Variation and Inheritance Flashcards
Define chlorosis.
When the leaves of a plant look pale because the cells are not producing the normal amount of chlorophyll and so the plant does not produce enough food
What environmental factors affect the phenotype of a plant?
- Lack of light
- Mineral deficiencies
- Virus infections
What is a source of genetic variation?
- Random fertilisation
- Independent assortment
- Crossing over
- Mutations
Define genotype.
The combination of alleles an organism inherits for a characteristic
Define phenotype.
The observable characteristics of an organism that are often aff teed by the environment
Define dominant allele.
The version of a gene that will always be expressed if present
Define recessive allele
The version of a gene only expressed if two copies of the allele same recessive allele are present
Define homozygous
When there are two identical allele for a characteristic
Define heterozygous
When there are two different alleles present for a characteristic, and the dominant one is expressed
Define continuous variation
A characteristic that can take any value in a range, there are many genes for a characteristic
Define discontinuous variation.
A characteristic that can only occur in specific values, there are one or two genes for a characteristic
Define monogenic inheritance.
The inheritance of a single gene
What does a homozygous genetic cross result in?
100% heterozygous genotype
What does a heterozygous genetic cross result in?
50% heterozygous genotype = dominant phenotype
25% recessive genotype = recessive phenotype
25% dominant genotype = dominant phenotype
Define codominance.
When two different alleles for a gene are equally dominant and thus both alleles will be expressed in the phenotype of the organism present
How do you set up a genetic cross for codominant alleles?
Capital letter with a superscript
What will a homozygous genetic cross of codominant alleles result in?
100 % heterozygous genotype = 100% codominant phenotype
What will a heterozygous codominant genetic cross result in?
50% heterozygous genotype - 50% codominant genotype ab
25% homozygous dominant genotype a - 25% genotype a
25% homozygous dominant genotype b - 25% genotype b
Define multiple alleles.
Genes that have more than two versions e.g. Tall, intermediate and short vs tall and short
How many alleles of any gene can be present in an individual?
2, one from each parent
What does the immunoglobulin gene code for?
The different antigens in a RBC
What are sex linked chromosomes?
Characteristics carried on the sex chromosomes
Does the chromosome carry less genes? And if so then why?
Yes because it is smaller
What is haemophilia?
When the patients blood clots very slowly due to a lack in Factor VIII
What happens if a male inherits the recessive haemophiliac gene?
They will have the disorder as they cannot have the corresponding dominant allele on their Y chromosome
Define a carrier
An individual that has the gene for a genetic condition but does not necessarily express it
What is a dyhybrid cross?
Shows the inheritance of two different characteristics caused by two genes which may be located on different pairs of homologous chromosomes
What is the ratio commonly found in a F2 dihybrid cross
9:3:3:1
Why may the ratio differ?
- the fertilisation of gametes is a random process, small sample = skewed results
- the genes being studied are both on the same chromosome thus if no crossing over occurs the alleles for the two characteristics will always be inherited together
Define autosomal linkage.
When the genes that are linked are in non-sex chromosomes
Recombination frequency equation
Number of recombinant offspring / Total number of offspring
What percentage suggests gene linkage?
50 or more
Define epistasis.
The interaction of genes at different loci
What results in epistasis occurring?
Continuous and multiple genes
Define hypostatic.
A gene that is affected by another gene
Define epistatic
A gene that affects the expression of another gene
What is recessive epistasis?
If the presence of two recessive alleles results in the lack of an enzyme
What is dominant epistasis?
When a dominant allele results in a gene having an effect on another gene by coding for an enzyme which modifies one of the precursor molecules in the pathway
Define population genetics
The investigation of how allele frequencies within populations changes over time
Define gene pool
The sum of the total gene sin a population at any given time
Define allele frequency
The relative frequency of a particular allele in a population
Calculation allele frequency
P+Q=1
What is the Hardy-Weinberg principle.
In a stable population with no disturbing factors, the allele frequencies will remain constant from one generation to the next and the will be no evolution
What is the Hardy-Weinberg equation?
p^2+2pq+q^2=1
What assumption is made by the Hardy-Weinberg principle?
A theoretical breeding population of diploid organisms that is large and isolated
Random mating
No mutations
No selection pressures
What factors lead to changes in allele frequency?
- Mutation
- Sexual selection
- Gene flow
- Genetic drift
- Natural selection
Which populations cannot adapt to change as easily?
Small populations with limited genetic diversity
Which factors affect the size of populations?
Density dependent factors
Density independent factors
Define density dependent factors.
Dependent on population size and competition, predation, parasitism, and communicable disease
Define density independent factors.
Affect all populations of all sizes
- Climate change
- Natural disasters
- Seasonal change
- Human activities
What are population bottlenecks?
Large reductions in population size over t least one generation
What is a pro and con of a genetic bottleneck?
Pro: a beneficial mutation will have a much greater impact and lead to the quicker development of new species
Con: the gene pool is greatly reduced and the effects will be seen in future generations
What is the founder effect?
The occurrence of a small population due to he establishment of new colonies by a few individuals
What is the effect of rare alleles in a small population?
They will become more frequent and have a much bigger impact on natural selection in the population
What is a normal distribution?
The bell shaped curved which forms form the distribution of different variants
What is stabilising selection?
The norm or average is selected for - increase in frequency of alleles
The extremes are selected against - decrees in frequency of alleles
What is directional selection?
When there is a change is the environment and the normal phenotype is no longer advantageous
It is therefore selected against and the allele frequency shifts to the more extreme phenotypes, resulting in evolution
What is disruptive selection?
When the extremes are selected for and the norm is selected against
Define speciation
The formation of new species through the process of evolution
Define allopatric speciation
The more common form of speciation when some members of a species are separated from the rest by some physical barrier
What causes allopatric speciation?
Different environment
Different election pressures
Different physical adaptations
.. Founder effect -> genetic drift
Define sympatric speciation
When members of two different species interbreed to produce fertile offspring
It may have a different number of chromosomes to the parents and so not breed with those species
= stop in gene flow + causes reproductive isolation of hybrid species
What cause sympatric speciation?
Disruptive selection
Mating preferences
Behaving real differences
= reproductive isolation, and although they live in the same habitat gene flow will still interfere with the process of speciation
What is a prezygotic reproductive barrier?
A barrier preventing fertilisation and the formation of a zygote
What is a postzygotic reproductive barrier?
A reduction in the viability or reproductive potential of an offspring
Define polymorphic
Display more than one distinct phenotype for most characteristics
What is the wild type allele?
The allele coding for the most common characteristic
What are mutants?
Alleles arising from mutations
What is artificial selection?
Instead of changes in the environment leading to survival of the fittest, the selection pressure for breeding plants with desirable characteristics is applied.
Define inbreeding
The breeding of closely related individuals
What are the problems caused by inbreeding?
It limits the gene pool which reduces genetic diversity and thus the ability of an organism to evolve and adapt to changes in its environment
Genetic disorders arise due to the accumulation of recessive of alleles when inbreeding occurs = less biologically fit organisms
What are seed banks?
Banks that keep samples of seeds of different species of plants
What are gene banks?
Store biological samples, other than seeds
What is outbreeding?
The breeding of unrelated or distant plants species - reduces the occurrence of homozygous recessive and increases the potential of plant species to adapt to changes in the environment
