In sweet peas, the synthesis of purple anthocyanin pigment...

In sweet peas, the synthesis of purple anthocyanin pigment in the petals is controlled by two genes, $B$ and $D$ The pathway is a. What color petals would you expect in a purebreeding plant unable to catalyze the first reaction? b. What color petals would you expect in a purebreeding plant unable to catalyze the second reaction? c. If the plants in parts $a$ and $b$ are crossed, what color petals will the $\mathrm{F}_{1}$ plants have? d. What ratio of purple: blue:white plants would you expect in the $\mathrm{F}_{2}$ ?

In sweet peas, the synthesis of purple anthocyanin pigment in the petals is controlled by two genes, $B$ and $D$ The pathway is
a. What color petals would you expect in a purebreeding plant unable to catalyze the first reaction?
b. What color petals would you expect in a purebreeding plant unable to catalyze the second reaction?
c. If the plants in parts $a$ and $b$ are crossed, what color petals will the $\mathrm{F}_{1}$ plants have?
d. What ratio of purple: blue:white plants would you expect in the $\mathrm{F}_{2}$ ?

Key Concepts

Dihybrid Cross Analysis

Dihybrid cross analysis involves examining the genetic outcomes from a cross between individuals that are heterozygous for two traits. It is used to predict phenotypic ratios in the offspring, particularly when dealing with two interacting genes or enzyme steps in a pathway, and helps in understanding the segregation and interaction of genetic factors.

Epistasis

Epistasis is the interaction between genes where the effect of one gene is masked or modified by one or more other genes. In multistep pathways, a mutation in an early step gene can prevent the expression of phenotype even if later steps are functional, which is critical for interpreting modifications in pigment synthesis.

Complementation

Complementation refers to the phenomenon in which two organisms with different homozygous recessive mutations in the same pathway can produce offspring with a wild?type phenotype when crossed, if each provides the functional gene product missing in the other. This concept demonstrates how separate gene mutations may compensate for one another when combined in a heterozygous state.

Biosynthetic Pathways

A biosynthetic pathway is a sequence of enzyme?catalyzed reactions in which the product of one reaction is the substrate for the next. Understanding these pathways is essential for grasping how organisms synthesize complex molecules like pigments and how mutations affecting enzymes can alter the final product.

Loss-of-Function Mutations

Loss-of-function mutations are genetic changes that result in reduced or abolished enzyme activity. In the context of enzyme-controlled pathways, such mutations can block a step in the synthesis process, leading to the accumulation of intermediates or the complete absence of the final product, which is important for analyzing phenotypic outcomes.

Transcript

00:01 In sweet peas, we can see that the color pathway goes like this.

00:07 It's white, and then you have enzyme b from gene b to produce blue.

00:18 And then from blue, you have enzyme from gene d to produce anthocyanin, which is purple.

00:34 And so if we know a couple other things, if enzyme b is missing, you can't produce blue because you're missing this step right here.

00:47 And so those flowers will accumulate white in their petals.

00:52 If enzyme d is missing here, then you will accumulate blue pigment in the flowers.

01:06 And so you can see that depending on which enzyme is mutated, you will get a different color in the color pattern.

01:14 And so what happens when you take a parent, who is homozygous recessive for gene b and homozygous dominant for gene d.

01:26 And you breed that parent to a flower who is homozygous for gene b and homozygous recessive for gene d.

01:36 The f1, we don't even need to do a cross because there's only one possibility for the four different allele combinations.

01:44 The f1 progeny will all be heterozygous for gene b and heterozygous for gene d.

01:53 And consequently, because they have a functional enzyme at both stops in the pathway, they will be purple or anthracyanide.

02:04 Now, in the f2, if you breed two heterozygotes b, d, i'm sorry, big b, little b, big d, big, d, little d.

02:15 Crossed with the same, you expect a 933 phenotypic ratio in the f2.

02:25 And so we can start to look at this, but we need to remember our pathway as we go...

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