Exam 2 Flashcards
Mendels Law of Segregation
2 alleles of each gene separate/segregate during gamete formation, and then unite at random (1 from each parent) at fertilization
- MI separates homologs; then MII separates sisters. Each gamete ends up with 1 copy of each allele
What happens in meiosis that underlies law of segregation?
Homologous chromosomes align in metaphase I and segregate into separate daughter cells
Mendel’s Law of Independent Assortment
During gamete formation, different pairs of alleles segregate independently of each other
50% chance of receiving alleles from mother vs father
what happens in meiosis that underlies law of IA?
Homologous chromosomes align in MetaphaseI with independent orientation; the orientation of 1 tetrad does not influence the orientation of another
independent assortment on same vs different chromosomes
- alleles on different chromosomes = always independently assort
- alleles on the same chromosome may/may not independently assort
independent assortment on the same chromosomes
2 alleles on the same chromosome will assort into the same gamete unless crossing over swaps one onto the homologous chromosomes
therefore, IA on the same chromosome may or may not occur
test cross
cross recessive genotype with mystery genotype
- all dominant –> homozygote dom
- half dominant –> heterozygote dom
why do a test cross?
figure out the genotype of an individual
true-breeding/pure-breeding
homozygous individuals whose line produces the same phenotype when selfed 100% of the time
**can assume genotype is homozygous
how to figure out which is the dominant individual?
look at heterozygous
-cross 2 pure-breeding individuals to get all heterozygous F1 generation and analyze the phenotypes
monohybrid self cross
heterozygotes of 1 gene crossed with each other
Ex/ Aa x Aa
1:2:1 genotypic ratio
3:1 phenotypic ratio
/
alleles on different homologs of the same chromosomes
Ex/ A/a
;
alles on different chromosomes
Ex/ A/a;B/b
dihybrid test cross
2 genes controlling 2 traits
-heterozygotes crossed with recessive homozygotes
Ex/ A/a;B/b x a/a;b/b
genotypic: 1:1:1:1
phenotypic: 1:1:1:1
dihybrid self cross
selfing of dihybrid
genotypic: 9:3:3:1
phenotypic: 9:3:3:1
9 - both dom
3 - 1 dom; 1 rec
3 - 1 rec; 1 dom
1 - both rec
product rule
AND
- the probability that 2 or more independent events occurring together is the product of the probabilities that each will occur by itself
sum rule
OR
- the probabilities of 2 mutually-exclusive events occurring is the sum of their individual probabilities
a scientific hypothesis makes — predictions and is —-.
testable and is falsifiable.
*null hypothesis must make a testable prediction.
ex/ IA will occur.
null hypothesis
there is no significant difference between the observed and expected frequencies
must be very certain that you can reject the null hypothesis
5%
chi-square tests
determine p-value using a formula
total = (observed - expected)^2 /expected
compare values in chart
p value
represents the probability that the null hypothesis is TRUE
p > 0.05
fail to reject the null hypothesis
p < 0.05
reject the null hypothesis with 95% certainty
- there is a greater than 95% chance that the null hypothesis is not true
degrees of freedom
the number of values observed/expected minus 1
how to find expected values for chi square?
look at total and use ratio expected based on the type of cross
ex/ monohybrid self cross = 3:1
out of 400
300 and 100 are expected values
genes controlled by single genes…
display characteristic inheritance patterns
*though most traits are not controlled by a single gene
genetic diseases
- typically involve 2 possible alleles: “disease” and “wild-type”
autosomal recessive disorders
- males and females equally affected
- unaffected individuals can have affected children via heterozygous carriers
- can skip generations
- rare
- becomes more common with inbreeding
rareness in diseases stipulation
when discussing diseases, you can assume these traits are rare so people entering the pedigree do NOT carry the disease allele
- unless you have info to suggest otherwise
why does disease become more common with inbreeding?
may be rare in the general population, but may not be rare in family –> so the homozygote recessive genotype becomes more common
tay-sachs symptoms
- affects babies a few months old
- lose vision and react abnormally when startled
- paralysis
- deafness
- seizures
- inability to breath/swallow
tay-sachs causes
- healthy neuron lysosomes act as the waste processing center of the cell
- With TS, lysosome enzymes cannot properly break down fatty cell products (gangliosides).
- Products build up and destroy cells
- recessive autosomal disease
tests for pregancy with genetic counselors
carrier test of parents (genotypes)
prenatal/preimplantation tests
PGD
Preimplantation Genetic Diagnosis
- blastomere from invitro fertilized embryos are removed and tested for disease gene
- embryos with disease are discarded/donated to science
- embryos without are implanted or frozen
autosomal dominant disorders
- males and females are equally affected
- affected individuals always have an affected parent (no heterozygous carriers bc they are affected too)
- does not skip generations
huntington’s disease
rare, fatal, degenerative neurological disease caused by a dominant disease allele
- start showing symptoms in 40s
- death within 15 years
- mutation in Huntingtin gene
- Huntington’s aggregates in neurons, shaking, personality changes
x-linked inheritance rules
- males inherit Y from their father and MUST inherit X from their mother
- females inherit one X from father and one X from mother
x-linked recessive disorders
- males more frequently affected
- never transmitted from fathers to daughters
- All sons of affected mothers will also be affected by the trait
- can “skip generations” via female carriers
x-linked dominant disorders
- females more frequently affected
- ALL of the daughters and NONE of the sons of affected fathers have the trait
- does not “skip generations”
penetrance
the percentage of individuals with a particular genotype that demonstate the expected phenotype
complete penetrance
100%
incomplete penetrance
1-99%
penetrance calculations over or underestimate?
overestimate
- there could be other nonpenetrant individuals that we are not certain about
- the demoninator is larger so the overall fraction/percentage will be smaller than originally estimated
variable expressivity
for individuals with the same genotype, there is a range of phenotype severity/expression
** the degree with which a genotype is expressed as a phenotype (how much phenotype is shown)
complete dominance
the heterozygous phenotype is the same as homozygous dominant
incomplete dominance
the heterozygous phenotype is intermediate between the two homozygotes
codominance
the heterozygous phenotype is a mizture of the 2 homozygotes
– aspects of both homozygotes are shown
incomplete and codominance
the same genotypic but different phenotypic ratios as complete dominance
monomorphic traits
have single “wild type” allele
variant alleles
- rare (<1%)
- classified as mutant
polymorphic traits
have multiple common allele variants
- There is no single wild type allele
trait classifications are NOT static
- classifications can change
- a mutant allele can become a common varient over time or a common varient can be lost from the population
AB blood type
The IAIB heterozygotes express two functional varients of the enzyme (Type A and B enzyme)
IAIB
***polymorphic
O blood type
The i allele encodes a null version of the enzyme (non-functional enzyme)
ii
B blood type
The IB allele encodes a version of the enzyme that adds B sugars
IBIB
IBi
A blood type
The IA allele encodes a version of the enzyme that adds A sugars
IAIA
IAi
how does red blood cells get their type?
Red blood cells have a protein on their cell membranes that can be modified by an enzyme
- enzyme decides phenotype (blood type)
with regard to blood type, IA is
codominant to IB and dominant to i
how to know if 1 or 2 genes are involved?
look at ratios
pleiotrophic allele
affects several properties of an oranis,
pleiotrophy in cystic fibrosis
single gene defect leads to lung disease, sterility, pancreatic dysfunction
BC the gene product (CFTR protein) has important functions in the cells of the respiratory, digestive, and reproductive tracts
dominance relationships are specific to..
each phenotype an allele affects
lethality and the pelger anomaly
recessive for pelger is lethal phenotype
lethal means that they have severe defects or are never born
changes the phenotypic ratios
with regard to the nuclear morphology phenotype, the “pelger” allele is…
dominant
with regard to the lethality phenotype, the “pelger” allele is…
recessive
when 2 genes impact the same trait…
we can observe the same familiar ratios or see modified rations
fewer phenotypic classes than expected indicate…
epistasis
epistasis
the effect of one gene masks the effect of another
- often occurs when 2 genes encode members of the same biochemical pathway
bombay phenotype
recessive epistasis
hh genotype
- overrides the blood type for an O blood type
- doesn’t matter what parental blood types are
- 2 recessive h alleles mask the IA alleles phenotypic effect
epistasis is a…
gene-gene interaction (relationship between 2 genes vs 2 alleles of same gene like in dominance relationships)
duplicate/redudant genes
collapse all phenotypic categories with 1+ dominant allele for either gene
15:1 ratio
dihybrid cross ratio
9:3:3:1
complementary ratio
9:7
duplicate genes ratio
15:1
recessive epistatis ratio
9:3:4
dominant epistatis ratio
12:3:1
complementation
when 2 individuals with the same mutant phenotype but different homozygous recessive genotypes produce offsprng with the wild-type phenotype when crossed
complementation test
only works for recessive mutants
- always cross homo recessive mutants that are mutant for only one gene
-no complementaion if mutant trait appears (same gene)
- complementation if mutatnt trait does not appear (mutations in different genes)
- failure to complement with itself bc if has mutant and crosses with itself, the mutant phenotype will remain.
melanocytes
- found in skin
- produce pigmented melanosomes that give skin its color
MC1R1/MC1R1
can produce different doses of eumelanin vs phenomelanin depending on the available receptor variants
purebreeding lines
organisms that produce offspring with specific parental traits that remain constant from generation to generation
(often homozygous)
dihybrids
individual that is heterozygous at 2 different genes
parental types
phenotypes that reflect a previously existing parental combination of alleles that is retained during gamete formation
recombinant types
phenotypes reflecting a new combo of alleles that occurred during gamete formation
hemizygote
genotype for genes present in only one copy in an otherwise 2n organism
ex/ x-linked genes in a male
polymorphic
describes a locus with 2 or more distinct alleles in a population
common variants
high-frequency alleles of a polymorphic gene or other chromosomal locus
pleiotrophy
single gene determines a number of distinct and seemingly unrelated characteristics
advantageous vs disadvantageous alleles
an allele that is advantageous in one environment may be disadvantageous in another
advantageous vs disadvantageous alleles example
- UV damages folate for neural birth defects
— alleles that increase pigmentation would be advantageous - UV is required for vitamin D production and without causes rickets in bone
— alleles that increase pigmentation would be disadvantageous in a sunny environment
— alleles that decrease pigmentation would be advantageous in a less sunny envt.
frequencies of alleles/phenotypes vary across populations due to…
generations of different selection pressures and limited gene flow
crossing over between 2 genes generates
recombinant chromosomes
if the ratio is not 1:1:1:1….
the more common of gamete will be the parental types
when crossing over occurs
crossing over between 2 genes does not always happen…
so >50% of gametes will be parental. Fewer offspring will be recombinant.
cis dihybrids
dominant alleles are on the same homolog
AB/ab
trans dihybrids
dominant alleles are on different homologs
Ab/aB
cis and trans dihybrids differ in…
what allele combinations are parental vs recombinant
(parental vs recombinant combos swap with cis and trans switch)
parental offspring are…
more common than recombinant in terms of expected ratios
– exact ratio depends on the distance
the farther apart genes are on a chromosome…
the more likely that they are to be affected by a crossover
(recombination is common)
when genes are close together…
fewer crossover events occur and recombination is rare.
calculation recombination frequency:
- do a test cross of dihybrid (heterozygous with recessive genotype)
- determine which offspring are recombinant
- add up recombinants and divide by the total offspring
RF = 0%
complete linkage
(only parental genotypes)
RF < 50%
linkage
(parental genotypes are more common)
RF = 50%
independent assortment
(parental and recomb are equally likely)
genes are UNLINKED and independently assort despite being on the same chromosome
RF will never…
exceed 50%
recombination frequencies
measurements of “genetic distance” between genes
aka 1% RF = 1 map unit (m.u. OR centiMorgan cM)
map unit
unit of measure of genetic distance correlating with 1% RF = 1 m.u. OR centiMorgan cM
determine location of genes with RF %
- the largest % will be the farthest apart
- convert % to mu and add together to get distances between genes
getting probability of recombinance with map unit
6 mu = 6% will be recombinant
- do a test cross and determine the parental and recomb genotypes
- divide % by 2 and assign % to each recomb.
- remaining % out of 100% divided by 2 will be the parental types
ex/
3% each recomb
47% each parental
1/2 of each recombinant will be of each recombinant type.
recessive lethal allele
an allele that prevents survival of homozygotes
- although heterozygotes carrying the allele survive
incomplete-/co-dominance changes only…
phenotypic ratios NOT genotypic ratios
complex traits
traits controlled by multiple genes and often also by environmental factors
- discrete or continuous
temperature sensitive alleles
function depends on the environmental temperature
(permissive conditions allows for allele; restrictive does not)
conditional lethal
allele that is lethal under certain conditions (permissive vs restrictive)
hypostatic gene
gene (and its genotypic effects) that is being masked by the epistatic allele
recessive epistasis
the effects of recessive alley at one gene hid the effects of alleles at another another gene
dominant epistasis
the effects of a dominant allele at one gene hide the effects of alleles at another gene
what does recessive epistasis indicate?
the dominant allele of the 2 genes function in the same pathway to achieve a common outcome
what does dominant epistasis indicate?
the dominant alleles of the 2 genes have antagonistic functions
redundant gene action
only 1 dominant allele of either of the 2 genes is necessary to produce phenotype
15:1 ratio
complementation test defintion
methods of discovering whether 2 mutations are in the same or separate genes
complementation test results
all WT (complementation occurs)
- strains had mutations in different genes
all mutant (complementation DNE)
- strains had mutations in the same gene
linked
genes whose parental allele configurations are inherited more often than not; typically located close together on the same chromosome
limitations of crosses to determine gene positions….
- difficult to determine gene order if some gene pairs lie very close together
- actual distances on map may not always add up
bombay phenotype order
no antigen (FUT 2 gene) –> H antigen (glycotransferase A or B) –> A or B (attached to H)
recessive epistasis
when recessive allele of one gene controls the expression of all alleles of the second gene.
9:3:4
dominant epistasis
when the dominant allele of one gene hides the expression of all alleles of another gene
12:3:1
complementary epistasis
need at least one dominant allele of both genes to get one phenotype and all other combinations give another phenotype
- both dominant alleles of both genes is necessary for dominant phenotype — otherwise shows recessive genotype
9:7 (dominant both: recessive either or both).
duplicate gene epistasis
whenever there is a dominant allele concealing the expression of recessive alleles at two loci
– meaning at least one dominant allele of either of the 2 genes will cause a dominant phenotype
15:1
linkage
the closeness of genes or other DNA sequences to one another on the same chromosome.
The closer two genes or sequences are to each other on a chromosome, the greater the probability that they will be inherited together
if unlinked test cross…what proportion will have certain genotype?
25%
1:1:1:1 ratio
