Key Points

Heredity is the passing of traits from parents to offspring. Variations are the differences among individuals, which are maximized in sexual reproduction and provide the basis for evolution.

Variations accumulate over generations. Advantageous variations help individuals survive better in their environment, forming the basis for natural selection.

Gregor Mendel studied inheritance in pea plants and established the basic rules of heredity. He used contrasting traits like plant height (tall/short) and seed shape (round/wrinkled).

When Mendel crossed pure tall ($TT$) and pure dwarf ($tt$) pea plants, all offspring in the first generation ($F_1$) were tall ($Tt$), showing only the dominant trait.

A dominant trait (like tallness, 'T') is expressed even if only one copy of its gene is present. A recessive trait (like shortness, 't') is only expressed when two copies are present ($tt$).

Self-pollinating the $F_1$ plants produced a second generation ($F_2$) with both tall and short plants in a phenotypic ratio of approximately $3:1$ (3 Tall : 1 Short).

The genetic makeup (genotype) ratio in the $F_2$ generation of a monohybrid cross is $1:2:1$, corresponding to the combinations $TT$ (homozygous tall), $Tt$ (heterozygous tall), and $tt$ (homozygous short).

During gamete formation, the two copies (alleles) of each gene segregate from each other so that each gamete receives only one copy. This explains the reappearance of recessive traits.

When crossing plants with two different traits (e.g., seed shape and color), the traits are inherited independently of each other. This is known as the Law of Independent Assortment.

Genes, which are segments of DNA, contain the information for making proteins. These proteins control cell functions and physical characteristics (traits), such as the production of hormones that regulate plant height.

Genes are located on chromosomes. In sexually reproducing organisms, cells have two copies of each chromosome, one inherited from each parent, ensuring equal genetic contribution.

Human sex is determined by a pair of sex chromosomes. Females have two X chromosomes ($XX$), and males have one X and one Y chromosome ($XY$).

A child inherits an X chromosome from the mother. The sex is determined by the chromosome from the father; an X-carrying sperm results in a girl ($XX$) and a Y-carrying sperm results in a boy ($XY$).