Mendelian genetics

Mendelian Genetics

Mendelian genetics is the study of how traits are inherited from one generation to the next. This field of genetics is based on the work of Gregor Mendel, who is known as the father of genetics. Mendel conducted experiments on pea plants to understand how traits are passed down through generations.

Key Concepts in Mendelian Genetics

There are several important concepts in Mendelian genetics:

• Genes: These are units of heredity that determine specific traits. Each gene can exist in different forms called alleles.
• Alleles: These are different versions of a gene. For example, a gene for flower colour in pea plants can have a purple allele and a white allele.
• Genotype: This is the genetic makeup of an organism, represented by the alleles it possesses. For example, a plant may have a genotype of PP (homozygous dominant), Pp (heterozygous), or pp (homozygous recessive).
• Phenotype: This refers to the observable characteristics or traits of an organism, such as flower colour, which results from the genotype.
• Homozygous: An organism is homozygous for a trait if it has two identical alleles (e.g., PP or pp).
• Heterozygous: An organism is heterozygous for a trait if it has two different alleles (e.g., Pp).

Mendel's Laws of Inheritance

Mendel proposed two main laws of inheritance based on his experiments:

1. Law of Segregation

This law states that during the formation of gametes (sperm and egg cells), the two alleles for a trait separate from each other. Each gamete receives only one allele for each gene. For example, if a plant has the genotype Pp, it can produce gametes with either the P allele or the p allele.

2. Law of Independent Assortment

This law states that the alleles for different traits are inherited independently of each other. For example, if we consider two traits, such as flower colour and seed shape, the inheritance of one trait does not affect the inheritance of the other trait.

Monohybrid Crosses

A monohybrid cross is a genetic cross that examines the inheritance of a single trait. To illustrate this, consider a cross between two pea plants, one homozygous dominant for purple flowers (PP) and one homozygous recessive for white flowers (pp).

Step 1: Parental Generation (P Generation)

In this example, the P generation consists of:

• Plant 1: PP (purple flowers)
• Plant 2: pp (white flowers)

Step 2: Gamete Formation

Each parent produces gametes:

• Plant 1 can only produce P gametes.
• Plant 2 can only produce p gametes.

Step 3: F1 Generation

The offspring (F1 generation) will receive one allele from each parent:

• F1 genotype: Pp (all offspring will have purple flowers).

Step 4: F1 Self-Cross

If you allow the F1 generation to self-fertilize, you can predict the F2 generation:

Pp × Pp

The possible gametes produced by the F1 plants are P and p. The Punnett square for this cross is:

|   | P | p |
|---|---|---|
| P | PP| Pp|
| p | Pp| pp|

The resulting genotypes in the F2 generation are:

• 1 PP (homozygous dominant)
• 2 Pp (heterozygous)
• 1 pp (homozygous recessive)

This gives a phenotypic ratio of 3:1, where three plants will have purple flowers and one will have white flowers.

Dihybrid Crosses

A dihybrid cross examines the inheritance of two traits at the same time. Let’s consider a cross between two pea plants that differ in two traits: seed shape (round or wrinkled) and seed colour (yellow or green).

Step 1: Parental Generation (P Generation)

Consider the following genotypes:

• Plant 1: RrYy (round yellow seeds)
• Plant 2: rryy (wrinkled green seeds)

Step 2: Gamete Formation

Each parent can produce four types of gametes:

• Plant 1 can produce: RY, Ry, rY, and ry
• Plant 2 can only produce: ry

Step 3: F1 Generation

The F1 generation will have the following genotypes:

|   | RY | Ry | rY | ry |
|---|----|----|----|----|
| r | RrYy | Rryy | rrYy | rryy |

Step 4: F2 Generation

When the F1 generation is self-fertilized, the Punnett square will show the combinations of alleles:

|   | RY | Ry | rY | ry |
|---|----|----|----|----|
| RY | RRYY | RRYy | RrYY | RrYy |
| Ry | RRYy | RRyy | RrYy | Rryy |
| rY | RrYY | RrYy | rryy | rryy |
| ry | RrYy | Rryy | rryy | rryy |

From this cross, you can determine the phenotypic ratio for the two traits. The expected ratio is 9:3:3:1 for round yellow, round green, wrinkled yellow, and wrinkled green seeds respectively.

Conclusion

Mendelian genetics provides a foundation for understanding how traits are inherited. By studying monohybrid and dihybrid crosses, you can predict the genotypes and phenotypes of offspring based on the parental traits.

Check your understanding

1. What is the difference between genotype and phenotype?
2. Explain Mendel's Law of Segregation.
3. In a monohybrid cross between a homozygous dominant and a homozygous recessive plant, what will be the genotype of the F1 generation?
4. What is the expected phenotypic ratio in the F2 generation of a dihybrid cross?