Genetics: Genotype and Phenotype

• Genetics is the study of heredity or the inheritance of traits
• Traits are determined by genes
• There are two copies of every gene, one from one's mother and one from one's father
• Two copies of similar genes are called alleles
• Gregor Mendel, an Austrian monk, did the first detailed studies in genetics
• In the mid-1800s Mendel worked with pea plants and followed the patterns of inheritance for a variety of pea plant traits (color, height, flowers, etc.)
• Mendel concluded that some traits are dominant and some are recessive
• Dominant traits are always expressed if a dominant allele is present
• Two recessive copies must be present for a recessive trait to be expressed
• Example: The gene for tall is dominant over the gene for short pea plants
• Mendel came up with the terms genotype and phenotype
• The genotype is the actual genetic make up in an organism
• The phenotype is the observed characteristic (what is actually seen)
• The genotype determines the phenotype
• Example: TT & Tt = Genotypes for Tall, tt = Genotype for short
• Tall and short are phenotypes
• When the alleles are the same, the genotype is called homozygous (TT or tt)
• When the alleles are different, the genotype is called heterozygous (Tt)
• A punnett square can be used to predict the outcome of a "cross" between two types of pea plants
• Crossing a single trait like plant height is called a monohybrid cross
• The parent generation TT and tt are known as the P1 Generation
• The offspring are known as the first filial or F1 Generation
• Performing a testcross by crossing the plant with a short plant and then looking at the offspring determines if a tall plant is TT or Tt
• The offspring will indicate the genotype of the tall parent
• If there are any short plants, then the tall plant had to be Tt
• Some human traits follow these rules, but most are polygenic, meaning many genes influence one trait, making them much more difficult to predict
• Mendel created what are now known as Mendel's Principles of Genetics

Mendel's Principles of Genetics

• Segregation of Alleles: During gamete formation, paired alleles separate
• Independent Assortment: Genes on the same chromosome sort themselves independently of each other during meiosis
• Genes located on the same chromosome are called linked genes, predicting outcomes of linked genes as well
• Example: Plant height and plant color are located on the same chromosome
• T - allele for tall
• t - allele for short
• G - allele for green
• g - allele for yellow

Crossing a tall, green plant with a similar tall, green plant

• Parent genotypes = TtGg x TtGg
• The combination of alleles that each parent could create must first be determined.
• TtGg would create TG, Tg, tG, and tg, similar to "foiling” in math

Phenotypic Ratio

• 9/16 Tall green
• 3/16 Tall yellow
• 3/16 Short green
• 1/16 short yellow