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Revision Notes Genetics and Evolution: Basic Genetics Molecular Genetics Biodiversity and Evolution Kendy Tsang Hin Fung 2 Mo Man Ying Contents Chapter 8 Basic Genetics 9 Molecular Genetics 18 Biodiversity and Evolution 37 10 1 1 8 Basic Genetics In this chapter, you should learn: Basic genetics: 1. Mendel’s laws of inheritance 2. Inheritance in humans (Multiple alleles ABO blood groups, sex linkage and sex determination) 3. Pedigree analysis 4. Variations in characteristics (Continuous variation, discontinuous variation and the causes of variation) A. Revision Notes 1. Basic concepts in genetics: Nearly all sexually reproducing organisms have paired sets of chromosomes, one set from each parent. The equivalent chromosomes that form pairs during cell division are called homologous chromosomes. They contain equivalent sets of genes for the same biological features such as eye colour. A gene is the basic unit of inheritance. Locus (plural: loci) is the position where a gene located on chromosome. A gene may have more than one form. Genotype is the genetic make-up of an organism. Phenotype is the characteristics or external appearance of an organism. between genotype and environment. It is a segment of DNA which controls hereditary character. Allele is one of the alternative forms of a gene on a locus. It is the result of the interaction 3 4 5 2 3 gene 1 a A allele a 4 allele A gene 2 B B gene 3 c c 1 2 1 5 locus is the position where a gene located on chromosome homologous chromosomes 1 8 Basic Genetics Homozygous and Heterozygous: Homozygous condition is a condition in which both homologous chromosomes possess identical copies of allele of a gene. The homozygous organism is called homozygote. Heterozygous condition is a condition in which both homologous chromosomes possess different copies of allele of a gene. The heterozygous organism is called heterozygote. a A B B c c homologous chromosomes Allele expression: The alleles which can fully express themselves in the phenotype even though different alleles are present (heterozygous conditions) are said to be dominant. The alleles which can only fully express themselves in the phenotype in the absence of different alleles or in the presence of identical alleles (homozygous conditions) are said to be recessive. Incomplete dominance occurs when neither one of the alleles in heterozygous condition is dominant. The phenotype of the heterozygote is the intermediate between two homozygotes. For example, a plant with red flowers and a plant with white flowers may give an offspring with pink flowers. Codominance occurs when both alleles in heterozygous condition can fully express themselves in the phenotype. Human ABO blood group system is an example. Some genes may have more than two alleles or alternative forms in the population as a whole. alleles is called multiple alleles. Human ABO blood group system is an example. Sex linkage is the phenotypic expression of allele related to the sex of organism. For example, some of the genes on X chromosome don’t have corresponding alleles on Y chromosome. Therefore, a single allele can be expressed in phenotype. A series of 2. Mendel’s laws of inheritance: Monohybrid inheritance: Inheritance of one well defined characteristic was studied in the experiments. Based on the results of the study of monohybrid inheritance, Mendel’s first law of inheritance was formulated. 2 8 Basic Genetics Usually, capital form is used to represent dominant character while small form is used to represent recessive character. Punnett square can also be used to show the genotype of offspring 75% of the F2 generation are tall while 15% the F2 generation are dwarf, phenotypic ratio of offspring is 3:1. Mendel’s first law (Law of segregation): Alleles exist in pairs. The characteristics of an organism are determined by a pair of alleles of genes. Alleles separate during gamete formation and enter different gametes. Each gamete possesses only one of the alleles of each gene. 3 8 Basic Genetics Dihybrid inheritance: Inheritance of two pairs of contrasting characteristics was studied in the experiments. Based on the results of the study of dihybrid inheritance, Mendel’s second law of inheritance was formulated. R = dominant round r = recessive wrinkled Y = dominant yellow y = recessive green parents Law of segregation gametes Only one type of gamete is produced from each parent. F1 generation All F1 generation must be round and yellow (RrYy). F1 gametes Each individual of F1 generation can produce four different kinds of gametes. (Independent assortment) F2 generation: F1 generation self-fertilized: RrYy x RrYy F1 generation is self-fertilized to give F2 generation Gametes RY RY RRYY RRYy RrYY RrYy Round, yellow Round, yellow Round, yellow Round, yellow Ry rY ry Ry rY ry RRYy RRyy RrYy Rryy Round, yellow Round, green Round, yellow Round, green RrYY RrYy rrYY rrYy Round, yellow Round, yellow Wrinkled, yellow Wrinkled, yellow RrYy Rryy rrYy rryy Round, yellow Round, green Wrinkled, yellow Wrinkled, green 4 16 possible offsprings produced from different combinations of F1 gametes