Extending Mendel's Genetics
Dominance does not always apply the way Mendel charcterized it, and alleles may be incompletely dominant or codominant. Each chromosome behaves genetically as if it were composed of genes linked together in a linear order. Genes that are in the same chromosome are said to be linked and may not undergo independent assortment. Recombination of linked genes can occur as a result of crossing over (breaking and joining of homologous chromosomes during prophase I of meiosis).
1. Some heterozygous genotypes result in incomplete dominance. Such individuals have an appearance that is intermediate between the phenotypes of the two parents.
2. There are other variations in dominance/recessiveness relationships. In codominance, a heterozygous organism expresses both phenotypes of its two alleles. The type of dominance we observe often depends on the size scale used for examination.
3. Many genes have multiple (more than two) alleles, like the gene for the human ABO blood groups which occupies a single locus on the chromosome. (see handout)
4. Pleiotropy is the ability of a single gene to affect multiple phenotypic traits. Most genes have many different effects on an organism
5. In epistasis, one gene interferes with, or some way regulates the expression of another gene.
6. Certain characters, such as human skin color or height, are quantitative characters that vary in a continuous fashion, indicating polygenic inheritance, an additive effect of two or more genes on a single phenotypic character. [Multiple independent pairs of genes may have similar and additive effects on a particular phenotype.] In polygenic inheritance, the F1 generation is intermediate between the two parental types and shows little variation. The F2 generation shows wide variation between the two parental types.
7. Environment also influences quantitative characters. Such characters are said to be multifactorial.
Mendelian Inheritance in Humans
The sex of humans and many other animals is determined by the X and Y sex chromosomes or their equivalent. Chromosomes that are not sex chromosomes are called autosomes. Normal female mammals have two X chromosomes; normal males have one X and one Y. The Y chromosome in mammals appears to be responsible for determining male sex. (See article on SRY gene.) The X chromosome contains many important genes that are unrelated to sex determination and are required by both males and females. A male receives all of his X-linked genes from his mother. A female receives X-linked genes from both parents.
1. Family pedigrees can be used to deduce the possible genotypes of individuals and make predictions about future offspring. Any predictions are usually statistical probabilities rather than absolute statements.
2. Certain genetic disorders&emdash;such as sickle--cell anemia, Tay-Sachs disease, and cystic fibrosis&emdash;-are inherited as simple recessive traits from phenotypically normal, heterozygous carriers.
3. Consanguineous matings (inbreeding) between close relatives may increase the chance that the offspring will be homozygous for a rare deleterious allele.
4. Although they are far less common than the recessive type, some human disorders are due to dominant alleles. Dominant alleles that are lethal may kill the organism as an embryo or act later, as in Huntington's disease. Outbreeding, mating of totally unrelated individuals, increases the probability that the offspring will be heterozygous at many loci. These heterozygous individuals may be stronger and better able to survive than either parent (hybrid vigor).
5. Using family histories, genetic counselors aid couples in determining the odds that their children will have genetic disorders. For certain diseases, tests that can identify carriers can more accurately define those odds.
6. Once a child is conceived, the techniques of amniocentesis and chorionic villi sampling can help determine whether a suspected genetic disorder is present.
7. Medical researchers are just beginning to sort out the genetic and environmental components of multifactorial disorders, such as heart disease and cancer.