Model Answer
0 min readIntroduction
Incomplete dominance is a form of intermediate inheritance where the heterozygous genotype expresses a phenotype that is intermediate between the two homozygous phenotypes. This differs from complete dominance where one allele masks the expression of the other. Understanding the principles of incomplete dominance is crucial in predicting inheritance patterns and assessing the probability of specific traits appearing in offspring. This question requires us to determine the percentage of normal offspring resulting from a cross exhibiting incomplete dominance.
Understanding the Genetic Cross
To determine the percentage of normal offspring, we first need to establish the genotypes of the parents. Assuming 'normal' is the dominant trait and 'abnormal' is the recessive trait, and given the scenario involves incomplete dominance, let's denote the alleles as follows:
- NN: Represents the homozygous genotype for the normal trait.
- nn: Represents the homozygous genotype for the abnormal trait.
- Nn: Represents the heterozygous genotype, which expresses an intermediate phenotype (neither fully normal nor fully abnormal).
The question doesn't explicitly state the parental genotypes. However, to arrive at a meaningful percentage of normal offspring, we must assume a specific cross. Let's consider a cross between two heterozygous individuals (Nn x Nn). This is a common scenario used to illustrate incomplete dominance.
Punnett Square Analysis
To determine the genotypic and phenotypic ratios, we can construct a Punnett square:
| N | n | |
|---|---|---|
| N | NN | Nn |
| n | Nn | nn |
From the Punnett square, we can determine the following:
- Genotypic Ratio: 1 NN : 2 Nn : 1 nn
- Phenotypic Ratio: 1 Normal (NN) : 2 Intermediate (Nn) : 1 Abnormal (nn)
Calculating the Percentage of Normal Offspring
Based on the phenotypic ratio, the proportion of normal offspring (NN) is 1 out of 4. Therefore, the percentage of normal offspring is:
(1/4) * 100% = 25%
Alternative Scenarios
If the cross were between a normal homozygous individual (NN) and an abnormal homozygous individual (nn), the F1 generation would all be heterozygous (Nn), resulting in 0% normal offspring in the F1 generation. If the cross were between a normal homozygous (NN) and a heterozygous (Nn) individual, the percentage of normal offspring would be 50%.
Importance of Incomplete Dominance
Incomplete dominance demonstrates that inheritance isn't always a simple dominant-recessive relationship. It highlights the blending of traits in heterozygotes, leading to a wider range of phenotypic expressions. This concept is vital in understanding the genetic basis of various traits in plants, animals, and humans.
Conclusion
In conclusion, assuming a cross between two heterozygous individuals (Nn x Nn) exhibiting incomplete dominance, the percentage of normal offspring is 25%. The outcome is directly dependent on the parental genotypes and the principles of Mendelian inheritance. Understanding incomplete dominance is crucial for accurately predicting phenotypic ratios and interpreting inheritance patterns in biological systems. Further research into complex inheritance patterns, such as codominance and polygenic inheritance, provides a more comprehensive understanding of genetic variation.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.