Biology 12. Mendelian Genetics

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1 Mendelian Genetics Genetics: the science (study) of heredity that involves the structure and function of genes and the way genes are passed from one generation to the next. Heredity: the passing on of characteristics from one generation to the next Hereditary trait: is a characteristic under control of the genes that is transmitted from one generation to another. Allele: a hereditary trait, represented by a letter Mendelian genetics: A simplified method for describing the heredity of a characteristic Key to Mendelian genetics is an idea that each trait has a particular set of alleles that is located on a set of chromosomes. All autosomal chromosomes (body chromosomes) normally occur in pairs; called homologous pairs. Homologous pair: chromosomes that have genes in the same locations or site on the chromosome Human Karyotype: 22 somatic chromosome pairs, 1 sex chromosome pair X and Y are not homologous; Y-chromosome is smaller, therefore whatever traits are on the X or Y chromosome will be expressed if the individual is a male Gregor Mendel Augustinian monk, 1866 described the inheritance of traits in pea plants Established rules for Mendelian Genetics Seed statistical analysis for determining relationship between traits and inheritance Mendel s Laws for genetics; 1. Law of Segregation - during gamete formation the pair of genes responsible for each trait separates so that each gamete receives only one gene for each trait 2. Law of Independent Assortment - genes for different traits segregate independently during gamete formation. 3. Law of Dominance one allele for the trait will be dominant and one recessive, a masked allele is recessive, an allele that is shown is dominant. Mendelian Genetics Terms to know for completing crosses. o Cross: combining of DNA from 2 individuals o Pure strain: offspring that are identical to the parents o P generation: parent generation o F 1 generation: first generation (made by crossing Pure parents) o F 2 generation: second generation (made by crossing two F 1 individuals)

2 Mendel s Experiment Examined: flower colour, pod colour, seed colour, seed shape, etc Allowed pea plants to self-fertilize and developed pure breeding strains o Pure breeding strains give offspring that are the same as the parents when crossed with themselves. E.g. o Mendel crossed a pure smooth pea plant with a pure wrinkled pea plant o All offspring (F 1 generation) were smooth Punnet Square Smooth Wrinkled Smooth o He then crossed the F 1 generation with each other and found that the offspring were smooth and wrinkled. Punnet Square Smooth Smooth Smooth Smooth Smooth Smooth Smooth Wrinkled Ratio of offspring were 3smooth: 1wrinkled o He discovered that some traits can be masked or hidden while other traits are shown. Traits that are masked are called recessive traits Traits that mask or cause another trait not to be shown are called dominant traits Describing Characteristics Allele: the inherited characteristic (either dominant(caps) or recessive(not caps)) Dominant: those traits that can mask other traits Recessive: those traits that can be masked Phenotype: a description of how the trait is expressed Genotype: a description of the alleles that make up a characteristic Homozygous: an individual that has both alleles the same Heterozygous: an individual that has both dominant and recessive alleles expressed. Types of Traits: Pleiotropic trait: when one gene influences more than one trait ie. Phenylketonuria(PKU): Is a genetic disease caused by a mutation in the gene for the enzyme phenylalanine hydroxylase that stops the changing of phenylalanine to tyrosine. The mutation affects hair and skin pigmentation and causes mental retardation.

3 Polygenic trait: a trait that is controlled by more than one gene i.e. Hair colour: controlled by two genes; Skin colour is thought to be controlled by twelve genes. Height is controlled by three genes Mendelian trait: when one trait is controlled by one gene located in one place on a set of corresponding chromosomes. i.e wet or dry ear wax albinism (non-pigmented) Brachydactyly (shortness of fingers and toes)

4 Mendelian Crosses: ie. Mendel s Pea plants Alleles: F 1 generation = cross of pure strains of parents S -> Smooth s -> wrinkled (*recessive version must have the same letter as the dominant trait) SS X ss; homozygous smooth crossed with homozygous wrinkled. Cross s s S Ss Ss Punnett Square S Ss Ss F 1 Offspring: Alleles Phenotype Genotype Ss Smooth Heterozygous Smooth F 2 generation= F 1 X F 1 Ss X Ss Cross S s S SS Ss Punnett Square s Ss ss F 2 Offspring: Alleles Phenotype Genotype SS Smooth Homozygous Smooth Ss Smooth Heterozygous Smooth ss wrinkled Homozygous wrinkled

5 Types of crosses Monohybrid cross: between individuals focusing on the expression of 1 characteristic Dihybrid cross: between individuals focusing on the expression of 2 characteristics Incomplete Dominance: when the dominant trait cannot completely mask the recessive trait Codominance: when two distinct traits are both expressed. Linked traits: when the alleles of focus are on the same chromosome, but not the X chromosome. Sex-linked: When the alleles of focus are found on the X chromosome Multiple Allele: When more than 2 types of alleles for a characteristic. Making Predictions When completing genetic problems, complete the following steps; 1. Declare alleles for problem 2. Show parents 3. Complete Punnet square 4. Re-read problem and give response that answers the question. Applications for Genetics Recombinant DNA: DNA constructed of genetic information from different sources creating codes that would not normally be found in nature. Virology: study of viruses for understanding their spread, life cycle, use in treating cancers, development of vaccines Gene Manipulation: changing of genetic material of organisms to develop preferred characteristics Predicting the inheritance of genetic mutations within populations/ offspring Tracing Ancestry: finding family history, evolution of organisms