Genes Lecture 2

Question 1

features of monohybrid cross

Answer 1

1 gene; 2 alleles
principle of dominance and segregation
F2 gen 3:1 phenotypic ratio

Question 2

features of a dihybrid cross

Answer 2

2 genes
2nd Law: a principle of independent assortment of genes
9:3:3:1 phenotypic ratio

Question 3

what does Mendel say about inheritance

Answer 3

inheritance is particulate and traits are passed on through genes that come in pairs

Question 4

effect of point mutations

Answer 4

nonsense and missense mutations may cause loss of function alleles (reduced function alleles or null function alleles); silence has no effect on mutations
usually recessive
rarely result in gain of functioning alleles or neomorphic activity (usually dominant)

Question 5

incomplete dominance

Answer 5

heterozygous phenotype is intermediate between the two other homozygotes

Question 6

example of incomplete dominance

Answer 6

FH (familial hypercholesterolemia) which affects LDL (low density lipoprotein) receptors
HH- dominant and fully functioning receptors
Hh- hetero and results in a few but not all receptors being made ( increase in cholesterol)
hh- no receptors made at all which is fatal
-affects 1 in 500

Question 7

co-dominance

Answer 7

heterozygotes show phenotype of both alleles

Question 8

example of co-dominance

Answer 8

blood groups

Question 9

multiple alleles

Answer 9

multiple alleles form a single gene
-alleles can be organized in a series of dominance (depending on how alleles interact with one another)
-e.g. in agouti mice

Question 10

pleiotropy

Answer 10

gene influences more than one trait

Question 11

example of pleiotropy

Answer 11

primary ciliary dyskinesia (PCD): lack/ dysfunction of protein dynein (essential for cilia and flagella beating movements)
-respiratory problems: fail to clear airways
-infertility: non-motile sperm
-situs inversus: organs are on the wrong side of the body due to impacted cilia unable to detect the flow of organization (50% of PCD)

Question 12

lethal alleles

Answer 12

skewed phenotypic ratios

Question 13

example of lethal alleles in mice

Answer 13

single Ay allele dominant for fur colour whereas Ay is recessive for lethality

Question 14

example of lethal alleles in humans

Answer 14

achondroplasia
dominant GOF mutation FGFR3 growth receptor results in premature conversion of cartilage into bone preventing bones from fully forming
however recessive for lethality: stillborn babies or early death in infants

Question 15

penetrance

Answer 15

percentage of individuals with a given genotype who exhibit the expected phenotype

Question 16

expressivity

Answer 16

the extent to which a genotype is expressed at a phenotypic level

Question 17

incomplete penetrance example

Answer 17

30-40% of individuals with breast cancer mutations don’t develop BC

Question 18

variable expressivity example

Answer 18

insertion of a transposable element in the promotor of the agouti gene may lead to siblings showing variation of expression of yellow coat color

Question 19

what may lead to incomplete penetrance or variable expressivity

Answer 19

gene modifiers or somatic mutations

Question 20

Alfred Knudson

Answer 20

hypothesized that familiar cancer syndromes caused by mutations in tumour suppressor genes as a result of a loss of heterozygosity

Question 21

loss of heterozygosity

Answer 21

loss of healthy allele due to somatic mutation

Question 22

examples of loss of heterozygosity syndromes

Answer 22

neurofibromatosis type 1
retinoblastoma
breast cancer

Question 23

what is neurofibromatosis type 1 caused by

Answer 23

LOF mutations in the NF1 gene
effects vary in individuals with the same NF1 mutation

Question 24

neurofibromatosis type 1

Answer 24

dominant familial cancer syndrome
affects 1 in 3500 individuals
characterized by benign neurofibromas under the skin and brown spots on the skin