SASS Bio Heredity Notes

6093 Biology (2022) Chapter 19 Heredity SASS_PJY 1 ST ANDREW’S SECONDARY SCHOOL Secondary 4 Biology (6093) Chapters 19: Heredity Name: _______________________ ( ) Class: _________ Date: __________ 15. Inheritance Content • The Passage of Information from Parent to Offspring • The Nature of Genes and Alleles, and their Role in Determining the Phenotype • Monohybrid Crosses • Variation • Natural and Artificial Selection Learning Outcomes Candidates should be able to: define a gene as a unit of inheritance and distinguish clearly between the terms gene and allele explain the terms dominant, recessive, codominant, homozygous, heterozygous, phenotype and genotype predict the results of simple crosses with expected ratios of 3:1 and 1:1, using the terms homozygous, heterozygous, F1 generation and F2 generation explain why observed ratios often differ from expected ratios, especially when there are small numbers of progeny use genetic diagrams to solve problems involving monohybrid inheritance (genetic diagrams involving autosomal linkage or epistasis are not required) explain co-dominance and multiple alleles with reference to the inheritance of the ABO blood group phenotypes (A, B, AB and O) and the gene alleles (IA, IB and Io) describe the determination of sex in humans – XX and XY chromosomes describe mutation as a change in the structure of a gene such as in sickle cell anaemia, or in the chromosome number, such as the 47 chromosomes in the condition known as Down syndrome name radiation and chemicals as factors which may increase the rate of mutation describe the difference between continuous and discontinuous variation and give examples of each state that variation and competition lead to differential survival of, and reproduction by, those organisms best fitted to the environment give examples of environmental factors that act as forces of natural selection explain the role of natural selection as a possible mechanism for evolution give examples of artificial selection such as in the production of economically important plants and animals

6093 Biology (2022) Chapter 19 Heredity SASS_PJY 2 19.1 Heredity ● A hereditary trait is a characteristic that can be passed on from one generation to another. ● Genetics is the study of the inheritance of characteristics, or traits, by transmission of genetic material from one generation to another. ● Monohybrid inheritance refers to the inheritance of one characteristic or trait that has two contrasting forms. (dihybrid inheritance not in syllabus) 19.2 Basic Knowledge for Studying Heredity Chromosome, Gene and Alleles ● A chromosome is a highly coiled and condensed structure visible during cell division. It is made up of the molecule DNA (deoxyribonucleic acid). ● A gene locus (plural: loci) is the position on the chromosome where a gene is located. ● A gene is a basic unit of inheritance, born on a particular locus of a chromosome. It is a segment of DNA in a chromosome in which specific genetic information is stored and controls a particular characteristic or protein in an organism. ● Alleles are different forms of the same gene. They occupy the same relative position (gene locus) on a pair of homologous chromosomes. alleles N2020/P2/A3(d) Distinguish between the terms gene and allele with an illustrated example. State: A gene is a segment of DNA in a chromosome while alleles are different forms of the same gene. Elaborate: An individual can possess two alleles of one gene. Exemplify: The gene for the height of the pea plant has two alleles, short and tall.

6093 Biology (2022) Chapter 19 Heredity SASS_PJY 3 Phenotype and Genotype ● The phenotype of an organism refers to its observable traits. (e.g. blue eye color, black hair) ● The genotype is the genetic make-up of an organism, that is, the combination of genes in an organism which is inherited from its parents. o Genotypes are written in the form of letter coding, for which each letter represents one of the two alleles. ● The phenotype of an organism is influenced by its genotype and the environment. Dominant and Recessive allele ● Alleles can exist in dominant or recessive forms. ● A dominant allele expresses itself in both homozygous dominant and heterozygous conditions. ● A recessive allele will only express itself in a homozygous recessive genotype. N2020/P2/A3(d) Distinguish between the terms gene and allele with an illustrated example. State: A gene is a segment of DNA in a chromosome while alleles are different forms of the same gene. Elaborate: An individual can possess two alleles of one gene. Exemplify: The gene for the height of the pea plant has two alleles, short and tall. genes Homologous chromosomes ttT

6093 Biology (2022) Chapter 19 Heredity SASS_PJY 4 Example: T represents dominant allele for tallness while t represents recessive allele for dwarfism o A genotype of TT and Tt will both produce tall plants o Only a genotype of tt will produce dwarf plants. Homozygous and Heterozygous ● An organism is homozygous for a trait if the two alleles controlling the trait are the same. ● An organism is heterozygous for a trait if the two alleles controlling the trait are the different. Genetic diagrams ● Genetic diagrams can be used to explain how alleles are passed on to offspring or predict the traits that will be displayed by an offspring.

6093 Biology (2022) Chapter 19 Heredity SASS_PJY 5 In a breeding experiment, Mendel crossed a pure-bred tall pea plant with a pure- bred dwarf pea plant. All the F1 offspring were tall. Explain this information by means of a genetic diagram. Let T represent the dominant allele for tall pea plants and t represent the recessive allele for dwarf pea plants. If the F1 offspring were crossed amongst themselves, what proportion of the F2 generation would be expected to be heterozygous?

6093 Biology (2022) Chapter 19 Heredity SASS_PJY 6 19.3 Determining Genotypes ● ______________experiments are used to identify genotype of an organism. ● To identify the genotype of an organism showing the dominant trait, a ____________ cross is performed. This involves crossing the organism with a homozygous ____________ organism. o If the organism is homozygous dominant, all the offspring should show the dominant trait. o If the organism is _______________, half the number of offspring should show the dominant trait. The remaining half should show the recessive trait. Why is it not necessary to perform a test cross to test for the recessive genotype? Practice Question In Drosophila flies, wing shape is controlled by a gene with two alleles found on chromosome 2. Curly wings were found to be dominant over normal wings. Using genetic diagrams, determine the genotype of a curly-winged male. Highlight the important information in the question and the objective of the question before you start. Inferences: _________________________________________________________ Concepts: __________________________________________________________ Case 1: In a breeding experiment, Mendel crossed a pure-bred black guinea pig with a pure-bred white guinea pig. All the F1 offspring were black. Explain this information by means of a genetic diagram. If the F1 offspring were crossed amongst themselves, what proportion of the F2 generation would be expected to be heterozygous?

6093 Biology (2022) Chapter 19 Heredity SASS_PJY 7 Punnett square ● Punnett square is a simpler method to determine or explain genetic crosses. ● The diagrams below are Punnett squares for Mendel’s monohybrid experiment. Parents F1 hybrid self-cross TT X tt Tt X Tt Gametes t t Gametes T t T Tt Tt T TT Tt T Tt Tt t Tt tt 19.3 Determining Genotypes ● Breeding experiments are used to identify genotype of an organism. ● To identify the genotype of an organism showing the dominant trait, a test-cross is performed. This involves crossing