Genetics and Evolution - Principles of Inheritance and Variation

Mendel's Laws: Includes the Law of Dominance (dominant allele masks recessive), the Law of Segregation (alleles separate during gamete formation), and the Law of Independent Assortment (genes for different traits segregate independently during the formation of gametes).

Incomplete Dominance: A condition where the $F_1$ phenotype does not resemble either parent and is an intermediate, such as the pink flowers in $Antirrhinum$ $majus$ resulting from a cross between red ($RR$) and white ($rr$) flowers.

Codominance: Occurs when both alleles express themselves fully in the heterozygote, e.g., the $AB$ blood group in humans where both $I^A$ and $I^B$ alleles are dominant.

Multiple Allelism: When more than two alleles govern the same character, such as the $ABO$ blood grouping system in humans ($I^A$, $I^B$, and $i$).

Pleiotropy: A single gene exhibiting multiple phenotypic expressions, such as Phenylketonuria ($PKU$) or Sickle-cell anemia.

Polygenic Inheritance: Traits controlled by three or more genes, where the effect of each allele is additive, such as human skin color or height.

Sex Determination: Different mechanisms including $XX/XY$ (humans, $Drosophila$), $XX/XO$ (grasshoppers), and $ZZ/ZW$ (birds, where females are heterogametic).

Mendelian Disorders: Genetic disorders caused by mutation in a single gene, following Mendelian inheritance patterns (e.g., Hemophilia - $X$-linked recessive, Sickle-cell anemia - Autosomal recessive).

Chromosomal Disorders: Caused by the absence or excess of one or more chromosomes, such as Down's Syndrome (Trisomy 21: $2n + 1$), Klinefelter's Syndrome ($47, XXY$), and Turner's Syndrome ($45, XO$).