Gregor Mendel The basic laws of heredity were first formed during the mid-1800’s by an Austrian botanist monk named Gregor Mendel.

Mendel’s paper published in 1866, but was not recognized by Science until the early 1900’s. Because his work laid the foundation to the study of heredity, Mendel is referred to as “The Father of Genetics”

Mendel’s Pea Plants Mendel based his laws on his studies of garden pea plants. Mendel was able to observe differences in multiple traits over many generations because pea plants reproduce rapidly, and have many variable traits such as:

Mendel’s Experiments First: Mendel noticed that some plants always produced offspring that had a form of a trait exactly like the parent plant. He called these plants purebred plants. For instance, purebred purple flowered plants always produced purple flowered offspring and purebred white flowered plants always produced white flowered offspring.

Parent: Offspring: Parent: Offspring:

Mendel’s Second Experiment Mendel Crossed purebred plants with opposite forms of a trait. He called these plants the parental genera, or P generation. For instance, purebred purple flowered plants were crossed with purebred white flowered plants. X= Parent (P) Generation Offspring (F1 Generation) Mendel observed that all of the offspring grew to have purple flowers. None resembled the white flower. He called this generation of offspring the first filial, or F1 generation.

Dominant and Recessive Genes Mendel went on to reason that one factor (gene) in a pair may mask, or hide, the other factor. For instance, in his first experiment, when he crossed a purebred purple flowered plant with a purebred white flowered plant, all offspring were purple flowered. Although the F1 offspring all had both purple and white factors, they only displayed the purple factor. He concluded that the purple factor masked the white factor.

Dominant Trait: mask or hide other alleles, such as the “purple flower”. Recessive Trait: is masked, or covered up, whenever the dominant allele is present, such as the white flower allele.

Mendel’s Third Experiment Mendel then crossed two of the offspring (F1) purple plants produced from his first experiment. X = Offspring (F2) 3/4 Purple Parent (F1) 1/4 White Mendel called this second generation of plants (F2). To his surprise, Mendel observed that this generation has a mix of purple and white flowered plants. This occurred even though none of the F1 parents had white flowers.

Mendel’s Law of Segregation Mendel’s first law, the Law of Segregation, has three parts. From his experiment, Mendel concluded that: 1. Plant traits are handed down through “hereditary factors” in the sex cells. 2. Because offspring obtain hereditary factors from both parents, each plant must contain two factors for every trait. 3. The factors in a pair segregate (separate) during the formation of the sex cells, and each sperm or egg receives only one member of the pair.

Today, scientists refer to the “factors” that controls traits as genes. The different forms of a gene are celled alleles. Alleles that mask or hide other alleles, such as the “purple flower” allele, are said to be dominant. A recessive allele, such as the white flower allele, is masked, or covered up, whenever the dominant allele is present.

Dominant/Recessive Alleles Mendel observed a variety of dominant and recessive alleles in pea plants other that the purple/white flower allele. Dominant Recessive

Homozygous Genes What Mendel referred to as a “purebred” plant we now know this to mean that the plant has two of the same alleles for a particular trait. X = P generation (Both white flowers) Offspring (F1) ALL white flowers According to Mendel’s Law of Segregation, each parent donates one flower color gene to the offspring. Since each parent had only white flower genes to donate, all offspring will have white flower genes (homozygous) and will therefore have white flowers.

Heterozygous When both alleles for a trait are present or both alleles are different. X = P generation one of each color F1 generation all purple flowers Although the offspring have both a purple flower and a white flower allele, only the purple flower allele is expressed and is therefore dominant over the white flower allele.

Law of Independent Assortment Mendel’s second law, the Law of Independent Assortment, states that each pair of genes separate independently of each other in the production of sex cells.

Genotype: the genetic makeup of an organism with reference to a singe trait, set of traits, or all of the organisms traits. Phenotype: The appearance of an organism resulting from the interaction of the genotype and the environment.