BIO 101: General Biology - Genetics and Heredity (PDF)

Nile University Nigeria DEPARTMENT OF BIOLOGICAL SCIENCES GENERAL BIOLOGY BY MR. UMAR FARUOK IBRAHIM Genetics and Heredity  History Genetics The study of heredity, or how the characteristics of living things are transmitted from one ge...

Nile University Nigeria DEPARTMENT OF BIOLOGICAL SCIENCES GENERAL BIOLOGY BY MR. UMAR FARUOK IBRAHIM Genetics and Heredity  History Genetics The study of heredity, or how the characteristics of living things are transmitted from one generation to the next Inheritance is how traits, or characteristics, are passed on from generation to generation. Chromosomes are made up of genes, which are made up of DNA. Genetic material (genes,chromosomes, DNA) is found inside the nucleus of a cell. Gregor Mendel is considered “The Father of Genetics"  Gregor Mendel Austrian Monk. Experimented with “pea plants”. Used pea plants because: – They were available – They reproduced quickly – They showed obvious differences in the traits Understood that there was something that carried traits from one generation to the next- “FACTOR”. Mendel cont…… In the mid-1800s, the rules underlying patterns of inheritance were uncovered in a series of experiments performed by an Austrian monk named Gregor Mendel. Mendel's Plant Breeding Experiments Gregor Mendel was one of the first to apply an experimental approach to the question of inheritance. For seven years, Mendel bred pea plants and recorded inheritance patterns in the offspring. Particulate Hypothesis of Inheritance Parents pass on to their offspring separate and distinct factors (today called genes) that are responsible for inherited traits.  Mendelian Genetics Dominant traits- traits that are expressed. Recessive traits- traits that are covered up. Alleles- An allele is one of the possible forms of a gene. Punnett Squares- is a square diagram that is used to predict an outcome of a particular cross or breeding experiment Probability- the chances/ percentages that something will occur. Genotype- genotype is the set of genes in our DNA which is responsible for a particular trait Phenotype- is the physical expression, or characteristics, of that trait. Homozygous- When an individual has two of the same allele, whether dominant or recessive, they are homozygous. E.g TT and tt Heterozygous-Heterozygous means having one each of two different alleles. Tt Mendel was fortunate he chose the Garden Pea Mendel probably chose to work with peas because they are available in many varieties. The use of peas also gave Mendel strict control over which plants mated. Fortunately, the pea traits are distinct and were clearly contrasting. To test the particulate hypothesis, Mendel crossed true-breeding plants that had two distinct and contrasting traits—for example, purple or white flowers. A true-breeding plant is one that, when self-fertilized, only produces offspring with the same traits. The alleles for these type of plants are homozygous. For each monohybrid cross, Mendel cross-fertilized true-breeding plants that were different in just one character—in this case, flower color. He then allowed the hybrids (the F1 generation) to self-fertilize.  Typical breeding experiment P generation (parental generation) F1 generation (first filial generation, the word filial from the Latin word for "son") are the hybrid offspring. Allowing these F1 hybrids to self-pollinate produces: F2 generation (second filial generation). It is the analysis of this that lead to an understanding of genetic crosses. Mendel studies seven characteristics in the garden pea : Statistics indicated a pattern.  Mendel’s law of inheritance Law Definition During gamete formation, the alleles for each gene segregate from each other so Law of segregation that each gamete carries only one allele for each gene. Genes for different traits can segregate Law of independent assortment independently during the formation of gametes. Some alleles are dominant while others are recessive; an organism with at least Law of dominance one dominant allele will display the effect of the dominant allele. Law of Dominance In the monohybrid cross (mating of two organisms that differ in only one character), one version disappeared. What happens when the F1’s are crossed? The F1 crossed produced the F2 generation and the lost trait appeared with predictable ratios. This led to the formulation of the current model of inheritance. Probability and Punnett Squares Punnett square: diagram showing the probabilities of the possible outcomes of a genetic cross Genotype versus phenotype. How does a genotype ratio differ from the phenotype ratio? Punnett squares - probability diagram illustrating the possible offspring of a mating. Ss X Ss gametes Testcross A testcross is designed to reveal whether an organism that displays the dominant phenotype is homozygous or heterozygous. How Does it Work? The Importance of the Environment The environmental influences the expression of the genotype so the phenotype is altered. Hydrangea flowers of the same genetic variety range in color from blue-violet to pink, depending on the acidity of the soil. Multifactorial; many factors, both genetic and environmental, collectively influence phenotype in examples such as skin tanning  Chromosomes Homologous chromosome: one of a matching pair of chromosomes, one inherited from each parent. Sister chromatids are identical  Homologous Chromosomes Homologous autosomes are identical in length, size, shape, and gene sequence Sex chromosomes are nonidentical but still homologous Homologous chromosomes interact, then segregate from one another during meiosis DNA DNA and proteins Nucleosome arranged as cylindrical fiber Histone  Human Karyotype 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 XX (or XY) Alleles: alternative versions of a gene. The gene for a particular inherited character resides at a specific locus (position) on homologous chromosome. For each character, an organism inherits two alleles, one from each parent  How do alleles differ? Dominant allele Recessive allele Recessive allele Recessive allele Dominant - a term applied to the trait (allele) that is expressed irregardless of the second allele. Recessive - a term applied to a trait that is only expressed when the second allele is the same (e.g. short plants are homozygous for the recessive allele).  Pedigree Chart that shows genetic connections among individuals Standardized symbols Knowledge of probability and Mendelian patterns used to suggest basis of a trait Conclusions most accurate when drawn from large number of pedigrees Pedigree analysis reveals Mendelian patterns in human inheritance In these family trees, squares symbolize males and circles represent females. A horizontal line connecting a male and female (--) indicates a mating, with offspring listed below in their order of birth, from left to right. Shaded symbols stand for individuals with the trait being traced.  Autosomal Recessive Inheritance Patterns If parents are both heterozygous, child will have a 25% chance of being affected Disorders Inherited as Recessive Traits Over a thousand human genetic disorders are known to have Mendelian inheritance patterns. Each of these disorders is inherited as a dominant or recessive trait controlled by a single gene. Most human genetic disorders are recessive. A particular form of deafness is inherited as a recessive trait.  Autosomal Recessive Diseases Galactosemia Sickle cell anemia Tay-Sachs disease Albinism  Sickle-Cell Anemia (Sickle-Cell Disease) The most common genetic disorder among black people About 1 in 500 African Americans has sickle-cell anemia. Carriers are said to have sickle-cell trait  Sickle-Cell Anemia Caused by an abnormal gene on chromosome 11 The gene is for one of the polypeptide chains in hemoglobin, a protein found in red blood cells that is responsible for transporting oxygen through the bloodstream Normal red blood cells. (©Phototake NYC.) Sickle blood cells. (©Photo Researchers, Inc.)  Autosomal Dominant Inheritance Trait typically appears in every generation  Autosomal Dominant Transmission of a dominant allele 50% chance of being affected Disease appears in every generation Males and females equally being affected Figure 5-3: Transmission of autosomal dominant disorders. (50% chance for an affected child). Dominantly Inherited Disorders Achondroplasia, a form of dwarfism with an incidence of one case among every 10,000 people. Heterozygous individuals have the dwarf phenotype. Huntington’s disease, a degenerative disease of the nervous system, is caused by a lethal dominant allele that has no obvious phenotypic effect until the individual is about 35 to 45 years old. Figure 5-4: A 12-year old Achondroplastic dwarf. Note the disproportion of the limbs to the trunk, the curvature of the spine, and the prominent buttocks.  X-Linked Recessive Inheritance Males show disorder more than females Son cannot inherit disorder from his father  Sex-Linked Inheritance Defective gene on X chromosome Defective X on male is unmasked and the trait is expressed. Female is carrier for the disease; heterozygous Male transmits the defective allele to his daughters. Figure 5-7: Transmission of sex-linked disorders.  Examples of X-Linked Traits Color blindness – Inability to distinguish among some of all colors Hemophilia – Blood-clotting disorder – 1/7,000 males has allele for hemophilia A – Was common in European royal families Sex-Linked Disorders in Humans Duchenne muscular dystrophy, affects about one out of every 3,500 males born in the United States. People with Duchenne muscular dystrophy rarely live past their early 20s. The disease is characterized by a progressive weakening of the muscles and loss of coordination. Researchers have traced the disorder to the absence of a key muscle protein called dystrophin and have tracked the gene for this protein to a specific locus on the X chromosome. Posture changes during progression of Duchenne muscular dystrophy. Color Blindness In Humans: An X-Linked Trait Numbers That You Should See If You Are In One Of The Following Four Categories: [Some Letter Choices Show No Visible Numbers] Sex-Linked Traits: 1. Normal Color Vision: A: 29, B: 45, C: --, D: 26 2. Red-Green Color-Blind: A: 70, B: --, C: 5, D: -- 3. Red Color-blind: A: 70, B: --, C: 5, D: 6 4. Green Color-Blind: A: 70, B: --, C: 5, D: 2 Pattern Baldness In Humans: A Sex Influenced Trait Baldness is an autosomal trait and is apparently influenced by sex hormones after people reach 30 years of age or older. In men the gene is dominant, while in women it is recessive. A man needs only one allele (B) for the baldness trait to be expressed, while a bald woman must be homozygous for the trait (BB). What are the probabilities for the children for a bald man and woman with no history of baldness in the family?  More Sex-Linked Recessive Inheritance Male-pattern baldness By age 50, nearly 60% of all men will experience some male pattern baldness. 35 million Americans experience some degree of hair loss, resulting in $900 million dollars a year being spent in efforts to grow it back.  Sex Limited traits Sex-hormones need to be present for a gene to function. E.g. beards Excessive or abnormal testosterone in female may cause bead growth.  Over secretion of testosterone in female  Practice Questions 1. Define the following a) Genetics b) Chromosome c) Inheritance d) Punnett square 2. Explain any three sex linked disorders in human 3. Define homologous chromosome and heterologous chromosome 4. State the 3 Mendel’s law of inheritance 5. Mendel studies seven characteristics in the garden pea. Name them