6.2 PATTERNS OF INHERITANCE AND VARIATION Flashcards
What is genetic variation?
Genetic variation is the variety of different combinations of alleles in a population.
e.g changes to the genetic code, hair and eye colour.
What is environmental variation?
Environmental variation is the differences in individuals that are not inherited but are caused by the environment that the organism lives in.
e.g tattoos, scars.
What is continuous variation?
Continuous variation is a characteristic that can take any value within a range. controlled by a number of genes (polygenes).
- can be graphed as a bell curve as individuals fall into a range and not into distinct categories
- often causes by a mixture of genes and environment. for example, genes that make you grow may be inherited but if you don’t eat nutritious food you may not grow as tall as potential
What is discontinuous variation?
Discontinuous variation is a characteristic that can only result in certain discrete values.
e.g blood type.
- can be graphed as a bar chart as individuals fall into distinct categories
- usually caused by one or two genes and is not effected by the environment
How are genes involved in the production of chlorophyll?
Genes are involved in the production of chlorophyll as most plants are genetically coded to produce large amounts of chlorophyll.
What are the symptoms of chlorosis?
Symptoms of chlorosis:
- leaves pale or yellow
- reduced ability to photosynthesise
Why does chlorosis in plants occur?
Chlorosis occurs because cells are not producing the normal amount of chlorophyll. Change in phenotype is due to environmental factors as most plants with chlorosis have normal genes coding for chlorophyll.
Why is chlorosis not beneficial to the plant?
Chlorosis is not beneficial to the plant as it reduces the ability to photosynthesise due to the lack of chlorophyll.
What environmental factors cause chlorosis?
Environmental factors that cause chlorosis:
1. lack of light - in the absence of light plants turn off chlorophyll production to conserve resources
2. mineral deficiencies - e.g lack of Fe or Mg. iron needed as a cofactor by some enzymes that make chlorophyll, magnesium found at the heart of the chlorophyll molecule. if either element is lacking in the soil, the plant cannot make chlorophyll and the leaves turn yellow
3. virus infections - when viruses infect plants they interfere with the metabolism of cells. common symptom is yellowing in the infected tissues as they can no longer support the synthesis of chlorophyll
What determines body mass in organisms?
Both genetic and environmental factors determines body mass in organisms.
What are the main environmental factors that affect body mass?
Environmental factors that affect body mass:
- amount and quality of food eaten
- quantity of exercise the organism gets
- presence of disease
When can obesity be caused by genetics?
Obesity can be caused by genetics by a mutation of chromosome 7 which causes the pattern of fat deposition in the body to be altered. The mutated gene acts in conjunction with other genes that regulate the energy balance in the body.
What is codominance?
Codominance occurs when two different alleles occur for a gene - both of which are equally dominant. as a result both alleles of the gene are expressed in the phenotype of the organism if present
. e.g the colour of snapdragon flowers = two equally dominant alleles exist, each of which codes for the colour of the flower
- an allele that codes for red flowers = codes for the production of an enzyme which catalyses the production of red pigment from a colourless precursor
- an allele that codes for white flowers = codes for an altered version of the enzyme which does not catalyse the production of the pigment, therefore the flowers are white
- three colours may be produced
1. red flowers = plant is homozygous for the allele coding for production of red pigment
2. white flowers = plant is homozygous for the allele coding for no pigment production
3. pink flowers = plant is heterozygous. single allele present which codes for red pigmentation produces enough pigment to produce pink flowers
. upper and lowercase letters are not used to represent the alleles. a letter is instead chosen to represent the gene e.g c^R for red flowers and c^W for white
What are multiple alleles?
Multiple alleles:
- more than two alleles for a gene
- organism carries only two versions of the gene (one on each of the homologous chromosomes) only two alleles can be present in an individual
. e.g blood group determined by a gene with multiple alles
- immunoglobin gene (gene 1) codes for the production of different antigens present on the surface of red blood cells. 3 alleles for the gene
1. I^A = results in production of antigen A
2. I^B = results in production of antigen B
3. I^O = results in production of neither antigen
- I^A and I^B are codominant whereas I^O is recessive to both other alleles. different combinations of these alleles result in the four blood groups
1. blood group A = I^A/I^A or I^A/I^O
2. blood group B = I^B/I^B or I^B/I^O
3. blood group AB = I^A/I^B
4. blood group O = I^O/I^O
How is sex determined in humans?
Determining sex:
- humans have 23 chromosomes of varying size and shape, in 22 of the pairs (autosomes) both members of the pair are the same, but in the 23rd pair (the sex chromosomes) are different
- human female have two x chromosomes, human males have an x and a y chromosome
- x chromosome is large and contains many genes not involved in sexual development
- y chromosome is very small, containing almost no genetic information, but does carry a gene that causes the embryo to develop as male
- sex of offspring determined by whether the sperm fertilising the egg will contain an x or a y chromosome
What is sex linkage?
Sex linkage:
- some characteristics are determined by genes carried on the sex chromosomes = genes are sex linked
- as the y chromosome is much smaller than the x chromosome, there are a number of genes in the x chromosome that males have only one copy of
= this means that any characteristic caused by a recessive allele on the section of the x chromosome, which is missing the y, occurs more frequently in males. this is because many females will also have a dominant allele present in their cells
What is haemophilia?
Haemophilia:
- sex linked genetic disorder
- blood which blood-clots extremely slowly due to the absence of a protein blood-clotting factor (in the majority of cases this is a factor VIII)
= as a result, injury can cause prolonged bleeding which is potentially fatal if left untreated
- 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
= as a result the vast majority of haemophilia sufferers are male
- females who are heterozygous for the haemophilia coding gene are known as carriers so do not suffer from the disease but can still pass it on to their children
- when showing the inheritance of a sex linked condition the alleles are shown linked to the chromosomes they are found on
= x^H is used to represent the dominant ‘healthy’ allele
= x^h is used to represent the recessive allele coding for haemophilia
= y is used to represent the y chromosome
- if a carrier female and normal male have children, then in theory half the male offspring produced will have the disorder, half the female offspring will be carriers
- an affected male can pass on the faulty allele to daughters, resulting in them becoming carriers
What is monohybrid inheritance?
Monohybrid inheritance:
- each character of a diploid organism is controlled by a pair of alleles
- e.g cystic fibrosis (cf)
= faulty allele causing cf is recessive
= normal allele is dominant
What is a homeobox gene?
Homeobox genes determines how how an organisms body develops as it grows from a zygote to a complete organism. Determines polarity (head and tail ends of each body segment) and positioning of organs.
What is a homeobox sequence?
A homeobox sequence is where each gene contains a homeobox (DNA sequence of about 180 nucleotides) which is highly conserved.
What are hox genes?
Hox genes are a class of homeobox genes only found in animals. Every hox gene is a homeobox gene, but not every homeobox gene is a hox gene.
- found in clusters
- mammals have four clusters on different chromosomes
- genes within these clusters are expressed in certain body segments at certain stages of development
- homology between homeobox genes in mice and humans (similarities)
- linked (or lined up) in a sequence which relates to the pattern the genes are expressed in
What are homeodomain proteins?
Homeodomain proteins are a product when a gene is transcribed. A transcription factor that switches particular genes on and off in certain body segments at particular stages of the development. The homeodomain is the part of the homeodomain protein that binds to DNA and is coded by the homeobox.
What are the effects of homeodomain proteins?
Effects of homeodomains:
activate a developmental gene
- switches on so RNA polymerase can bind (transcription occurs)
repress a developmental gene
- switches off so RNA polymerase cannot bind (transcription does not occur)
Why are fruit flies studied?
Fruit flies commonly studied by geneticists:
- genome quite small an well understood
- reproduces and develops rapidly
- each body segment is characterised by the structures it contains
How have homeobox genes evolved?
Homeobox genes and evolution:
- homeobox genes are highly conserved (if mutation occurs to them organism wont survive)
- homeobox genes are so fundamental that any mutation will mean that the organism will not survive
- each kingdom has its own set of homeobox genes
- plant homeobox genes are similar to those in other plants but different to those in animals
- homeobox genes haven’t really changed through evolution = importance
What is the evidence of homeobox genes?
Evidence for homeobox genes:
- if a mouse eye homeobox gene is inserted into the DNA of a fruit fly, a fruit fly eye will be produced even if it is in the wrong part of the body
- in a normal fly, a homeobox gene called Antp causes legs to develop in the necessary body segments
- in the head, this homeobox gene is turned off so that legs do not develop
- in some mutant flies, Antp is switched on and legs develop where antennae should be
- mice have a similar homeobox gene - if it is put into a fruit fly head it had the same effect as Antp