Lecture 5

Question 1

What does a gene code for?

Answer 1

polypeptide and RNAs 20000 genes code for polypeptides 5000 genes code for RNAs

Question 2

How was it proved that genes code for polypeptides?

Answer 2

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

Question 3

prototroph:

Answer 3

-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

Question 4

auxotroph:

Answer 4

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

Question 5

multiple alleles:

Answer 5

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

Question 6

On what chromosome are the ABO blood groups?

Answer 6

-9, one gene locus

Question 7

What 3 alleles does the ABO blood group system have?

Answer 7

IA IB IO - IA and IB are co-dominant -IA and IB are dominant to IO the IAIO- phenotype Type A…

Question 8

What is a phenocopy?

Answer 8

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!

Question 9

Give an example of a phenocopy.

Answer 9

-inherited vitamin D-resistant rickets and phenocopy: low vit D in diet and lifestyle(this is phenocopy to rickets)

Question 10

What is a pleiotropy?

Answer 10

-single gene affecting many features if the phenotype

Question 11

Give two examples of pleiotropy.

Answer 11

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

Question 12

What is meant by variable expression?

Answer 12

• 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

Question 13

Give an example of variable expression.

Answer 13

e.g.- polydactyly (extra fingers) -some offspring has 6 fingers, some 7…

Question 14

What is meant by reduced penetrance?

Answer 14

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

Question 15

Give an example of completely penetrant and reduced penetrance trait.

Answer 15

Huntington’s- completely penetrant trait Polydactyly- ifsome offspring don’t have extra fingers- reduced penetrance as well as variable expression

Question 16

Can penetrance and expressivity be affected by environmental factors? If then how?

Answer 16

yes: temperature. -Siamese cats, darker ears plus tails when colder… -Ziannias flowers-red when seeds raised in hotter environment

Question 17

What is lethality on genetics?

Answer 17

an allele, genotype or mutation which results in death (often before fully formed organism)

Question 18

What is recessive lethal?

Answer 18

: only lethal as homozygote aa BB (doesn’t kill as much!-recessive) - can be a dominant phenotype in the heterozygote

Question 19

What is dominant lethal?

Answer 19

: lethal as HH and Hh, so whenever present, even one so dominant killer! - must have a late onset in the phenotype otherwise it would not be sustained in the genotype (eg. Huntington’s onset in one’s 40s) -have to adjust the phenotypic ratios accordingly, since some genotypes die off, just cross of the ones that die but consider the age at which they do

Question 20

What do we have to add for the fungus to grow:

Answer 20

citrulline or arginine

Question 21

What do we have to add so the fungus grows?

Answer 21

ornithine, citrulline or arginine

Question 22

name structural and transport protein

Answer 22

fibrin and haemoglobin