INHERITANCE AND VARIATION Flashcards
Describe chlorosis
chlorosis is when the cells are not producing the normal amount of chlorophyll, reducing photosynthesis. Most plants with chlorosis have the normal genes coding for chlorophyll production. The change in their phenotype is a result of their environment.
What are the environmental factors causing chlorosis?
-lack of light: plants will turn off their 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 in chlorophyll molecule
-virus infections: viruses can interfere with metabolism of cells at the point they can no longer support the synthesis of chlorophyll.
Genetic variation is due to?
sexual reproduction involving meiosis (the formation of gametes) and the random fusion of gametes at fertilisation.
what is a genotype?
the genetic constitution of that organism (the alleles it has for a gene)
What is a phenotype?
the expression of the genes and its interaction with the environment
What is a dominant allele?
version of the gene that will always be expressed if present in an organism
What is a recessive allele?
version of gene that will only be expressed if two copies of this allele are present in an organism
what is homozygous?
a pair of homologous chromosomes carrying the same alleles for a single gene
what is heterozygous?
a pair of homologous chromosomes carrying two different alleles for a single gene
Describe continuous variation?
-a characteristic that can take any value within a range
-genetic and environmental causes
-polygenes (controlled by multiple genes)
-e.g animal mass, skin colour
Describe discontinuous variation?
-a characteristic that can only appear in specific (discrete) values
-mostly genetic causes
-one or two genes involved
-e.g blood group, albinism
How do you perform a genetic cross?
-state phenotype for both parents
-state genotype for both parents
-state gametes for each parent
-punnett square
-state proportion of each genotype which are produced in the offspring (ratio)
-state the corresponding phenotype for each possible genotype
what is codominance?
occurs when two equally dominant alleles occur for a gene. As a result both alleles for a gene are expressed in the phenotype of the organism if present.
Describe how blood group is determined by a gene with multiple alleles?
the immunoglobulin gene codes for the production of antigens on the surface of red blood cells. There are three alleles for this gene.
What is the male and female chromosome pair?
male- X Y
female- X X
Describe the X and Y chromosomes?
X chromosome is large and contains many genes not involved in sexual development. Y chromosome is very small, containing almost no genetic variation but it does carry the gene that causes the embryo to develop as a male
What is a sex-linked gene?
characteristics determined by genes carried on the sex chromosomes. As X chromosomes are much bigger than Y chromosomes, any characteristic caused by a recessive allele on the section of the X chromosome, which is missing on the Y chromosome, occurs more frequently in males
what is haemophilia?
sex-linked disorder in which blood clots extremely slowly due to the absence of a protein blood-clotting factor. This can lead to prolonged bleeding and if untreated can be fatal
How is haemophilia sex-linked?
if a male inherits the recessive allele that codes for haemophilia (on their X chromosome) they cannot have a corresponding dominant allele on their Y chromosome and so develop the condition. Most haemophilia sufferers are male
What is a dihybrid cross?
show the inheritance of two different characteristics, caused by two genes, which may be located on different pairs of homologous chromosomes. Each of these genes can have two or more alleles.
what is the expected ratio in a dihybrid cross between two heterozygous individuals?
9:3:3:1
why might the ratio not meet the expected ratio of 9:3:3:1?
-random fertilisation of gametes
-crossing over may have occurred disrupting the linked genes (genes on the same chromosome)
what is an autosome?
chromosome not concerned with sex determination
what is linkage?
genes are located on the same chromosome. Linked genes are inherited as one unit- there is no independent assortment during meiosis unless the alleles are separated by chiasmata.
What is autosomal linkage?
when the genes that are linked are found on one of the other pairs of chromosome
what are recombinant offspring?
have a different combination 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 fewer recombinant offspring produced.
what is recombination frequency?
a measure of the amount of crossing over that has happened in meiosis.
recombination frequency equation?
number of recombinant offspring/ total number of offspring
A recombination frequency of 50 percent indicates?
indicates that there is no linkage and the genes are on separate chromosomes.
A recombination frequency of less than 50 percent indicates?
indicates that there is gene linkage and the random process of independent assortment has been hindered.
the degree of crossing over is determined by what?
how close the genes are on a chromosome
Describe the chi squared test?
statistical test that measures the size of the difference between the observed and expected results, helps to determine significance.
How do you calculate degrees of freedom in chi squared test?
n(number of categories)-1
in a chi squared test, what is the null hypothesis?
there is no significant difference between the observed and expected results
If the chi squared value id less than the critical value found in the table at 5 percent significance (p=0.05)?
we do not have sufficiently strong evidence to reject null hypothesis therefore we accept null.
If the chi squared value is higher than the critical value found in the table at 5 percent significance (p=0.05)?
reject the null- some other factor is likely to be causing a significant difference between the expected and observed results.
what is epistasis?
interaction of genes at different loci
what is an epistatic gene?
gene that effects the expression of another gene
what is a hypostatic gene?
gene that is affected by another gene
what is recessive epistasis?
if the presence of two recessive alleles at a gene locus led to the lack of an enzyme
what is dominant epistasis?
the expression of the dominant allele of the epistatic gene prevents the expression of the hypostatic gene. This means that any genotypic combination with either one or two of the dominant alleles for the epistatic gene will suppress the expression of the hypostatic gene
what is the gene pool?
the sum of all the genes in a population at any given time
what is the allele frequency?
the relative frequency of a particular allele in a population
how do you calculate allele frequency?
p + q = 1
p- dominant genotype
q- recessive genotype
what does the Hardy-Weinburg principle state?
in a stable population with no disturbing factors, the allele frequencies will remain constant from one generation to the next and there will be no evolution.
The Hardy-Weinberg principle equation?
p(squared) + 2pq + q(squared) = 1
p(squared)- frequency of homozygous dominant genotype
2pq- frequency of heterozygous genotype
q(squared)- frequency of homozygous recessive genotype
Assumptions from the Hardy-Weinberg principle?
population of diploid organisms is:
-large
-isolated
-random mating
-no mutations
-no mutations (no evolution)
factors which lead to the changes in frequency of alleles within a population and so they therefore affect the rate of evolution?
-mutation
-sexual selection (improves mating success)
-gene flow (movement of alleles between populations, immigration and emigration)
-genetic drift (random mutation, appearance of a new allele which will have a greater impact on a smaller population).
-natural selection
what are density-dependent factors?
affect is dependent on population size and include competition, predation and disease
what are density-independent factors?
affect populations of all sizes in the same way including climate change, natural disasters, seasonal change and human activity (e.g deforestation)
what is a population bottle neck?
large reductions in population size which lasts for at least one generation. The gene pool and genetic diversity is greatly reduced.
what is a positive aspect of genetic bottleneck?
a beneficial mutation will have a greater impact and lead to the quicker development of a new species
what is genetic drift?
describes change in allele frequencies in the gene pool of a population (evolution) due purely to chance events and not selection pressures. It has a greater influence on smaller populations.
what is the founder effect?
When a new population is established by a small number of individuals, the founding population will have low genetic diversity and be heavily influenced by genetic drift.
what is stabilising selection?
in a normal distribution curve, the average is selected for (positive selection) and the extremes are selected against (negative selection). Therefore results in a reduction in the frequency of alleles at the extremes and an increase in the frequency if the average alleles
what is directional selection?
occurs when there is a change in the environment and the normal (most common) phenotype is no longer advantageous . The extreme phenotype is positively selected. The allele frequency then shifts towards the extreme phenotypes and evolution occurs
what is disruptive selection?
the extremes are selected for and the norm is selected against. This is the opposite of stabilising selection.
what is a species?
group of organisms in which two individuals can mate to produce fertile offspring
what is speciation?
formation of new species through the process of evolution
what events occur leading to speciation?
-members of a population become isolated and no longer interbreed with the rest of the population (no gene flow)
-alleles within the group undergo random mutations. The environment of each group may be different or change (resulting in different selection pressures) so different characteristics will be selected for and against.
-the accumulation of mutations and changes in allele frequencies over many generations. Members of different population become so different, they cannot interbreed. Reproductively isolated.
Describe allopatric speciation
-more common form of speciation
-speciation when a population is separated by a physical barrier (e.g river or sea)- geographical isolation. The environments are different meaning different selection pressures resulting in different physical adaptations. This will lead to founder effect leading to genetic drift.
Describe sympatric speciation
-occurs within populations that share the same habitat
-happens less frequently than allopatric speciation
-more common in plants than animals
-members of two different species interbreed and form fertile offspring. Hybrid formed is a new species which will have a different number of chromosomes to either parent and may no longer be able to interbreed with members of either parent population. This stops gene flow and reproductively isolates the hybrid organisms.
Describe prezygotic reproductive barriers?
prevent fertilisation and the formation of a zygote
Describe postzygotic reproductive barriers?
often produced as a result of hybridisation, reduce the viability or reproductive potential of offspring
what is a polymorphic population?
display more than one distinct phenotype for most characteristics
what is the wild type allele?
the allele coding for the most common characteristic is called the wild type allele.
what are mutant alleles?
other forms of allele, apart from wild type allele, resulting from mutations.
what is artificial selection (or selective breeding)?
selection of breeding plants or animals with desirable characteristics determined by humans (farmers or breeders)
what are the problems caused by inbreeding?
limited gene pool decreases genetic diversity and reduces chances of a population of inbred organisms evolving and adapting to changes in their environment.
how does inbreeding caused genetic disorders?
breeding of closely related organisms results in offspring with a greater chance of being homozygous for these recessive traits. They are less biologically fit and less likely to survive and reproduce.
describe seed banks?
keep sample of seeds from both wild type and domesticated varieties
Describe gene banks?
store biological samples which are usually frozen (e.g sperm and egg)
Describe outbreeding?
-e.g alleles from gene banks can be used to increase genetic diversity, owing to the problems caused by inbreeding
-e.g breeding unrelated or distantly related varieties
This reduces occurrence of homozygous recessives
what is sex linkage?
a gene whose locus is on the X chromosome
what is epistasis?
when one gene modifies or masks the expression of a different gene at a different locus
what is monohybrid?
genetic inheritance cross of a characteristic determined by one gene
what is dihybrid?
genetic inheritance cross for a characteristic determined by two genes
