[BIO] Chapter 18 - Inheritance

© Hee Xin Wei (Copyrighted) Topic 18: Inheritance

Chapter Analysis FOCUS EXAM WEIGHTAGE •Constitute to around 9.5% in Paper 2 in the past 5 years •commonly tested in MCQ and structured questions •tested twice in section B in the past 5 years •may be an abstract topic •application questions

Inheritance Inheritance is the process by which genetic information is passed on from parent to child Hereditary traits that can be passed down to you by your parents: ability to roll tongue dimples right or left handed attached or free earlobe ability to taste PTC (bitter compound) allergy

variation Variations are differences in traits between individuals of the same species. Discontinuous variation Continuous variation Phenotype Few clear-cut phenotypes with no intermediate Range of phenotypes. Environment influence Rarely affected by environmental conditions Greatly affected by environmental conditions. Genes Controlled by one or few genes Controlled by many genes Additive effect Not present The effect of many genes add together and contribute the phenotype Graph Discrete groups Normal distribution Examples Blood groups, eyelid, flower colour in pea plant Height, skin colour, weight, intelligence © Hee Xin Wei (Copyrighted)

monohybrid hesitance codominance sex determination Key Concept © Hee Xin Wei (Copyrighted)

terms definitions Gene Gene is sequence of DNA nucleotides that stores information used to make a polypeptide, therefore gene is a unit of inheritance passed from parents to offspring Chromosome •Chromosome is a compact structure visible in the nucleus during cell division and it is made up of DNA •The place on the chromosome where the gene is located is called the gene locus. Allele •Alleles are different forms of a gene. •Alleles of a gene occupy the same locus on a pair of homologous chromosomes. •For example, eye colour gene has brown allele and blue allele Dominant A dominant allele is the allele that is always expressed in the phenotype, no matter under homozygous or heterozygous condition. Recessive A recessive allele is the allele that is only expressed under homozygous recessive condition. Codominant •When both alleles have an equal effect on the phenotype of the offspring. •Both alleles are expressed in the phenotype. Homozygous Organisms having two identical alleles of a particular gene. Allele can be either both dominant or both recessive. HeterozygousOrganisms having two different alleles of a particular gene. Phenotype •Phenotype refers to the expressed trait in an organism. •The phenotype of an organism is the result of its genes and the effects of its environment. Genotype •A genotype is the genetic makeup of an organism. •An organism’s genotype is homozygous for a trait if the two alleles controlling the trait are identical, heterozygous for a trait if the alleles controlling the trait are different. © Hee Xin Wei (Copyrighted)

Monohybrid Inheritance •The gene for flower colour of pea plant has two alleles: dominant purple allele (P) and recessive white allele (p’) •When a homozygous purple plants (PP) is crossed with a homozygous white plant (p’p’), each organism inherits one allele from the mother and one allele from the father during sexual reproduction •The offspring generation consisted of all purple-flowered plants even though their genotype is heterozygous (Pp’) as the dominant purple allele (P) is expressed over recessive white allele •Self-pollination in the F1 generation produced a F2 generation where the phenotypic ratio of purple-flowered to white-flowered plants is 3:1 © Hee Xin Wei (Copyrighted) xParental Generation F1 Generation F2 Generation

Genetic Diagram 3 purple flower: 1 white flower is the expected ratios, but the actual observed ratio can be different especially when there are small numbers of progeny, because •Fertilisation of the ova and sperms is a random event.  •Therefore the expected ratios are only based on chance and probabilities. With small number of offspring, the observed ratios often differ from expected ratios. But, with large number of offspring (large sample size), the observed ratios will be closer to expected ratios.

codominance Complete dominance is when the heterozygote has the same phenotype as the dominant homozygote. In pea plant flower example, Pp has the same phenotype as pp. The recessive allele present in the heterozygote is masked by the dominant allele. Co-dominance is when both alleles contribute equally to the phenotype. ABO blood group is determined by 3 alleles: -IA: Allele for the production of Type A antigen (Blood Group A) -IB: Allele for the production of type B antigen (Blood Group B) -IO: Allele that produces neither antigen (Blood Group O) IA and IB are codominant, while IO is recessive to both •For IAIB genotype, both antigen A and antigen B are expressed since they are codominant and each of the alleles produces its own antigen. Both alleles contribute to the phenotype, which is blood group AB. •For IAIO and IBIO genotype, IO is recessive to IA and IB thus, the phenotype is blood group A and B respectively. ABO blood group Blood group phenotype Homozygous Genotype Heterozygous Genotype A IAIA IAIO B IBIB IBIO AB IAIB O IOIO © Hee Xin Wei (Copyrighted)

codominance abo blood group example parents with blood group B can produce offsprings with blood group O © Hee Xin Wei (Copyrighted)