Mendel’s Theory

1. Mendel’s Theory Section 8-2

2. Mendel’s Work Became a Theory of Heredity • Mendel’s experiments changed people’s belief that an offspring got his/her characteristics from blending of the two parents’ characteristics. • Mendel stated that each parent has two separate “heritable factors” for each trait – one from each parent. • When gametes form, only one of the two factors for each trait is given to a gamete. • When gametes fuse during fertilization, the resulting offspring has two factors for each trait.

3. Mendel’s Hypotheses • Mendel developed four hypotheses that were based directly on the results of his experiments. • These four hypotheses now make up the Mendelian theory of heredity.

4. Hypothesis One • For each inherited trait , an individual has two copies of the gene – one from each parent.

5. Hypothesis Two • There are alternative versions of genes. Today these are called alleles. • In the case with flower color, the alleles would be purple or yellow. • One allele comes from each parent.

6. Hypothesis Three • When two different alleles occur together, one of them may be completely expressed, while the other may have no observable effect on the organism’s appearance. • Dominant – the expressed form of the trait • Recessive – the form of the trait that was not expressed when the dominant form is present.

7. Hypothesis Four • When gametes are formed, the alleles for each gene in an individual separate independently of one another. Thus, gametes carry only one allele for each inherited trait. • When gametes unite during fertilization, each gamete contributes one allele. • Parents can only contribute one allele because of the way gametes are formed in meiosis.

8. Mendel’s Findings in Modern Terms • Dominant alleles are represented by the first letter of the trait as a capital letter. • Recessive alleles are represented by the same letter as the dominant trait, but in lower case. • Homozygous – when two alleles are the same for a particular gene • Heterozygous – when the two alleles for a particular gene are different

9. In heterozygous individuals, only the dominant allele is expressed. • The recessive allele is present, but not expressed. • The set of alleles an individual has is called its genotype. • The physical appearance of a trait is called a phenotype. • Phenotypes (physical appearance) are determined by the genotype (set of alleles). • Dominant form of trait is written first, then the recessive form.

10. If purple is the dominant trait (P) and white is the recessive trait (p) what would the following be? • Homozygous Recessive pp and white • Homozygous Dominant PP and purple • Heterozygous Pp and purple

11. Mendel’s Ideas Gave Rise to the Laws of Heredity • Mendel’s ideas or hypotheses are often referred to as the Laws of Heredity • The Law of Segregation • Describes the behavior of chromosomes during meiosis, when homologous chromosomes and then chromatids are separated • States that the two alleles for a trait segregate (separate) when gametes are formed

12. The Law of Independent Assortment • Mendel studied whether the inheritance of one trait influenced the inheritance of a different trait. • He conducted dihybrid crosses to determine this. • Dihybrid cross – a cross that considers two pairs of contrasting traits • Found that the inheritance of one trait does not influence the inheritance of another trait • States that the alleles of different genes separate independently of one another during gamete formation • We now know this only applies to genes located on different chromosomes or that are far apart on the same chromosome.

13. Summary • Different versions of a gene are called alleles. An individual usually has two alleles for a gene, each inherited from a different parent. • Individuals with the same two alleles for a gene are homozygous; those with two different alleles for a gene are heterozygous. • The law of segregation states that two alleles for a trait separate when gametes are formed. The law of independent assortment states that two or more pairs of alleles separate independently of one another during gamete formation.

14. Homework • Section 8-2 Review Questions p. 167 #1-5