Gregor Mendel discovered the principles of heredity. His success can be attributed to his choice of the pea plant as an experimental organism, the use of characteristics with a few, easily distinguishable phenotypes, his experimental approach, the use of mathematics to interpret his results, and careful attention to detail.
Genes are inherited factors that determine a characteristic. Alternative forms of a gene are called alleles. The alleles are located at a specific place, called a locus, on a chromosome, and the set of alleles that an individual organism possesses is its genotype. Phenotype is the manifestation or appearance of a characteristic and may refer to a physical, physiological, biochemical, or behavioral characteristic. Only the genotype—
The principle of segregation states that a diploid individual organism possesses two alleles encoding a trait and that these two alleles separate in equal proportions when gametes are formed.
The concept of dominance indicates that when two different alleles are present in a heterozygote, only the trait of one of them, the dominant allele, is observed in the phenotype. The other allele is said to be recessive.
The two alleles of a genotype are located on homologous chromosomes. The separation of homologous chromosomes in anaphase I of meiosis brings about the segregation of alleles.
Probability is the likelihood that a particular event will take place. The multiplication rule states that the probability of two or more independent events taking place together is calculated by multiplying the probabilities of the independent events. The addition rule states that the probability of any of two or more mutually exclusive events taking place is calculated by adding the probabilities of the events.
A testcross, which can reveal the genotype (homozygote or heterozygote) of an individual with a dominant trait, consists of crossing that individual with one that has the homozygous recessive genotype.
The principle of independent assortment states that genes encoding different characteristics separate independently when gametes are formed. Independent assortment is based on the random separation of homologous pairs of chromosomes in anaphase I of meiosis; it takes place when genes encoding two characteristics are located on different pairs of chromosomes.
Observed ratios of progeny from a genetic cross may deviate from the expected ratios owing to chance. The chi-
Pedigrees are often used to study the inheritance of traits in humans. Autosomal recessive traits typically appear with equal frequency in both sexes and often skip generations. They are more likely to appear in families with consanguinity (mating between closely related persons). Autosomal dominant traits also appear with equal frequency in both sexes but do not skip generations. Unaffected people do not normally transmit an autosomal dominant trait to their offspring.