Chapter 7, Problem 32P

In corn snakes, the wild-type color is brown. One autosomal recessive mutation causes the snake to be orange, and another causes the snake to be black. An orange snake was crossed to a black one, and the F₁ offspring were all brown. Assume that all relevant genes are unlinked.

a..	Indicate what phenotypes and ratios you would expect in the F2 generation of this cross if there is one pigment pathway, with orange and black being different intermediates on the way to brown.
b..	Indicate what phenotypes and ratios you would expect in the F2 generation if orange pigment is a product of one pathway, black pigment is the product of another pathway, and brown is the effect of mixing the two pigments in the skin of the snake.

Summary Introduction

a.

To determine:

The expected ratios and phenotypes in the F₂ generation of a cross between an orange snake and black snake if single pigment pathway is present.

Introduction:

In corn snakes, the wild-type color is brown. One autosomal recessive mutation results in the orange snake color. Another autosomal recessive mutation causes the snake to be black color.

Explanation of Solution

As per the given information, there is a single pigment pathway which is as follows:

$\text{Orange}\stackrel{A}{\to }\text{Black}\stackrel{B}{\to }\text{Brown}$

In this pathway, the gene aa snakes are orange, and bb snakes are black. The initial cross is as follows:

$\begin{array}{c}aaBB\times AAbb\\ \downarrow \\ AaBb\end{array}$

The F₂ generation is as follows:

	AB	Ab	aB	ab
Ab	AABb	AAbb	AaBb	Aabb
AB	AABB	AABb	AaBB	AaBb
aB	AaBB	AaBb	aaBB	aaBb
ab	AaBb	Aabb	aaBb	aabb

The phenotypic ratios are as follows:

$\begin{array}{rllll}AB& =& \text{brown}& =& \frac{9}{16}\\ Abb& =& \text{black}& =& \frac{3}{16}\\ aaB& =& \text{orange}& =& \frac{3}{16}\\ aabb& =& \text{orange}& =& \frac{1}{16}\end{array}$

Summary Introduction

b.

To determine:

The expected ratios and phenotypes in the F₂ generation of a cross between an orange snake and black snake if separate pigment pathway is present for orange and black color.

Introduction:

Autosomal recessive is a way in which a trait, disease, or disorder can be passed from one generation to the next generation. An autosomal recessive disease contains the two copies of an abnormal gene that must be present to develop that disorder.

Explanation of Solution

As per the given information, there is a separate pigment pathway which is as follows:

$\left[\begin{array}{l}\to \stackrel{A}{\to }\to Black\\ \to \to \stackrel{B}{\to }Orange\end{array}\right]Brown$

The initial cross is as follows:

$\begin{array}{c}aaBB\times AAbb\\ \downarrow \\ AaBb\end{array}$

The F₂ generation is as follows:

	AB	Ab	aB	ab
Ab	AABb	AAbb	AaBb	Aabb
AB	AABB	AABb	AaBB	AaBb
aB	AaBB	AaBb	aaBB	aaBb
ab	AaBb	Aabb	aaBb	aabb

The phenotypic ratios are as follows:

$\begin{array}{rllll}AB& =& \text{brown}& =& \frac{9}{16}\\ Abb& =& \text{black}& =& \frac{3}{16}\\ aaB& =& \text{orange}& =& \frac{3}{16}\\ aabb& =& \text{white}& =& \frac{1}{16}\end{array}$