You are handed a mystery pea plant with tall stems and axial...

You are handed a mystery pea plant with tall stems and axial flowers and asked to determine its genotype as quickly as possible. You know that the allele for tall stems ( $T$ ) is dominant to that for dwarf stems $(t)$ and that the allele for axial flowers ( $A$ ) is dominant to that for terminal flowers ( $a$ ). (a) Identify all the possible genotypes for your mystery plant. (b) Describe the one cross you would do in your garden to determine the exact genotype of your mystery plant. (c) While waiting for the results of your cross, you predict the results for each possible genotype listed in part a. Explain how you do this and why this is not called "performing a cross." (d) Explain how the results of your cross and your predictions will help you learn the genotype of your mystery plant.

You are handed a mystery pea plant with tall stems and axial flowers and asked to determine its genotype as quickly as possible. You know that the allele for tall stems ( $T$ ) is dominant to that for dwarf stems $(t)$ and that the allele for axial flowers ( $A$ ) is dominant to that for terminal flowers ( $a$ ).
(a) Identify all the possible genotypes for your mystery plant.
(b) Describe the one cross you would do in your garden to determine the exact genotype of your mystery plant.
(c) While waiting for the results of your cross, you predict the results for each possible genotype listed in part a. Explain how you do this and why this is not called "performing a cross."
(d) Explain how the results of your cross and your predictions will help you learn the genotype of your mystery plant.

Key Concepts

Predictive Modeling in Genetics

This involves using known genetic principles to forecast the results of various crosses without physically performing them. While predictions based on Punnett squares and allele dominance are theoretical, they are not considered experimental crosses because they involve calculation rather than performing an actual biological mating.

Test Cross

A test cross involves mating an individual of unknown genotype with an individual that is homozygous recessive for the trait in question. This method is used to reveal whether the unknown genotype is homozygous dominant or heterozygous by observing the phenotypes in the offspring.

Homozygous vs Heterozygous

These terms describe whether an organism has two identical alleles (homozygous) or two different alleles (heterozygous) for a particular trait. This distinction is crucial when determining the full genotype of an organism, especially in cases where dominant traits may conceal a recessive allele.

Punnett Square Analysis

Punnett squares provide a visual tool to predict the genotypic and phenotypic outcomes of a cross. By listing all possible allele combinations from the parents, one can forecast the distribution of traits in the progeny, which is especially useful in predicting the outcomes of test crosses.

Genotype vs Phenotype

Genotype refers to the genetic makeup of an organism, consisting of the alleles present, while phenotype is the observable expression of these genes. This distinction is key to interpreting how dominant and recessive alleles influence the appearance of traits, as seen with tall stems versus dwarf stems or axial versus terminal flowers.

Mendelian Inheritance

This concept explains how alleles segregate and assort independently during the formation of gametes. It establishes that traits are inherited in predictable ratios based on the dominant and recessive nature of alleles, setting the foundation for understanding genotype-to-phenotype relationships.

Allelic Dominance

Dominance refers to the relationship between two alleles of a gene, where the dominant allele masks the expression of the recessive allele in the phenotype. This idea is central to predicting which traits will be expressed when an organism has one dominant and one recessive allele at a locus.

Transcript

00:01 In this problem, we're looking at two characteristics in a plant.

00:05 So a die hybrid cross is what we're going to be taking a look at.

00:14 And this one, the plant, has tall axial flowers.

00:25 Tall and axial flowers.

00:40 We'll look at our first characteristic.

00:45 So tall is dominant here to wharf.

01:00 For these characters six, and axial is dominant to terminal.

01:16 And that's indicated by the capital letter for our t and our a.

01:29 And so this individual here then is displaying both dominant traits.

01:38 In order to have and display the dominant phenotype, your genotype, in your genotypes, you have to have at least one, right? at least one dominant allele.

02:00 So in our first part, we're going to identify the possible genotypes for this plant because we know the phenotypes, but we, again, can have different combinations in our genotype.

02:19 So again, we just need one tall teeth, but we could have, that individual could have two dominant for tall and axial.

02:42 They could be, right, we just need one tall, one dominant, one recessive.

02:47 So they could be heterozygous for both.

02:52 That still would make them tall and axial.

02:58 They could be heterozygous for the teeth and homozygous for the teeth.

03:09 The position of the flowers, or that could be reversed.

03:13 They could be homozygous dominant for the height, and they could be heterozygous or the position.

03:28 So we can see in here we have four possible genotypes for that plant.

03:48 In order to determine the exact genotype of that mystery plant, what you would want to do is cross a short or a dwarf plant, a dwarf terminal plant, which has the genotype t -t -t -a -a -a, all homozygous recessive all aleals.

04:36 So we're crossing it with there a homozygous recessive.

04:44 Recessive for each of those traits because the offspring can be predicted using a dihybrid test cross.

05:08 So, and this is the same for one or two traits, a test cross is used when you know the phenotype, but not the genotypes of a dominant individual.

05:21 That test cross is used because, the offspring can be predicted from that.

05:36 So let's take a look at what those possible genotypes are from that, from that in particular cross.

05:46 So if we crossed our first individual, which is homozygous dominant for each trait, we crossed that with our homozygous recessive individual, our possible offspring genotype is an individual that is heterozygous for each trait.

06:20 And so they all show the dominant characteristics here because we can see that big t would go only with little t, big a with only little a...

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