YIJC [H2] CI2.12 Inheritance (N)(S)(vetted)

2025 JC2 BIOLOGY LECTURE NOTES CORE IDEA 2: GENETICS & INHERITANCE TOPIC 2.12: INHERITANCE Learning Outcomes (u) Explain the terms, locus, allele, dominant, recessive, codominant, incomplete dominance, homozygous, heterozygous, phenotype and genotype and linkage. (v) Explain how genes 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, autosomal 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 ratios from dihybrid crosses. (aa) Describe the interaction between loci (epistasis) and predict phenotypic ratios in problems involving epistasis (knowledge of the expected ratio for various types of epistasis is not required; focus of this section is on problem solving). (bb) Explain how the environment may affect the phenotype (including how diet affects the differentiation of honeybees and how temperature affects fur colour of Himalayan rabbits). (cc) Explain the difference between genetic variation that is continuous (many, additive genes control a characteristics) and genetic variation that is discontinuous (one or a few genes control a characteristics). (dd) Use the chi-squared test to test the significance between observed and expected results. Use the knowledge gained in this section in new situations or to solve related problems. Textbooks and References Campbell, Urry, Cain, Wasserman, Minorsky, Reece (2018) Biology – A Global Approach (11 th Edition)(Global Edition) Chapter 14: Mendelian Genetics pg. 319 – 340, Chapter 15: Linkage & Chromosomes pg. 344 - 361 (Pearson Publication) ISBN-10 1-292-17043-3 Note: This textbook is available in our library. You may wish to borrow them to supplement your reading when necessary. H2

2 Contents No. Sub-Topics 1 Introduction 2 Glossary of Terms 3 Genotype Influence on Phenotype 4 Inheritance of Gene from Parents 5 Mendel’s Experiment 6 Monohybrid inheritance 6.1 Mendel’s First Law: Law of Segregation 7 Dihybrid inheritance 7.1 Mendel’s Second Law: Law of Independent Assortment 8 Test Cross 9 Non-Mendelian Inheritance 9.1 Incomplete Dominance 9.2 Codominance 9.3 Multiple Alleles 9.4 Sex Linkage (includes Pedigree Analysis) 9.5 Autosomal Linkage 9.6 Epistasis 10 Environmental Influence on Phenotype 11 Variation 12 Chi-Squared (2) Statistical Test Summary Annex

3 1. Introduction • Different cells in a multicellular organism contains different number of sets of chromosomes inside the nucleus. Cells are either: o haploid - containing 1 set of chromosomes - e.g. gametes like sperm and ovum - note that haploid ≠ monoploid o diploid - containing 2 sets of chromosomes - e.g. somatic cells like skin cell - diploid number varies among organisms. e.g. cucumber 2n = 14, dog 2n = 78 Fig. 1 Human karyotype CHECKPOINT 1: In the next section, you will learn that the genes at these loci can slightly differ, bringing about variation. • The complete set of chromosomes is displayed in the _______________ (arrangement of chromosomes based on length and number). • Each chromosome contains hundreds to thousands of genes, which code for the different proteins and RNA products. The total genetic information carried by a cell, or an organism is known as the _______________. It is equivalent to the DNA of a haploid set of chromosomes from that organism. • The _______________ pattern for each pair of chromosomes observed in the karyotype is similar. That is because the genes within these chromosomes are at similar positions (_______________). Hence, they are known as _______________pair of chromosomes (homologous = “same locus”).

4 2. Glossary of terms Term Explanation Gene • A gene is a specific nucleotide sequence of DNA that occupies a specific locus in a chromosome. • It codes for a functional protein or RNA product, and hence determines a specific phenotype of an individual. • It is a discrete unit of inheritance o E.g. gene coding for height in plants Allele • An allele is an alternative form of a gene and is responsible for determining the contrasting characteristics of a gene. o For instance, there is one gene responsible for the stem height of pea plants; however, there are 2 forms of it: ▪ allele for tall stem (T) ▪ allele for short stem (t) • Occupy the same gene locus in a diploid cell • Different alleles of the same gene have their own unique nucleotide sequence and therefore codes for a different form of the gene product. o Alleles are found to be identical in most of their sequences and differ only at one/few nucleotides of the thousand nucleotides that make up the gene. o The following diagram represents the DNA of two alleles of one gene and their difference in the nucleotide sequence. Allele 1 Allele 2 • A pair of homologous chromosomes has a pair of alleles, occupying the same gene locus. Fig. 2 A pair of homologous chromosomes Locus • It refers to the specific location of a gene along the length of a chromosome. Learning Outcome 2(u): Explain the terms, locus, allele, dominant, recessive, codominant, incomplete dominance, homozygous, heterozygous, phenotype and genotype and linkage. AACCG AATTG

5 Term Explanation Homozygous • The condition in which a pair of identical alleles for a gene are present at the same gene locus on a pair of homologous chromosomes in a diploid cell. o E.g. TT or tt Heterozygous • The condition in which two different alleles for a gene are present at the same gene locus on a pair of homologous chromosomes in a diploid cell. o E.g. Tt Dominant *Represented by capital letter • An allele which can exhibit its phenotypic effect regardless of the other allele at the same locus. • Dominant allele is fully expressed in the phenotype in a heterozygote (Tt) o E.g. The tall allele, T is dominant to the short allele, t. Recessive *Represented by lower case letter • An allele whose phenotypic effect is completely masked in the presence of a dominant allele. • Recessive alleles’ phenotypic effect can only be observed when two copies are present in homozygous recessive (tt) or in the absence of a dominant allele. o E.g. The short allele, t is recessive to the tall allele, T. o E.g. The sex-linked recessive haemophilia allele, XhY Genotype • Refers to the allelic makeup of various genes, found in a cell / organism. Phenotype • Observable trait (physical or physiological) of an organism. • The phenotype of an individual is determined by the genotype and may be influenced by environmental factors. o E.g. physical traits like hair colour o E.g. physiological traits like blood group Codominant alleles • An allele that can exhibit its phenotypic effect with another allele. • In a heterozygote, codominant alleles are simultaneously and fully expressed in the phenotype. o IAIB results in blood group AB Incomplete dominance • Occurs when alleles do not show complete dominance over each other. • Heterozygote displays a blended, intermediate phenotype between the homozygous dominant and recessive traits. o Presence of allele for red flower and white flower in snapdragon results in pink flowers Linkage • When two genes are located on the same chromosome, they are described as being linked o E.g. Gene for plant height and flower colour found on the same chromosome

6 CHECKPOINT 2 1 Which of the following descriptions matches X, Y and Z in the diagram? X Y Z A Locus Allele