The contributions of Gregor Mendel are referred to as Mendelian genetics.
It involves the basic laws of inheritance and some general principles about the relationship between the genetic code and the traits that are the end product of that code.
The genetic principles described by Mendel form the basis of modern genetics.
Although farmers and herders realized they could manipulate the frequency and expression of desired traits in plants and animals, no one previous to Mendel could explain how these traits were affected through selective breeding.
The predominant belief centered on the blending of parental traits in the offspring.
Even Darwin believed in some aspects of blending inheritance, since he was unaware of Mendel's work.
The Genetic Principles Discovered by Mendel
Gregor Mendel (1822-1884) developed his theory of heredity while working with garden pea hybrids.
Purebred strains were crossed to produce hybrids, and Mendel calculated the frequencies of traits in each generation.
These results were the empirical basis for his theory.
Mendel conducted 8 years of extensive breeding experiments.
He crossed plants that exhibited different expressions of a trait and then crossed hybrids with each other.
He used only traits that were monogenic—a trait coded for by a single gene.
He reach the conclusion that each organism possess two genes from each trait—one from each parent .
Not only does each organism posses two of each gene, but genes may come in different versions. These are called alleles .
Alleles: variants of a gene.
The parental (P) generation was crossed to produce the first filial (F 1 )generation.
The F 1 generation did not have intermediate traits.
The F 1 generation was then crossed to produce the F 2 generation.
One expression of the trait, shortness of the stem or wrinkling of the seeds, for example, disappeared in the F 1 generation, but reappeared in the F 2 generation.
The expression that was present in the F 1 generation occurred more often in the F 2 generation (in a 3:1 ratio).
Mendel concluded that discrete units, occurring in pairs and separating into different sex cells, must control the traits.
This is Mendel's principle of segregation.
Some alleles are dominate and some are recessive
Dominate: the allele of a pair that is expressed in the phenotype.
Recessive: the allele of a pair that is only expressed if homozygous.
Homozygous: having two of the same allele in a gene pair.
Heterozygous: having tow different alleles in a gene pair.
Dominate alleles are not necessarily better or more common.
They simply mean that if two alleles in the relationship are in a heterozygous genotype, the action of the dominate will be expressed and the action of the recessive will be hidden.
Dominance and Recessiveness
EX: PTC a chemical that people can either taste or not. For those who can, it has a bitter flavor.
The taster trait is monogenic, but the trait has two alleles: T, which codes for the ability to taste PTC, and t , which codes for the inability to taste the chemical.
Two genes for the taster trait—one from the father and one from the mother—with three possible combinations
Mendel made crosses with two traits simultaneously, such as plant height and seed color.
The results indicated that the proportion of F 2 traits did not affect each other.
Mendel stated this relationship as the principle of independent assortment.
The loci coding for height and seed color happened to be on different chromosomes that assort independently of each other during meiosis and were therefore not linked.
How Inheritance Works
Genes come in pairs, so to do the chromosomes that carry these genes.
Thus, the members of a pair of genes are found on a pair of chromosomes.
When a cell divides, each chromosome copies itself.
Mendelian Inheritance in Humans
Mendelian traits are also called discrete traits or traits of simple inheritance.
There are over 9,600 Mendelian traits in humans.
Most are biochemical in nature and the result of harmful alleles.
Traits may be inherited either as dominant or recessive alleles. Recessive conditions are typically associated with the lack of a substance.
Individuals who are heterozygous are termed carriers.
The probability of having an affected child when both parents are carriers is 25%.
The ABO blood groups are inherited in a Mendelian fashion.
Dominance, recessiveness, as well as codominance are illustrated in this system.
Alleles for most traits do not work this neatly.
In most cases, heterozygous genotypes result in phenotypes that exhibit some action of both alleles.
codominate -- blood types
codominate: when both alleles of a pair are expressed in the phenotype.
Genotype Antigens on Phenotype
Red blood cells
AA, AO A A
BB, BO B B
AB A and B AB
OO none O
Misconceptions Regarding Dominance and Recessiveness
Some traits, such as eye color, are mistakenly described as having Mendelian inheritance.
Eye color is in fact determined by alleles occurring at two or three loci.
Dominance and recessiveness are not all-or-nothing situations.
Recessive alleles may have an effect on the phenotype in the heterozygous condition.
Several alleles are known to have effects on the phenotype at the biochemical level.
Dominant alleles are not "stronger", "better", or more common than recessive alleles.
Patterns of Inheritance
Six different modes of Mendelian inheritance have been identified in humans through the use of pedigree analysis: autosomal dominant, autosomal recessive, X-linked recessive, X-linked dominant, Y-linked, and mitochondrial.
Autosomal dominant traits are governed by loci on the autosomes.
All affected family members have at least one affected parent.
Males and females are equally affected.
Autosomal recessive traits are also influenced by loci on autosomes.
Pedigrees for autosomal recessive traits differ from those for autosomal dominant traits.
Recessive traits may appear to skip generations if both parents are carriers.
Most affected individuals have unaffected parents.
The frequency of affected offspring from most matings is less than 50%.
As in autosomal dominant traits, males and females are equally affected.
Sex-linked traits are affected by loci on either the X or Y chromosome.
Most of the approximately 250 known sex-linked traits have loci on the X chromosome.
Because females have two X chromosomes, they have an autosomal-like pattern of expression.
Males, having only one X chromosome, are hemizygous, and cannot express dominance or recessiveness for X-linked traits.