can cause the
pattern of fat deposition in the body
to be altered
7
Q
How is genetic variation
created?
A
• For most genes there are many different possibles alleles or variants • Combination is determined by sexual reproduction involving meiosis (formation of gametes), and the random fusion of gametes at fertilisation
8
Q
What are modifications?
A
Changes to a person’s phenotype due to the environment • Not inherited • Only mutations (changes to the DNA) in the gamete can be passed on to the offspring
9
Q
What are the 2 different types
of allele?
A
• Dominant allele - the version of the genes that will always be expressed if present in an organisms • Recessive allele - will only be expressed 2 copies of this allele are present in an organism
10
Q
What are the 2 different types
of genotype?
A
• Homozygous - they have 2 identical alleles for a characteristic. Either homozygous dominant, or homozygous recessive • Heterozygous - the have 2 different alleles fora characteristic. The allele for the dominant phenotype will be expressed
11
Q
Compare continuous and
discontinuous variation
A
12
Q
What are the steps in drawing
a genetic cross diagram?
A
1. State the phenotype of both parents 2. State the genotype of both parents. Capital letter for dominant allele, and corresponding lowercase letter for recessive allele 3. State the gametes of each parent. Common practice is to circle the letters 4. Use a Punnett square to show the results of random fusion of gametes during fertilisation 5. State the proportion of each genotype which are produced among the offspring 6. State the corresponding phenotype for each of the possible genotypes
13
Q
What is codominance?
A
Codominance occurs when two different alleles occur for a gene - both of which are equally dominant • As a result both alleles of the alleles are expressed in the phenotype of the organism if present • e.g. Red and White allele make Pink flower • CR = red, CW = white
14
Q
Give an example of a
characteristic that is
determined by a gene with
multiple alleles?
A
Blood group • The immunoglobulin gene (Gene I) coders for the production of different antigens present on the surface of red blood cells • IA = antigen A • IB = antigen B • IO = antigen O
15
Q
Describe the alleles involved in
determining blood group
A
IA and IB are codominant, whereas IO is recessive to both of the other alleles. This leads to 4 blood groups: • Blood group A = IAIA or IAIO • Blood group B = IBIB or IBIO • Blood group AB = IAIB • Blood group O = IOIO
16
Q
How is sex determined?
A
• The 23rd pair of chromosomes in humans are sex chromosomes • Female = XX • Male = XY • X-chromosome is large and contains many genes not involved in sexual development • Y-chromosomes is very small containing almost no genetic information, but does carry a genes that causes the embryo to develop as a male
17
Q
What are sex linked
characteristics?
A
Characteristics determined by genes carried on the sex chromosomes • Y-chromosomes is much smaller, so there are many genes in the Xchromosome that males have 1 copy of • Any characteristic caused by a recessive allele on the section of the X-chromosome which is missing in the Y-chromosome occurs more frequently in males
18
Q
What is haemophilia?
A
Sex-linked genetic disorder • Blood clots extremely slowly due to the absence of a protein bloodclotting factor (usually factor VIII) • If a male inherits the recessive allele that codes for haemophilia, they cannot have a corresponding dominant allele on their Y chromosome, so they develop the condition • Females who are heterozygous for the haemophilia coding gene are known as carriers
19
Q
What is dihybrid inheritance?
A
The inheritance of two genes
20
Q
What is the ratio for dihybrid
crosses with heterozygous
parents?
A
9 : 3 : 3 : 1
21
Q
What can the actual ratio differ
from the expected ratio?
A
• The fertilisation of gametes is a random process so in a small sample a few chance events can lead to a skewed ratio • The genes being studied are both on the same chromosome. These are known as linked genes. If no crossing over occurs, the alleles for the two characteristics will always be inherited together
22
Q
What is autosomal linkage?
A
• When the genes that are linked are
found one of the other pairs of
chromosomes
23
Q
Describe linked genes
A
• Inherited as one unit • No independent assortment during meiosis unless the alleles are separated by chiasmata • Tend to be inherited together • Cannot undergo normal allele ‘shuffling’ in meiosis • Expected ratios will not be produced in the offspring
24
Q
What are recombinant
offspring?
A
They have different combinations of alleles than either parent • The closer the genes are on a chromosome, the less likely they are to be separated during crossing over, and the fewer recombinant offspring produced
25
What is the recombination
| frequency?
Number of recombinant offspring
______________________________
Total number of offspring
26
What do different
recombination frequencies
indicated?
```
• 50% indicates there is no linkage
and the genes are on separate
chromosomes
• Less than 50% indicates that there
is gene linkage and the random
process of independent
assortment has been hindered
• Recombination frequency of 1%
relates to a distance of one map
up on a chromosome
```
27
How does crossing over affect
| recombination frequency?
```
As the degree of crossing over
reduces, the recombination
frequency also gets smaller
• Degree of crossing over is
determined by how close the
genes are on a chromosome
• The closer they are, less likely they
will be separated during crossing
over
```
28
What is the chi-squared test?
```
A statistical test the measures the
size of the difference between the
results you actually get (observe),
and those you expected to get
• Helps determine whether the
differences in the expected and
observed results are significant or
not
• Null hypothesis: there is no
significant difference between
what we expect and what we
observe
```
29
What is the chi-squared test
| formula?
30
What do large chi-squared
| values mean?
```
There is a significant difference
between the observed and expected
results
• The probability that these
differences are due to chance is
low
```
31
How does the number of
categories being compared
affect the size of the chisquared
value?
```
• The degrees of freedom is the
number of values being made, and
is calculated as n-1
• n is the number of categories or
possible outcomes present in the
analysis
• e.g. if looking a yellow and green
peas, there would be 2 categories,
and so one degree of freedom
```
32
```
• The degrees of freedom is the
number of values being made, and
is calculated as n-1
• n is the number of categories or
possible outcomes present in the
analysis
• e.g. if looking a yellow and green
peas, there would be 2 categories,
and so one degree of freedom
```
```
If the value is less than the critical
value, there is no significant
difference. Reject null hypothesis
• If the value is greater than or equal
to the critical value, there is a
significant difference. Accept null
hypothesis
```
33
What is epistasis?
```
The interaction of genes at different
loci
• Gene regulation is a form of
epistasis with regulatory genes
controlling the activity of structural
genes e.g. the lac operon
• The characteristics of plants and
animals that show continuous
variation involve multiple genes,
and epistasis occurs frequently
```
34
What is dominant and
| recessive epistasis?
```
• Recessive epistasis occurs if the
presence of two recessive alleles
at a gene locus led to the lack of
an enzyme
• Dominant epistasis occurs if a
dominant allele results in a gene
having on another gene
```
35
List the factors affecting
| evolution
```
Mutation causes the existence of
different alleles, and the formation
of new alleles leads to genetic
variation
• Sexual selection leads to alleles
coding for characteristics which
improve mating success
increasing in frequency
• Gene flow - the movement of
alleles between populations.
Immigration and emigration result
in changes of allele frequency
within a population
• Genetic drift - change in allele
frequency due to the random
nature of mutation. Occurs in small
populations. The appearance of a
new allele will have a greater
impact (is more likely to increase in
number) in a smaller population
than in a much larger population
• Natural selection
```
36
What are the 2 types of limiting
factors for the size of a
population?
```
Density-dependent factors
• Dependent on populaation size
• e.g. competition, predation,
parasitism and communicable
disease
Density-independent factors
• Affect population of all sizes in the
same way
• e.g. climate change, natural
disasters, seasonal change and
human activities
```
37
What are genetic bottlenecks?
Large reductions in population size
which last for at least one generation
• Gene pool and genetic diversity is
greatly reduced
38
What is the founder effect?
```
An extreme example of genetic drift
• Small populations can arise due to
the establishment of new colonies
by a few isolated individuals
• These small populations have
much smaller gene pools than the
original population and display
less genetic variation
• If carried to the new population,
the frequency of any alleles that
were rare in the original population
will be much high in the new,
smaller population and so will
have a much larger impact during
natural selection
```
39
What happens in stabilising
| selection?
```
• The average is selected for
(positive selection), and the
extremes are selected against
(negative selection)
• Results in a reduction in the
frequency of alleles at the
extremes, and an increase in the
frequency of average alleles
• e.g. natural selection in baby
weight
```
40
What is directional selection?
```
When there is a change in the
environment and the normal (most
common) phenotype is no longer the
most advantageous
• Organisms which are less common
and have more extreme
phenotypes are positively selected
• The allele frequency then shifts
towards the extreme phenotypes
and evolution occurs
• e.g. peppered moths during the
industrial revolution
```
41
What is disruptive selection?
```
When the extremes are selected for
and the norm is selected against
• The opposite to stabilising
selection when the norm is
positively selected
• e.g. feather colour in lazuli bunting
birds
```
42
What is speciation?
```
The formation of new species
through the process of evolution
• The organisms belonging to the
new species will no longer be able
to interbreed to produce fertile
offspring with organisms belonging
to the original species
```
43
What are the events that lead
| to speciation?
```
• Members of a population become
isolated and no longer interbreed
with the rest of the population
resulting in no gene flow between
the 2 groups
• Alleles within the group continue
to undergo random mutations. The
environment of each group may be
different, resulting in different
selection pressures, so different
characteristics will be selected for
and against
• The mutations and changes in
allele frequencies over many
generations will eventually lead to
large changes in phenotype. The
members of the different
population become so different
that they are no longer able to
interbreed
```
44
What is allopatric speciation?
```
When some members of a
population are separated from the
rest of the group by a physical
barrier e.g. a river or the sea
• More common form of speciation
• Geographically isolated
• Environments will be different and
so there will be different selection
pressure resulting in different
physical adaptations
• Separation of a small group will
often result in the founder effect
leading the genetic drift
• E.g. the finches of the Galapagos
Islands
```
45
What is sympatric speciation?
```
Occurs when members of two
different species interbreed and
form fertile offspring. The hybrid
formed (which is a new species) will
have a different number of
chromosomes to either parent and
may longer be able to interbreed
with members of either parent
population. This stops gene flow
and reproductively isolates the
hybrid organism
• Happens less frequently than
allopatric speciation
• Occurs within populations that
share the same habitat
• More common in plants than
animals
• E.g. fungus-farming ants and blind
mole rats
```
46
What are reproductive
| barriers?
```
Prezygotic reproductive barriers
prevent fertilisation and the
formation of a zygote
• Postzygotic reproductive barriers,
often produced as a result of
hybridisation, reduce the viability
or reproductive potential of
offspring
```
47
What does polymorphic mean?
```
Displaying more than one phenotype
for a characteristic
• The allele coding for the most
common, or normal, characteristic
is called the wild type allele
• Other forms of that allele, resulting
from mutations, are called mutants
```
48
What is artificial selection
| (selective breeding)?
```
The same as natural selection,
except for the nature of the selection
pressure applied
• Instead of changes in the
environment leading to survival of
the fittest, it is the selection for for
breeding of plants or animals with
desirable characteristics by
farmers or breeder
```
49
Describe the process of
| artificial selection
```
1. Individuals with the desired
characteristics are selected and
interbred
2. Offspring from this cross
showing the best examples of
the desired traits are then
selected to breed
3. The breeding of closely related
individuals is called inbreeding
4. The process is repeated over
many generation resulting in
changes to the frequency of
alleles within the population, and
eventually speciation
```
50
What are the problems called
| by inbreeding?
```
Limits the gene pool and so
decreases genetic diversity which
reduces the chance of the
population evolving and adapting
to changes in their environment
• Many genetic disorders are
caused by recessive alleles -
organisms that are closely related
are genetically similar and likely to
have the same recessive alleles
```
51
What do seed banks do?
```
The store samples of seeds from
both wild type and domesticated
varieties
• They are an important genetic
resource
```
52
What do gene banks do?
They store biological samples, other
than seeds, such as sperm or eggs.
They are usually frozen
53
How are the problems of
| inbreeding tackled?
```
Alleles from gene banks are used
to increase genetic diversity in a
process called outbreeding
• Breeding unrelated or distantly
related varieties is also a form of
outbreeding
• This reduces the occurrence of
homozygous recessives, and
increases the potential to be able
to adapt to environmental change
in the future
```
