Dihybrid Cross Problems: | Lecture notes Genetics | Docsity - Free Printable

Problem Analysis and Solution

The image contains two dihybrid cross problems. Let's solve each problem step by step using Punnett squares.

---

#### Problem 1: Dogs (Barking and Ear Traits)

Traits and Genes:
- Barking trait:
- Dominant allele: \( B \) (bark)
- Recessive allele: \( b \) (silent)
- Ear trait:
- Dominant allele: \( S \) (standing ears)
- Recessive allele: \( s \) (droopy ears)

Cross Description:
- Heterozygous standing-eared barker: \( SsBb \)
- Homozygous droopy-eared silent trailer: \( ssbb \)

Step 1: Set up the Punnett Square
The parents are:
- Parent 1: \( SsBb \)
- Parent 2: \( ssbb \)

The gametes from Parent 1 (\( SsBb \)) are:
- \( SB \), \( Sb \), \( sB \), \( sb \)

The gametes from Parent 2 (\( ssbb \)) are:
- \( sb \)

Now, fill in the Punnett square:

\[
\begin{array}{c|cccc}
& SB & Sb & sB & sb \\
\hline
sb & SsBb & Ssbb & ssBb & ssbb \\
\end{array}
\]

Step 2: Determine Phenotypes and Genotypes

- Phenotypes:
- Standing-eared barkers: \( S\_B\_ \)
- Standing-eared silent: \( S\_bb \)
- Droopy-eared barkers: \( ssB\_ \)
- Droopy-eared silent: \( ssbb \)

From the Punnett square:
- \( SsBb \): Standing-eared barker
- \( Ssbb \): Standing-eared silent
- \( ssBb \): Droopy-eared barker
- \( ssbb \): Droopy-eared silent

Phenotypic Ratio:
- Standing-eared barkers: 1
- Standing-eared silent: 1
- Droopy-eared barkers: 1
- Droopy-eared silent: 1

So, the phenotypic ratio is:
\[
\boxed{1:1:1:1}
\]

- Genotypes:
- \( SsBb \): 1
- \( Ssbb \): 1
- \( ssBb \): 1
- \( ssbb \): 1

So, the genotypic ratio is:
\[
\boxed{1:1:1:1}
\]

---

#### Problem 2: Guinea Pigs (Coat and Hair Traits)

Traits and Genes:
- Coat trait:
- Dominant allele: \( R \) (rough coat)
- Recessive allele: \( r \) (smooth coat)
- Hair trait:
- Dominant allele: \( S \) (short hair)
- Recessive allele: \( s \) (long hair)

Cross Description:
- Homozygous rough and short-haired guinea pig: \( RRSS \)
- Smooth long-haired guinea pig: \( rrss \)

Step 1: Set up the Punnett Square
The parents are:
- Parent 1: \( RRSS \)
- Parent 2: \( rrss \)

The gametes from Parent 1 (\( RRSS \)) are:
- \( RS \)

The gametes from Parent 2 (\( rrss \)) are:
- \( rs \)

Now, fill in the Punnett square:

\[
\begin{array}{c|c}
& RS \\
\hline
rs & RrSs \\
\end{array}
\]

Step 2: Determine Phenotypes and Genotypes

- Phenotypes:
- All offspring will have the dominant traits for both coat and hair because they inherit at least one dominant allele for each trait.
- Rough coat (dominant \( R \))
- Short hair (dominant \( S \))

So, all offspring will be:
- Rough, short-coated guinea pigs.

Phenotypic Ratio:
- Rough, short-coated: 1

So, the phenotypic ratio is:
\[
\boxed{1:0}
\]

- Genotypes:
- All offspring will have the genotype \( RrSs \).

So, the genotypic ratio is:
\[
\boxed{1:0}
\]

---

Final Answers:

1. For the dog problem:
- Phenotypic ratio: \(\boxed{1:1:1:1}\)
- Genotypic ratio: \(\boxed{1:1:1:1}\)

2. For the guinea pig problem:
- Phenotypic ratio: \(\boxed{1:0}\)
- Genotypic ratio: \(\boxed{1:0}\)