expressing the genome

how genes interact

HideShow resource information

expression of genes

  • many charactersitics depend on a single gene that has multiple alleles, or are polygenic traits in which phenotypic features result from the interactions of a number of different genes  in gene expression.
1 of 12

epistasis meaning to "stand over"


  • happens between two genes coding for seperate phenotype features when one locus masks the expression of the other
  • e.g coat colour in domestic cats
  • cats have many genes effecting coat colour, two of which code for coat pattern. The aguti gene causes banding and is associated with tabby coat pattern of any colour, unless this allele is present the cat will have a solid coat colour. the agutio alleles are A and a . AA or Aa gives an aguti coat aa combination gives pure black.
  • There are other genes which effect th pattern e.g macrle gene spots and ticks but without the dominant aguti gene the pattern will not show up because the cat will be all one colour.

dominance is different

  •  occurs between two alliels coding for the same gene
2 of 12

interaction between genes and the environment

an example f the impact the environmnet has on the interaction of genes and physical appearence is the siamese cat and certain rabbit breeds have dark ears and feet and tails

the genotype of these animals suggest that they should have dark fur all over their bodies, as a result of melinin production in the process involving the enzyme tyrosonase

however the mutation in the cats results in a version of this enzyme being inactive at normal body temperature and only works at lower temperatures so at normal body temp it denatures , so in siamese cats the majority of the body is pale and the extremities such as ears and nose which are colder are a darker colour as the enzyme have not denatured.

3 of 12

the lac operon effect - showing environment can ha

  • lactose a disaccharide found in milk that is broken own in galactose and glucose by the enzyme B-galactosidase in bacteria scientists found 

 lactose absent

  • when Lactose is absent a special repressor gene attatches to the area of DNA which codes for the lactose
  •  RNA polymerases chain then cannot bind to the area of DNA so lactose digesting enzymes cannot be transcribed (stops waste of resources when lactose is absent , no need for enzyme)

Lactose is present

  • the lactose binds to the receptors changing its shape and inactivating it so it can't fit onto the dna and mask it
  • RNA polymerase binds to the DNA and transcribes the genes for the enzyme so lactose can be made and lactoses can be broken down into galactose and glucose
4 of 12

studying human variation

Difficulties in studying the interatction of genotypes and environment in humans

  • important that all organisms are subjected to the same conditions so any difference between them can be see as a pure result of genetics but this is hard

ways of studying effects of nature vs nurture

  • Twin studies, identical twins are copies of the same DNA, non identical twins are siblings with different DNA but same age, so more likely similar environmental upbringing, normal siblings are a good control.
  • twins seperated at birth also give an indication into the importance of nature interacting with nurture
  • epidemiological studies, one way of seperating environmental and genetic influences in humans is by looking at huge numbers of individuals and consider a genetic feature along with information about lifestyle. e.g the impact of mothers smoking when pregnant and the difference in birth weights of the baby
5 of 12

conclusions drawn from impact of nature and nurtur

stong environmnetal influence weak genetic influence  (in order down)

  • multiple scleosis
  • cancer
  • cardiac conditions
  • asthma
  • diabetes
  • neurotic extrovert
  • IQ
  • schizophrenia
  • major depression
  • soriasis

strong genetic influence weak environmental influence (in order up)

6 of 12

variation - Discontinious and continious variation

Discontinious variation

  • is shown by the features that are present or not e.g blood group or sex
  • generally determined by one or at most a few genes
  • environmnet doesn't usually have an effect
  • very clear cut

Continious variation

  • variations which change and continue to develope
  • e.g height, weight or number of leaves on a plant
  • factors often determined by a number of genes being polygenic
  • but they are also greatly effected by the environment
7 of 12

studying continious variation

polygenic features - are different genes effecting different factors relating to size or weight etc. They also have environmetal factors such as balanced diet - this is continious variation.

when studying you need

  • large sample group because by sheer change if you had a small group you might pick a region of relatively short individuals
  • randomly pick subjects from as much of the organisms habitat as possible ( if you collect from only one area you might pick a place effected by disease change in weather conditions or diet any one of these variables effects the findings)
  • once data is collected it must be presented in a graph or histogram, to show the frequency distribution of the characteristics clearly
8 of 12

Distribution curve and standard divation

If the features of the animal and plant shows continious variation, the frequency distribution will be a normal distribution curve (bell shaped curve).

  • at the extreams there will be few individuals .
  • mean measurement is in the exact centre of a normal distribution curve and the heights are distributed evenly around the mean.
  • help judge amount of variation in a sample the standard divation is used which is a statistical measure of the amout of differences in the sample, if it is small and clustered together then its more likely theres a corrilation and there is little variety. If it is large then there is alot of variety it will more likely be random rather than a common link between results.


  • normal divation is 68%within one standard divation , 95% within 2 and all within 3
9 of 12

genes and environment human features Monoamine Oxi

Monoamine Oxidase A (MAOA) is an enzyme found in the nervous system, the liver and the gut where it breaks down monoamines. they are eaten as part of the diet but also many of the chemicals involved in nerve impulses transmitting across the synapses (neurotransmitters ) are also monoamines e.g seritonin, noroadrenilin and dopimine. Once they have been released into the synapses they are broken down by monoamine oxidase.

Too much of the enzyme leads to break down of too many neurotransmitter which is linked to depression ( a traetment is a drug which inhibit the enzyme MAOA allowing higher levels of neorotransmitters e.g seritonin to build up)

levels of monoamine oxidase enzyme is genetically determined but environmental factors e.g stress also combine with this to determin the phenotype (characteristic)

low levels of MAOA seem to be linked also to addictive behaviour e.g drinking

high levels are linked to risk taking and aggressive behaviour

10 of 12

cancer genes and the environment

cancers don't respond to the normal mechanisms that controll the cell cycle, they divide rapidly forming a tumour which invades the surrounding tissue. a tumour may split (metastasis) releasing small clumps to the blood stream where they circulate and become lodged in different areas of the body they are then  malignant. Cancer cells live longer and destroy surrounding tissue.

Causes of cancer

genetic factors

  • virual infections, viral genetic material triggers uncontrolled cell growth
  • many causes come from mutations in the DNA of the cells as they reproduce and divide, this usually takes several mutations for the cell to become cancerous
  • some cancers are caused by a single gene mutation. genes known as protooncogenes code for the proteins that stimulate the cell cycle, if one gene mutates its called an oncogene. Oncogene produce uncontrollable amounts of the protein which stimulates the cell cycle - gene is dominant.
11 of 12

another gene associated with cancer

tumour suppressor genes, these normally produce chemicals which supress the cell cycle. When there not present uncontrollable division happens.

Environmental factors

these can play a part in increasing the likelihood of cancer causing mutations occuring e.g tar in cigarette smoke, the chemicals in alchol, asbestos and ionising radiation aswell as uv light from the sun.( uv leads to melanoma)

12 of 12


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all DNA, genetics and evolution resources »