A pure-breeding tall plant producing oval fruit as described i...

Question

A pure-breeding tall plant producing oval fruit as described in Problem 2 is crossed to a pure-breeding short plant producing round fruit.

If the F₁ identified in part (a) are crossed to one another, what proportion of the F₂ are expected to be short and produce round fruit? What proportion are expected to be tall and produce round fruit?

Accepted Answer

Hi, everyone. Let's look at our next question. It says, what is the percentage of offspring with phenotype? Dwarf plant with yellow fruit? If a plant with Big R little R, big T little T genotype is crossed with a plant that is little r, little R, little T little T in which the red fruit, big R is dominant over yellow fruit, little R and tallness. Big tea is dominant over dwarf ness little tea. And our answer choices are a 50% will be dwarf plants with yellow fruit. B 25% will be dwarf plants with yellow fruit. C 75% will be dwarf plants with yellow fruit or d all the offspring will be dwarf plants with yellow fruit. So we're looking for the percent of the offspring that are dwarfed with yellow fruit. So that would be homozygous recessive little R, little r, little T little T since dwarf and yellow are both the recessive traits. The cross we have here is the heterozygous parent in both traits crossed with a homozygous recessive parent. So essentially a test cross. Now, since these genes independently assort to look at the percent of the offspring or probability of the offspring having those traits, we can get the product or use the product of the probabilities of each gene having that outcome. So for the color, we can look at what the cross. Big R, little R cross with little R, little R is and say with that cross, what's the probability of having the homozygous recessive offspring that plant with the yellow fruit? Well, when you have a cross like this, the homozygous recessive parent can only give that recessive gene. Where is the heterozygous parent? Half the time gives the dominant all half the time gives the recessive. So that means that half the offspring of this cross are heterozygous and half the offspring are homozygous recessive. You'll have the exact same result looking at big T little D cross with little T little T, half the offspring heterozygous, half the offspring homozygous recessive. So the probability of the offspring being little R, little R, little T, little T is the product of the two probabilities. So one half the probability of being little R little, R multiplied by one half, the probability of being little T little T that gives us the probability of both of those being true, which is 1/4 and as a percent that is of course, 25%. So back to our answer choices that will be choice B 25% will be dwarf plants with yellow fruit. See you in the next video

Key Concepts

Mendelian Genetics

Punnett Square

Phenotypic Ratios

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