Genetics: The Science of Heredity Chapter 3 Sections 1-2

Mendel’s Peas Gregor Mendel: Young priest Gardener Studied math and Science in Vienna

Mendel’s Peas Curious why some plants had different characteristics. Observed that traits were similar to parents Traits: physical characteristics of an organism Heredity: passing of traits from parents to offspring

Mendel’s Peas Peas are easy to study Have many traits that show in 2 forms Stems: tall or short Produce large number of offspring in one generation Self pollinating Pollen of one plant lands on the pistil of the same plant.

Mendel’s Peas Mendel devised a way to “cross pollinate” plants. Used a paint brush to take pollen from one plant and brush it on the pistil of another.

Mendel’s Experiment Crossed plants with opposite traits Started with purebreds Short plants always produced short plants Allowed peas with one trait to self pollinate for several generations (purebred) Purebred: plant that always produces off spring with the same form of a trait as parent

Mendel’s Experiement First experiment Crossed tall pea plants with short pea plants P generation (parents) Offspring F1 Generation (filial)

Mendel’s Experiment F1 Generation All Tall Crossed these plants- offspring F2 Generation Mix of tall and short plants

Other Traits

Dominant and Recessive Alleles Factors that control traits occur in pairs. Female contributes one trait and the male the other Individual alleles control inheritance Some are dominant and others are recessive Gene- factors that control traits Alleles- different forms of a gene Dominant alleles- traits that always show up when the allele is present Recessive alleles- traits masked when recessive allele is present

Dominant and Recessive Alleles

Mendel’s Crosses Purebred plants have two identical alleles Hybrid plant have two different alleles

Using Symbols in Genetics Standard shorthand method Capital letter represents dominant alleles Lower case letter represents recessive alleles

Punnett Square Cross between two hybrids Show possible outcomes of a genetic cross

Probability and Genetics The likelihood that a particular event will occur Principles of Probability Coin toss Heads and tails- 1 in 2 chances of either happening or 1:2 or ½ or 50% Toss a coin 20 times You would expect it to be heads or tails 10 times Did that happen?

Mendel and Probability The first to recognize that the principles of probability can be used to predict the results of genetic crosses. He used Punnett Squares to show this F1 was always tall- 100% F2 was 75% tall and 25% short

Phenotype and Genotype The physical appearance, or visible traits Genetic makeup, or allele combinations

Codominance Alleles are neither dominant or recessive. Both alleles are expressed in the offspring See page 92-93