Chapter 5.3, Problem 3TPSQ

Summary Introduction

To determine:

The possibilities of obtaining a beardless male offspring in a cross between two bearded goats.

Introduction:

In goats, presence of beard is a sex influenced character, as male and female have different genotypes. Allele $B^b$ is responsible for the beard and allele $B^{+}$ is responsible for beardless phenotype. In males, $B^b$ allele is dominant over $B^{+}$. In females, recessive allele is responsible for beard.

Explanation of Solution

Genotypes responsible for beard and beardless phenotype in goats are given in the following table:

Genotype	Males	Females
$B^{+}{B}^{+}$	Beardless	Beardless
$B^{+}{B}^b$	Bearded	Beardless
$B^b{B}^b$	Bearded	Bearded

The possibilities of obtaining a beardless male offspring in a cross between two bearded goats:

If the parent goats have genotype: $B^{+}{B}^b$ for father and $B^b{B}^b$ for mother

$\begin{array}{l}{B}^{+}{B}^b\times B^b{B}^b\\ \downarrow \end{array}$

	$B^{+}$	$B^b$
$B^b$	$B^{+}{B}^b$ (Bearded male)	$B^b{B}^b$ (Bearded male)
$B^b$	$B^{+}{B}^b$ (Bearded male)	$B^b{B}^b$ (Bearded male)

The possibilities of obtaining a beardless male offspring in a cross between two bearded goats, if the parent goats have genotype: $B^{+}{B}^b$for father and $B^b{B}^b$for mother is $0\%$

If the parent goats have genotype: $B^b{B}^b$ for father and $B^b{B}^b$ for mother

$\begin{array}{l}{B}^b{B}^b\times B^b{B}^b\\ \downarrow \end{array}$

	$B^b$	$B^b$
$B^b$	$B^b{B}^b$ (Bearded male)	$B^b{B}^b$ (Bearded male)
$B^b$	$B^b{B}^b$ (Bearded male)	$B^b{B}^b$ (Bearded male)

The possibilities of obtaining a beardless male offspring in a cross between two bearded goats, if the parent goats have genotype: $B^b{B}^b$for father and $B^b{B}^b$for mother is $0\%$

Summary Introduction

To determine:

The reason why or why not it is possible to obtain a beardless male offspring in a cross between two bearded goats.

Introduction:

In goats, phenotype for beard is determined by the genotypes: $B^{+}{B}^{+}$ is responsible for beardless male and female, $B^{+}{B}^b$ is responsible for breaded male and beardless female, and $B^b{B}^b$ is responsible for bearded male and female.

Explanation of Solution

Beard in goats is a sex influenced phenotype, as male and female have different genotypes. Allele $B^b$ is responsible for the beard and allele $B^{+}$ is responsible for beardless phenotype. $B^b$ allele is dominant over $B^{+}$. In males, the presence of dominant allele is responsible for the occurrence of beard whereas in females, occurrence of recessive allele is responsible for beard.

Hence, if the parent goats have genotype $B^{+}{B}^b$ for father and $B^b{B}^b$ for mother, all male progenies produced are beardless.

$\begin{array}{l}{B}^{+}{B}^b\times B^b{B}^b\\ \downarrow \end{array}$

	$B^{+}$	$B^b$
$B^b$	$B^{+}{B}^b$ (Bearded male)	$B^b{B}^b$ (Bearded male)
$B^b$	$B^{+}{B}^b$ (Bearded male)	$B^b{B}^b$ (Bearded male)

If the parent goats have genotype $B^b{B}^b$ for father and $B^b{B}^b$ for mother, all male progenies produced are beardless.

$\begin{array}{l}{B}^b{B}^b\times B^b{B}^b\\ \downarrow \end{array}$

	$B^b$	$B^b$
$B^b$	$B^b{B}^b$ (Bearded male)	$B^b{B}^b$ (Bearded male)
$B^b$	$B^b{B}^b$ (Bearded male)	$B^b{B}^b$ (Bearded male)

Summary Introduction

To determine:

The possibilities of obtaining a bearded female offspring in a cross between two bearded goats.

Introduction:

Explanation of Solution

Genotypes responsible for beard and beardless phenotype in goats are given in the following table:

Genotype	Males	Females
$B^{+}{B}^{+}$	Beardless	Beardless
$B^{+}{B}^b$	Bearded	Beardless
$B^b{B}^b$	Bearded	Bearded

The possibilities of obtaining a bearded female offspring in a cross between two bearded goats:

If the parent goats have genotype $B^{+}{B}^b$ for father and $B^b{B}^b$ for mother

$\begin{array}{l}{B}^{+}{B}^b\times B^b{B}^b\\ \downarrow \end{array}$

	$B^{+}$	$B^b$
$B^b$	$B^{+}{B}^b$ (Beardless female)	$B^b{B}^b$ (Beardless female)
$B^b$	$B^{+}{B}^b$ (Beardless female)	$B^b{B}^b$ (Beardless female)

The possibilities of obtaining a bearded female offspring in a cross between two bearded goats, if the parent goats have genotype $B^{+}{B}^b$for father and $B^b{B}^b$for mother is $50\%$

If the parent goats have genotype: $B^b{B}^b$ for father and $B^b{B}^b$ for mother:

$\begin{array}{l}{B}^b{B}^b\times B^b{B}^b\\ \downarrow \end{array}$

	$B^b$	$B^b$
$B^b$	$B^b{B}^b$ (Bearded female)	$B^b{B}^b$ (Bearded female)
$B^b$	$B^b{B}^b$ (Bearded female)	$B^b{B}^b$ (Bearded female)

The possibilities of obtaining a bearded male offspring in a cross between two bearded goats, if the parent goats have genotype $B^b{B}^b$for father and $B^b{B}^b$for mother is $100\%$

Conclusion

The possibilities of obtaining a beardless male offspring in a cross between two bearded goats, if the parent goats have genotype: $B^{+}{B}^b$ for father and $B^b{B}^b$ for mother is $0\%$, and if the parent goats have genotype: $B^b{B}^b$ for father and $B^b{B}^b$ for mother, it is $0\%$. The possibilities of obtaining a bearded female offspring in a cross between two bearded goats, if the parent goats have genotype: $B^{+}{B}^b$ for father and $B^b{B}^b$ for mother is $50\%$ ,and if the parent goats have genotype: $B^b{B}^b$ for father and $B^b{B}^b$ for mother, it is $100\%$