Mendel, Genes and Gene Interactions

§The study of inheritance is called genetics. A monk by the name of Gregor Mendel suspected that heredity depended on contributions from both parents and that specific characteristics from each parent were passed on to their offspring. Early theories suggested that offspring were a blend of their parents factors (called the “Blend Theory”). This could not explain the appearance of recessive traits from one generation to the next.

Mendel studied pea plants. He would breed these and then observed offspring produced. Above, a homozygous spherical seed plant is crossed with a homozygous wrinkled seed plant. Each parent produces gametes of only one kind, either S or s, producing hybrid offspring with the genotype Ss and the spherical seed phenotype.

When the F1 plants self-pollinate they produce three different genotypes and two phenotypes "spherical seed" and "wrinkled seed” in a classic 3:1 ratio. Mendel often learned most from offspring produced in the F 2 cross.

Recessive traits - The wrinkled seed phenotypes can only correspond to the "ss" genotype. What did Mendel figure out without knowing about genes and chromosomes? Dominant Traits - The spherical seed phenotype corresponds to offspring with one S allele "SS" or “Ss”genotypes. The spherical seed character is dominant and the character for "wrinkled seed" is termed recessive.

Mendel’s experiments

Cross between varieties

Alleles of a gene at a locus

Law of segregation

Genotype v. phenotype

Testcross

Independent assortment of genes

Segregation by chance

Traits can be inherited as a result of possessing specific pairs of alleles at a gene locus. For traits controlled by only one gene, inheritance is best explained by Mendelian rules of inheritance. The law of segregation results from the randomness of meiosis I separation of maternal and paternal chromosomes into different gametes. Some (dominant) alleles do not affect phenotype when paired with some other (recessive) alleles. The law of independent assortment results from the fact that some gene loci are located on different chromosomes, so the fact that a gamete gets a chromosome that is paternal, does not mean that the other chromosomes it receives with be paternal. Even alleles on the same chromosome can be “independent” if they are far enough apart that crossing over almost always occurs somewhere between those two loci. Overall lessons: