Understanding basic Mendelian genetics is fundamental to grasping the complexities of heredity. This practice session will reinforce key concepts like alleles, genotypes, phenotypes, and Punnett squares, equipping you with the tools to solve genetics problems. Let's delve into the foundational principles and work through some examples.
Understanding Mendelian Inheritance
Gregor Mendel's experiments with pea plants laid the groundwork for modern genetics. His work revealed the basic principles of inheritance:
- Alleles: Different versions of a gene. For example, a gene for flower color might have an allele for purple flowers and an allele for white flowers.
- Genotype: The genetic makeup of an organism, represented by the combination of alleles. For example, PP (homozygous dominant), Pp (heterozygous), or pp (homozygous recessive).
- Phenotype: The observable characteristics of an organism, determined by its genotype. For example, purple flowers or white flowers.
- Homozygous: Having two identical alleles for a particular gene (e.g., PP or pp).
- Heterozygous: Having two different alleles for a particular gene (e.g., Pp).
- Dominant Allele: An allele that masks the expression of another allele (represented by a capital letter).
- Recessive Allele: An allele whose expression is masked by a dominant allele (represented by a lowercase letter).
Punnett Squares: Predicting Genotypes and Phenotypes
Punnett squares are a valuable tool for predicting the probability of different genotypes and phenotypes in offspring. They visually represent the possible combinations of alleles from each parent.
Example 1: Monohybrid Cross
Let's consider a monohybrid cross involving flower color in pea plants. Purple (P) is dominant over white (p). If we cross two heterozygous plants (Pp x Pp), what are the expected genotypes and phenotypes of their offspring?
| P | p | |
|---|---|---|
| P | PP | Pp |
| p | Pp | pp |
Results:
- Genotypes: 25% PP, 50% Pp, 25% pp
- Phenotypes: 75% Purple flowers, 25% White flowers
Example 2: Dihybrid Cross
A dihybrid cross involves two different genes. Let's consider pea plants with two traits: flower color (purple, P, dominant; white, p, recessive) and seed shape (round, R, dominant; wrinkled, r, recessive). If we cross two heterozygous plants (PpRr x PpRr), what are the expected genotypes and phenotypes of their offspring? (This requires a larger 4x4 Punnett square, which is best worked out step-by-step).
Practice Problems
-
In humans, brown eyes (B) are dominant over blue eyes (b). What is the probability of a child having blue eyes if one parent is homozygous dominant (BB) and the other is heterozygous (Bb)?
-
In a certain breed of dogs, short hair (S) is dominant over long hair (s), and black fur (K) is dominant over brown fur (k). If two dogs with the genotype SsKk are crossed, what is the probability of their offspring having long hair and brown fur?
-
Explain the difference between a genotype and a phenotype, providing an example for each.
Further Exploration
This practice session covers the basics of Mendelian genetics. To deepen your understanding, explore more complex inheritance patterns like incomplete dominance, codominance, and sex-linked inheritance. Numerous online resources and textbooks can provide further examples and explanations. Understanding Mendelian genetics is crucial for more advanced concepts in genetics, like population genetics and molecular genetics.
