Topic 7 - Human Genetics Past Paper AY23-24 PDF

Topic 7: Human Genetics Mendelian Genetics GRD101_ Human Genetics Reading Material Text Book: Campbell, N., Urry, L., Cain, M., Wasserman, S., Minorsky, P., Reece, J., & Orr, R. (2020). Biology: A global approach, global edition. Pearson Education, Limited. Link: https://ebookc...

Topic 7: Human Genetics Mendelian Genetics GRD101_ Human Genetics Reading Material Text Book: Campbell, N., Urry, L., Cain, M., Wasserman, S., Minorsky, P., Reece, J., & Orr, R. (2020). Biology: A global approach, global edition. Pearson Education, Limited. Link: https://ebookcentral.proquest.com/lib/fchsae/reader.action?do cID=6191695 Chapter 14: Mendelian Genetics GRD101_ Human Genetics 2 Course Learning Outcomes CLO 5: Recognize the different patterns of inheritance, identify common genetic disorders, and understand the basics of genetics Learning Objectives – Understand Mendel’s Laws of Inheritance. – Understand the pedigree analysis of human traits (Distinguish between recessively and dominantly inherited disorders) – Explain molecular basis of different inheritance pattern GRD101_ Human Genetics 3 How to check the Important Vocabularies for topic 7? For Vocab Self-Quiz, check the following link : https://media.pearsoncmg.com/intl/ge/2017/ge_ campbell_biology_11/msa/vocab/flashcards.php Select “chapter 13” → Select “Create Deck” GRD101_ Human Genetics 4 Define Heredity Heredity, or inheritance, is the passing of traits from parents to offspring. The science of heredity is called genetics. Parents Offspring Male and female The Babies GRD101_ Human Genetics 5 Mendel’s Laws of Inheritance Genetics is the study of heredity (the transmission of traits from generation to generation). Genetics began with the work of Gregor Mendel. Mendel developed basic principles of heredity with NO knowledge of genes or chromosomes. Mendel worked with pea plants. Why peas? GRD101_ Human Genetics 6 Garden Pea, Pisum sativum Model organism Short Observable Controlled Fast Growth generation Traits Mating time Only two clear Grows traits(phenotype) fast Makes many offspring Makes many offspring 7 GRD101_ Human Genetics Mendel’s experimental Approach Mendel tracked seven characters that occurred in two distinct alternative forms (traits) 1. Flower color 2. Seed color 3. Seed shape 4. Pod shape 5. Pod color 6. Flower position 7. Hight, stem length GRD101_ Human Genetics 8 Crossing Technique of pea plants In a typical experiment, Mendel cross-pollinated two contrasting, true-breeding traits/characters. The true-breeding parents are the P generation (Parental generation) The hybrid offspring of the P generation are called the F1 generation (for the first family/filial generation) When F1 individuals self-pollinate or cross pollinate with other F1 hybrids, the F2 generation (2nd family/filial generation) is produced. 9 GRD101_ Human Genetics Crossing Technique of pea plants If the flower is allowed to fertilize itself, this is called self-pollination. If the pollen is taken from one flower to fertilize another flower, this is called cross-pollination. GRD101_ Human Genetics 10 Mendel’s Experiment He noticed that white flowers had offspring that were white. purple flowers had offspring that were purple. So, he cross-fertilized white with purple. to see what will happen? GRD101_ Human Genetics 11 Mendel’s Experiment’s results He crossed He crossed true-bred true purple bred purple with withbred true true-bred white white flower. flowers. What did he What did gethe get in F1? in F1? 100% purple He self-fertilized the purple flowers from (F1). What do you think he got? Some white flowers appear. He got 75% purple flowers and 25% white flowers. What is the ratio? 3:1 12 Mendel’s Experiment’s results Mendel performed similar experiments with another observable trait (seed colour, seed shape etc.) After growing thousands of pea True breeding X True breeding plants and testing multiple traits, Mendel realized why these 100% (trait 1) results continued to happen. His analysis of these experiments still influences genetics today. That is 75% trait 1 vs 25% trait 2 why he is named the father of 3:1 genetics. GRD101_ Human Genetics 13 Alleles Each trait is controlled by a gene. Genes come in different forms, called alleles. Alleles come in pairs - one from the female and one from the male during fertilization. Define… An allele is a specific form of a single gene passed from generation to Blue eye Brown eye generation. GRD101_ Human Genetics 14 Dominance One allele is more dominant than the other and causes the dominant trait to show. A genetic trait is considered dominant if it is expressed in an organism that has Recessive Dominant one or two copies of the dominant allele. The purple flower color is the A genetic trait is considered recessive if it dominant trait for pea plants, while the white flower color is is only expressed in an organism that has a recessive trait. two copies of the recessive allele GRD101_ Human Genetics 15 Genotype vs Phenotype Each gene has two alleles. The gene combinations are called genotypes. In a genotype, the dominant allele is represented by a capital letter, and the recessive allele is represented by the same letter but in lowercase. Based on the genotype, specific traits will be observable. The observable traits (flower color, seed shape) are called phenotypes. GRD101_ Human Genetics 16  Homozygous vs Heterozygous Homozygous Heterozygous (same alleles) (different alleles) When a genotype has two identical When a genotype has one alleles, it is called homozygous. dominant allele and one recessive allele, it is Can be homozygous dominant or called heterozygous. homozygous recessive Heterozygous genotypes are Homozygous genotypes are written as written as one capital and one two capital letters or two lowercase lowercase letter. letters. i.e.,(Aa) i.e., (AA) or (aa) GRD101_ Human Genetics 18 Law of Segregation When Mendel crossed true-breeding white- and purple- flowered pea plants, the F1 offspring (hybrids) were all purple. Surprisingly, when the F1 hybrids with purple flowers were crossed or self-fertilized, many of the F2 plants had purple flowers, but some had white. Mendel discovered a ratio of about 3:1, purple to white flowers, in the F2 generation. GRD101_ Human Genetics 19 Punnett Square: Genotype and Phenotype P represents a dominant allele, and p represents a recessive allele Possible combinations of alleles of sperm and egg in fertilization can be shown using a Punnett square. Therefore, Punnett square shows the genotypes and phenotypes of the offspring. GRD101_ Human Genetics 20 How to solve a Punnett Square Punnett square – used to predict the outcome of genetic crosses GRD101_ Human Genetics 22 For example, Let’s do a test for eye color Bb or BB B=Brown, b=blue TT TT BB x bb Bb x bb GRD101_ Human Genetics 23 Law of Segregation – In a diploid organism, e.g., humans, the two alleles (of a gene) on every homologous chromosome segregate (separate) in anaphase I during meiosis. – Each Sperm or egg carries half the number of chromosomes; accordingly, it has one allele for a given trait. – In fertilization, the zygote then inherits two alleles of each gene, one from each parent via the sperm and the egg. Figure 14.5: Mendelian law of segregation  The Testcross An individual with the dominant phenotype could be either homozygous or heterozygous dominant. The genotype of an individual with a dominant trait can be detected by a testcross with the individual with a homozygous recessive. (e.g., PP with pp OR Pp with pp) If any offspring display the recessive phenotype, the parent with the unknown genotype must be heterozygous. ✓ To determine the organism’s genotype, geneticists can perform a testcross. Figure 14.7 The Testcross The Testcross – A dwarf pea plant must be tt. – A tall pea plant could be either TT or Tt, so genotype must be determined by a testcross – Cross the unknown individual (TT or Tt) to a homozygous recessive individual (tt) ❑ If some offspring are dwarf, an unknown individual must have been Tt ❑ If all offspring are tall, the unknown individual was TT GRD101_ Human Genetics 26 The Law of Independent Assortment Mendel derived the law of segregation by following examining a single character (color, shape, texture, etc.) producing monohybrids (offspring, each expressing one trait; purple or white). Mendel derived the law of independent assortment by following the inheritance of two characters (height and color) in dihybrids. GRD101_ Human Genetics 27 The Law of Independent Assortment The Law of Independent Assortment: two characters at the same time Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters. A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently. GRD101_ Human Genetics 28 The Law of Independent Assortment The law of independent assortment states that allele pairs separate independently during the formation of gametes. Therefore, traits are transmitted to offspring independently of one another. GRD101_ Human Genetics 29 The dihybrid bennet square GRD101_ Human Genetics 30 How to read a dihybrid Punnett square GRD101_ Human Genetics 31 Pedigree Analysis A pedigree is a family tree that describes the interrelationships of parents and children across generations Is used to check the inheritance of characters or diseases Diseases can be dominant or recessive GRD101_ Human Genetics 32 Recessive diseases Individuals who are homozygous exhibit disease symptoms. Parents of the disease have one diseased allele but are normal -carriers GRD101_ Human Genetics 33 Recessive diseases: Cystic Fibrosis (CF) Normal CF allele codes for transporter protein that regulates chloride ion balance Mutation diminishes the function of the transporter protein, causing multiple pleiotropic effects: ―Thickmucus in the lungs is due to water imbalance caused by ion balance ―Sweat is very salty because salt cannot be recycled back into body without a transporter ―Some males are infertile because Cl- transporter is needed for proper development of vas deferens GRD101_ Human Genetics 34 Recessive diseases: Cystic Fibrosis (CF) GRD101_ Human Genetics 35 Recessive diseases: Albinism One of the two sisters shown here does not have albinism; the other does. Most recessive homozygotes are born to parents who are carriers of the disorder, but themselves have a normal phenotype, the case shown in the Punnett square. Albinism: homozygous autosomal recessive Recessive diseases: Albinism 37 Is this disease dominant or recessive and why? GRD101_ Human Genetics 38 Dominant diseases: Huntington For example, Huntington’s disease and causes the degeneration of brain cells involved in emotions, intellect, and movement. Huntington’s disease has an autosomal dominant inheritance pattern The pattern of the most dominant disorders is that every affected individual has one affected parent. Different Inheritance Patterns 1. Simple Mendelian inheritance ― Recessive allele does not affect the phenotype of heterozygote ― Single copy of the dominant allele makes enough functional protein to provide a normal phenotype, masking recessive allele ― Sometimes, the heterozygote will upregulate the dominant allele to provide higher expression More complex forms of inheritance 2. Incomplete dominance (intermediate phenotype) 3. Codominance (both alleles are expressed individually) 4. Sex Linked Traits 5. Environment affects the phenotype 40 1. Simple Mendelian Inheritance Example: Purple pigment, P One P allele makes enough functional protein to provide a normal phenotype 41 2. Incomplete dominance ❑ Heterozygote shows intermediate phenotype ❑ Neither allele is dominant ❑ Example: Pink four-o’clock 50% of normal protein is not enough to give a red color. GRD101_ Human Genetics 42 incomplete dominance - sickle cell anemia ❑ In homozygous individuals, all haemoglobin is abnormal (sickle-cell) ❑ Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms Copyright ©2021 John Wiley & Sons, Inc. 43 3. Multiple Alleles and Codominance – Multiple alleles – three or more variants in a population – Phenotype depends on which two alleles are inherited – example: ABO blood types in humans – Type AB is codominant – expresses both alleles equally 44  Multiple Alleles – some traits are controlled by more than 2 different allele types. Ex. Human blood types – The inheritance of blood types in humans can be explained by a model in which there are 3 alleles for blood type. IA =A protein IB = B protein i = no protein IA and IB are both dominant (codominance) over i Blood Type Genotypes A IAIA, IA I B IBIB, IB i AB IA IB O ii GRD101_ Human Genetics 45 ABO Blood Group – Multiple Alleles Copyright ©2021 John Wiley & Sons, Inc. 46  4. Sex-Linked Traits ― Sex-linked traits involve genes that are carried only on the sex chromosomes (X or Y). ― The female genotype is XX, while the male genotype is XY. ― Males, who have only a single copy of the X chromosome, are more likely to be affected by a sex-linked disorder (X- linked) than females, who have two copies. ― In females, the presence of a second, non-mutated copy may not cause any symptoms and are called carriers. Copyright ©2021 John Wiley & Sons, Inc. 47 5. Role of Environment ― The environment plays a vital role in phenotype ― Genotype provides the plan to create a phenotype; the environment provides nutrients and energy to carry out the plan ― Norm of reaction – effects of environmental variation on a phenotype ― example: hydrangea plants produce different flower colours depending on soil conditions. Basic soil Acidic soil GRD101_ Human Genetics 48 Role of Environment and Genes Traits that depend on multiple genes combined with environmental influences are called multifactorial Example: Phenylketonuria (PKU) disease ― A genetic disorder caused by mutation ― Patients cannot make the enzyme phenylalanine hydroxylase, which is needed to break down phenylalanine ― they can develop normally if given a diet free of phenylalanine ― If the diet contains phenylalanine, symptoms include mental retardation, underdeveloped teeth and foul-smelling urine GRD101_ Human Genetics 49