Lesson 6 Mendelian Genetics (Part 1 & 2) PDF

Mendelian Genetics (PART 1)  Gregor Johann Mendel He conducted groundbreaking experiments with pea plants (Pisum sativum) and laid the foundation for understanding how traits are inherited from generation to generation. There are distinct units of inheritance, and their behavi...

Mendelian Genetics (PART 1)  Gregor Johann Mendel He conducted groundbreaking experiments with pea plants (Pisum sativum) and laid the foundation for understanding how traits are inherited from generation to generation. There are distinct units of inheritance, and their behavior during the formation of gametes Mendel’s postulates were accepted as the basis for the study of what is known as transmission genetics  Garden Pea easy to grow and hybridize artificially self-fertilizing in nature seven visible features (unit characters), each represented by two contrasting forms or traits Character: stem height: traits (two contrasting forms): tall and dwarf  Garden Pea Mendel's simplest crosses involved one pair of contrasting traits from two parent strains, each exhibiting one of the two character forms under study. Initially, During examination the first generation of offspring of such a cross, and then we consider the results of selfing The original parents constitute the P1 or parental generation , their offspring are the F1 or first filial generation, and the individuals resulting from the selfed F1 generation are the F2 or second filial generation.  Garden Pea When Mendel crossed tall plants with dwarf plants, the resulting F1 generation consisted only of tall plants. (gibberellin) When members of the F1 generation were selfed, Mendel observed that 787 of 1064 F2 plants were tall, while the remaining 277 were dwarf. To explain these results, Mendel proposed the existence of particular unit factors for each trait. The R (allele) gene is believed to be the basic units of heredity, determining the traits of individual plants, such as "round" and "wrinkled" peas..  Mendel’s First Three Postulates 1. Unit Factors in Pairs: Genetic characters are controlled by unit factors that exist in pairs in individual organisms. ⚬ Because the factors occur in pairs, three combinations are possible: ￭ two factors for tall = TT ￭ two factors for dwarf = tt ￭ one factor for each trait = Tt  Mendel’s First Three Postulates 2. Dominance/Recessiveness: When two unlike unit factors responsible for a single character are present in a single individual, one unit factor is dominant to the other, which is said to be recessive. ⚬ the trait expressed in the F1 generation is controlled by the dominant unit factor ⚬ the trait not expressed is controlled by the recessive unit factor ⚬ pertains only when unlike unit factors are present in pairs  Mendel’s First Three Postulates 3. Segregation: During the formation of gametes, the paired unit factors separate or segregate randomly so that each gamete receives one or the other with equal likelihood.  Modern Genetic Terminology Phenotype refers to the observable physical or biochemical characteristics of an organism ⚬ Dominant phenotype: the phenotype seen when two alternative alleles are present together (Pp: purple) ⚬ Recessive phenotype: the phenotype that is only seen when two identical alleles are found together (pp: white) Genes are the fundamental units of heredity and are responsible for passing on traits from one generation to the next.  Modern Genetic Terminology Allele is one of the variant forms of a gene that can exist at a specific locus (location) on a chromosome. ⚬ Dominant allele: the form of the gene that is expressed when two alternative alleles are present together (P>p) ⚬ Recessive allele: the form of the gene that is not expressed when two alternative alleles are present together. ⚬ Homozygous: having two identical alleles (PP = homozygous dominant or pp = homozygous recessive) ⚬ Heterozygous: having two different alleles (Pp)  Modern Genetic Terminology Genotype is the genetic makeup of an organism, which includes the specific combination of alleles (gene variants) an individual possesses for a particular set of genes. ⚬ Genotype: the genetic constitution of an individual (PP, Pp, pp) When both alleles are the same (DD or dd), the individual is homozygous or a homozygote; when the alleles are different (Dd), we use the term heterozygous or a heterozygote.  PUNNETT SQUARES Female parent It is a graphical tool used in genetics to predict the possible combinations of alleles resulting from a genetic cross between two individuals. Male parent The vertical column represents the female parent, and the horizontal row represents those of the male parent. This process thus lists all possible random fertilization events. The genotypes and phenotypes of all potential offspring are ascertained  The Testcross: One Character It is a genetic cross used to determine an individual’s genotype with a dominant phenotype. To distinguish the genotype, Mendel devised the testcross method. The organism expressing the dominant phenotype, but of unknown genotype, is crossed to a known homozygous recessive individual. PART 2 Mendel’s Dihybrid Cross Generated a Unique F2 Ratio  Mendel’s Dihybrid Cross Generated a Unique F2 Ratio Selection of Parental Plants: Mendel started with two pea plants that were true-breeding for two traits.  Mendel’s Dihybrid Cross Generated a Unique F2 Ratio First Filial Generation (F1): Mendel crossed the two true-breeding parental plants. The F1 generation consisted of all heterozygous individuals (Yy and Ss), with yellow and smooth seeds.  Mendel’s Dihybrid Cross Generated a Unique F2 Ratio Second Filial Generation (F2): Mendel allowed the F1 generation to self-fertilize. The F2 generation resulted from this cross, and it revealed the inheritance patterns of the two traits in combination.  Mendel’s Dihybrid Cross Generated a Unique F2 Ratio Based on similar results in numerous dihybrid crosses, Mendel proposed a fourth postulate called independent assortment During gamete formation, segregating pairs of unit factors assort independently of each other. As a result of random segregation, each gamete receives one member of every pair of unit factors.  4th Postulates: Independent assortment Thus, according to the postulate of independent assortment, all possible combinations of gametes are formed in equal frequency The testcross can also be applied to individuals that express two dominant traits but whose genotypes are unknown. The Trihybrid Cross Demonstrates That Mendel’s Principles Apply to Inheritance of Multiple Traits Three pairs of contrasting traits, in what is called a trihybrid cross, or three-factor cross. When F1 individuals serve as parents, each produces eight different gametes in equal frequencies. At this point, we could construct a Punnett square with 64 separate boxes and read out the phenotypes.  Pedigrees Reveal Patterns of Inheritance of Human Traits Pedigrees The traditional way to study inheritance has been to construct a family tree, indicating the presence or or absence of the trait in question for each member of each generation Such a family tree is called a pedigree. By analyzing a pedigree, we may be able to predict how the trait under study is inherited— for example, is it due to a dominant or recessive allele?  Pedigrees Reveal Patterns of Inheritance of Human Traits When many pedigrees for the same trait are studied, we can often ascertain the mode of inheritance. ⚬ ✓ Forked-Line Method or Branch Diagram  Pedigrees Reveal Patterns of Inheritance of Human Traits Autosomal Recessive Trait. Since neither II-3 nor II-4 is affected, this must be a recessive trait that has skipped a generation; and II-3 and II-4 must both be heterozygous. Approximately 1/4 of their offspring should be affected, and II- 3 and II-4 individuals can be represented by a shaded dot within their symbols.  Pedigrees Reveal Patterns of Inheritance of Human Traits Autosomal dominant is a term used in genetics to describe a pattern of inheritance in which a specific genetic trait or disorder is caused by a dominant allele located on one of the autosomal chromosomes  F1 cross: Stem Length and Flower color TtVv (Tall and violet) x ttVV (dwarf and violet) Pedigree Analysis  Pedigree Analysis Pedigree analysis is a tool that studies an individual's family tree to determine inheritance patterns of genetic traits or disorders. Pedigree Conventions A pedigree chart displays a family tree and shows the members of the family who are affected by a genetic trait. Consanguineous It is a term used to describe a type of marriage or union between individuals who share a common ancestor or blood relationship. ⚬ The parents are related (first cousins) ⚬ They are connected by a double line Sibs ⚬ short for siblings ⚬ connected by a horizontal sibship line Monozygotic refers to a type of twins, commonly known as identical twins. ￭ for identical twins ￭ diagonal lines are linked by a horizontal line Dizygotic often referred to as "DZ" twins, are a type of twins known as fraternal twins. ￭ for fraternal twins ￭ same as monozygotic but lacks a connecting line  Proband Refers to the individual within a family or group who is the first to come to the attention of healthcare professionals for a particular medical condition or genetic disorder. ￭ individual whose phenotype first brought attention to the family ￭ indicated by an arrow connected to the designation p ￭ can be applied to either male or female Pedigree Analysis Autosomal Dominant Trait/Dse Autosomal Recessive Trait/Dse ￭ X-linked Dominant Trait/Dse ￭ X-linked Recessive Trait/Dse ￭ Y-linked Trait/Dse Autosomal Dominant Trait/Dse UNAFFECTED FEMALE (10) UNAFFECTED MALE (4) I: 2 II: 4,6 II: 2,3,5 III: 6,8 III: 1,4,5,7,9,10 Autosomal Dominant Trait/Dse AFFECTED FEMALE AFFECTED MALE II: 7 I: 1 III: 2 II: 1, III: 3,11 X-Linked Dominant Trait X-Linked Dominant Trait UNAFFECTED FEMALE: xx (normal phenotype) AFFECTED FEMALE: XX AND Xx UNAFFECTED MALE: xY Affected male : XY X-Linked Recessive Trait Carrier: Female Unaffected male: XY Affected male: xY Unaffected female: XX Carrier: XX (normal) Affected female: xx X-Linked Recessive Trait Y Linked Trait