Reading: Chapter 11.1-11.4

Summary of Major Sections

11.1 Gregor Mendel

Gregor Mendel used the garden pea as the subject in his genetics studies. In contrast to preceding plant breeders, his study involved nonblending traits of the garden pea. Mendel applied mathematics, followed the scientific method very closely, and kept careful records. His results supported a particulate theory of inheritance, effectively disproving the blending theory of inheritance.

11.2 Mendel’s Laws

When Mendel crossed heterozygous plants with other heterozygous plants, called a monohybrid cross, he found that the recessive phenotype reappeared in about 1/4 of the F₂ plants. There was a 3:1 phenotypic ratio. This allowed Mendel to propose his law of segregation, which states that the individual has two factors for each trait (the genotype), and the factors segregate with equal probability into the gametes.

Although Mendel didn’t know it, the variations in the traits were due to variations in genes, called alleles. Each gene has a specific location, or locus, on a chromosome. Dominant alleles mask the expression of recessive alleles.
Mendel conducted dihybrid crosses, in which the F₁ individuals showed both dominant characteristics, but there were four phenotypes in a 9:3:3:1 ratio among the F₂ offspring. This allowed Mendel to deduce the law of independent assortment, which states that the members of one pair of factors separate independently of those of another pair. Therefore, all possible combinations of parental factors can occur in the gametes.
The laws of probability can be used to calculate the expected phenotypic ratio of a cross. A large number of offspring must be counted in order to observe the expected results, and to ensure that all possible types of sperm have fertilized all possible types of eggs, as is done in a Punnett square. The Punnett square uses the product law of probability to arrive at possible genotypes among the offspring, and then the sum law can be used to arrive at the phenotypic ratio.
Mendel also crossed the F₁ plants having the dominant phenotype with homozygous recessive plants. The 1:1 results indicated that the recessive factor was present in these F₁ plants (they were heterozygous). Today, we call this a testcross, because it is used to test whether an individual showing the dominant characteristic is homozygous dominant or heterozygous. The two-trait testcross allows an investigator to test whether an individual showing two dominant characteristics is homozygous dominant for both traits or for one trait only or is heterozygous for both traits.

11.3 Mendelian Patterns of Inheritance and Human Disease

Studies have shown that many human traits and genetic disorders can be explained on the basis of simple Mendelian inheritance. When studying human genetic disorders, biologists often construct pedigrees to show the pattern of inheritance of a characteristic within a family. The particular pattern indicates the manner in which a characteristic is inherited, such as autosomal dominant or autosomal recessive.

Autosomal recessive: An individual must possess two copies of the recessive allele to express the trait. Heterozygous individuals may be carriers for the trait.
Autosomal dominant: An individual needs to possess only a single copy of the dominant allele to express the trait. Homozygous dominant and heterozygous individuals both express the trait.

11.4 Beyond Mendelian Inheritance

Other patterns of inheritance have been discovered since Mendel’s original contribution.

For example, some genes have multiple alleles, although each organism has only two alleles, as in the inheritance of blood type in humans. Inheritance of blood type also illustrates codominance. With incomplete dominance, the phenotype of F₁ individuals is intermediate between the parent phenotypes; this does not support the blending theory, because the parent phenotypes reappear in F₂. With incomplete penetrance, some traits that are dominant may not be expressed due to unknown reasons.
Gene interactions, such as those in metabolic pathways, are examples of epistatic interactions. In pleiotropy, one gene has multiple effects, as in Marfan syndrome and sickle-cell disease. Polygenic traits are controlled by several genes that have an additive effect on the phenotype, resulting in quantitative variations within a population. A bell-shaped curve is seen, because environmental influences bring about many intervening phenotypes, as in the inheritance of height in humans. Skin color and eye color are also examples of multifactorial traits (multiple genes plus the environment).
In Drosophila, as in humans, the sex chromosomes determine the sex of the individual, with XX being female and XY being male. Experimental support for the chromosome theory of inheritance came when Morgan and his group were able to determine that the gene for a trait unrelated to sex determination, the white-eyed allele in Drosophila, is on the X chromosome.
Alleles on the X chromosome are called X-linked alleles. Therefore, when doing X-linked genetics problems, it is the custom to indicate the sexes by using sex chromosomes and to indicate the alleles by superscripts attached to the X. The Y is blank because it does not carry these genes. Color blindness, Menkes syndrome, adrenoleukodystrophy, and hemophilia are X-linked recessive disorders in humans.