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Chapter 15: Q1ITD (page 294)

The results in the data table are from a simulated F1 dihybrid test cross. The hypothesis that the two genes are unlinked predicts that the offspring phenotypic ratio will be 1:1:1:1. Using this ratio, calculate the expected number of each phenotype out of the 900 total offspring, and enter the values in that data table.

Short Answer

Expert verified

F1test hybrid is obtained from the crossing made between the cosmos plants. The expected number of each phenotype, out of the 900 offspring, is 225.

Step by step solution

01

Genes and their inheritance

Gene is the basic genetic material found in every organism. There are two types of alleles present in it such as linked alleles and unlinked alleles. Linked alleles are present in the chromosomes at a closer distance within a gene, and unlinked alleles are found in the different chromosomes.

02

Calculation of the expected from the given conditions

The expected number of unlinked genes is calculated as follows:

Total offspring\( = 900\)

Offspring phenotypic ratio\( = 1:1:1:1\)

Since the ratio equals the four different offspring, the total value divided by four will give the final expected number.

Expected number\( = \frac{{total{\rm{ }}number}}{{no.{\rm{ }}of{\rm{ }}offsprings}} = \frac{{900}}{4} = 225\)

The expected number of each offspring is 225.

03

Tabulation with the expected number results

Offspring from test cross of AaBb(F1)* aabb

Purple stem/short petals(A-B-)

Green stem/short petals (aaB-)

Purple stem/long petals(A-bb)

Green stem/ long petals(aabb)

Expected ratio if the genes are unlinked

1

1

1

1

Expected number of offspring (of 900)

225

225

225

225

The observed number of offspring (of 900)

220

210

231

239

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Most popular questions from this chapter

Using the information from problem 4, scientists do a further testcross using a heterozygote for height and nose morphology. The offspring are tall upturned snout, 40; dwarf upturned snout, 9; dwarf downturned snout, 42; tall downturned snout, 9. Calculate the recombination frequency from these data, and then use your answer from problem 4 to determine the correct order of the three linked genes.

Two genes of a flower, one controlling blue (B) versus white (b) petals and the other controlling round (R) versus oval (r) stamens, are linked and are 10 map units apart. You cross a homozygous blue oval plant with a homozygous white round plant. The resulting F1 progeny are crossed with homozygous white oval plants, and 1,000 offspring plants are obtained. How many plants of each of the four phenotypes do you expect?

The\({\chi ^2}\)value means nothing on its own- it is used to find the probability that, assuming the hypothesis is true, the observed data set could have resulted from random fluctuations. A low probability suggests that the observed data are consistent with the hypothesis, and thus the hypothesis should be rejected, A standard cutoff point used by biologists is a probability of 0.05(5%). If the probability corresponding to the\({\chi ^2}\)value is 0.05or considered statistically significant, the hypothesis (that the genes are unlinked) should be rejected. If the probability is above 0.05, the results are not statistically significant: the observed data are consistent with the hypothesis.

To find the probability, locate your\({\chi ^2}\)value in the\({\chi ^2}\)Distribution table in Appendix F. The “degree of freedom” (pdf) of your data set is the number of categories (here,4 phenotypes), minus 1, so df=3.

(a). Determines which values on the df =3 line of the table your calculated\({\chi ^2}\)value lies between.

(b). The column headings for these values show the probability range for your\({\chi ^2}\)number. Based on whether there is non-significant (p\( \le \)0.05) or significant (p>0.05) difference between the observed and expected values, are the data consistent with the hypothesis that the two genes are unlinked and assorting independently, or is there enough evidence to reject this hypothesis?

Pseudohypertrophic muscular dystrophy is an inherited disorder that causes gradual deterioration of the muscles. It is seen almost exclusively in boys born to apparently unaffected parents and usually results in death in the early teens. Is this disorder caused by a dominant or a recessive allele? Is its inheritance sex-linked or autosomal? How do you know? Explain why this disorder is almost never seen in girls.

Assume that genes A and B are on the same chromosome and are 50 map units apart. An animal heterozygous at both loci is crossed with one that is homozygous recessive at both loci. What percentage of the offspring will show recombinant phenotypes resulting from crossover? Without knowing these genes are on the same chromosome, how would you interpret the results of this cross?
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