Exceptions To Mendel's Laws
The seven traits that Mendel evaluated all assort independently, but not all sets of traits do. Independent assortment is true for the seven traits that Mendel evaluated, and holds generally true for traits (genes) found on non-homologous chromosomes. Any chromosome carries a collection of traits located on a long string of DNA, and the traits are therefore physically linked in a series or sequence. A non-homologous chromosome carries a unique collection of traits on a long string of DNA, that is different from the gene collection of an other non-homologue. Normal nonhomologous chromosomes are not attached to each other during meiosis, and move independently of one another, each carrying their own gene collection. Each chromosome, composed of a long string of DNA, carries a collection of genes, with each gene showing up in a particular form or type. Seven chromosomes reside within a pea gamete, and each of the traits Mendel chose to study lie on a different (non-homologous) chromosome.
Independent assortment is not true for the collection of traits that are located on a homologous chromosome. In eukaryotes, homologues come in pairs, one donated from each parent. Two homologous chromosomes carry the same collection of genes, but each gene can be represented by a different allele on the two homologues (a heterozygous individual). A gamete will receive one of those homologues, but not both. Genes or alleles that travel together on a chromosome do not show independent assortment, because they do not move independently of each other into a gamete.
Punnett and William Bateson (1861-1926), an English biologist, published the first report of gene linkage in peas. A comparison between the ratios at which certain genes were inherited and the expected Mendelian ratios showed that the traits did not assort independently.
Sometimes two traits on non-homologous chromosomes affect each other's phenotypic expression. Purple flowers, for example, occur only with the presence of at least one dominant allele from two different genes. Off-spring of two parents who are heterozygotes for both genes produce flowers that are purple or white at a ratio of nine to seven. The genotypes of the purple offspring are either aaB-or A-bb. (A dash indicates that the individual could have either a dominant or a recessive allele.)
Incomplete dominance occurs when a heterozygote has a unique phenotype. Pink flowers, for example, result when one parent is homozygous white and the other homozygous red. Neither allele hides the other, and their appearance together creates a unique intermediate phenotype.
Alleles are said to be codominant when heterozygotes express both alleles but neither affects the other's character. Individuals who have the allele for blood type A and the allele for blood type B, for example, have the characteristics of both blood types and are referred to as being of blood type AB.
Modern studies of genetic diseases use Mendel's ratios to determine whether or not genes are linked to certain chromosomes. Family histories are converted into pedigrees to help understand inheritance patterns. A disease might skip generations as expected of recessive alleles, or be linked to other traits. Deafness, white hair, and blue eyes are linked in cats, for example. A disease's symptoms might also become more severe with successive generations, as is the case with some dominant alleles.
Mendelian genetics and molecular biology together can elucidate the function of genes that are critical for development and life, in both experimental animals and human beings. Understanding of genetic processes can help to cure diseases.
Susanne D. Dyby
Mendel, Gregor. Trans. C. T. Druery, and William Bateson. "Experiments in Plant Hybridization." (1866). MendelWeb. http://www.netspace.org/MendelWeb>.