Mendel and Monohybrid Inheritance
The basic genetic principles first worked out and described by Gregor Mendel
in his classic experiments on the common garden pea have been found to apply to many inherited traits in all sexually reproducing organisms, including humans. Until the work of Mendel, plant and animal breeders tried to formulate laws of inheritance based upon the principle that characteristics of parents would be blended in their offspring. Mendel’s success came about because he studied the inheritance of contrasting or alternative forms of one phenotypic trait at a time. The phenotype of any organism includes not only all of its external characteristics but also all of its internal structures, extending even into all of its chemical and metabolic functions. Human phenotypes would include characteristics such as eye color, hair color, skin color, hearing and visual abnormalities, blood disorders, susceptibility to various diseases, and muscular and skeletal disorders.
Mendel experimented with seven contrasting traits in peas: stem height (tall vs. dwarf), seed form (smooth vs. wrinkled), seed color (yellow vs. green), pod form (inflated vs. constricted), pod color (green vs. yellow), flower color (red vs. white), and flower position (axial vs. terminal). Within each of the seven sets, there was no overlap between the traits and thus no problem in classifying a plant as one or the other. For example, although there was some variation in height among the tall plants and some variation among the dwarf plants, there was no overlap between the tall and dwarf plants.
Mendel’s first experiments crossed parents that differed in only one trait. Matings of this type are known as monohybrid crosses, and the rules of inheritance derived from such matings yield examples of monohybrid inheritance. These first experiments provided the evidence for the principle of segregation and the principle of dominance. The principle of segregation refers to the separation of members of a gene pair from each other during the formation of gametes (the reproductive cells: sperm in males and eggs in females). It was Mendel who first used the terms “dominant” and “recessive.” It is of interest to examine his words and to realize how his definitions remain appropriate: “Those characters which are transmitted entire, or almost unchanged by hybridization, and therefore in themselves constitute the characters of the hybrid, are termed the dominant and those which become latent in the process recessive.” The terms dominant and recessive are used to describe the characteristics of a phenotype, and they may depend on the level at which a phenotype is described. A gene that acts as a recessive for a particular external trait may turn out not to be so when its effect is measured at the biochemical or molecular level.
An Example of Monohybrid Inheritance
The best way of describing monohybrid inheritance is by working through an example. Although any two people obviously differ in many genetic characteristics, it is possible, as Mendel did with his pea plants, to follow one trait governed by a single gene pair that is separate and independent of all other traits. In effect, by doing this, the investigator is working with the equivalent of a monohybrid cross. In selecting an example, it is best to choose a trait that does not produce a major health or clinical effect; otherwise, the clear-cut segregation ratios expected under monohybrid inheritance might not be seen in the matings.
Consider the trait of albinism, a phenotype caused by a recessive gene. Albinism is the absence of pigment in the hair, skin, and eyes. Similar albino genes have been found in many animals, including mice, buffalo, bats, frogs, and rattlesnakes. Since the albino gene is recessive, the gene may be designated with the symbol c and the gene for normal pigmentation as C. Thus a mating between a homozygous normal person (CC) and a homozygous albino person (cc) would be expected to produce children who are heterozygous (Cc) but phenotypically normal, since the normal gene is dominant to the albino gene. Only normal genes, C, would be passed on by the normally pigmented parent, and only albino genes, c, would be passed on by the albino parent. If there was a mating between two heterozygous people (Cc and Cc), the law of segregation would predict that each parent would produce two kinds of gametes: C and c. The resulting progeny would be expected to appear at a ratio of 1CC: 2Cc: 1cc. Since C is dominant to c, ¾ of the progeny would be expected to have normal pigment, and ¼ would be expected to be albino. There are three genotypes (CC, Cc, and cc) and two phenotypes (normal pigmentation and albino). By following the law of segregation and taking account of the dominant gene, it is possible to determine the types of matings that might occur and to predict the types of children that would be expected (see the table “Phenotype Predictions: Albino Children”).
Because of dominance, it is not always possible to tell what type of mating has occurred. For example, in matings 1, 2, and 4 in the table, the parents are both normal in each case. Yet in mating 4, ¼ of the offspring are expected to be albino. A complication arises when it is realized that in mating 4 the couple might not produce any offspring that are cc; in that case, all offspring would be normal. Often, because of the small number of offspring in humans and other animals, the ratios of offspring expected under monohybrid inheritance might not be realized. Looking at the different matings and the progeny that are expected, it is easy to see how genetics can help to explain not only why children resemble their parents but also why children do not resemble their parents.
Modification of Basic Mendelian Inheritance
After Mendel’s work was rediscovered early in the twentieth century, it soon became apparent that there were variations in monohybrid inheritance that apparently were not known to Mendel. Mendel studied seven pairs of contrasting traits, and in each case, one gene was dominant and one gene was recessive. For each trait, there were only two variants of the gene. It is now known that other possibilities exist. For example, other types of monohybrid inheritance include codominance (in which both genes are expressed in the heterozygote) and sex linkage (an association of a trait with a gene on the X chromosome). Nevertheless, the law of segregation operates in these cases as well, making it possible to understand inheritance of the traits.
Within a cell, genes are found on chromosomes in the nucleus. Humans have forty-six chromosomes. Each person receives half of the chromosomes from each parent, and it is convenient to think of the chromosomes in pairs. Examination of the chromosomes in males and females reveals an interesting difference. Both sexes have twenty-two pairs of what are termed “autosomes” or “body chromosomes.” The difference in chromosomes between the two sexes occurs in the remaining two chromosomes. The two chromosomes are known as the sex chromosomes. Males have an unlike pair of sex chromosomes, one designated the X chromosome and the other, smaller one designated the Y chromosome. Females, on the other hand, have a pair of like sex chromosomes, and these are similar to the X chromosome of the male. Although the Y chromosome does not contain many genes, it is responsible for male development. A person without a Y chromosome would undergo female development. Since genes are located on chromosomes, the pattern of transmission of the genes demonstrates some striking differences from
that of genes located on any of the autosomes. For practical purposes, “sex linked” usually refers to genes found on the X chromosome since the Y chromosome contains few genes. Although X-linked traits do not follow the simple pattern of transmission of simple monohybrid inheritance as first described by Mendel, they still conform to his law of segregation. Examination of a specific example is useful to understand the principle.
The red-green color-blind
gene is X-linked and recessive, since females must have the gene on both X chromosomes in order to exhibit the trait. For males, the terms “recessive” and “dominant” really do not apply since the male has only one X chromosome (the Y chromosome does not contain any corresponding genes) and will express the trait whether the gene is recessive or dominant. An important implication of this is that X-linked traits appear more often in males than in females. In general, the more severe the X-linked recessive trait is from a health point of view, the greater the proportion of affected males to affected females.
If the color-blind gene is designated cb and the normal gene Cb, the types of mating and offspring expected may be set up as they were for the autosomal recessive albino gene. In the present situation, the X and Y chromosomes will also be included, remembering that the Cb and cb genes will be found only on the X chromosome and that any genotype with a Y chromosome will result in a male. (See the table “Phenotype Predictions: Color Blindness.”)
“Carrier” females are heterozygous females who have normal vision but are expected to pass the gene to half their sons, who would be color blind. Presumably, the carrier female would have inherited the gene from her father, who would have been color blind. Thus, in some families the trait has a peculiar pattern of transmission in which the trait appears in a woman’s father, but not her, and then may appear again in her sons.
Impact and Applications
The number of single genes known in humans has grown dramatically since Victor McKusick published the first Mendelian Inheritance in Man
catalog in 1966. In the first catalog, there were 1,487 entries representing loci identified by Mendelizing phenotypes or by cellular and molecular genetic methods. In the 1994 catalog, the number of entries had grown to 6,459. Scarcely a day goes by without a news report or story in the media involving an example of monohybrid inheritance. Furthermore, genetic conditions or disorders regularly appear as the theme of a movie or play. An understanding of the principles of genetics and monohybrid inheritance provides a greater appreciation of what is taking place in the world, whether it is in the application of DNA fingerprinting in the courtroom, the introduction of disease-resistant genes in plants and animals, the use of genetics in paternity cases, or the description of new inherited diseases.
Perhaps it is in the area of genetic diseases that knowledge of monohybrid inheritance offers the most significant personal applications. Single-gene disorders usually fall into one of the four common modes of inheritance: autosomal dominant, autosomal recessive, sex-linked dominant, and sex-linked recessive. Examination of individual phenotypes and family histories allows geneticists to determine which mode of inheritance is likely to be present for a specific disorder. Once the mode of inheritance has been identified, it becomes possible to determine the likelihood or the risk of occurrence of the disorder in the children. Since the laws governing the transmission of Mendelian traits are so well known, it is possible to predict with great accuracy when a genetic condition will affect a specific family member. In many cases, testing may be done prenatally or in individuals before symptoms appear. As
knowledge of the human genetic makeup increases, it will become even more essential for people to have a basic knowledge of how Mendelian traits are inherited.
Key terms
allele
:
one of the pair of possible alternative forms of a gene that occurs at a given site or locus on a chromosome
dominant gene
:
the controlling member of a pair of alleles that is expressed to the exclusion of the expression of the recessive member
recessive gene
:
an allele that can be expressed only when the controlling or dominant allele is not present
Bibliography
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McKusick, Victor A., comp. Mendelian Inheritance in Man: A Catalog of Human Genes and Genetic Disorders. 12th ed. Baltimore: Johns Hopkins UP, 1998. Print.
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