What Traits Mendel Studied and His Genetic Terms

• Mendel worked with pea varieties that differed in seven easily recognizable traits, this can also be referred to as an individual’s phenotype (a trait that can be observed.)

• Genotype refers to the two alleles an individual has for a specific trait. The two categories of genotypes are homozygous and heterozygous.

• Mendel’s pea population had two distinct phenotypes for each of the seven traits.

• A trait is a variant for a character, for example: brown.

• True breeding is a term that describes how all of the offspring are of the same variety.

• A monohybrid cross describes the mating between two parents that differ in only one trait.

• We label the different generations of a cross as: P generation (parents), F1 generation (first filial generation), and F2 generation (second filial generation).

• Testcross is a process that is used to assess whether an individual displaying a dominant trait is heterozygous or homozygous

The Monohybrid Cross

• Mendel crossed plants with round seeds and plants with wrinkled seeds.

• He observed that all of the F1 offspring had round seeds, and this contradicted the hypothesis of blending inheritance.

• The genetic determinant for wrinkled seeds seemed to have disappeared.

• Mendel allowed the F1 progeny to self-pollinate and the wrinkled seed trait reappeared in the F2 generation.

Mendel’s Law of Segregation

• Mendel’s Law of Segregation how each individual has a pair of factors for each trait called alleles. The alleles segregate during gamete formation.

• each gamete contains only one allele from each pair of factors.

• Fertilization gives the offspring two factors for each trait.

Mendelian Genetics

• Each trait in a pea plant is controlled by two alleles (alternate forms of a gene).

• Alleles can be dominant or recessive.

• A dihybrid cross was performed involving pea plants that differed in two traits.

• The two possible results were: the F2 generation with two phenotypes (identical to the parents) or the F2 generation with four phenotypes.

Mendel’s Law of Assortment

• Each pair of alleles segregates independently of the other pairs.

• All possible combinations of alleles can occur in the gametes.

• The dihybrid cross always yields a phenotypic ratio of 9:3:3:1 in the F2 generation.

Human Genetic Disorders

• Genetic disorders can be either autosomal or sex-linked.

• Autosomal disorders affect autosomes and sex-linked disorders affect the X and Y chromosomes.

• To have an autosomal dominant disorder, the individual who has it would either have to have AA or Aa.

• Examples of autosomal dominant disorders would be Osteogenesis Imperfecta (“brittle bone disease”) and Hereditary Spherocytosis (causes abnormal red blood cells that hurt easily).

• To have an autosomal recessive disorder, the individual who has it would have homozygous recessive alleles (aa).

• Examples of autosomal recessive disorders would be Methemoglobinemia and Cystic Fibrosis.