A pure breeding plant with red dot on leaves was crossed with pure breeding white dotted plants. The heterozygotes have both red and white dots. On self pollination of `F_(1)`, what fraction of plants show both red and white dots on leaves

Options

A. `(2)/(4)`
B. `1/4`
C. `2/3`
D. `3/4`

Answer

To solve the problem step by step, we will follow the principles of Mendelian genetics, particularly focusing on the inheritance patterns of traits.

### Step 1: Define the Traits
We have two traits:
- Red dots on leaves (dominant trait) represented by the allele **R**.
- White dots on leaves (recessive trait) represented by the allele **r**.

### Step 2: Parental Generation
We start with two pure breeding plants:
- One plant with red dots (homozygous dominant): **RR**
- One plant with white dots (homozygous recessive): **rr**

### Step 3: F1 Generation
When we cross these two plants, the offspring (F1 generation) will all be heterozygous:
- F1 genotype: **Rr**
- Since **R** is dominant, all F1 plants will show both red and white dots on their leaves.

### Step 4: Self-Pollination of F1 Generation
Next, we self-pollinate the F1 generation (Rr x Rr). We can use a Punnett square to determine the genotypes of the F2 generation.

### Step 5: Constructing the Punnett Square
The Punnett square for the cross Rr x Rr looks like this:

```
R r
----------------
R | RR | Rr |
----------------
r | Rr | rr |
----------------
```

### Step 6: Analyzing the F2 Generation
From the Punnett square, we can see the following genotypes:
- **1 RR** (homozygous dominant) - red dots only
- **2 Rr** (heterozygous) - both red and white dots
- **1 rr** (homozygous recessive) - white dots only

### Step 7: Fraction of Plants with Both Red and White Dots
Now, we need to find the fraction of plants that show both red and white dots. In the F2 generation:
- Total plants = 4 (1 RR + 2 Rr + 1 rr)
- Plants showing both red and white dots = 2 (the 2 Rr plants)

Thus, the fraction of plants showing both red and white dots is:
\[ \text{Fraction} = \frac{2}{4} = \frac{1}{2} \]

### Final Answer
The fraction of plants that show both red and white dots on their leaves is **1/2**.

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