Genes Lecture 2 Flashcards
features of monohybrid cross
1 gene; 2 alleles
principle of dominance and segregation
F2 gen 3:1 phenotypic ratio
Mendel’s law of dominance
when an individual is heterozygous for a particular trait, the dominant allele will always be expressed
Mendel’s law of segregation
allele pairs separate during gamete formation
features of a dihybrid cross
2 genes
2nd Law: a principle of independent assortment of genes
9:3:3:1 phenotypic ratio
Mendel’s law of independent assortment
alleles for different traits are passed independently of one another, resulting in a 9:3:3:1 ratio
what does Mendel say about inheritance
inheritance is particulate and traits are passed on through genes that come in pairs
effect of point mutations
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)
incomplete dominance
heterozygous phenotype is intermediate between the two other homozygotes
example of incomplete dominance
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
co-dominance
heterozygotes show phenotype of both alleles
example of co-dominance
blood groups
multiple alleles
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
pleiotropy
gene influences more than one trait
example of pleiotropy
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)
lethal alleles
skewed phenotypic ratios
example of lethal alleles in mice
single Ay allele dominant for fur colour whereas Ay is recessive for lethality
example of lethal alleles in humans
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
penetrance
percentage of individuals with a given genotype who exhibit the expected phenotype
expressivity
the extent to which a genotype is expressed at a phenotypic level
incomplete penetrance example
30-40% of individuals with breast cancer mutations don’t develop BC
variable expressivity example
insertion of a transposable element in the promotor of the agouti gene may lead to siblings showing variation of expression of yellow coat color
what may lead to incomplete penetrance or variable expressivity
gene modifiers or somatic mutations
Alfred Knudson
hypothesized that familiar cancer syndromes caused by mutations in tumour suppressor genes is a result of a loss of heterozygosity
loss of heterozygosity
loss of healthy allele due to somatic mutation
examples of loss of heterozygosity syndromes
neurofibromatosis type 1
retinoblastoma
breast cancer
what is neurofibromatosis type 1 caused by
LOF mutations in the NF1 gene
effects vary in individuals with the same NF1 mutation
neurofibromatosis type 1
dominant familial cancer syndrome
affects 1 in 3500 individuals
characterized by benign neurofibromas under the skin and brown spots on the skin
