Mendelian Inheritance I 17 October, 2005 Text Chapter 14

character: a heritable feature, like flower color. Gregor Mendel’s experiments used pea plants as a model system. He examined the inheritance of characters like flower color and seed shape by mating plants and observing the offspring. character: a heritable feature, like flower color. trait: a variant of a character, like purple or white flowers. true-breeding: plants that, when self pollinated, produce offspring of the same variety Mendel’s Experiments

Mendel followed heritable characters for three generations. Mendel’s results refuted the blending hypothesis. He proposed a particulate theory of inheritance where characters are determined by genes (recipes for a character) that come in different versions (alleles).

Alleles are different versions of a gene. Diploid organisms have two copies of each gene. These copies can be the same or different. One copy was inherited from each parent. If the two alleles differ, then one, the dominant allele determines the appearance of the organism. During gamete formation, the two alleles segregate into gametes.

Mendel’s Rules of Inheritance Each parent has two alleles. Gametes contain only one allele. Offspring have two alleles - one allele from each parent. When both alleles are present, the dominant allele determines appearance. Gametes contain only one allele. Offspring have two alleles - one allele from each parent. When both alleles are present, the dominant allele determines appearance. This leads to a 3:1 ratio of offspring.

Important terms: homozygous: a diploid organism that has two copies of the same allele for a given gene. heterozygous: a diploid organism that has two different alleles for a given gene. phenotype: an organism’s appearance. genotype: an organism’s genetic makeup, its collection of alleles.

Testcross We cannot be sure of the genotype of an individual with a dominant phenotype. That individual could be homozygous or heterozygous. A testcross can reveal the genotype of the individual in question. A homozygous dominant individual will produce all dominant phenotype offspring in a testcross. A heterozygote will produce a 1:1 ratio of offspring (dominant to recessive phenotype).

Independent Assortment