Model Answer
0 min readIntroduction
Polygenic inheritance refers to the inheritance of traits that are controlled by multiple genes, each contributing a small, additive effect to the phenotype. Unlike Mendelian inheritance, which deals with discrete traits, polygenic inheritance results in continuous variation within a population. Skin colour in humans is a classic example of a polygenic trait, influenced by at least three genes (though more are likely involved). This question asks us to predict the outcome of a mating between individuals with extreme phenotypes – a ‘pure’ black individual and a ‘pure’ white individual – and analyze the resulting F1 and F2 generations, both qualitatively and quantitatively.
Understanding the Genetic Basis of Skin Colour
Skin colour is determined by the amount of melanin, a pigment produced by melanocytes. The more melanin, the darker the skin. We will assume a simplified model with three genes (A, B, and C) controlling melanin production, where each dominant allele (A, B, C) contributes one unit of melanin, and each recessive allele (a, b, c) contributes no melanin. Therefore, the genotype with the maximum melanin production would be AABBCC, and the minimum would be aabbcc.
Parental Generation (P)
Let's define our parental genotypes:
- Negro man (pure black): AABBCC (6 units of melanin)
- White woman (pure white): aabbcc (0 units of melanin)
First Filial Generation (F₁)
The F1 generation will be heterozygous for all three genes:
- F1 genotype: AaBbCc (3 units of melanin)
- F1 phenotype: Intermediate skin colour – a shade between the parents. Qualitatively, the F1 generation will exhibit a mixed skin tone.
Second Filial Generation (F₂)
To determine the F2 generation, we need to consider all possible combinations of alleles from the F1 cross (AaBbCc x AaBbCc). This is equivalent to a trihybrid cross. The total number of possible genotypes is 33 = 27, but due to the additive nature of the genes, we can group these genotypes based on the number of dominant alleles they possess.
Quantitative Analysis of F₂ Generation
The F2 generation will exhibit a range of skin colours, following a bell-shaped curve distribution. The number of dominant alleles determines the skin colour intensity:
| Number of Dominant Alleles | Genotype Combinations | Phenotype (Melanin Units) | Approximate Proportion |
|---|---|---|---|
| 0 | aabbcc | 0 (Very Light) | 1/64 |
| 1 | Aabbcc, aaBbcc, aabbCc | 1 (Light) | 3/64 |
| 2 | AAbbcc, AabBcc, AabbCc, aaBBcc, aaBbCc, aabbCC | 2 (Intermediate Light) | 6/64 |
| 3 | AABbcc, AAbbCc, AabBcc, AabCc, aaBBcc, aaBbCc, aabbCC | 3 (Intermediate) | 7/64 |
| 4 | AABBcc, AABbCc, AAbbCC, AaBBcc, AaBbCc, AabbCC | 4 (Intermediate Dark) | 6/64 |
| 5 | AABBCc, AABBcc, AAbbCc, AaBBCc, AaBBcc, AabBCC | 5 (Dark) | 3/64 |
| 6 | AABBCC | 6 (Very Dark) | 1/64 |
This distribution approximates a normal distribution with a peak at 3 units of melanin (the F1 phenotype). Qualitatively, the F2 generation will show a continuous spectrum of skin colours, ranging from very light to very dark, with the most individuals having intermediate shades.
Important Considerations
- This is a simplified model. More genes are involved in skin colour determination.
- Environmental factors (sun exposure) also influence skin colour.
- Gene interactions (epistasis) can further complicate the inheritance pattern.
Conclusion
In conclusion, the mating between a ‘pure’ black and a ‘pure’ white individual, based on a simplified polygenic model of three genes, results in an F1 generation with intermediate skin colour. The F2 generation exhibits a continuous range of skin colours, following a bell-shaped distribution, demonstrating the additive effect of multiple genes. This example highlights the complexity of human traits and the limitations of applying simple Mendelian principles to polygenic inheritance. Understanding polygenic inheritance is crucial for comprehending the genetic basis of many complex human characteristics.
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.