1. A true breeding tall pea line is crossed with a true breeding short line. The
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Question
1. A true breeding tall pea line is crossed with a true breeding short line. The F1 plants are all short. The F1 are crossed with themselves to produce an F2. If the difference (tall vs. short) is due to a single locus, what proportion of the F2 individuals do you expect to be tall?
2. Imagine that a researcher crossed the peas in the example above, and counted 100 F2. 86 were short. 14 were tall. Do a chisquare test to see whether the results conform to a single locus cross with the allele coding for short plants dominant to the allele coding for tall plants.
3. A clematis from a true-breeding line with white flowers and short corollas (line A) is crossed to a clematis from a true-breeding line with deep red flowers and long corollas (line B). The resulting offspring, the F1, are all pink-flowered, with corollas of medium length. An individual from the F1 is crossed to an individual from line A. The resulting F2 offspring are as follows: 25% white flowers and short corollas, 25% white flowers with medium corollas, 25% pink flowers with short corollas, 25% pink flowers with medium corollas.
What is the most likely genetic mechanism determining flower color and corolla length? If a plant from the F1 listed above were selfed (crossed with itself), what proportion of the F2 offspring would you expect to be white flowered with short corollas?
4. A man and a woman walk into a genetic counselor’s office. They have just had a child diagnosed with situs inversus, a complete transposition of the internal organs from left to right, and vice versus. This condition usually produces no obvious symptoms. It is caused by an autosomal recessive allele. What is the chance that their next child will have the same condition? What might this suggest about the parents?
Another man and woman walk into the same office. The father is an achondroplasic dwarf. The mother is of normal stature. (Look up achondroplasia, it is a lethal dominant). What is the chance that their first child will be a dwarf?
Please show work! (:
Explanation / Answer
1. If the trait is due to a single locus, the proportion of F2 progeny which are tall would be 25% or 1/4 of the number of progeny.
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2. chi square = sum[ (Observed-expected)2 /Expected value] + [(Observed-expected)2 /Expected value]
= (14-25)2 / 25 + (86-75)2 /75
= 121/25 + 121/75
= 4.8 + 1.6
= 6.4
The degree of freedom would be 2-1 = 1. Checking in the probability values table, the value is 3.84. Since the value we got is greater, the null hypothesis is rejected.
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3. Taking red as dominant and long as dominant:
line A : rrll
line B- RRLL
F1 - RrLl -pink and medium length
This combination of both characters of parents indicate incomplete dominance in both the characteristics.
F1 is crossed with line A : RrLl x rrll
Gives 4 categories of 25% each.
RrLl - pink n medium
Rrll - pink and short
rrLl - white n medium
rrll - white n short
IF F1 is self crossed - RrLl x RrLl, this cross is the regular dihybrid cross with 9:3:3:1 ratio. Hence the proportion of the progeny with white and short corollas with genotype of rrll would be 1/16.
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4. Since the condition is autosomally recessive it indicates that the child has 2 defective alleles. This indicates that both the parents are carriers of the defective gene.The chances that the next child will have the same disease is 25%.
Crossing Ss x Ss gives SS, Ss, Ss and ss. The last category of ss gives the disease. The Ss is genotype for carriers. The SS genotype are the normal people.
Since achondroplasia is dominant in a heterozygote but lethal in the homozygous condition, the father is heterozygous. Since mother is normal, the cross would be:
NN x Nn
The progeny would be: 50% NN and 50% Nn.
Hence the child will have a 50% chance of being a dwarf. But there wont be any lethality since the mother is normal.
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