Linkage and crossing over, polygenic inheritance

1. Introduction

1. Genetics is the study of heredity and variation in organisms.
2. Linkage and crossing over explain how genes are inherited together or separated during meiosis.
3. Polygenic inheritance describes traits controlled by multiple genes, leading to a wide range of phenotypic variations.

2. Linkage

1. Linkage is the tendency of genes located on the same chromosome to be inherited together.
2. First discovered by Thomas Hunt Morgan in fruit flies (Drosophila melanogaster).
3. Genes that are closely located on the chromosome are less likely to separate during meiosis.
4. Types of linkage:
   • Complete linkage: Genes are always inherited together (no crossing over).
   • Incomplete linkage: Genes are inherited together but can be separated by crossing over.

3. Crossing Over

1. Crossing over is the exchange of genetic material between homologous chromosomes during prophase I of meiosis.
2. It leads to genetic recombination and increases genetic diversity.
3. Occurs at specific regions called chiasmata.
4. The frequency of crossing over is directly proportional to the distance between genes.
5. Crossing over helps in mapping genes on a chromosome.

4. Polygenic Inheritance

1. Polygenic inheritance refers to the inheritance of traits controlled by multiple genes.
2. Unlike Mendelian traits, these do not show a clear dominant-recessive pattern.
3. Results in continuous variation in traits.
4. Examples of polygenic traits:
   • Human skin color: Controlled by multiple genes affecting melanin production.
   • Height: Influenced by several genes as well as environmental factors.
   • Eye color: Determined by multiple genes affecting pigment deposition.
5. Follows a bell-shaped curve in population distribution.

5. Differences Between Linkage, Crossing Over, and Polygenic Inheritance

6. Significance of These Concepts

1. Linkage helps explain why some traits are inherited together.
2. Crossing over increases genetic variation, which is essential for evolution.
3. Polygenic inheritance explains complex traits in humans and plants.
4. Used in genetic mapping and breeding programs.