Mendelian Genetics

• Mid—1800s—Gregor Mendel studied how traits were passed from one generation of pea plants to the next. He discovered the basic laws of genetics.

• Purebred—Organisms that produced offspring identical to themselves. Pea plants can self-pollinate.

• Mendel crossed plants that were purebred for different traits. For example, he crossed tall plants and short plants.

Mendelian Genetics

• When he crossed purebred tall with purebred short plants, all of the offspring were tall. This helped him to discover the following:– Genes come in different varieties called alleles (tall allele, short

allele)– Most traits are controlled by a pair of alleles. One allele is

inherited from each parent.– Some alleles are dominant, while others are recessive.

Dominant alleles are symbolized with capital letters. Recessive alleles are symbolized with lowercase letters. If an organism has one or two dominant alleles, they will have the dominant appearance or phenotype.

– Purebred plants are homozygous—They have two identical alleles for a trait.

– Heterozygous organisms have two different alleles for a particular trait.

Mendelian Genetics

• During Meiosis, the two alleles for a trait are separated. One allele ends up in each gamete (sex cell).

• This is called the law of segregation.

Punnett Squares

• Table used to demonstrate the possible combinations of alleles from genetic crosses.

• Genotype—actual alleles possessed by an organisms (examples: Tt; TT; tt)

• Phenotype—The traits displayed because of certain combinations of alleles. The physical appearance. (examples: tall; short)

• Genotypic ratios• Phenotypic ratios

Mendelian Genetics

• P generation—original purebred parents that started a cross.

• F1 generation—the offspring of the P generation• F2 generation—the offspring of the F1 generation.• Monohybrid cross—cross in which only one trait is

studied.• Dihybrid cross—cross in which two traits are studied.• Independent Assortment—the alleles for traits found on

different chromosomes separate independently.

Genetics

• Incomplete Dominance

• Codominance

• Polygenic traits

• Nature versus nurture