JWSS Biology Chapter 14 Heredity

4G3 Sci Biology Chapter 16 Notes 1 Prepared by Ms Pui LX JURONG WEST SECONDARY SCHOOL SECONDARY FOUR – Science BIOLOGY (5088) Chapter 16: HEREDITY LEARNING OUTCOMES: Candidates should be able to: a) distinguish between the terms gene and allele b) distinguish between continuous and discontinuous variation and give examples of each c) explain the terms dominant, recessive, homozygous, heterozygous, phenotype and genotype d) 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 e) explain why observed ratios often differ from expected ratios, especially when there are small numbers of progeny f) use genetic diagrams to solve problems involving monohybrid inheritance g) describe the determination of sex in humans – XX and XY chromosomes h) 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 a condition known as Down Syndrome i) name ionising radiation (e.g. X-ray) and chemical mutagens as factors which may increase the rate of mutation Introduction What is heredity? • The term heredity refers to the passing down of genes or genetic information from the parents to their offspring. • The offspring will either display the physical traits from either of their parents or a mixture of both. • Examples: How do genes get passed down? • Recall that the nucleus of a cell contains genetic information. • These genetic information are in the form of chromosomes. 1. What other examples of physical traits did you inherit from your parents? 2. Did you inherit all your physical traits from either of your parents or a mixture of both?

4G3 Sci Biology Chapter 16 Notes 2 Prepared by Ms Pui LX a) distinguish clearly between the terms gene and allele c) explain the terms dominant, recessive, homozygous, heterozygous, phenotype and genotype GENETIC TERMS (Glossary) • Genetics is the scientific study of heredity. • Chromosome is made up of the molecule DNA (deoxyribonucleic acid) which are visible during cell division. • A gene is a unit of inheritance born on a particular locus of a chromosome. It is a sequence of DNA nucleotides that controls the formation of a single polypeptide. • Alleles (for example, T or t) are different forms of the same gene. They occupy the same relative positions on a pair of homologous chromosomes. • Homologous chromosomes are chromosomes that are similar in size and shape (except for sex chromosomes). In a single pair of homologous chromosomes, one comes from the male parent and one comes from the female parent. • Phenotype refers to the expressed trait of an organism. E.g. blood group, double eyelids. • Genotype is the genetic make-up of an organism, i.e. of genes in an organism. It is normally expressed by two letters (e.g. TT, Tt or tt). For a single trait, o AA – homozygous dominant (because both are dominant alleles) o aa – homozygous recessive (because both are recessive alleles) o Aa – heterozygous • Homozygous means the organism carries two copies of the same allele. I.e. homozygous dominant means TT but homozygous recessive means tt. • Heterozygous means the organism carries one copy of two different alleles e.g. Tt • Dominant allele (T) is expressed which results in the same phenotype in both homozygous and heterozygous organisms. Tt or TT • Recessive allele (t) is not expressed in the heterozygous condition, which expressing itself only in the homozygous condition, tt. *Please use letters where the capital and small letters look different!* e.g. Aa, Bb, Tt. DO NOT USE X and Y because they are reserved for sex chromosomes only!

4G3 Sci Biology Chapter 16 Notes 3 Prepared by Ms Pui LX THINK & EXPLORE What is the difference between gene and allele (JUST DEFINE!)? Do both terms mean the same thing? Gene is a small segment of DNA but allele is an alternative form of the same gene. A gene codes for a specific characteristic such as eye colour but alleles codes for different traits such as brown eye colour or black eye colour. Offspring inherit genes from their parents but the traits they display are dependent on the alleles. AfL in Class [Individual work]: Use your knowledge about the new genetic terms and test yourself without reference. Use the information provided and your knowledge of genetics to answer each question. 1. For each genotype below, indicate whether it is heterozygous OR homozygous. 2. Circle the genotype that is considered as purebred. TT Homozygous dominant Bb heterozygous DD Homozygous dominant Ff hetero BB Homozygous dominant Dd heterozygous ff Homozygous recessive Tt heterozygous bb Homozygous recessive FF Homozygous dominant 3. Determine the phenotype for each genotype. Let B be allele for brown. Let b be allele for black. ﹡ Brown body color is dominant to black. BB ___brown____ Bb __brown__ bb Black ﹡ Round shape is dominant to square. RR Round Rr Round rr square

4G3 Sci Biology Chapter 16 Notes 4 Prepared by Ms Pui LX (d) 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 (e) Explain why observed ratios often differ from expected ratios, especially when there are small numbers of progeny (f) use genetic diagrams to solve problems involving monohybrid inheritance GENETIC CROSS Genetic crosses are performed to show how two different individuals bring about offspring that carry part of genetic material of each parent. It shows the probability of the offspring attaining certain traits. There are two methods of genetic cross: 1. Genetic diagram 2. Punnett Square 1. GENETIC DIAGRAM Mendel’s pea plants (read textbook for the full story) • Mendel crossed tall homozygous dominant and dwarf homozygous recessive pea plants • The F1 generation are all heterozygous. Worked example: Parental Phenotype Tall x short Parental Genotype TT x tt Gametes T T t t F1 Genotype Tt Tt Tt Tt F1 Phenotype Tall Tall Tall Tall Phenotypic ratio All Tall Think! 1. Does the term, heterozygous dominant/recessive, exist? Nope! Heterozygous already means the organism carries one copy of each of the two different alleles e.g. Tt 2. If all the offspring are TALL, should we write the ratio as 4 tall: 0 short? nope! Please write it as all tall. Remember to circle gametes! Commented [PLX1]: https://www.youtube.com/watch?v=Meh z7tCxjSE

4G3 Sci Biology Chapter 16 Notes 5 Prepared by Ms Pui LX 2. PUNNETT SQUARE Using the same question as shown previously, here is how a Punnett square is drawn: Gametes T T t Tt (tall) Tt (tall) t Tt (tall) Tt (tall) F1 phenotypic ratio: All tall AfL in Class [Individual work / Pair work]: Use your knowledge about the new concept and test yourself without reference. Q1. In humans, dark eye colour is dominant (B) while blue eye colour is recessive (b). Use a genetic diagram to show the cross between a blue-eyed mother and a homozygous dark-eyed father. Q2. A mouse homozygous for white fur (white is recessive) is mated with a mouse homozygous for grey fur. Draw a genetic diagram. What are the different types of fur that we expect to see? Grey =_____ White = _____ Number of grey mice _____, number of white mice_____. If there are 24 offspring how many grey mice do we expect to see? _____ How many white?___

4G3 Sci Biology Chapter 16 Notes 6 Prepared by Ms Pui LX Q3. In the botanical gardens, pink tiger lilies are dominant t