Starch synthesis in pea seeds: A single gene controls both seed shape and starch grain size.

Dominant allele (B) produces larger starch grains and round seeds.
Recessive allele (b) produces smaller starch grains and wrinkled seeds (due to less starch synthesis, leading to less water retention and shrinkage upon drying).

Sex Determination

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.
4. Haplodiploidy (e.g., Honeybees):
- Females (queen, workers) develop from fertilized eggs (diploid, $2n=32$ chromosomes).
- Males (drones) develop from unfertilized eggs via parthenogenesis (haploid, $n=16$ chromosomes).
- Males have no father and thus no sons, but they have a grandfather and can have grandsons (through their daughters).
- Sex determined by the number of chromosome sets, not sex chromosomes.
Environmental Sex Determination:
- In some reptiles (e.g., turtles, crocodiles), temperature during egg incubation determines sex.

Mutation

Definition: A sudden, heritable change in the genetic material (DNA sequence or chromosome structure).
Mutations are the ultimate source of new alleles and genetic variation. Recombination shuffles existing alleles.
Can be spontaneous (due to errors in replication) or induced (by mutagens).
Types of Mutations:
- A. Gene/Point Mutations:
  - Change in a single base pair of DNA.
  - Substitution: One base replaced by another.
    - Silent: No change in amino acid sequence.
    - Missense: Changes amino acid. (e.g., Sickle-cell anemia - Glu to Val).
    - Nonsense: Changes to a stop codon, premature termination of protein synthesis.
  - Frameshift: In