Mendel’s Principles of Heredity

Chapter Mendel’s Principles of Heredity Chapter of Genetics: From Genes to Genomes, 4th edition (2011) Hartwell L H et al Chapter outline 2.1 Background: The Historical Puzzle of Inheritance 2.2 Mendel’s approach to genetic analysis 2.3 Mendelian Inheritance in Humans A family portrait with members of four generations Fig 2.1 • Why some of the children look like only one of the parents, while some of the other children look more like the great grandparents? • What causes the similarities and differences of appearance and the skipping of generations? Gregor Mendel discovered the basic principles of genetics • Mendel was the first scientist to combine data collection, analysis, and theory to understand heredity • He inferred genetic laws about the appearance and disappearance of traits during different generations Gregor Mendel Genetics explains the mechanisms that determine the inheritance of traits Genes are the basic units of heredity • Heredity is the way that genes transmit traits from parents to offspring • Genes are passed from one generation to the next Genes underlie the formation of every heritable traits, such as, cleft chin, hair loss, color of hair, skin, and eyes Genetic variation exists even within dog breeds Mendel's laws explain why two black Labradors could have a litter of black, brown, and golden puppies Fig 2.3 2.1 Background: The Historical Puzzle of Inheritance • Artificial selection has been an important practice since before recorded history – Domestication of animals – Selective breeding of plants • 19th century – precise techniques for controlled matings in plants and animals to produce desired traits in many offspring Critical questions about selective breeding before Mendel's studies Concluding remarks by Abbot C Napp at 1837 annual meeting of the Moravian Sheep Breeders Society, three basic questions must be answered: – What is inherited? – How is it inherited? – What is the role of chance in heredity? Historical theories of inheritance before Mendel’s study (1) One parent contributes most features (e.g., homunculus, N Hartsoiker, 1694) A misconception Well into the nineteenth century, N Hartsoiker and other microscopists believed they saw a fully formed, miniature fetus crouched within the head of a sperm The homunculus: Historical theories of inheritance before Mendel’s study (2) Blending inheritance – parental traits become mixed and forever changed in offspring Parents Offspring Before Mendel’s study, scientists could not explain why traits would sometimes disappear and then reappear in subsequent generations 10 Predicting proportions of progeny from multihybrid crosses – example P F1 RRYYTTSS × rryyttss RrYyTtSs × RrYyTtSs What is the probability of obtaining the genotype RrYyTtss? Rr × Rr Yy X Yy Tt × Tt Ss × Ss 1RR:2Rr:1rr 1YY:2Yy:1yy 1TT:2Tt:1tt 1SS:2Ss:1ss 2/4 Rr 1/4 ss 2/4 Yy 2/4 Tt Probability of obtaining individual with Rr and Yy and Tt and ss 2/4 × 2/4 × 2/4 × 1/4 = 8/256 (or 1/32) 33 P F1 RRYYTTSS × rryyttss RrYyTtSs × RrYyTtSs What is the probability of obtaining a completely homozygous genotype? 34 P RRYYTTSS × rryyttss F1 RrYyTtSs × RrYyTtSs What is the probability of obtaining a completely homozygous genotype? Genotype could be RRYYTTSS or rryyttss Rr × Rr Yy × Yy Tt × Tt Ss × Ss 1RR:2Rr:1rr 1YY:2Yy:1yy 1TT:2Tt:1tt 1SS:2Ss:1ss 1/4 RR 1/4 rr 1/4 SS 1/4 ss 1/4 YY 1/4 yy 1/4 TT 1/4 tt (1/4 × 1/4 × 1/4 × 1/4) + (1/4 × 1/4 × 1/4 × 1/4) = 2/256 = 1/128 35 1900 - Carl Correns, Hugo deVries, and Erich von Tschermak rediscover and confirm Mendel’s laws 36 2.3 Mendelian inheritance in humans • Most traits in humans are due to the interaction of multiple genes and not show a simple Mendelian pattern of inheritance • In 2009, there were ~ 4300 single-gene traits known in humans Table below shows some of the common single-gene traits (caused by recessive alleles or dominant alleles in humans) 37 Some of the most common single-gene traits caused by recessive alleles in humans 38 Some of the most common single-gene traits caused by dominant alleles in humans 39 Mendelian inheritance in humans However, even with single-gene traits, determining inheritance pattern in humans can be tricky: - Long generation time - Small numbers of progeny - No controlled matings - No pure-breeding lines In humans, pedigrees can be used to study inheritance Pedigrees are orderly diagrams of a family's relevant genetic features, includes as many generations as possible (ideally, at least both sets of grandparents of an affected person) 40 Symbols used in pedigree analysis 41 A vertical pattern of inheritance indicates a rare dominant trait Every affected person has at least one affected parent Mating between affected person and unaffected person is effectively a testcross Pedigree of Huntington’s disease 42 How to recognize dominant traits in pedigrees Three key aspects of pedigrees with dominant traits: • Affected children always have at least one affected parent • As a result, dominant traits show a vertical pattern of inheritance • Two affected parents can produce unaffected children, if both parents are heterozygotes 43 A horizontal pattern of inheritance indicates a rare recessive trait Parents of affected individuals are unaffected but are heterozygous (carriers) for the recessive allele Pedigrees of cystic fibrosis disease 44 How to recognize recessive traits in pedigrees Four keys aspects of pedigrees with recessive traits: Affected individuals can be the children of two unaffected carriers, particularly as a result of consanguineous matings All the children of two affected parents should be affected Rare recessive traits show a horizontal pattern of inheritance Recessive traits may show a vertical pattern of inheritance if the trait is extremely common in the population 45 Connections with Mendel's work Mendel answered the three basic questions about heredity as follows: • To “What is inherited?” he replied, “alleles of genes.” • To “How is it inherited?” he responded, “according to the principles of segregation and independent assortment.” • To “What is the role of chance in heredity?” he replied, “for each individual, inheritance is determined by chance, but within a population, this chance operates in a context of strictly defined probabilities.” 46 Chapter outline 2.1 Background: The Historical Puzzle of Inheritance 2.2 Genetic Analysis According to Mendel 2.3 Mendelian Inheritance in Humans 47 ... probability of event + probability of event 19 Probability and Mendel’s Results From a cross of Yy x Yy peas • What is the chance of getting YY offspring?  Chance of Y pollen is 1/2  Chance of Y ovule... around 1862 holding one of his experimental plants Mendel’s garden: Gregor Mendel’s garden of Pisum sativum was part of his monastery’s property in Brno 11 Keys to the success of Mendel’s experiments... mechanisms that determine the inheritance of traits Genes are the basic units of heredity • Heredity is the way that genes transmit traits from parents to offspring • Genes are passed from one generation