Mendel's Experiments and Heredity
12.1 Mendel’s Experiments and the Laws of Probability
Based on Gregor Mendel's work with pea plants, the principles of modern genetics were developed.
Mendel’s Model System
The garden pea has several advantageous characteristics that allowed Mendel to develop the laws of modern genetics.
Mendel's crosses involved mating two true-breeding organisms that had different traits to produce new generations of pea plants.
Garden Pea Characteristics Revealed the Basics of Heredity
Mendel's experiments with peas revealed the presence of dominant and recessive traits in the filial generations.
The rules of probability can be applied to Mendelian crosses to determine the expected phenotypes and genotypes of offspring.
12.2 Characteristics and Traits
Phenotypes and Genotypes
The observable traits expressed by an organism are referred to as its phenotype and its underlying genetic makeup is called its genotype.
The Punnett Square Approach for a Monohybrid Cross
A Punnett square applies the rules of probability to predict the possible outcomes of a monohybrid cross and their expected frequencies.
Alternatives to Dominance and Recessiveness
With the inclusion of incomplete dominance, codominance, multiple alleles, and mutant alleles, the inheritance of traits is complex process.
A gene that is present on the X chromosome, but not on the Y chromosome, is said to be X-linked.
Human Sex-Linked Disorders
Inheriting two copies of mutated genes that are nonfunctional can have lethal consequences.
12.3 Laws of Inheritance
Pairs of Unit Factors, or Genes
Genes exist in pairs within an organism, with one of each pair inherited from each parent.
Alleles Can Be Dominant or Recessive
In a heterozygote, the allele which masks the other is referred to as dominant, while the allele that is masked is referred to as recessive.
Equal Segregation of Alleles
Mendel's Law of Segregation states that a diploid organism will randomly pass one of its pair of alleles for a trait to its offspring.
Independent assortment allows the calculation of genotypic and phenotypic ratios based on the probability of individual gene combinations.
Linked Genes Violate the Law of Independent Assortment
Genes that are on the same chromosome, or "linked", do not assort independently, but can be separated by recombination.
Epistasis occurs when one gene masks or interferes with the expression of another.