Genetics and Heredity

History Genetics is the scientific study of genes, heredity and variation in living organisms. Heredity is the passing of traits to offspring from its parents or ancestor.  Inheritance is how traits, or characteristics, are passed on from generation to generation.

Gregor Mendel Austrian Monk. Experimented with “pea plants”. Used pea plants because: They were available and were pure breeding (offspring identical to themselves). They reproduced quickly They showed obvious differences in the traits Understood that there was something that carried traits from one generation to the next- “FACTOR”.

Mendel cont…… In the mid-1800s, the rules underlying patterns of inheritance were uncovered in a series of experiments performed by an Austrian monk named Gregor Mendel.

Mendel's Plant Breeding Experiments Gregor Mendel was one of the first to apply an experimental approach to the question of inheritance. For seven years, Mendel bred pea plants and recorded inheritance patterns in the offspring. Particulate Hypothesis of Inheritance Parents pass on to their offspring separate and distinct factors (today called genes) that are responsible for inherited traits.

To test the particulate hypothesis, Mendel crossed true-breeding plants that had two distinct and contrasting traits (a specific characteristic that varies from one plant to another)—for example, purple or white flowers. What is meant by “true breeding?”

Monohybrid Cross: represents mating between two plants which have different alleles at one gentic location. For each monohybrid cross, Mendel cross-fertilized true-breeding plants that were different in just one character—in this case, flower color. He then allowed the hybrids (the F1 generation) to self-fertilize.

Typical breeding experiment P generation (parental generation) F1 generation (first filial generation, the word filial from the Latin word for "son") are the hybrid offspring. Allowing these F1 hybrids to self-pollinate produces: F2 generation (second filial generation). It is the analysis of this that lead to the formulation of Principle of Dominance states that some alleles are dominant and some are recessive.

Mendel studies seven characteristics in the garden pea

Statistics indicated a pattern. : Statistics indicated a pattern.

Chromosomes Homologous chromosome: one of a matching pair of chromosomes, one inherited from each parent. Sister chromatids are identical

Alleles: alternative versions of a gene. The gene for a particular inherited character resides at a specific locus (position) on homologous chromosome. For each character, an organism inherits two alleles, one from each parent

How do alleles differ? Dominant allele Recessive allele Recessive allele Dominant - a term applied to the trait (allele) that is expressed irregardless of the second allele. Recessive - a term applied to a trait that is only expressed when the second allele is the same (e.g. short plants are homozygous for the recessive allele).

Segragation: Mendel wanted the answer to another question: Had the recessive alleles disappeared or they were still present in the F1 plants: To answer this question he allowed all seven kinds of F1 hybrid plants to produce an F2 generation by self pollination. The results were that the traits controlled by recessive allele reappeared in F2 generation. Mendel formulated his second law, The law of Segregation: When each F1 plant flowers and produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of the gene. Therefore, each F1 plant produces two types of gametes- those with the allele for tallness and those with the allele for shortness.

Probability and Punnett Squares Punnett square: diagram showing the probabilities of the possible outcomes of a genetic cross Probability is the likelihood that a particular event will occur.

Genotype versus phenotype. How does a genotype ratio differ from the phenotype ratio?

Punnett squares - probability diagram illustrating the possible offspring of a mating. Ss X Ss gametes

Independent Assortment : After showing that the alleles segregate during formation of gametes, Mendel wondered if they did so independently. Or, does the segregation of one pair of alleles affect the segregation of another pair of alleles. To test this he did a Two Factor cross or a Dihybrid Cross: He crossed true breeding plants that produced round yellow peas with wrinkled green

Principle of Independent Assortment: This principle states that the genes for different traits can segregate independently during the formation of gametes. Independent assortment helps to account for the many genetic variations observed in plants, animals, and other organisms. After seeing the results of F2 cross Mendel knew that the F1 plants had the genotype of RrYy, they were all Hetrozygous for both seed shape and seed color genes. The plants in the F1 generation were formed by the fusion of gametes carrying the RY allele with gamete carrying the ry allele. Did this mean that the two dominant alleles would always stay together or they would segregate independent of each other. The F2 plants showed results that had phenotypes different from the parents. This indicated that the alleles for seed shape and alleles for seed color segregated independently of each other.

Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.

Incomplete Dominance: One allele is not completely dominant over the other. The heterozygous phenotype is somewhere in between the two homozygous phenotypes.

Codominance Both alleles contribute to the phenotype .

Multiple alleles: Many genes have more than two alleles Multiple alleles: Many genes have more than two alleles. But an individual can have only two allels

Polygenic Traits Traits produced by interaction of several genes. The wide range of skin colour in humans comes about partly because more than four different genes control this trait