alleles particular forms of genes heterozygotes individuals whose genetic information contains two different forms (alleles) of a particular gene progeny offspring phenotype observable characteristics of an organism genotype set of genes present

Classical Hybrid Genetics


AA x aa homozygous homozygous dominant recessive

Aa x Aa heterozygotes






A a





a a





All offspring are heterozygotes, whose phenotype matches the AA parent.

Offspring genotypes:

1AA : 2Aa : 1aa Offspring phenotypes: 3 dominant: 1 recessive

If these offspring show the dominant phenotype, then ? = A. If they show the recessive phenotype, then ? = a.

Punnett squares are used to track the inheritance of traits. The upper- and lowercase letters represent dominant and recessive alleles, respectively. P1 is the parental generation, and F1 is the first filial, or offspring, generation. A test cross is performed to determine the genotype of an organism showing the dominant trait, but whose genotype is unknown.

form, then the parent with the dominant phenotype had to carry one recessive and one dominant allele for that trait. In contrast, if all its offspring showed only the dominant form of the trait, the plant with the dominant phenotype must have contained only dominant alleles.

By categorizing and counting offspring of several generations of plants, Mendel discovered two laws of inheritance, described as segregation and independent assortment. Segregation meant that a gamete or reproductive cell received only one allele out of a choice of two alleles carried for each trait (gene) within a parent cell. The gamete had an equal chance of receiving either allele. Independent assortment indicated that traits entered into a gamete independently of each other. This is only true for gene or trait collections located on long strings of hereditary material (chromosomes) that are now termed "nonhomologous" (not alike). Nonhomologous chromosomes carry unique collections of specific genes or traits, whereas two homologous chromosomes carry the same collection of genes, but may carry two distinct alleles for a specific trait or gene.

Armed with his laws, Mendel was able to predict the frequency at which various alleles for several traits would co-appear in descendents of hybrids of the common garden pea. Mendel's principles were not appreciated for about thirty years, until Walter Sutton (1877-1916), an American physician and geneticist, described chromosome movements during meiosis and identified chromosomes as the carriers of genes and heredity.

Back crosses, which are based on Mendel's principles, are used to develop commercially useful plant or animal varieties. In a back cross, a heterozygous plant and its offspring are crossed repeatedly to one of their parents to develop a line of plants that mostly resemble one parent but that have an allele of interest from the other. For instance, a rare flower color is integrated into a line of nematode-resistant stock of roses, joining beauty and disease-resistance. see also Genetics; Inheritance Patterns; Mendel, Gregor; Mendelian Genetics; Probability.

Susanne D. Dyby


Sutton, Walter S. "The Chromosomes in Heredity." Biological Bulletin 4 (1903): 231-251.

-. "On the Morphology of the Chromosome Group in Brachystola Magna."

Biological Bulletin 4 (1902): 24-39.

Internet Resource

Mendel, Gregor. "Experiments in Plant Hybridization." (1866). C. T. Druery and William Bateson, trans. MendelWeb. <>.

Clinical Geneticist

Medical genetics is the application of genetics to the study of human health and diseases. As a profession, medical genetics is usually a mixture of both clinical services and research. Worldwide, services can include diagnosis, counseling, and management of birth defects and genetic disorders. How medical genetics is actually practiced depends on several factors, including the expertise and training of the professionals involved, the expertise available within any given medical facility, and the structure of the practice of medicine within a given society.

In the United States, the practice of medical genetics includes two different career tracks, both requiring certification by the American Board of Medical Genetics (ABMG): the medical geneticist and the clinical geneticist. A medical geneticist holds a Ph.D. and is certified in medical genetics. Typically a medical geneticist is a highly trained research laboratory professional who can additionally take on the role of consultant to physicians. A clinical geneticist is a physician (either a doctor of osteopathy or a medical doctor) involved in all parts of clinical practice related to genetic disorders. Working closely with patients, clinical geneticists identify, diagnose, determine the prognosis of, develop predictive tests for, treat, and manage genetic diseases. They can also be active in conducting research on genetic disorders and studying theoretical genetics, and they usually help to administer and set policies for the clinical genetics profession and for medical centers in general.

Clinical geneticists also can be involved in the bioethical debates and policy-making issues concerning how genetic information is gathered, who has access to it, and how that access should be regulated. This role is becoming increasingly important as society struggles to deal with the tremendous explosion of genetic information arising as a consequence of the Human

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