Introduction to Genetics How are traits passed on?

Gregor Mendel Austrian monk who studied pea plants He chose these plants to study because they were: Easy to grow Mature quickly Have sharply contrasting traits

Traits of Pea Plants

Pea Plants Naturally, pea plants self-pollinate. Mendel observed that some plants, when they were allowed to self-pollinate, always “bred” true. This means that these plants were “true-breeding” (Ex. Tall plants always produced tall plants.) Mendel cross-pollinated some plants to observe the results.

When he crossed true-breeding tall plants and true breeding short plants, he found that he always got tall plants Then, he crossed the resulting tall plants, and found that the short plant reappeared. P = Parental Generation F1 = first filial generation F2 = Second filial generation

Why did Mendel get these results? All living things have genes, or sections of chromosomes that have instructions for making different characteristics There are always two versions of these genes, one from each parent Called Alleles Represented by letters

Alleles Alleles, or the two forms of each gene, can be Examples of Dominant – this trait will always show up if the dominant allele is present Represented by capital letters Recessive – this trait will only show up in the absence of the dominant allele Represented by lowercase letters Examples of Dominant traits – tongue rollers, brown eyes, dark hair, left thumb hand clasping Recessive traits – non-tongue rollers, light eyes, light hair, right thumb hand clasping

Allele Combinations Depending on the two alleles present, an organism can be considered homozygous or heterozygous for a trait Homozygous – two alleles are the same, either both dominant or both recessive (TT or tt) Heterozygous/Hybrid – two alleles are different, one dominant and one recessive (Tt) The different arrangements of alleles is called an organism’s genotype

Phenotype An organism’s genotype will determine its phenotype, or physical characteristic For example: A plant with the alleles TT is Homozygous Dominant & one with the alleles Tt is heterozygous, and tt is read Homozygous Recessive Both plants have the dominant gene for tallness (T) and will therefore have a tall phenotype

Back to Mendel’s Plants Why did the trait for short plants disappear in the F1 generation? Why did it reappear in the F2 generation? Punnett squares can be used to demonstrate the different possible combinations of alleles…..

Punnett Squares T T t T t T t F1 results crossed  t T t T t T t

Mendel’s Laws

Law of Dominance When there are two forms of a gene for one trait, one form may be dominant over the other

Law of Segregation Two forms of each gene are segregated during the formation of reproductive cells

Law of Independent Assortment Genes for different traits may assort independently of one another