Understanding Complete Dominance in Mendelian Genetics
Complete dominance is a fundamental concept in Mendelian genetics, which explains how different versions of a gene interact to produce specific traits. In this worksheet, we will explore the concept of complete dominance, its key characteristics, and how to apply it to solve genetics problems.
What is Complete Dominance?
Complete dominance occurs when one allele (a variant of a gene) completely masks the effect of the other allele. This means that the dominant allele will always be expressed, while the recessive allele will not be visible in the phenotype.
Key Characteristics of Complete Dominance:
- One allele is dominant (B), while the other is recessive (b)
- The dominant allele will always be expressed in the phenotype
- The recessive allele will only be expressed in the phenotype if an individual is homozygous recessive (bb)
Genotype and Phenotype in Complete Dominance
To understand how complete dominance works, let’s consider a simple example. Suppose we have a gene that controls flower color, with two alleles: B (dominant) and b (recessive). The B allele codes for red flowers, while the b allele codes for white flowers.
| Genotype | Phenotype |
|---|---|
| BB | Red flowers |
| Bb | Red flowers |
| bb | White flowers |
In this example, the B allele is dominant, and the b allele is recessive. When an individual has the genotype BB or Bb, the dominant B allele will always be expressed, resulting in red flowers. Only when an individual is homozygous recessive (bb) will the recessive b allele be expressed, resulting in white flowers.
Solving Genetics Problems with Complete Dominance
Now that we understand the basics of complete dominance, let’s practice solving some genetics problems.
Problem 1:
A cross is made between two pea plants, one with red flowers (Bb) and the other with white flowers (bb). What is the probability that their offspring will have red flowers?
Solution:
To solve this problem, we need to consider the possible genotypes and phenotypes of the offspring. The parent with red flowers is Bb, while the parent with white flowers is bb.
| Parent 1 (Bb) | Parent 2 (bb) |
|---|---|
| B | b |
| b | b |
The possible genotypes of the offspring are:
- Bb (50% chance)
- bb (50% chance)
Since the B allele is dominant, the offspring with genotype Bb will have red flowers. The probability of the offspring having red flowers is therefore 50%.
Problem 2:
A cross is made between two individuals, one with the genotype BB and the other with the genotype bb. What is the probability that their offspring will be homozygous dominant (BB)?
Solution:
To solve this problem, we need to consider the possible genotypes of the offspring. The parent with genotype BB can only contribute a B allele, while the parent with genotype bb can only contribute a b allele.
| Parent 1 (BB) | Parent 2 (bb) |
|---|---|
| B | b |
| B | b |
The possible genotypes of the offspring are:
- Bb (100% chance)
Since the offspring can only inherit a B allele from the BB parent, they will always be heterozygous (Bb). The probability of the offspring being homozygous dominant (BB) is therefore 0%.
Problem 3:
A cross is made between two individuals, one with the genotype Bb and the other with the genotype Bb. What is the probability that their offspring will be homozygous recessive (bb)?
Solution:
To solve this problem, we need to consider the possible genotypes of the offspring. Both parents are heterozygous (Bb), so they can contribute either a B allele or a b allele.
| Parent 1 (Bb) | Parent 2 (Bb) |
|---|---|
| B | B |
| B | b |
| b | B |
| b | b |
The possible genotypes of the offspring are:
- BB (25% chance)
- Bb (50% chance)
- bb (25% chance)
The probability of the offspring being homozygous recessive (bb) is therefore 25%.
🔥 Note: In complete dominance, the recessive allele will only be expressed in the phenotype if an individual is homozygous recessive (bb).
Conclusion
Complete dominance is a fundamental concept in Mendelian genetics that explains how different versions of a gene interact to produce specific traits. By understanding the key characteristics of complete dominance and how to apply it to solve genetics problems, we can better appreciate the complexity of inheritance and the role of genetics in shaping the traits of organisms.
What is complete dominance in genetics?
+Complete dominance occurs when one allele completely masks the effect of the other allele, resulting in a specific phenotype.
How does complete dominance affect the phenotype of an individual?
+The dominant allele will always be expressed in the phenotype, while the recessive allele will only be expressed if an individual is homozygous recessive.
What is the probability of an offspring being homozygous dominant (BB) when both parents are heterozygous (Bb)?
+25%
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