1.1-1.2 Flashcards
(14 cards)
1.1 Puzzle of Inheritance
mendel was the first person to combine data collection, analysis and theory in succesful prusit of basis of heredity
many years before, only g enetic pratcice was seltvie breeding on domesticated plants and animals with desirable characteristics - artifical selection
AS was helpful in creating plants etc that turned into foods and also used to make valuable herds of sheep, pigs, dogs etc
AS was limited by hit or miss nature; ppl didnt necesarily understand heredity
2 errors in intrepreting results of selective breedng were misleading
ppl first thought that one parent contributes most to offsprings inherited features and usually thought of as male by way of a fully formed homunculus inside teh sperm
another deceptive notion was concept of blended inheritence, the idea that aprental traits become mixed and forevr chainged in the offspring - but this couldnt explain differences in siblings nor the eprsitence of variation within etxended families
what mendel did diff:
first he chose teh garden pea as the organism- it has a sperm and egg producing organ allowing it to self fertalize and cross fertalize
in self fertlization the egg and pollen come from same plant and sometiems flower
to cross fertalize mendel would remove stamens from teh flower to prevent selfing and then rbushed pollen of otehr plant onto pistils of first plant
peas also have many generations in a growing season providing advantage
second he examined the inhertcen of clear-cut alternatives ofr traits; purple vs white flower, yellow vs green pea colour
he could trace unambigosly the transmision of either or traits bc no intermediate forms existed - these are known as discrete traits
continuous traits have many intermediate forms - ex. skin colour or height
third, mendel collected and perpetuated lines of peas that bred true
matings within such pure breeding or true breeding lines produce offpsing carrying specifci parental charetriscs that remain constant from gen to gen
these lines also known as inbred bc they mated w eachother for many gens
plants w white flowers always produced white flowers etc
mendel called constant but mutally exlcusive alternatives like purple vs white flowers antagonistc pairs and he settled on seven pairs of rthe study (seed colour, seed shape, flower colour, pod colour, pod shape, stem length, flower position)
mendel cross fetrlized pairs of plants to produce hybrids, offspring of genetically dissimalr parents for eahc antagonisc pair
fourth, mendel made reciprocal crosses in which he reversed the charcterics of the male and female parents, conrolling whetehr charcetrics was transmitted via egg or sperm - bc the progeny of recpircoal corsses were similar, mendel demonstarted that teh two parents contribute equally to inheritcne
fifth, mendel worked with large numebrs of plants, counted all offpsirng, subjected findings to analysis and then compared results with predictions
he studies inherticen in qunatitive manner and revealed basic laws of heredity
mendel was pratical experimentalist - he set up black and white experiment systems and figrued out how it worked
1.2 Genetic Analysis According to Mendel
(split into specific sections of 1.2)
Mendel described transmission of visible charcterics in pea plants, defines unseen but logically deduced units (genes) that determined when and how often the traits appaear and analzyes behaviour of genes in simple matehmtical terms to reveal perviosuly unsuspected principles of heredity
Monohybrid crosses reveal law of segregation
once mendel isolated pure breeding lines for charcterics he carried out series of mating b/w indivduals that differed in only one trait, such as seed color, stem length etc
in each cross, one parent had one form of the trait and the other parent had the antagonsitc trait
mendel planted pure breeding green peas and pure breeding yellow peas and allowed them to grow into parental P gen
when plants had flowered he brushed stigma of green with pollen from yellow and the reciprocal cross
he found in both cases the peas produced were all yellow
the yellow peas, progeny of p gen were the first filial F1 gen
Mendel planted them to obtain mature F1 plants that he allowed to self fertlize
expeirments involing hyrbids for a isngle trait are called monohybrid crosses
he harveted and counted peas of the second filial F2 gen, progeny of F1
the porgeny of one series of F1 self fertliztions were 6022 yellow and 2001 green F2 peas which is 3:1 yellow green ration
f1 of the reciprocal orgnal cross has a similar ratio
Reappearence of recessive characetric
the presence of green peas in the f2 gen was evidence that blending didnt occur
info remianed intact and was able to direct formation of green peas in the second filial gen
these green peas were indistibglisble from the green grandparents
mendel concluded that 2 types of yellow peas must exist: those that are true bred like P gen and those that can yield some green offpsirng like the yellow F1 hybrid
the second type contained info for the green pease
he called the yellow seeds in teh F1 hyrbid a dominant charcetrics and the green pea charcetric that was hiden in f1 but present in f2 the recessive
Genes: Discrete units of inheritence
to account for observations mendel proposed that for each trait, two copies of a unit of inhertince (gene) were received one from each parent
the pea plants had two copies of a gene for seed colour, two copies for seed shape etc
mendel proposed that each gene comes in alt forms and combinations of the alt forms determines the contrasting charcetrics he was studing
the lat forms of a single gene are called alleles
the gene for pea colour has a yellow and green alllele etc
in mendels monohybrid crosses, one allele of each gene was dominat and the otehr recesive
in p gene one parent carried two dominant alleles and the otehr had two recssive
the f1 gen hyrbids carried one domiant and one recessive allele of the egen
indivduals having two diff alleles of a single gene are monohybrids
law of segregation
mendel answered questions in terms of two mechanisms behind reproductin: gamete formation and the random union of gametes at fertalization
gametes are specilized cells- eggs adn sperm- that carry genes between gens
mendel imagined that at formation of the gametes, two copies of each gene in the parent seperatge/segregate so that each sperm and egg reveives only one allele for each trait
so each egg and each sperm receive only one allele for pea colour (y or g)
at fertalization, a sperm with on or the otehr allele units at random with egg caryng one or the other allele, restoring the two copies of the gene for each trait in the fertalized egg or zygote
if the sperm carries yellow and the egg green, result is a hyrbid yellow pea like F1 monohyrbids
if yellow carying sperm units with yellow carying egg, result is yellow pea that grows into pure breedin plant like that of P gen
if spemr carring allele for green fetrlizes egg carying green then progeny is a pure breeding green pea
mendels law of segregation encapsulates this general princple of herdity: the two alleles of each gene seperate/segrate during gamete formation, and then unite at random, one from each parent at fertlization
term segretain referens to equal segregation in which one allele of eahc gene goes to eahc gamete
note this doesnt apply to somatic cells which have two copies of a gene
Punnett square
mendel invented system of symbols that allowed him to analyze all his crosses in the same way
he designated dominant alleles with upercase A,B, or C and recessive with lowercase lettes
modern genetics adopted this convention for naming genes but the letter relates to the characterics being looked at like Y for yellow
pure breeding of parental gen are eitehr YY (yellow) or yy (green) thats why each cross between them resulted in Yy hyrbid
to visualize what happens whne Yy hyrbids self fertlize we set up a punnett square
the square provids method to track gametes produce and possible combinations
as punnet shows the first column and first row of the hrybids produces two gametes, Y and y in ratio of 1:1 so half sperm and hald egge carry Y while otehr half carry y
each box in the square reps one posisble fetrlization event
at fertlization 1/4th of porgeny will be YY, 1/4 Yy, 1/4 yY and 1/4 yy
bc the gametic source of allele for traits mendel studies had no infleunce on alleles effect the Yy and yY are equivalent meaning a 2/4 possibility/ratio
Mendels results reflect basic rules of probability
the punnett square illustrates two simple rules of probability - product and sum rule that are central to analysis of genetic crosses
these rules predict the likelihood that a particular combo of events will occur
product rule:
the product rule states that the probability of two or more idnependt events occuring together is the product of the probabilities that each event will occru by itself
with independent events: probability of event 1 and event 2 = probaility of event 1 x probability of event 2
consectuve coin tosses are indepednet: heads doesnt inc nor dec the proability of heads in the next toss
like wise tossing two coins doesnt infleucne result of either
the proabbilty of given combo is the product of their indepdnt probabilities
simialrly the formation of egg and sperm are indepdent events; in hyrbid plant, the probabilty is 1/2 that a gievn gamete will carry Y and 1/2 that itll cary y
bc fertlizaton happens at random the proability that a combo of maternal and patenral alleles will occur simluanetly in the same zygote is the product of the idnepdent probailities of these alleles being packaged in egg and sperm
to find the chance of Y egg uniting with Y spemr you need to multiply 1/2 x 1/2 and get 1/4 which is the same fraction of YY seen in punnet square
each box in the square reps an equally likely oytcome of the cross only bc each of the two types of sperm and eggs (Y and y) are rpdicued at equal frquencies
the sum rule:
while we can describe the moment of random fertlization as teh simulatnous occurnce of two independt events, we can also say that two diff fertluaton events are mutually exlcusive
for instance, if Y combines with Y it cant combine with y in the same zygote
second rule of proabilty is the sum rule which states that the probability of eitehr of two mutually exlcusive events occuring is the sum of theri individual probabilities: probability of event 1 or event 2 = probability of event 1 + probaility of event 2
to find liklihood of Yy hyrbid self fertlization will be a hyrbid like parents you add the 1/4 probability of Y with y and the 1/4 proability of Y with y and get 1/2 like the punnet square
iin anotehr ue of sum rule you can predict the ratio of yellow to green F2 progeny
the fraction of F2 peas that will be yellow is sum of 1/4 (event of YY) plus 1/4 (event of Yy) plus 1/4 (event of yY) to get 3/4 and the remaining 1/4 of F2 is green so its a 3:1 ratio
Further crosses verify law of segregation
the law of segregation was a hypothesis that explained the data from simple crosses involving monohybrid peas but mendel needed to perfom more experiments to confimr validity
mendels hypothesis made the testable prediction that the F2 should have two kinds of yellow peas YY and Yy but only one type of green - he predicted that YY and Yy yellow should be present in 1:2 ratio
to confirm this, mendel allowed self-fertalization of all the lants in F2 gen and countd the types of F3 progeny
he found that the plants that developed from F2 green peas produced green peas in F3 and green. continued to be producd - this is whats expected in pure breeding yy lines
yellow peas were diff - 1/3 were pure breeding yellow and 2/3 were hyrbids bc they gave yellow and green peas
so YY to Yy ratio was 1:2
it took mendel many years with his experiments but he conled that segregation of dominant and recessive alleles during gamete formation and theri random union at fertlization could explain the ratios
but it raisd the question - how can you know if its a pure breeding dominant or a hybrid
for self fertilizin plants the answer is to observe next gen but its diff for non self fertlziing
Testcrosses: establishing genotype
Observable characetric is a phenotype while pair of allleles present in indivdual is genotype
YY or yy is homozygous while genotype with two diff alleles for the trait is heterozygous or hybrid for the trait
indivdual w homozygous genotype is homozygote while one with heterozygous genotype is a heterozygote
the phenotype of hertozygote defines which allele is dominant - if u know the genotype and the dominance relation of alleles you can predict the phenotype but the reverse isnt true bc some phenotypes can dervie from mutliple pheontypes
we can look at the method mendel devised for deciphering the unkown genotype responible for dominant phenotype
ex. Y-; the method testcross is a mating in which an indidual showing the dominant phenotype is crosed with indivdual with recesisve phenotype like yy - if the dominat phenotype derives from a homozygous genotype all offpsirng will show dominant yellow phenotype but if dominat parent ofu unknwn genotype is hyrbid there should be green and yellow
mendels cocnpet of the gene and law of segregation can be genrlized to almost all sexually reproducing organisms
Dihybrid crosses reveal the law of indepent assortment
having determined from monohyrbid corsses that genes are inherited accordng to the law of segregation, mendel looked at simulatnous inehrticn of two unrelated traits in peas
he aksed how two pairs of alleles can segretae in a dihybrid inividual
to contruct such a dihybrid, mendel mated true-breeding plants grown from yellow round peas with tue breeding plants rown from green wrinkled peas - he obrtained the dihybrid F1 gen (YyRr) showing the two dominant phenotypes
then he allowd them to self-fertlize to produce the F2 gen
would all the F2 progeny be parental types thta looked either like the orginal or would their be new combos - the new phenotypic combos like these are called recombinant types
when mendel counted the F2 gen - he found the shuffling of alleles of diff genes did occur - for ex. yellow wrinkled or green round
law of indepdent assortment:
from the observed ratios, mendel inffered the biologila mehcnaism of that shuffling - the indepdent assortment of gene pairs during gamete formation
the gens of pea colour and shape assort idnepdently (one allele of a gene doesnt predict teh allele of a diff gene)
each dihyrbid of the F1 gen can form 4 types of gametes: YR, Yr, yR, and yr
at fetrlization then, in a mating of dihybrids, 4 diff kinds of eggs can combine with any 1 of 4 diff kinds of sperm producing 16 zygotes
punnet square shows this
each of the 16 box reps an equally likley fertlziation event
each box is an equally likely outcome onlybc each of the four diff gamete types is produced at equal frequency in each parent - therefore suing the product rule, the frquency of progeny type in each box is 1/4 x 1/4 - 1/16
some of the 16 allele combos are identifcal bc of the source of the allele doesnt make it any diff
theres only 4 phenotype poissibilities in a ratio of 9:3:3:1
but the idnidual trait is still inhertied in a 3:1 ratio
the ratio of each dominat charcteric to its antagonisc recessive chareic is 12:4 or 3:1
so inherticna of gene colour is unaffected by inherticane of other gene and vice versa
the preceding analysis became the basis of mendels second general gentic principle - the law of indepdent assortment: during gamete formation, diff pairs of alleles segregate idnepdently of each other
the indepdence of their segregtaion and the subseqeunct random union of gamtes at fetrlization detrmine the phenotypes observed
using the product rule ofr assesing proability of indepdent events, you can see the ratio observed in dihybrid cross dervies from two seperate 3:1 phenotypic ratios
if the two sets of alleles assort idnepently, yellow to green ratio n the F2 gen would be 3/4:1/4 and liklewise for round to wirnkled - you can use these proabilityvalues and product rule to get ratio of the dihyrbid cross (ratio of 3/4 and 1/4 is dihybrid cross of one gene)
branches line diagrams:
a convinent way to keep track of the proabilites of each potential outcome in a gentic cross is to cosntruct a branched line diagram which shows all the possible genotypes of phenotypes for each gene in a seuence of columns
the first column can be the two psosible pea colour pheontypes and the second is that each pea colour can occur with eitehr of two pea shapes and thenagain the 9:3:3:1 ratio becomes apparent
testcrosses with dihybrids:
understanding of dihyrbid crosses has many apps
one answer is to cross the two of pure breeding strains to generate a dihyrbid and then self cross dihyrbid and the yellow round peas - only one out of 9 such phonegy would be approate to get the yellow round
to find these plants you could subject each yellow round candiate to a test cross witha green wrinkled
if testcross yeils all yellow round offspring then you can sell the test plant bc its homozygous for both traits
if the cross yields 1/2 yellow round and 1/2 yellow wrinkeld etc then uou know it is homozygous for one trait and getrozygous for the other
if the cross yeilds 1/4 yllow round and 1/4 yellow wrinkeld and 1/4 green round and 1/4 green wirnkled then plant is hetrozygote for both traits
Mendels laws predict probabilities not specific outcomes
mendelian analysis makes accurate predictions abt the offpsring of complex multihybrid crosses: matings between the f1 progeny of pure breeding parents that differ in three or more traits
suppose you want to predict the occuranc of one specific genotype in a cross of indepnt assorting alleles of several genes
ex. if hyrbids of heterozygous for four geens self fertlize - Aa Bb Cc Dd - what proposution of progeny has AA bb Cc Dd
2 ^ 4 (4 traits) = 16 so 16*16 is 256 boxes - too much for punnet square
can anazye by breaking down the multihybrid cross into four indepdent asorting monhybrid crosses
1:2:1 ratio for monohyrbid cross of the heterozygous trait
so probaility is found by multiply proability of each indepdent event: AA (1/4 of progeny proeuced by Aa x Aa); bb (1/4); Cc (2/4) and Dd (2/4)
so multiply all and you get 1/64 probability
if instead of genotype you want the proability of certin phenotype you need product rule but need to know phenotpic ratios produced by each pair of alleles in the cross
ex. in multihyrbid cross of the mix you want to know how many show dominant A charcterics (genotyoe AA or Aa = 1/4 + 2/4 = 3/4), teh recesibe b charcetric (bb = 1/4), domiant C charcteric (CC or Cc = 3/4) and dominat D (3/4) so multiply all and get 27/256
punnet squares are great for visualizing some genes but when more than two genes are used clculations of probability are useful for chances of one or a few outcomes but if all outcomes are wanted to know - a branched line diagram is best
mendel realized that bc of the effects of chance, his laws only make good predictions when examining large number of plants
the larger number of trials, the lower proability that chance skews the data
Mendels genuis was unapreciated
he showed the existence of genes - indepdent units thatd etrmine observale patterns of inehrtance for traits
he showed reapprence of hiden traits, disproved blended inheritce, showed that parent makes equal contirbution to next gen
Recessive alleles are most often nonfunctional whle domaint alleles are usually functional
most genes specfiy the proetins that dictate the structure and function of cells
recently two genes were idenifed that are likely to correspond to mendels genes for seed shape and seed colour
the
pea shape gene specifies enyzme sbe1
sbe1 ctalyzes conversion of amylose to amylopetcin
dominant R allele of pea shape gene detrmines normal fucntioning sbe1
ithe recessive r allele correspond to no sbe1 enzyme
but the single domannt allele in Rr herterozytes is enough that they dont lose water and has nromal sbe1 actvity
pea colour gene detemrines enzyme sgr
sgr perfoms one step in the pathway leading to the breakdown of the green pigrment chrolsphyll
a process that occurs natrually in peas as they mature
the dominat U species sgr and recessive y doesnt
homozygous yy stay green bc no sgr and chrlsophyll remains
- a specific gene detrmines a specific protein - activty of the proetin may affetc the phenotype of the plant in many ways dependng on the biochm pathway it functions
- the dominant allele detrmines annromally functioning protein while recessibe protein doesnt speficy a functinal protein - usually common but not the only explantion for why one allele is dominat to another allele
