Key Points

Genetics is the study of inheritance, the process of passing characters from parents to offspring, and variation, the degree by which progeny differ from their parents. These are the fundamental concepts of heredity.

In a heterozygous pair of alleles, one allele (dominant) expresses itself and masks the effect of the other (recessive). This explains why the $F_1$ generation in a monohybrid cross shows only one parental trait.

During gamete formation, the two alleles for a heritable character segregate from each other so that each gamete ends up with only one allele. This law is universally applicable and explains the reappearance of the recessive trait in the $F_2$ generation.

A cross involving one pair of contrasting traits results in a phenotypic ratio of $3:1$ (dominant:recessive) in the $F_2$ generation. The corresponding genotypic ratio is $1:2:1$ (homozygous dominant:heterozygous:homozygous recessive).

When two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair. A dihybrid cross results in a phenotypic ratio of $9:3:3:1$ in the $F_2$ generation.

A test cross involves crossing an organism with a dominant phenotype to a homozygous recessive parent. This cross determines if the dominant organism is homozygous (all dominant offspring) or heterozygous (a $1:1$ ratio of dominant to recessive offspring).

In incomplete dominance, the $F_1$ hybrid exhibits a phenotype that is intermediate between the two parental phenotypes. The phenotypic and genotypic ratios in the $F_2$ generation are both $1:2:1$.

In co-dominance, both alleles in a heterozygote express themselves fully. Human ABO blood groups are a classic example, controlled by three alleles ($I^A$, $I^B$, and i), where $I^A$ and $I^B$ are co-dominant.

Pleiotropy is when a single gene controls multiple phenotypic traits, such as in phenylketonuria. Polygenic inheritance is when a single trait, like human skin color, is controlled by multiple genes.

Proposed by Sutton and Boveri, this theory states that genes are located on chromosomes. The behavior of chromosomes during meiosis, including segregation and independent assortment, provides the physical basis for Mendel's laws.

Genes located on the same chromosome are linked and tend to be inherited together, deviating from independent assortment. Recombination, due to crossing over, generates non-parental gene combinations, and its frequency is proportional to the distance between genes.

Sex determination can be chromosomal, as in the XY system in humans (Male XY, Female XX) and the ZW system in birds (Male ZZ, Female ZW). In honeybees, it is a haplodiploid system where males are haploid and females are diploid.

Pedigree analysis is a tool used to study the inheritance of traits in human families over generations using a standardized set of symbols. It helps determine if a trait is dominant, recessive, autosomal, or sex-linked.

These are genetic disorders caused by a mutation in a single gene. Examples include autosomal recessive disorders like Sickle-cell anemia and Phenylketonuria, and X-linked recessive disorders like Haemophilia and Colour blindness.

This is an autosomal recessive disorder caused by a point mutation in the beta-globin gene, changing GAG to GUG. This substitution of glutamic acid with valine causes red blood cells to become sickle-shaped under low oxygen. The affected genotype is $Hb^S Hb^S$.

Aneuploidy is a condition caused by the gain or loss of one or more chromosomes. Common examples include Down's Syndrome (Trisomy of chromosome 21), Turner's Syndrome (monosomy, 45 with XO), and Klinefelter's Syndrome (trisomy, 47 with XXY).