BIOL 226 MIDTERM Flashcards
Define and explain what P0 is
P0 is the parental cross
will be homozygotic for traits
known as true breeding
Define and explain F1
First generation of offspring from P0, will be all heterozygotic
shows the dominant phenotypes
phenotypes will not equal genotypes
F1 generation suggests dominance
Define and explain F2
Second generation of offspring/ hybrids
will show all possible phenotypes and can infer all genotypes
F2 generation shows evidence for the independent segregation of alleles
What is a test cross?
Used to determine an unknown genotype by crossing with a homozygous recessive
genotypic and phenotypic frequencies will be identical
Define and explain the first mendelian law
The law of segregation –> alleles of a gene separate independently from each other during transmission from parent to offspring
In a basic monohybrid cross the dominant phenotype will appear 100% of the time in the F1
F2 will show a 3:1 ratio for phenotype and a 1:2:1 ratio for genotype
P0:(AAxaa) = F1(all Aa) F2(1AA, 2Aa, 1aa)
In a dihybrid cross of F1 will show 9331 in F2
Probability of independent events rule
Multiplicitive rule
If a and b are are independent p(a and B) = Pa x Pb
Additive rule
if a and b are independent probability of only one occurring p(a or b) is
Pa + Pb -(Pa xPb)
However, if two events do NOT overlap they are mutually exclusive
Pa x Pb = 0 and P(a or b) = Pa + Pb
Define hetero and homozygote
Heterozygote –> two different alleles at the same locus
Homozygote –> two copies of the same allele at the same locus
Define and explain the second mendelian law
Law of independent assortment
Alleles of two or more genes (loci) segregate independently during transmission from parent to offspring
the two dominant phenotypes appear at 100% in F1
In F2, 4 phenotypes are present —> two parental, two recombinant
expected frequency of phenotypes is 9331
ex) if YyRr x YyRr would have 3/4 Y, 1/4y , 3/4R, 1/4r = 3/4Y x 3/4R = 9/16 YR
Explain how to calculate the # of phenotypes for nth hybrid crosses
dihybrid - 2 loci = 2^2 =4 –> 9:3:3:1
trihybrid - 3 loci = 2^3 =8 –> 27:9:9:9:3:3:3:1
How many gametes (egg/sperm = haploid) can form from x genotype?
total possible allele combinations = 2^n where n= number of segregating loci
segregating being key term here, if homozygous for any allele that allele effectively doesn’t segregate because there is only one possible outcome
wtf is a chi^2
Value testing to determine if data is consistent
The sum of observed - expected^2 / expected
Then take the df (#of categories -1)
If the p value is larger,than 0.05 accept the null hypothesis, the data is not strange and makes sense, there is no significant difference
However if p value is smaller than 0.05 then there IS a significant difference and the data cannot be explained by the null hypothesis (like cannot be explained by mendelian cross etc/)
Explain sex ratios in drosophila
Sex determination is based on the ratio of x chromosomes to sets of autosomal (A) chromosomes
if X/A = 1 then female
if X/A = 0.5 then male
In between 1 and 0.5 is intersex
greater than one is metafemale
less than 0.5 is meta male
How can you tell if a trait is x linked recessive?
Nearly all affected people are male
all his daughters are carriers, no sons will be affected
Carrier females are phenotypically normal but heterozygous for the allele
half her sons are affected
half her daughters are carriers
all sons of an affected female will be affected
not always seen in every generation
How can you tell if a trait is x linked dominant?
affected males transmit the trait to all of their daughters but none of their sons
affected heterozygous females transmit the trait to half of their children regardless of sex (like autosomal dominant)
affected homozygous females transmit the trait to all of their children
because females can be hetero or homo, more females than males will have the trait
unaffected individuals cannot be carriers
define Karyotype
A collection of chromosomes with specific number and structure of a species / individual in a condensed state
Define diploid
2n
normal for most eukaryotes
Define Monoploid
1n
the number of unique chromosomes in a set
Define Haploid
1n
only one set of chromosomes - no homologous chromosomes
male bees, gametes
Define Euploid
having a ‘normal’ number of each chromosome
Define polyploid
having extra copies of each chromosome
common in plants and certain tissues
causes larger cells and increases cellular metabolism
Triploid = 3n
Only possible because gene dosage is preserved
Plants look the same but polyploid are bigger - same number of cells, but each cell is bigger
Lowers chances of inbreeding risk - buffered against deleterious alleles
Lessen selection on individual gene copies
Associated with asexual reproduction
Some tissues
Liver
Heart
bone marrow
Define allo and autopolyplopid
Allopolyploid – mixing two uneven genomes via hybridization
combination of alloploidy and chromosome loss or gain creates new species
Usually sterile
Autopolyploid – multiplication of chromosomes from the same organism
Bigger plants = bigger fruits
Define Aneuploid
does not equal 2n – abnormal number of one or more chromosomes but not all
Caused by nondisjunction - homologs don’t separate
Monosomic –> one missing chromosome - 2n-1
XO sex determination – bees, bats, snails
All lethal in humans except XO (turner syndrome)
Trisomic – one extra chromosome compared to wold type - 2n+1
Most are lethal in humans except
Kleinfelter syndrome (XXY)
Jacob syndrome (XYY)
Trisomy X (XXX)
Trisomy 21 - Down syndrome
Define nondisjunction
Gametes have an incorrect number of chromosomes (either carrying both homologs or none)
Can happen in meiosis1 or 2
Rare and often lethal
Caused by lack of crossing over in meiosis 1
Caused by defect in chromosome cohesion in meiosis 2 – sister chromatids don’t separate
What is the difference between intraallelic and interallelic effects?
Intraallelic effects are the affects of different alleles on the proteins produced
Interallelic affects are how different proteins interact
Define pseudodominance
If chromosome deletion removes haplosufficient genes, any homologous haploinsufficient genes will have a phenotypic effect
Pseudodominance is when recessive alleles uncovered by the deletion will appear dominant in pedigree
Define hypomorphic
An allele in the middle of the dominance hierarchy of multiple allels
Has partial function
Define incomplete dominance
Appearance of a third phenotype that blends the parental phenotypes
no clear dominance in the heterozygote
new phenotype not present in parents
Non mendelian relationship
skin pigmentation in humans
intraallelic
Define codominance
More than one allele is dominant
Heterozygote displays both parental phenotypes
variegation
Non mendelian relationship
found in ABO blood type system
intraallelic
Define pleiotropy
one allele that affects two or more phenotypes
Example of coat color in mice, expected ratio is 3 yellow - 1 brown (yellow dominant over WT) but is actually 2-1 because AyAy is lethal, because gene codes for both fur color and lipid metabolism
This resulted due to chimerism where a piece of another gene was merged with the WT coat color gene
Explain variable penetrance and expressivity
Variable penetrance is when individuals with the same genotype might not always express the phenotype, same genotype, different phenotypes
Polydactyly - 50-80% of people with dominant mutant allele develop malformations
Determined by modifier genes, environmental factors and complex interactions
All or none
Variable expressivity is the degree or intensity with which a genotype is expressed – all individuals express the mutation but the ‘severity’ is variable
ex Marfan syndrome
All or some
Can be both
All, some, or none
Define polygenic inheritance
Where many genes / alleles affect the same phenotype
Interallelic affect
Explain the complementation test
crossing homozygous recessive mutants together to hide mutations and show wild type
Used to determine which alleles causing the phenotype are on the same gene
If cross between two homozygous recessive mutants does not result in the WT and F1 are parental then there is no complementation, meaning the mutations are on the same gene – monogenic, allelic mutation
opposite will be polygenic, F1 is WT, complementation, nonallelic mutation
Often deals with the lack of production of substrates or enzymes in an earlier step
Define additive gene action
alleles of two genes generate four phenotypes in a single trait
2 loci, 1 trait, 4 phenotypes
ex a 2 colour snake, can have both pigments present, one or the other, or neither
Cannot be codominance because there is four phenotypes not 3
Non mendelian
Still produces 9331 ratio in phenotypes, but only deals with one trait instead of 2 (dihybrid)
Define complimentary gene action
2 loci, 1 trait, 2 phenotypes
homozygosity in mutations of either gene will lead to an identical mutant phenotype
Has a modified phenotypic ratio of 9:7
Define redundant / duplicate gene action
2 loci, 1 trait, 2 phenotypes
Only two genes
Mutant only appears ion double homozygotes aabb 1/16
Dominant alleles of both genes overpower recessive alleles
modified ratio of 15:1
called pseudoalleles
Overlapping roles for two genes which independently can make up for the absence of function of the other
Define epistasis
the phenotype of a gene is masked by alleles of a separate gene
One gene overrides the phenotypic effect of an allele of another gene when both are present in the same phenotype
The phenotype that is present over the other is epistatic to the other
the phenotype that is hidden is hypostatic to the other
interallelic
different locus (unlike standard dominance which is at the same locus)
Recessive epistasis – when the recessive allele of one gene masks the effect of the second gene (either dominant or recessive)
Dominant epistasis – when the dominant allele of one gene masks the effect of the second gene (dominant or recessive)
Explain recessive epistasis - supplementary interaction
a recessive allele of a gene overpowers the effect of an allele of a second locus
2 loci, 1 trait, 3 phenotypes
Modified ratio of 9:3:4
Any genotype with a homozygous recessive epistatic gene shows the epistatic phenotype but a dominant copy of epistatic gene allows the phenotype of the hypostatic gene to appear – in coat color, think of the double recessive allele as blocking any pigment at all, resulting in yellow, but if its not double recessive than the other two colours can show through
Explain dominant epistasis
The dominant allele of one gene hides the effect of the other gene
2 loci, 1 trait, 3 phenotypes
Modified ratio of 12:3:1
What is crossing over and where does it occur?
Crossing over is the cellular mechanism that generates recombination
Chiasmata are the sites of crossing over
Happens in the prophase of meiosis 1
half parental gametes, half recombinant
What is recombination frequency?
The distance between two markers on a chromosome is directly related to how often crossover events occur between them
= the percentage of offspring that is recombinant
Better to use a test cross, less of an underestimation of recombination
Max Rf is 50%, because the likelihood for crossing over is 50%
Min RF is 0%
For finding the trihybrid RF, least common genotypes are recombinant double crossover, the gene that is ‘switched’ compared to the parental(F1) will be the middle gene, because of this, multiply the RF X2 for the gene pair that DOES NOT include the middle gene
**Genes further apart are more likely to recombine
Quick way to tell if genes are linked or unlinked?
Look at which non parental genotypes are common, then look for the two genes that “move” together
What are Barr Bodies?
dense chromatin that acts as a marker for silenced X chromosome in cells, have to have equal gene dosage across sex chromosomes, for humans, male X cells is always active, whereas one of the X in female cells will be silenced so both sexes contain the same amount of X chromosome –> bc they are already compensating, makes trisomies or monosomies less deleterious in sex chromosomes
SRY gene –> without SRY female is default
when is multiplication rule used, when is addition?
Multiplication for independent events
Addition for mutually exclusive events
Define syntenic genes
syntenic genes are genes on the same chromosome
Linked genes are always syntenic
Max rf of syntenic genes is 50%, but they can be further apart
Define sister chromatids
Have identical copies
Same genes in same order
Same alleles in same order
Define homologous chromosomes
Same chromosomes from different parents
Have genes in same order
Homozygous - same alleles in same order
Heterozygous - different alleles in same order
Note that an allele is a different version of the same gene
How does a deletion occur
Repair errors after breakage
How does a translocation occur?
Caused by a recombination mistake
Crossover between repeated sequences that are not homologous
3 kinds
Non viable - lacks correct number of centromeres or arrangement of telomeres - nonfunctional
Balanced - reciprocal - does not change gene or centromere number can often go undetected
Unbalanced - non reciprocal - change in gene or centromere mumber
How does gene duplication occur?
Naturally, creates duplicates for evolution to act on
What happens when genes are inverted?
Depends on breakpoint of inverted section — if genes are truncated or not
Inversions can distrupt chromosome pairing in meiosis
