Three dominant traits of corn seedlings, tunicate seed (...

Question

Three dominant traits of corn seedlings, tunicate seed (T-), glossy appearance (G-), and liguled stem (L-), are studied along with their recessive counterparts, nontunicate (tt), nonglossy (gg), and liguleless (ll). A trihybrid plant with the three dominant traits is crossed to a nontunicate, nonglossy, liguleless plant. Kernels on ears of progeny plants are scored for the traits, with the following results:

Phenotype Number
Tunicate, glossy, liguled 102
Tunicate, glossy, liguleless 106
Tunicate, nonglossy, liguled 18
Tunicate, nonglossy, liguleless 20
Nontunicate, glossy, liguled 22
Nontunicate, glossy, liguleless 23
Nontunicate, nonglossy, liguled 99
Nontunicate, nonglossy, liguleless 110
500

Using the gene symbols given above, write the genotypes of F₁ and F₂ plants.

Table showing phenotypes and counts of corn seedlings in genetics study.

Accepted Answer

Hey, everyone. Let's take a look at this question together. Assuming we have three linked genes for a specific plant. The allele for purple flowers are dominant over the white flowers. The allele for tall plants is dominant over short plants and the allele for smooth stems is dominant over the hairy stems. A test cross is performed by crossing a plant with heterozygous P, heterozygous T and heterozygous S genotype with the true breeding plant with homozygous recessive P homozygous recessive T and homozygous recessive S genotype. They yield the following progeny. What are the genotypes of the offspring? That have a double crossover allele combination. Is it answer choice. A homozygous dominant, P. Homozygous dominant T, homozygous dominant S and homozygous recessive P. Homozygous recessive T homozygous recessive S is it answer choice? B heterozygous P, heterozygous T, heterozygous S and Homozygous recessive P. Homozygous recessive T homozygous recessive S is it answer choice? C homozygous dominant P, heterozygous T, heterozygous S and heterozygous P. Homozygous dominant T and heterozygous S or answer choice. D heterozygous P. Homozygous recessive T, homozygous recessive S and homozygous recessive P, heterozygous T and heterozygous S let's work this problem out together. To try to figure out which of the following genotypes represent the offspring that have a double crossover allele combination. So we know that from the test cross, that was performed, the offspring that have the parental alleles can be distinguished since they are the ones that have the least number of offspring in the progeny, which we know that the ones with the least number of offspring are the ones that are purple, short and hairy as well as the ones that are white, tall and smooth. So the purple, short and hairy are our double crossover one. Any white, tall and smooth are the double crossover too. But because we only know that they are purple, short and hairy as well as white, tall and smooth, we do not know the full genotype or the double crossover, all little combinations. And so the shorthand combination or the phenotype of purple, short and hairy can be denoted by the uppercase dominant P or purple. The lowercase recessive T for short and the lowercase recessive S for Harry and then for the white, tall and smooth, the lowercase recessive P represents that white color. The uppercase dominant T represents the tall height and the upper case dominant S represents the smooth stems. And so our double cross over one is uppercase P, lowercase T and lowercase S. And our double crossover two is lowercase P, uppercase T and upper case S. And so we have that double crossover one and double crossover two. And the full genotype for the double crossover one is P T and S and then double crossover two is P. The S which we can fill in the blanks for the rest of the genotype to get the full complete genotype by using the parents. Since we know that the alleles contributed by one parent are all recessive and we are able to fill in the blanks. So we can go ahead and fill in all of the blanks with those recessive alleles. To give us the complete genotypes for our double crossover one and double crossover two, which means that we have that heterozygous P, Homozygous recessive T and Homozygous recessive S four, our double crossover one and for double crossover two, we have Homozygous recessive P. Heterozygous T and heterozygous S which means that answer choice D that states the genotypes of the offspring that have a double crossover allele combination are heterozygous P. Homozygous recessive T and Homozygous recessive S and Homozygous recessive P. Heterozygous T and heterozygous S which is the correct answer because from the progeny, we are able to determine the offspring with the parental alleles which are the purple, short and hairy and the white, tall and smooth offspring because they are the least numerous. So our purple, short and hairy is our double crossover one and our white, tall and smooth is our double crossover two which the purple, short and hairy, double cross over one can be represented by the shorthand uppercase P, lowercase T, lowercase S. And our double crossover two that is white, tall and smooth can be represented by the shorthand lowercase P, uppercase T uppercase S. And since one of the parents in the cross is homozygous recessive for all three genes, we can go ahead and fill out the rest of the blanks to give us the full genotype combinations or the offsprings that have that double crossover allele combination, which results in the combination that is found in answer choice D. So answer choice D is the correct answer. I hope you found this video to be helpful. Thank you and goodbye.

Key Concepts

Dominant and Recessive Traits

Punnett Square and Genetic Crosses

Phenotypic Ratios

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