Gregor Mendel•Founder of genetics•Augustinian monk (1822-1884)•First to use scientific method to study inheritanceBlending theory of inheritance• Popular belief until about 1900-Hereditary traits blend evenly in offspring through mixing of parents’ blood• Does not explain some observations:-Extremes do not gradually disappear-Offspring sometimes have different traits than either parentMendel’s experimental model – the garden pea• In 1866, Mendel-correctly argued that parents pass on to their offspring discrete “heritable factors” and-stressed that the heritable factors (today called genes), retain their individuality generation after generation.• A heritable feature that varies among individuals, such as flower color, is called a character.• Each variant for a character, such as purple or white flowers, is a trait.• True-breeding varieties-Self-fertilized plants (same trait each generation)• Easy to cross-Cross-pollination between parents • Offspring of two different varieties are hybrids.• True-breeding parental plants are the P generation (parents).• Hybrid offspring are the F1 generation (filial).• A cross of F1 plants produces an F2 generation.Mendel - inheritance of a single character• A cross between two individuals differing in a single character is a monohybrid cross.• Mendel crossed a plant with purple flowers and a plant with white flowers.-The F1 generation produced all plants with purple flowers.-A cross of F1 plants with each other produced an F2 generation with ¾ purple and ¼ white flowers.Mendel’s law of segregation describes the inheritance of a single character• The all-purple F1 generation did not produce light purple flowers, as predicted by The blending hypothesis.• Mendel needed to explain why:-white color seemed to disappear in the F1 generation and-white color reappeared in one quarter of the F2 offspring.• Mendel observed the same patterns of inheritance for six other pea plant characters.• Mendel developed four hypotheses:1. Alleles are alternative versions of genes that account for variations in inherited characters.2. For each characteristic, an organism inherits two alleles, one from each parent. The alleles can be the same or different.• A homozygous genotype has identical alleles.• A heterozygous genotype has two different alleles.3. If the alleles of an inherited pair differ, then one determines the organism’s appearance and is calledthe dominant allele. The other has no noticeable effect on the organism’s appearance and is called therecessive allele.• The phenotype is the appearance or expression of a trait.
• The genotype is the genetic makeup of a trait.• The same phenotype may be determined by more than one genotype.