Lecture 5 Flashcards
What does a gene code for?
polypeptide and RNAs 20000 genes code for polypeptides 5000 genes code for RNAs
How was it proved that genes code for polypeptides?
demonstrated this relationship using Neurospora crassa -grows on minimal medium (prototroph) -made mutants (by radiation) that needed additional supplements on minimal medium (auxotrophs) -some F2 mutants prototrophs some auxotrophs -the auxotrophs grew when arginine added so radiation caused damage to arginine producing gene -these arginine deficient mutants would sometimes grow in compounds closely related to arginine -several intermediates on the pathway to arginine, at each step a different enzyme neede to produce the next step—- hypothesis: each enzyme coded for by a different gene -some proteins made up of more than one unit for each of which a different gene is needed
prototroph:
-With reference to bacteria/fungi an organism that can grow and divide on a substrate of minimal medium which contains inorganic salts, a carbon source for energy and water -usually fungi/bacteria that grow on minimal media, can synthesise all the the nucleic acids it needs
auxotroph:
A mutant organism (as above) which requires a compound to be added to minimal media to grow and divide. -can’t make all the nucleic acids
multiple alleles:
more than two alternatives are possible for a gene locus (most genes) -when gene has more than two forms but each organism will only have two alleles - some genes have very high numbers ctfr gene over a 1000 - one always from mother the other from father
On what chromosome are the ABO blood groups?
-9, one gene locus
What 3 alleles does the ABO blood group system have?
IA IB IO - IA and IB are co-dominant -IA and IB are dominant to IO the IAIO- phenotype Type A…
What is a phenocopy?
copy of an inherited condition but results from an environmental factor -occurs when environment modifies the phenotype to mimic the effect of genotype -not inherited!
Give an example of a phenocopy.
-inherited vitamin D-resistant rickets and phenocopy: low vit D in diet and lifestyle(this is phenocopy to rickets)
What is a pleiotropy?
-single gene affecting many features if the phenotype
Give two examples of pleiotropy.
e.g.- sickle cell anaemia -primary effect of mutation is to alter the production of beta globin but results in many different effects: heart failure, pneumonia, paralysis, kidney failure etc -chain reaction e.g. albinism -primary effect is a change in enzyme—no melanin -other consequences: pink iris, red pupil, (those two resulting in astigmatism, photophobia, nystagmus), white skin, unpigmented hair
What is meant by variable expression?
- the degree or range in which a particular phenotype is expressed by individuals -can have the same gene but displayed differently in different individuals due to interactions with other genes -everyone displays the phenotype but with varying severity
Give an example of variable expression.
e.g.- polydactyly (extra fingers) -some offspring has 6 fingers, some 7…
What is meant by reduced penetrance?
some with the genotype don’t have the phenotype -proportion of genotypes that show the expected phenotype -85% penetrant trait- if 15% don’t show the phenotype
Give an example of completely penetrant and reduced penetrance trait.
Huntington’s- completely penetrant trait Polydactyly- ifsome offspring don’t have extra fingers- reduced penetrance as well as variable expression
What do we have to add for the fungus to grow:
citrulline or arginine
What do we have to add so the fungus grows?
ornithine, citrulline or arginine
