He reasoned that the higher the frequency of recombination, the greater the distance between two genes on a chromosome.

Unit of Distance: Recombination frequencies are expressed in centimorgans (cM), where 1 cM equals 1% recombination frequency.

Outcome: This led to the creation of the first genetic maps (linkage maps), showing the linear arrangement of genes along chromosomes.

Definition: Inheritance of traits controlled by two or more genes, where each gene contributes additively to the phenotype.
Also known as quantitative inheritance.
Results in a continuous variation of the phenotype (e.g., bell-shaped curve).
Examples:
- Human skin color: Controlled by 3 main genes (A, B, C), each with two alleles. Darker alleles (A, B, C) contribute to more melanin; lighter alleles (a, b, c) contribute to less.
- Human height
- Kernel color in wheat
Often influenced by environmental factors.

Definition: The mechanism by which the sex of an individual is established.
Early studies on sex determination primarily used insects.
Henking (1891): Discovered the 'X body' in insect testes, observing that some sperms received it while others didn't. This 'X body' was later identified as the X chromosome, laying the groundwork for chromosomal sex determination.
Different mechanisms exist:
1. XX-XY Type (e.g., Humans, Drosophila):
  - Females are homogametic (XX); Males are heterogametic (XY).
  - Sex of offspring determined by the male parent's sperm.
  - Humans: Presence of Y chromosome (specifically SRY gene) determines maleness.
  - Drosophila: The ratio of X chromosomes to autosomes (X:A ratio) determines sex. Y chromosome is needed for male fertility but not for maleness itself.
2. ZW-ZZ Type (e.g., Birds, some reptiles, moths):
  - Females are heterogametic (ZW), producing two types of gametes (Z and W).
  - Males are homogametic (ZZ), producing only one type of gamete (Z).
  - Sex of offspring determined by the female parent's egg.
3. XO-XX Type (e.g., Grasshoppers and a large number of insects):
  - Females are homogametic (XX).
  - Males are heterogametic (XO), possessing only one X chromosome and no Y.
  - Sex of offspring determined by the male parent, based on whether the sperm carries an X or no X.