Understanding Genetics through Mendel's Garden Pea Experiments

1. Genetics and Inheritance

2. Gregor Mendel and the Garden Pea • Genetics – the scientific study of heredity • Why the garden pea? • Easy to grow • Produce large numbers • Mature quickly • Reproductive organs in same flower (self fertilization or cross fertilization)

3. Mendel’s Experiments • Step 1 – produced pure breed plants – Parental or P generation • Self fertilized for many generations • Used seeds from these to produce offspring with one form of a particular trait • Step 2 – cross two varieties in the P generation • F1 generation – first filial • Crossed purple anthers and white stigmas • Produced only purple

4. Mendel’s Experiment • Step 3 – allow F1 to self-fertilize • F2 generation – second filial • White reappeared (3:1 ratio – purple to white) • Mendel developed a model to explain his results (Rules about inheritance) • Parents transmit information about traits to their offspring (factors) • Each individual has two factors from each trait (one from each parent) • Homozygous – same information • Heterozygous – different information

5. Mendel’s Experiments • Step 3 continued • Alternative forms of a factor – alleles • The alleles an organism possesses – genotype • Physical appearance (determined by the alleles) – phenotype • An individual possesses two alleles for each trait • The presence of an allele does not mean it will be expressed • Dominant – allele is expressed • Recessive – allele is NOT expressed (in heterozygous)

6. Visualizing Mendel’s Model • Punnett square – diagram used to predict the results of a cross • Probability – the likelihood that something will happen

7. Mendel’s Conclusions • Law of segregation – the members of each pair of alleles separate when gametes are formed • A gamete will receive one alleles or the other • Law of independent assortment – two or more pairs of alleles segregate independently of one another during gamete formation

8. Predicting the Outcome of a Cross • Genes combine according to the rules of probability • The probability of a gamete having an allele is 50% or one-half • Simple Punnett square is monohybrid • 4 offspring

9. Visualizing a Cross

10. Other Factors that Influence Heredity • Incomplete codominance (snapdragons) • Homozygotes – show red or white • Heterozygotes – show pink • Codominance • Both genes are fully expressed • Roan – red and white hairs

11. Other Factors that Influence Heredity • Traits that arise from multiple alleles • Blood types – three alleles IA, IB, and I – four blood groups (Table 7-2) • Polygenic – traits controlled by many genes • Examples – height, weight, hair and skin color • Gene expression affected by environmental conditions • Arctic fox – temperature affects coat color