GENETICS 1

1 EJC H2 Biology T4W1 Genetic Basis for Variation I Genetics & Inheritance – Genetic Basis for Variation I 1. Introduction Organisms inherit genetic information from their parents. To appreciate how characteristics or conditions are passed on in a family, it is important to understand the basic laws of inheritance. Gregor Mendel was the first person to describe the manner in which traits are passed on from one generation to the next. By experimenting with pea plant breeding, Mendel developed the principles of inheritance that described the transmission of genetic traits, before anyone knew genes exi sted. However, not all orga nisms pass their genes on the same way as the pea plant and therefore Mendel’s principles of inheritance only explain the most basic phenomena of inheritance. Other than Mendelian inheritance, you will also learn of non-Mendelian inheritance (e.g. autosoma l linkage, epistasis), as well as how the environment plays a role in determining the phenotype of an organism. Statistical tests, such as the chi -squared test, which allow us to test the significance of differences between observed an expected results of genetic crosses will also be covered. 2. Learning Outcomes u. Explain the terms: locus, allele, dominant, recessive, codominant, incomplete dominance, homozygous, heterozygous, phenotype, genotype and linkage. v. Explain how ge nes are inherited from one generation to the next via the germ cells or gametes. w. Explain how genotype is linked to phenotype. x. Use genetic diagrams to solve problems in dihybrid crosses, including those involving codominance, incomplete dominance, multiple alleles, sex linkage, autos omal linkage and epistasis. y. Use genetic diagrams to solve problems involving test crosses. z. Explain the meaning of the terms linkage and crossing -over and explain the effect of linkage and crossing-over on the phenotypic rations from dihybrid crosses. bb. Explain how the environment may affect the phenotype (including how the diet affects the differentiation of honey bees and how temperature affects fur colour of Himalayan rabbits). 3. References Campbell, N.A. and Reece, J.B. (2008). Biology, 9th edition. Pearson.

2 EJC H2 Biology T4W1 Genetic Basis for Variation I Contents 1. Introduction ............................................................................................................... 1 2. Learning Outcomes.................................................................................................... 1 3. References ................................................................................................................. 1 4. Common Terms to Understand ................................................................................. 3 5. Labelling Conventions for Alleles .............................................................................. 6 6. From Genotype To Phenotype .................................................................................. 7 7. Environmental Factors & Phenotypic Variation ........................................................ 8 8. Monohybrid Cross & The Law of Segregation ......................................................... 10 9. Dihybrid Cross & Law of Independent Assortment ................................................. 16 10. Other Allelic Interactions ..................................................................................... 21 a. Multiple Alleles ................................ ................................ ................................ .... 21 b. Codminance ................................ ................................ ................................ ......... 21 c. Incomplete Dominance ................................ ................................ ........................ 22 d. Lethal Alleles ................................ ................................ ................................ ........ 22 e. Sex Linkage................................ ................................ ................................ ........... 25 11. Pedigree Analysis ................................................................................................ 28

3 EJC H2 Biology T4W1 Genetic Basis for Variation I 4. Common Terms to Understand Term Explanation Homologous chromosome Recall: A diploid organism (2n) has 2 sets of chromosomes, one set from each parent. Homologous chromosomes are similar in size, shape, position of centromere and staining pattern. Homologous chromosomes have the same genes that determine a certain trait at corresponding loci . e.g. blood group. However, the genes may be of different alleles which give rise to different observed trait (phenotype), e.g. blood group A, B, O. Locus The position of a gene on a chromosome. Gene A gene is an ordered sequence of DNA nucleotides located at a particular locus on a particular chromosome that encodes a specific functional product, i.e. a protein or RNA molecule. Allele An allele is one of a number of alternative forms of a gene, each possessing a unique nucleotide sequence, only one of which can occur at a given locus. The differences in the nucl eotide sequences of each allele , when translated may produce a different gene product which result s in various phenotypes. Alleles are usually represented by the same letter of the alphabet with upper case for dominant alleles and lower case for recessive alleles (e.g. A and a respectively). Homozygous The diploid condition in which the alleles at a given locus on a pair of homologous chromosomes are identical, e.g. AA or aa. Heterozygous The diploid condition in which the alleles at a given locus on a pair of homologous chromosomes are different, e.g Aa. Notes to self

4 EJC H2 Biology T4W1 Genetic Basis for Variation I Genotype Genotype refers to the combination of alleles , situated on homologous chromosomes at corresponding loci (such as "Aa", "aa" or “AA”). The genotype determines a specific trait of an organism. Phenotype Phenotype refers to the characteristic (trait) of an organism that is expressed or observed. These characteristics may arise from the interaction between the genotype (i.e. the alleles on the homologous chromosomes) and the environment in which the organ ism develops. Hence, the environment may alter the appearance of the organism. For example, while Himalayan rabbits can produce black fur, the black fur is only produced on parts of the body that are cool enough. Heat from other warmer parts of the body result in white fur (Page 8). Dominant allele The allele that shows its phenotype regardless of whether it is homozygous or heterozygous. In cases involving heterozygotes, it is said that the expression of the gene product of the dominant allele masks the effect of the gene product expressed by the recessive allele. Recessive allele The allele that shows its phenotype only in the presence of another identical recessive allele (homozygous recessive). In cases involving heterozygotes, the effect of the gene product of the recessive allele is masked by the effect of the gene product expressed by the dominant allele. A B F D e a B F d e Homologous chromosomes Alleles of the same gene Different genes Homozygous dominant alleles Heterozygous alleles Homozygous recessive alleles Different gene loci Alleles on homologous chromosomes

5 EJC H2 Biology T4W1 Genetic Basis for Variation I Cross The mating of two organisms. Self- fertilisation Crossing of an organism with itself. This term is used for plants and not animals. Self-fertilisation over many generations can produce a pure line. Test cross Cross between an individual of unknown genotype or a heterozygote and a homozygous recessive individual: unknow