Mendelian Inheritance
Mendelian inheritance is the foundational concept of genetics that describes how traits are passed from parents to offspring. This topic is named after Gregor Mendel, a 19th-century scientist whose experiments with pea plants laid the groundwork for our understanding of heredity.
Key Concepts of Mendelian Inheritance
1. Genes and Alleles
Genes are segments of DNA that determine specific traits. Each gene can have different forms called alleles. For instance, the gene for flower color in pea plants has two alleles: purple (dominant) and white (recessive).2. Dominant and Recessive Alleles
In Mendelian inheritance, some alleles are dominant while others are recessive. A dominant allele will mask the effect of a recessive allele when both are present in an individual. For example: - Purple flower color (P) is dominant. - White flower color (p) is recessive.An individual with at least one purple allele (PP or Pp) will exhibit purple flowers, while only those with two recessive alleles (pp) will have white flowers.
3. Genotype and Phenotype
- Genotype refers to the genetic makeup of an organism (e.g., PP, Pp, pp). - Phenotype is the observable trait (e.g., purple or white flowers).4. Mendel’s Laws of Inheritance
Law of Segregation
This law states that during the formation of gametes (egg and sperm), the two alleles for a trait segregate from each other. For instance, a heterozygous plant (Pp) will produce two types of gametes: P and p.Law of Independent Assortment
This law states that the alleles of different genes assort independently of one another during gamete formation. For example, if we consider two traits, such as flower color (P/p) and seed shape (R/r), the alleles for these traits will segregate independently.5. Punnett Squares
Punnett squares are a useful tool to predict the genotypes and phenotypes of offspring from genetic crosses. For example, if we cross a homozygous dominant purple flower plant (PP) with a homozygous recessive white flower plant (pp), the Punnett square would look like this:`
P P
+-------+-------+
p | Pp | Pp |
+-------+-------+
p | Pp | Pp |
+-------+-------+
`
In this case, all offspring (100%) would have the genotype Pp, resulting in purple flowers (phenotype).
Practical Example of Mendelian Inheritance
Consider a breeding program for dogs where coat color is determined by a single gene with two alleles: black (B) is dominant and brown (b) is recessive. If we breed a homozygous black dog (BB) with a heterozygous black dog (Bb), the following outcomes are possible:Using a Punnett square:
`
B B
+-------+-------+
B | BB | BB |
+-------+-------+
b | Bb | Bb |
+-------+-------+
`
The results show: - 50% BB (black) - 50% Bb (black)
Thus, all offspring will be black, but 50% will carry the recessive allele for brown coat color.