Most flour beetles are black, but several color variants are...

Most flour beetles are black, but several color variants are known. Crosses of pure-breeding parents produced the following results (see table) in the $\mathrm{F}_{1}$ generation, and intercrossing the $\mathrm{F}_{1}$ from each cross gave the ratios shown for the $\mathrm{F}_{2}$ generation. The phenotypes are abbreviated Bl, black; Br, brown; Y, yellow; and W, white. $$\begin{array}{clll} \text { Cross } & \text { Parents } & \mathrm{F}_{1} & \mathrm{F}_{2} \\ \hline 1 & \mathrm{Br} \times \mathrm{Y} & \mathrm{Br} & 3 \mathrm{Br}: 1 \mathrm{Y} \\ 2 & \mathrm{Bl} \times \mathrm{Br} & \mathrm{Bl} & 3 \mathrm{Bl}: 1 \mathrm{Br} \\ 3 & \mathrm{Bl} \times \mathrm{Y} & \mathrm{Bl} & 3 \mathrm{Bl}: 1 \mathrm{Y} \\ 4 & \mathrm{W} \times \mathrm{Y} & \mathrm{Bl} & 9 \mathrm{Bl}: 3 \mathrm{Y}: 4 \mathrm{W} \\ 5 & \mathrm{W} \times \mathrm{Br} & \mathrm{Bl} & 9 \mathrm{Bl}: 3 \mathrm{Br}: 4 \mathrm{W} \\ 6 & \mathrm{Bl} \times \mathrm{W} & \mathrm{Bl} & 9 \mathrm{Bl}: 3 \mathrm{Y}: 4 \mathrm{W} \\ \hline \end{array}$$ a. From these results, deduce and explain the inheritance of these colors. b. Write the genotypes of each of the parents, the $\mathrm{F}_{1}$, and the $\mathrm{F}_{2}$ in all crosses.

Most flour beetles are black, but several color variants are known. Crosses of pure-breeding parents produced the following results (see table) in the $\mathrm{F}_{1}$ generation, and intercrossing the $\mathrm{F}_{1}$ from each cross gave the ratios shown for the $\mathrm{F}_{2}$ generation. The phenotypes are abbreviated Bl, black; Br, brown; Y, yellow; and W, white.
$$\begin{array}{clll}
\text { Cross } & \text { Parents } & \mathrm{F}_{1} & \mathrm{F}_{2} \\
\hline 1 & \mathrm{Br} \times \mathrm{Y} & \mathrm{Br} & 3 \mathrm{Br}: 1 \mathrm{Y} \\
2 & \mathrm{Bl} \times \mathrm{Br} & \mathrm{Bl} & 3 \mathrm{Bl}: 1 \mathrm{Br} \\
3 & \mathrm{Bl} \times \mathrm{Y} & \mathrm{Bl} & 3 \mathrm{Bl}: 1 \mathrm{Y} \\
4 & \mathrm{W} \times \mathrm{Y} & \mathrm{Bl} & 9 \mathrm{Bl}: 3 \mathrm{Y}: 4 \mathrm{W} \\
5 & \mathrm{W} \times \mathrm{Br} & \mathrm{Bl} & 9 \mathrm{Bl}: 3 \mathrm{Br}: 4 \mathrm{W} \\
6 & \mathrm{Bl} \times \mathrm{W} & \mathrm{Bl} & 9 \mathrm{Bl}: 3 \mathrm{Y}: 4 \mathrm{W} \\
\hline
\end{array}$$
a. From these results, deduce and explain the inheritance of these colors.
b. Write the genotypes of each of the parents, the $\mathrm{F}_{1}$, and the $\mathrm{F}_{2}$ in all crosses.

Key Concepts

Mendelian Inheritance

This is the foundational principle that traits are transmitted from parents to offspring through discrete factors, or genes, that segregate and assort independently. It emphasizes that each trait’s phenotype is the result of specific combinations of alleles inherited from each parent.

Dominance Relationships

This concept refers to how certain alleles can mask the expression of other alleles in heterozygous individuals. In many crosses, one phenotype appears in the offspring despite the presence of different alleles because the dominant allele determines the trait, while the recessive allele’s effect is only seen in a homozygous state.

Epistasis

Epistasis describes an interaction where one gene’s allele masks or modifies the expression of alleles at a second gene. This gene interaction can alter the typical Mendelian ratios, such as producing modified phenotypic ratios like 9:3:4 in dihybrid crosses, and indicates a hierarchical relationship in the control of a particular trait.

Dihybrid Crosses

A dihybrid cross involves two different genetic loci and the simultaneous segregation of two pairs of alleles. The resulting offspring ratios provide insight into how multiple genes interact, especially when one gene’s effect can mask or modify the expression of the other, as seen in altered phenotypic ratios due to epistasis.

Phenotypic Ratios

Observed ratios in offspring, such as the classic 3:1 in monohybrid crosses and the 9:3:4 in some dihybrid crosses, serve as clues to the underlying genetic mechanisms. These ratios help deduce whether traits are controlled by one gene, show complete dominance, or involve interactions like epistasis between two genes.

Transcript

00:01 Flower beetles are typically black, but they can also be brown, yellow, or white.

00:07 And so the data provided, we can sort of tease apart how this inheritance pattern works.

00:13 And so let's start with crosses 1 through 3.

00:18 Crosses 1 through 3 all yield a 3 to 1 ratio of phenotypes in the f2.

00:27 This tells us that these color alleles are determined by one gene.

00:36 So they're from a single gene.

00:41 Otherwise, you wouldn't get an f2 phenotypic ratio of 3 to 1.

00:44 The other thing we can determine is that there is a dominance hierarchy.

00:49 So if you look at the phenotypes in f1 and then you look at the phenotypic ratios in f2, you would see that black is dominant to brown and that brown is dominant to yellow.

01:04 And so we can use the following genotypes.

01:07 Let's use blbl to indicate black.

01:12 Let's use br br to indicate brown.

01:17 And let's use by, by to indicate yellow.

01:24 Okay.

01:25 Now, let's look at crosses 4 through 6 because those are different than the crosses 1 through 3.

01:33 All of these crosses indicate that there is something else going on because they all have a modified 9.

01:41 To three, three to one ratio in the phenotypes in the f2.

01:47 This tells us that there are at least two genes and that these two genes are exhibiting epistasis.

01:57 So they're epistatic and that one gene is influencing what happens to the other.

02:04 And if we notice, white is always in the least numbers.

02:08 And so that tells us that the presence of color dominates, no color.

02:26 And so let's determine, let's make color, big c, big c, or big c, little c, and no color to little c's.

02:46 And i'm going to underline my recessive traits because c's you can't tell apart.

02:52 The other thing we can determine from crosses two is that white is probably made of two phenotypes.

03:12 One is a homozygous, double dominant, double homozygous, i mean.

03:22 The other is a gene that's one heterozygous combined with one homozygous, and i'll show you this.

03:35 So for example, and i'm not going to use the real needles, but double homozygous would be two little as and two little bs.

03:40 One heterosigis and one homozygousigus would be a big a, little a, two little bs.

03:47 Although those are not the letters we're using, that's what we're going to look for when we talk about.

03:50 White.

03:52 And so let's start working on our crosses.

03:56 So i'm just going to move this up.

03:59 Cross one, i'm going to do parents, f1, and f2.

04:07 And i'll change colors so that you can see what we're doing.

04:10 Cross one was a brown parent, crossed with a yellow parent.

04:16 The f -1s were all brown, so we know brown is dominant over yellow.

04:21 And the f -2, we yielded three brown flower.

04:24 Beetles to one yellow.

04:28 Okay, so let's start with the parents.

04:31 We could start with the f1.

04:35 So if we look at the f1, we know we have a yellow in the f2.

04:40 So this f1 individual must be b -r -b -y, because you've got brown and the hidden yellow, and they are with color.

04:53 So two capital sees.

04:56 Now let's backtrack to the parents.

04:59 That means that, and we're assuming that all of these parents are true breeding flower wheels, otherwise this is a disaster and we can't do it.

05:07 Not easily anyway.

05:09 And so the brown parent must be big r, i'm sorry, big r, b, r with a color gene.

05:17 And the yellow parent must be homozygous for yellow and the color gene.

05:24 That means these brown flower beetles, there's three of them, but there are b are something with color gene, to one yellow flower beetle with a color gene.

05:38 And that's our f1 and f2 for cross one.

05:45 Cross two, we have a black flower beetle crossed with a brown flower beetle, and they yielded all black flower beetles.

05:55 So that tells us that black is dominant to brown, and that's how we derived our dominance hierarchy.

06:00 Black is dominant to brown, brown is dominant to yellow.

06:05 And then the f2, we have three black flower beetles to one brown flower beetle.

06:13 And so again, let's start with our f2, or f1.

06:16 We know there's a hidden brown.

06:18 So that means the f1 has to be a heterozygous.

06:21 Black, brown, in the color gene.

06:27 And that means the parents, again, assuming true breeding, black flower beetle, two bls with the color gene, cross with two brown alleles in the color gene.

06:39 And in the f2, this is easy.

06:41 Three bl something with the color gene, with one br br in the color gene.

06:50 Cross three.

06:55 The parents were a black flower beetle crossed with yellow.

06:58 All of the offspring were black, so we know black is dominant to yellow, but we knew that already.

07:05 And the f2 is three black flower beetles with one yellow.

07:09 And so again, let's backtrack...