Understanding Heredity: Molecular Genetics and Mendel's Laws

Unit 10: The Basis of Heredity - Molecular Genetics

Basics of Genetics

• Gene: Each piece of DNA containing information for one character.
• Alleles: Each form a gene can have.
• Locus: The physical location a gene occupies on a chromosome.
• Loci: The place occupied by several genes.
• Diploid Organisms: Individuals who have two alleles for each gene, one from the mother and one from the father. This is represented as 2n.
• Haploid Organisms: Individuals that have only one gene for each character. This is represented by n.
• Gamete: A sexual reproductive cell, haploid (n), produced by meiosis.
• Genotype: The set of alleles of an individual for one or more characters.
• Phenotype: Each of the aspects or observable manifestations of a character.
• Phenotype = Genotype + Environment
• Dominant Inheritance: One of the alleles is easier to demonstrate than the other. The strongest is called the dominant allele, and the weaker is called the recessive allele.
• Intermediate Inheritance: This is when the alleles have the same force to express themselves so that neither dominates the other.

Mendel's Laws and the Chromosome Theory of Inheritance

Mendel's 1st Law: The Law of Uniformity

When crossing two purebreds, an offspring is obtained which is identical to each other and identical to one parent.

Mendel's 2nd Law: The Law of Segregation

When crossing two hybrids, hereditary factors (alleles) of each individual are separated because they are independent and are combined with each other in every possible way.

Mendel's 3rd Law: The Law of Independent Assortment

When crossing two hybrids, each heritable trait separates independently and combines with each other in every possible way.

Chromosome Theory of Inheritance and Sex-Linked Inheritance

Boveri and Sutton, in 1902, created the "Chromosome Theory of Heredity."

• Genes are found on chromosomes.
• The place occupied by a chromosomal gene is called the "locus."
• The segregation of Mendelian factors (alleles) corresponds to the segregation of chromosomes during meiotic division (parallelism between chromosomes and genes).

Linkage and Recombination

Linked genes are those that are on the same chromosome.