A true-breeding strain of Virginia tobacco has dominant...

A true-breeding strain of Virginia tobacco has dominant alleles determining leaf morphology (M), leaf color (C), and leaf size ( $S$ ). A Carolina strain is homozygous for the recessive alleles of these three genes. These genes are found on the same chromosome as follows: An $F_1$ hybrid between the two strains is now backcrossed to the Carolina strain. Assuming no interference: a. What proportion of the backcross progeny will resemble the Virginia strain for all three traits? b. What proportion of the backcross progeny will resemble the Carolina strain for all three traits? c. What proportion of the backeross progeny will have the leaf morphology and leaf size of the Virginia strain but the leaf color of the Carolina strain? d. What proportion of the backcross progeny will have the leaf morphology and leaf color of the Virginia strain but the leaf size of the Carolina strain?

A true-breeding strain of Virginia tobacco has dominant alleles determining leaf morphology (M), leaf color (C), and leaf size ( $S$ ). A Carolina strain is homozygous for the recessive alleles of these three genes. These genes are found on the same chromosome as follows:
An $F_1$ hybrid between the two strains is now backcrossed to the Carolina strain. Assuming no interference:
a. What proportion of the backcross progeny will resemble the Virginia strain for all three traits?
b. What proportion of the backcross progeny will resemble the Carolina strain for all three traits?
c. What proportion of the backeross progeny will have the leaf morphology and leaf size of the Virginia strain but the leaf color of the Carolina strain?
d. What proportion of the backcross progeny will have the leaf morphology and leaf color of the Virginia strain but the leaf size of the Carolina strain?

Key Concepts

Dominance and True?Breeding Strains

This concept involves understanding that certain alleles mask the effects of others, where a true?breeding strain has homozygous alleles that consistently produce a phenotype. In genetics, dominant alleles determine the phenotype in hybrids when paired with recessive alleles, which appear only when an organism is homozygous recessive.

Genetic Linkage

Genetic linkage refers to genes located close to each other on the same chromosome that tend to be inherited together. This concept is key because linked genes do not assort independently, affecting the outcomes of crosses such as backcrosses, where the parental and recombinant types are influenced by the physical distance between genes.

Recombination Frequency

Recombination frequency measures the likelihood of a crossover event between two linked genes and is usually expressed in map units or centimorgans. It quantifies how often recombinants occur during meiosis, which directly impacts the proportions of various phenotypes in progeny from crosses involving linked genes.

Backcross Analysis

A backcross involves breeding a hybrid organism with one of its parents or a genetically similar individual, often a homozygous recessive parent. This type of cross is useful for analyzing recombination events, mapping gene order, and determining how linked genes are transmitted, especially when evaluating phenotypic ratios influenced by linkage and recombination.

Interference and the Assumption of No Interference

Interference is the phenomenon where the occurrence of one crossover event affects the probability of another nearby crossover. Assuming no interference means that crossover events occur independently of each other. This assumption simplifies the calculation of recombinant frequencies and the expected proportion of progeny with specific combinations of linked traits.

Transcript

00:01 So a true breeding strain of virginia tobacco has dominant allele determining leaf morphology mc and s capital letter.

00:10 A carilata strain is homozygous for the recessive allele of these three genes.

00:14 Now these three genes are found on the same chromosome as follows.

00:18 You can see that m and c has six map unit apart and c and s has 17 map unit apart.

00:27 So an f1 hybrid between two strains is back crossed to the carilata strain which means homozygous recessive.

00:34 Assuming no interference which means the crossover of two one region does not affect the crossover of the other region.

00:41 What proportion of the back cross progeny will resemble the virginia strain for all three traits? so let's take a look.

00:53 So as you can see the f1 must have m heterozygous, c heterozygous and s heterozygous and back crossed with homozygous lower m, lower c and lower s.

01:10 So if this region are linked so if there's no crossover between the two genes mc and cs you'll have parental mc i guess i would say cs dominant versus lower m, c and s recessive.

01:51 This is one of the progeny or the other case you have mcs lowercase with and then the other chromosome is mcs lowercase.

02:08 I'm going to use different color just to make sure.

02:23 So you can see that the progeny inherit one chromosome or one chromosome from each of the parent.

02:35 So we can see that this is called parental.

02:40 Parental progeny has the same phenotype as the parents and they are one to one ratio.

02:52 Now the other kind is that you have crossover.

02:57 So in that case you have crossover between m and c or c and s.

03:03 So you will have produced different kind of combination of the progeny but they are not the same as the parental.

03:13 So what we know is that there is six map unit between m and c and between c and s will have 17 map unit.

03:25 So the map unit or distance between the genes are actually calculated by the number of recombinant progeny.

03:46 So by looking at the crossover you can see that the crossover between m and c is six map unit.

03:54 So this means that between m and c there is six percent recombinant progeny.

04:13 This includes single crossover and double crossover.

04:19 And between c and s there are 17 map unit.

04:24 This means there are 17 percent recombinant progeny that has a crossover between c and s.

04:38 So they add up to a total of six plus 17 which is 23 percent of the recombinant progeny in total.

04:55 Then you also want to subtract the calculated double crossover because you add it twice which is six percent times 17 percent.

05:13 0 .06 times 0 .17 so which is 0 .01 or one percent.

05:28 So that means you add the one percent double crossover twice when you calculate a recombinant progeny in the map distance.

05:36 So the actual recombinant progeny is going to be 23 percent minus one percent.

05:53 Again the recombinant progeny is single crossover between m and c plus double crossover.

06:00 So you can see that is why we have to subtract double crossover because when we calculate the map unit or distance we add a dco twice.

06:11 So you get a total of 22 percent recombinant progeny.

06:19 So this means that the parental is going to be 100 percent minus recombinant progeny which is 100 percent minus 22 percent.

06:50 This will give you 78 percent.

06:54 So now if we go back to our first one.

06:56 So the parental now we have known that these two add up to 78 percent.

07:05 So 78 percent of chance the f1 cross is going to produce a parental progeny.

07:13 And again these two progeny has a one to one ratio.

07:19 So each has half of 78.

07:28 39 percent.

07:30 So 39 percent will have capital m, c, s which is the virginia tobacco.

07:38 And the other 39 percent is going to be homozygous recessive m, c and s which will be the same as carolina strain.

07:48 So the question asked about what portion of back cross will represent the virginia strain.

07:54 It's going to be 39 percent.

07:58 This is the virginia and this is carolina.

08:11 So again 39 percent of the progeny which are parental will be same as the virginia.

08:19 So that's the question number that the first question...

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