Patterns of Inheritance PDF
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This document provides an overview of inheritance patterns, covering classical genetics concepts like Mendel's experiments, and contemporary genetic screening techniques.
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PATTERNS OF INHERITANCE Chapter 9 Gregor Mendel and His Pea Plants An Augustinian monk living in an abbey in Brunn, Austria Studied pea plants in the garden Well versed in physics, mathematics, and chemistry His research on pea plants was based on large amounts of quantitative data. His...
PATTERNS OF INHERITANCE Chapter 9 Gregor Mendel and His Pea Plants An Augustinian monk living in an abbey in Brunn, Austria Studied pea plants in the garden Well versed in physics, mathematics, and chemistry His research on pea plants was based on large amounts of quantitative data. His research was also highly controlled. Pea plants were a great species to study – easy to grow, 7 easily distinguishable traits, and he could control pollination. History of Genetics Heredity – the study of the passing of traits from parents to offspring Genetics – the branch of biology that studies heredity Attempts to explain heredity date back to ancient Greece Earliest theories included pangenesis: life was contained within gemmules, tiny particles with heritable information, that migrated to the gonads; proposed by Greek philosophers & revisited by Charles Darwin blending hypothesis: offspring are a blend of parents; endured into the 19th century Gregor Mendel is the father of modern genetics due to his research showing the inheritance of discrete traits, not blended traits as previously thought. Gregor Mendel and His Pea Plants Mendel’s Classic Crosses Mendel would always start with true-breeding varieties (homozygous) The first generation was the P generation (parental) They produced the F1 (filial – kids) Which in turn produced the F2 (grandchildren of P) Mendel began by studying one trait at a time in monohybrid crosses P: cross a purebred purple with a purebred white F1: All the offspring were purple, when they were crossed- F2: approximately ¾ of the offspring were purple and ¼ white These results were repeatable for each of the seven traits studied in pea plants. Mendel’s Classic Crosses Rules of Inheritance Mendel’s results were so repeatable and predictable that he was able to develop four hypotheses (“rules”) regarding inheritance patterns 1. Each trait is controlled by a gene, each gene has two alternate forms called alleles. 2. For each trait, an organism inherits two alleles, one from each parent, can be the same (homozygous) or different (heterozygous). 3. Of the two alleles – one is dominant and is always expressed when present, the other is recessive and is only expressed in the absence of the dominant. 4. Each parent passes one of the two alleles at random (on sperm or egg) to each offspring = Law of segregation Punnett Square A mathematical model used to predict all possible outcomes of a cross between two parents Phenotype – the physical expression of a trait (purple or white) Genotype – actual genetic makeup – PP, Pp, pp https://www.youtube.com/watch?v=Y1PCwxUDTl 8 Meiosis and Heredity Meiosis is the mechanism behind the law of segregation. Each pair of homologous chromosomes have the same gene loci, but can have alternative alleles at those loci. The homologous chromosomes are split during meiosis and only one chromosome (with one allele) goes into each individual gamete. Dihybrid Crosses Mating parental varieties differing in two traits P – purebreds F1 – heterozygous dominant for both F2 – 9:3:3:1 phenotypic ratio Law of independent assortment – alleles for separate traits are inherited independently of each other This only works for genes inherited on separate chromosomes or far apart on the same c’some Test Cross Trying to determine an unknown dominant phenotype Cross the unknown with a known genotype (homozygous recessive) Study the phenotypes of the offspring to determine the unknown genotype of the parent Rules of Probability Rule of multiplication – the probability of two or more independent events occurring simultaneously is the product of their individual probabilities Rule of addition – the probability that an event can occur in two or more alternative ways is the sum of the separate probabilities https://www.youtube.com/watch?v=y4Ne9DXk_Jc Pedigree Can be used to study the passing of a trait or disorder through several generations of a family Can help in identification of carriers of recessive disorders (heterozygotes who are healthy but carry harmful recessive alleles) Simple Inheritance in Humans Traits that are governed by Mendel’s rules are said to follow simple inheritance patterns (only 2 alleles-1 completely dom & 1 rec). Several examples of simple dominant and recessive traits in humans (see the next slide) Dominant Recessive Recessive Dominant Genetic Screening Several tests are available to identify possible genetic abnormalities in unborn children Amniocentesis: 14-16 weeks; 4 tsp of fluid drawn from womb, cells are isolated, grown, karyotyped, and biochemical tests run; takes several weeks for results; risk of miscarriage is 0.1 to 0.3 % CVS: as early as 8 weeks, but typically 10-12; chorionic villus tissue (derived from the fertilized egg) is removed from placenta; karyotyped and biochemical tests run; results obtained quickly; risk of miscarriage is 0.5 to 1% Blood tests (15-20 weeks; no risk of miscarriage; used to determine if further screening is necessary) Ultrasound (imaging to look for anatomical abnormalities) Newborn screening – PKU (heel stick can sample blood for presence of enzyme that converts phenylalanine into tyrosine) Complex Patterns of Inheritance There are MANY traits that do not strictly follow the rules of Mendelian genetics and are governed by complex patterns of inheritance. Several different patterns: Incomplete dominance (F1 hybrids have intermediate phenotype) Codominance (F1 hybrids express both phenotypes; blood type) Multiple allele (more than 2 forms of a gene; blood type) Pleiotropy (a single gene affects multiple characteristics; CF & SC) Polygenic (multiple genes influence a trait, phenotypes occur along a continuum; skin color) Genetic linkage (genes located close together = inherited together) Sex-linked traits (y-linked genes help determine sex, x-linked genes contain many characters unrelated to gender) Incomplete Dominance Incomplete dominance in snapdragon flower color The heterozygote is an intermediate between the two homozygous traits. Incomplete dominance in hypercholesterolemia Codominance White coat Brown coat Both alleles are equally dominant and equally expressed if present in the heterozygote Roan coat Multiple Alleles More than two possible alleles exist in the population One individual will still only carry two of those alleles Universal recipient ABO blood group Universal donor in humans Important in transfusions and Clumping transplants reaction can kill IA and IB are also recipient. codominant over i Practice: A man with Type A blood marries a woman with Type B blood. They already have a child with type O blood. What genotypes would they be unable to produce in offspring? Pleiotropy One gene influences multiple characteristics Examples: CF, PKU, SC Sickle cell disease Also a codominant condition Most common in people of African descent (1/10 African-Americans are heterozygous; 1/400 have the disease) Heterozygote advantage – resistance to malaria Quick research: What is the condition called when an individual is heterozygous for sickle cell? Polygenic Inheritance Cross between two triple heterozygotes Many genes (with two AaBbCc X AaBbCc alleles each) control the expression of a single trait. Opposite of pleiotropy Leads to the expression of a trait along a continuum – Recessive, Dominant, height, skin color in very light (aabbcc) very dark (AABBCC) humans Quick Research: How many genes are thought to be responsible for the human height continuum? Genes and the Environment The environment has an impact on certain genetic traits – sun exposure & skin color, nutrition & height, exercise and build,etc. Nature vs. Nurture Some traits are strictly genetic (blood type), others are very strongly influenced by the environment and others fall somewhere in between. The Chromosomal Theory of Inheritance The behavior of chromosomes during mitosis and meiosis explain Mendel’s results. Genes occupy specific loci on chromosomes and the chromosomes undergo segregation and independent assortment. Linked Genes When Mendel studied dihybrids in pea plants he was studying traits that were on separate chromosomes so they always sorted independently. Genes that are close together on the same chromosome tend to be inherited together = linked genes. Crossing over can rearrange linked genes. Will not get results consistent with Mendel’s 9:3:3:1 ratio when studying linked traits. https://www.youtube.com/watch?v=rzF6nG4C0nc Crossing Over Max recombination frequency = Rearranges linked genes 50% If genes are linked most of the offspring will display parental phenotypes. Those that do not are called recombinants. Can calculate the recombinant frequency Thomas Hunt Morgan and fruit flies Recombinant frequencies can be used to map the relative location of genes on a chromosome. The higher the frequency, the greater the distance between the genes (50% would be the same as being on two different chromosomes). https://www.youtube.com/watch?v=TU44tR0hJ8A Sex Chromosomes 1st 22 pairs of chromosomes in humans are autosomes 23rd pair = sex chromosomes XX = female XY = male Sperm determine gender In other species the egg can determine gender - ZW in certain fishes, butterflies, and birds (Males are ZZ and females are ZW; eggs determine gender and will donate either a Z or a W) -XO in certain insects (O stands for the absence of a sex chromosome; females are XX, males are XO; sperm either donate an X chromosome or no sex chromosome) Some organisms like ants and bees do not have sex chromosomes and gender is determined by number of chromosomes (males = 16 and females = 32). In some reptiles, the temperature of egg incubation determines gender (for European pond turtles above 30°C= female; green sea turtles will be all male if above 30°C). Sex-Linked Traits Traits carried on sex chromosomes (most often the X chromosome only) Females carry two alleles (one on each X) Males only carry one allele on their only X Leads to unique inheritance patterns Males are much more likely to express sex-linked traits than females Red-green colorblindness Hemophilia Duchenne muscular dystrophy Quick Research: Which of the above disorders was called the “Royal Disease”? Sex-Linked Traits