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Mendel and Genetics. The appearance of an organism is the result of two factors: 1. the gene being expressed 2. the environment Mendel stated that each individual has two factors for each trait, one from each parent. The two factors may or may not contain the same information.
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Mendel and Genetics • The appearance of an organism is the result of two factors: 1. the gene being expressed 2. the environment • Mendel stated that each individual has two factors for each trait, one from each parent. • The two factors may or may not contain the same information. • The alternative forms of a factor are called alleles. • If the two factors are identical the individual is called homozygous for the trait (pure bred) • If the two factors have different information, the individual is called heterozygous. (hybrid) • In heterozygous individuals the only allele that in expressed is the dominant. • The recessive allele is present but its expression is hidden.
Mendel suspected that heredity depended on contributions from both parents and that characteristics from each parent were passed on, rather than being blended together in the offspring. Mendel began by studying a single trait using monohybrids. He used garden peas for his experiments by crossing pure-breeding varieties, e.g. smooth seed with wrinkled seed. The subsequent generations are known as filial (F) generations. Mendel and Genetics
The genotype of an individual are made up of the many alleles it possesses. An individual's physical appearance or phenotype is determined by its alleles. Genotype and Phenotype
An individual possesses two alleles for each trait; one allele is given by the female parent and the other by the male parent. When gametes form, the alleles separate randomly so that each gamete receives a copy of one of the two alleles. The presence of an allele doesn't promise that the trait will be expressed in the individual that possesses it. The genes that code for the traits being expressed can be located along the length of chromosomes and are composed of a specific sequence of nucleotides in DNA. The specific position of a gene is called its locus and each allele will be found at the same locus on each homologous chromosome. Mendel and Genetics
Inheritance Patterns • There are 3 different ways in which traits can be inherited • dominant or recessive traits • co-dominant traits • incomplete dominant traits.
One parent has a single, faulty dominant gene (D), which overpowers its normal counterpart (d), affecting that parent. When the affected parent mates with an unaffected and non-carrier mate (dd), the offspring are either affected or not affected, but they are not carriers. accessexcellence.org Dominant Inheritance
Both parents carry a normal gene (N), and a faulty, recessive, gene (n). The parents, although carriers, are unaffected by the faulty gene. Their offspring are affected, not affected, or carriers. This type of inheritance was first shown by Mendel. Recessive Inheritance
DOMINANT AND RECESSIVE CHARACTERISTICS Characteristics in the left-hand column dominate over those characteristics listed in the right-hand column. (blinn.edu)
In some organisms when both alleles are present, the organism will display an intermediate appearance which is often a combination of both parent phenotypes. An example of incomplete dominance appears in snapdragons. Incomplete Dominance
Co-dominance • In some organisms when both alleles are present, the organism will display both the characteristics in separate, distinguishable ways. • One example is in roan cattle
Co-dominance example 2 • Another example of codominance is ABO blood typing system used to determine the type of human blood. • A blood transfusion can only take place between two people who have compatible types of blood. Human blood is separated into different classifications because of the varying proteins contained in each blood type's red blood cells. These proteins are there to identify whether or not the blood in the individual's body is it's own and not something the immunity system should destroy.
Blood types • The gene for blood group has three alleles: IA, IB, IO. • An individual can only ever have any two of the three alleles in their genotype. A and B are both dominant to the recessive IO allele. • Of the six different genotypes, there are four different phenotypes. • With this type of inheritance there is: • a dominant/recessive genotype IAIO, IBIO • an incompletely dominant genotype IAIB • a recessive genotype IOIO.
