Genetic analysis predates Gregor Mendel, but Mendel's laws form the theoretical basis of our understanding of the genetics of inheritance.
Mendel made two innovations to the science of genetics:
1.developed pure lines
2.counted his results and kept statistical notes
Pure Line - a population that breeds true for a
particular trait [this was an important innovation because any
non-pure (segregating) generation would and did confuse the results
of genetic experiments].
Mendel's Conclusions
1.The hereditary determinants are of a particulate nature. These determinants are called genes.
2.Each parent has a gene pair in each cell for each trait studied. The F1 from a cross of two pure lines contains one allele for the dominant phenotype and one for the recessive phenotype. These two alleles comprise the gene pair.
3.One member of the gene pair segregates into a gamete, thus each gamete only carries one member of the gene pair.
4.Gametes unite at random and irrespective of
the other gene pairs involved.
Mendelian Genetics Definitions
Allele - one alternative form of a given allelic pair; tall and dwarf are the alleles for the height of a pea plant; more than two alleles can exist for any specific gene, but only two of them will be found within any individual.
Allelic pair - the combination of two alleles which comprise the gene pair.
Homozygote - an individual which contains only one allele at the allelic pair; for example DD is homozygous dominant and dd is homozygous recessive; pure lines are homozygous for the gene of interest.
Heterozygote - an individual which contains one of each member of the gene pair; for example the Dd heterozygote.
Genotype - the specific allelic combination for a certain gene or set of genes.
Gene - structural or regulatory. Composed of a linear sequence of nucleotides with structural of regulatory capacity. Structural genes encode for proteins or RNA molecules. Regulatory genes recognize proteins and are important for control of structural genes.
Gene--- the biological unit of inheritance. Contains the encoded information regulate the expression (reading) of and/or direct the synthesis of a product necessary for an observed phenotypic trait.
Alleles are simply different forms (or versions of the same genes) and may differ from each other by as little as a single nucleotide.
relationship of genes to chromosomes - Genes are specific sequences of DNA nucleodtides that are linearly arranged along the lenght of a chromosome, and whose sequences directs either a regulatory or structural messege.relation between individual genes and individual proteins - Structural genes are specific sequences of DNA nucleodtides that are linearly arranged along the lenght of a chromosome, and whose sequences can be "translated" into a specific linear sequence of amino acids.
locus - refers to the position on a chromosome where a gene is situated. Genes (or gene loci) are said to be mapped with respect to their relative positions along the length of a chromosome.
homologous chromosomes - maternal and pateral chromosomes that share common and corresponding alleles for different traits. These alleles may be the same or different and occupy the same relative position or locus on the different chromosomes.
heterologous chromosomes - different chromosomes in the complete set. Alleles do not correspond to each other on heterologous chromosomes, and in fact, encode for completely different genes for different traits.
A trait may be described by different alleles. With respect to seed color, for example, one allele may code for yellow and another allele (situated at the same relative position on the homologous chromosome) may code for for green. Alleles may be dominant, recessive or codominant.
Dominant allele - The allele which will be phenotypically expressed when present.
Dominance - the ability of one allele to express its phenotype at the expense of an alternate allele; the major form of interaction between alleles;generally the dominant allele will make a gene product that the recessivecan not; therefore the dominant allele will express itself whenever it is present
Recessive allele - that allelele which will not be expressed phenotypically unless present as both copies of the genetic information for the trait.
Recessive - an allele whose expression is suppressed in the presence of a dominant allele; the phenotype that disappears in the F1 generation from the cross of two pure lines and reappears in the F2 generation .
Homozygous for an allele - an individual may have identical copies (or versions) of a gene on their homologous chromosomes.
Homozygous--- in a diploid cell, having like (identical information) alleles for any given gene (trait).
Heterozygous for an allele - an individual may have different copies (or versions) of the gene on their homologous chromosomes.
Heterozygous--- in a diploid cell, having unlike (diffferent information) alleles for any given gene (trait).
Testcross - the cross of any individual to a homozygous recessive parent; used to determine if the individual is homozygous dominant or heterozygous So far, all the discussion has concentrated on monohybrid crosses.
Monohybrid - the offspring of two parents that are homozygous for alternate alleles of a gene pair.
Monohybrid cross - a cross between parents that differ at a single gene pair (usually AA x aa).
Dihybrid- an individual heterozygous for two pairs of alleles (AaBb)
Dihybrid cross - a cross between two parents that differ by two pairs of alleles (AABB x aabb)
Phenotype - literally means "the form that is shown"; it is the outward,physical appearance of a particular trait Mendel's pea plants exhibited the following phenotypes:
Mendles laws
1) Segregation of alleles - during gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete.
2) Independent assortment of alleles -the law of independent assortment; during gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair.
Codominance - a relationship among alleles where both alleles contribute to the phenotype of the heterozygote. Important example is blood groups.
Incomplete dominance - the F1 produces a phenotype quantitatively intermediate between the two homozygous parents; if the product is exactly intermediate between the two homozygous parents the relationship is termed no dominance (although some have tried to substitute the term no dominance for codominance, it has not been widely accepted)
Mutation - change - a change in nucleotide sequence of a gene. This change may be small and only involve a single nucleotide, or it may be significant. Both may have significant results affecting the behavior of the product of that gene.
Mutation--- a permanent transmissible change in the genetic material, either in the biochemical sequence within the DNA, the total quantity of information within a chromosome or in the number of chromosomes.
duplication - when regions or lengths of chromosomes are duplicated. This may involve a single nucleotide or a substantial region of the chromosome.
deletion - may involve the loss of a single nucleotide, or a substantial region of the chromosome.
inversion - when some linear sequence of a chromosomes flips.