The Antirrhinum commonly known as snapdragon is a fascinating flowering plant often used in genetic studies. One of the most intriguing experiments involves crossing a red Antirrhinum with a white Antirrhinum to observe flower color inheritance.
This cross does not follow the simple Mendelian dominant-recessive pattern. Instead it exhibits a phenomenon known as incomplete dominance where the offspring have an intermediate color rather than expressing the dominant trait fully.
In this topic we will explore:
- The genetic principles behind this cross
- The expected flower colors in the offspring
- How incomplete dominance works
- The significance of this cross in genetics
Understanding Incomplete Dominance
What Is Incomplete Dominance?
Incomplete dominance is a type of genetic inheritance where the heterozygous offspring show a blended or intermediate trait rather than fully expressing one dominant allele.
In the case of Antirrhinum (snapdragon) flowers:
- Red flowers (RR) have two dominant alleles for red pigmentation.
- White flowers (WW) have two recessive alleles producing no pigment.
- When a red (RR) snapdragon is crossed with a white (WW) snapdragon the result is a pink (RW) snapdragon.
How Does This Differ from Complete Dominance?
In complete dominance the dominant allele fully expresses itself in the heterozygous condition. For example in pea plants studied by Gregor Mendel purple flowers (dominant) completely masked the presence of white flowers (recessive).
In contrast with incomplete dominance:
- The dominant allele does not completely override the recessive allele.
- Instead of red or white the offspring appear pink a mixture of both parent traits.
Genetic Cross of Red and White Antirrhinum
To understand this cross let’s analyze the genetic combination using a Punnett square.
Parental Cross (P Generation)
- Red Antirrhinum (RR) × White Antirrhinum (WW)
F1 Generation (First Filial Generation)
Each parent contributes one allele to the offspring:
| R | R | |
|---|---|---|
| W | RW | RW |
| W | RW | RW |
Result: 100% Pink (RW) snapdragons.
F2 Generation (Second Filial Generation)
If two F1 pink snapdragons (RW) are crossed:
| R | W | |
|---|---|---|
| R | RR | RW |
| W | RW | WW |
Result:
- 25% Red (RR)
- 50% Pink (RW)
- 25% White (WW)
This means that the pink snapdragons from F1 can produce red pink and white flowers in the F2 generation in a 1:2:1 ratio.
Why Does This Happen? The Role of Pigments
The color of Antirrhinum flowers is determined by pigments called anthocyanins.
- Red flowers (RR): Produce a high amount of anthocyanin.
- White flowers (WW): Lack anthocyanin production leading to no color.
- Pink flowers (RW): Produce an intermediate amount of anthocyanin leading to a pink shade.
Since the RW genotype does not have enough pigment to make a fully red flower the result is an intermediate pink color.
Significance of the Red × White Antirrhinum Cross
1. Demonstrating Incomplete Dominance
This cross is a classic example of incomplete dominance where neither allele completely dominates over the other.
2. Challenging Traditional Mendelian Genetics
While Mendelian inheritance explains dominant and recessive traits incomplete dominance adds another layer of genetic variation in plants and animals.
3. Practical Applications in Horticulture
- Understanding flower color inheritance helps breeders develop new snapdragon colors.
- Hybridization techniques use incomplete dominance to create aesthetic variations in flowers.
Other Examples of Incomplete Dominance in Nature
The red × white Antirrhinum cross is not the only example of incomplete dominance. Other cases include:
1. Andalusian Chickens
When black-feathered chickens are crossed with white-feathered chickens the result is a blue-grey feathered hybrid.
2. Japanese Four O’Clock Flowers (Mirabilis jalapa)
Crossing red and white flowers results in pink offspring just like in snapdragons.
3. Human Hair Type
Straight-haired parents (SS) crossed with curly-haired parents (CC) can produce children with wavy hair (SC).
The cross between a red and white Antirrhinum flower is a perfect example of incomplete dominance where the offspring exhibit a blended trait (pink flowers) instead of expressing one dominant trait.
Through this cross we learn:
- Genetics is more complex than simple dominance-recessive patterns.
- Flower color inheritance in snapdragons follows a 1:2:1 ratio in the F2 generation.
- Incomplete dominance plays a key role in plant breeding and genetic studies.
This fascinating genetic principle extends beyond plants and can be observed in various organisms making it an essential concept in biology.
