Mendel’s “factors” are now known to be genes encoded by DNA, and the variations are called alleles. “T” and “t” are alleles of one genetic factor, the one that determines plant size.

Mendel called an individual that has one dominant allele and one recessive allele for a trait a purebred. Mendel said that the factors that control a trait exist in pairs. Mendel’s experiments showed that the traits of an offspring were a blend of the characteristics of the parents.

Gregor Mendel, through his work on pea plants, discovered the fundamental laws of inheritance. He deduced that genes come in pairs and are inherited as distinct units, one from each parent. Mendel tracked the segregation of parental genes and their appearance in the offspring as dominant or recessive traits.

Genetics is the study of how heritable traits are transmitted from parents to offspring. Humans have long observed that traits tend to be similar in families. It wasn’t until the mid-nineteenth century that larger implications of genetic inheritance began to be studied scientifically.

Gene, unit of hereditary information that occupies a fixed position (locus) on a chromosome. Genes achieve their effects by directing the synthesis of proteins. Genes are made up of promoter regions and alternating regions of introns (noncoding sequences) and exons (coding sequences).

Human genes vary widely in the number and size of their exons and introns. Note that these are not the largest genes in the genome, but they encode many of the largest proteins. Only one transcript from each gene was used. The total number of exons for the gene may be larger than shown.