Mendelian Genetics & Inheritance PDF

Mendelian Genetics & Inheritance Garden Pea, Pisum sativum Gregor Mendel, 1822-1884  Mendel, documented mechanisms of inheritance – decades before chromosomes and genes were observed Characters Traits (Variants)  What genetic principles a...

Mendelian Genetics & Inheritance Garden Pea, Pisum sativum Gregor Mendel, 1822-1884  Mendel, documented mechanisms of inheritance – decades before chromosomes and genes were observed Characters Traits (Variants)  What genetic principles account for the passing of traits from parents to offspring? The “blending” hypothesis Loading… The “particulate” hypothesis  Planned breeding experiments Organisms used should: – show variation in characteristics – produce large number of offspring – allow controlled matings to be conducted – have short life cycle – be convenient to handle  Mendel brought an experimental approach to genetics Mendel chose to work with garden peas because: – True breeding varieties exist – Variation exists for particular traits – Could control mating – Short life cycle – Large number of offspring Loading…  Mendel studied inheritance of 7 characters existing in 2 forms -- either/or traits Parental generation (P) true- breeding - Only purple flowers or only white flowers First filial generation (F1) hybrids - all purple flowers Fig. 14-3-3 EXPERIMENT P Generation (true-breeding parents) Purple White flowers flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation No blending of traits! 705 purple-flowered 224 white-flowered plants plants When plants of contrasting traits (that bred true) were crossed: purple flowers x white flowers The offspring (F1)all exhibited one of the traits The traits did not blend Traits that appear in all F1 = dominant Traits that are not expressed = recessive  All characters he observed behaved the same – P(parental) dominant x recessive F1: – all dominant trait – Monohybrids F2: – 3/4 dominant trait 1/4 recessive trait  The Law of Segregation Mendel developed a hypothesis to explain the inheritance pattern observed in F2 offspring Five concept model Loading… concepts can be related to what we now know about genes and chromosomes  First concept Parents don’t transmit traits directly to offspring but as “heritable factors” Mendel’s “heritable factors” are genes  Second concept Alternative versions of genes account for variations in inherited characters alleles locus  Third concept For each character an organism inherits two alleles, one from each parent Mendel made this deduction without knowing about chromosomes The two alleles on homologous chromosome may be identical or two alleles may differ Homozygous (dominant or recessive) Heterozygous  pair of homologous chromosomes Both chromosomes carry the same allele of this gene so the organism is homozygous at this locus This gene represents another homozygous locus The chromosomes carry different alleles of this gene so the organism is heterozygous at this locus  Fourth concept If alleles at a locus differ: dominant allele determines appearance while recessive allele has no noticeable effect – Genotype – Phenotype  Fifth concept The two alleles for a heritable character separate (segregate) during gamete formation egg or sperm (haploid) gets only one of the two alleles present in somatic cells (diploid) Mendel’s Law of Segregation Two copies of a gene segregate from each other during the transmission from parent to offspring True-breeding parents: homozygous for trait F1 generation: all heterozygous dominant genotype and phenotype F2 generation: heterozygous and homozygous genotypes variety of phenotypes  Punnett square - diagram for predicting results of a genetic cross Step 1. Write down genotypes of parents Male parent: Pp Female parent: Pp Step 2. Write down the possible gametes that each parent can make Male gametes: P or p Female gametes: P or p Step 3. Create an empty Punnett square. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Male ♂ P gamete p s ♀ P F e m a l p e g a Step 4. Fill in the possible genotypes. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Male ♂ P gamete p s ♀ P PP Pp F e m a l p Pp pp e g a Step 5. Determine relative proportions of genotypes and phenotypes. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Male ♂ P gamete p s Genotype ratio ♀ PP:Pp:pp P PP Pp F 1:2:1 e m Phenotype ratio a l p Pp pp purple:white e 3:1 g a Test cross: Determine unknown genotype by mating it with a known genotype  The Law of Independent Assortment Mendel derived the law of segregation by following a single character Cross between true-breeding individuals homozygous for one character is monohybrid – F1 plants were monohybrids, heterozygous for a single character (Pp)  What would happen if he followed two characters at the same time? Seeds may be either yellow or green and may be either round (smooth) or wrinkled Were these two characters transmitted from parents to offspring as a package? Will the Y and R alleles always stay together, generation after generation? – Mendel crossed two true-breeding peas differing in both of these characters yellow-round seeds (YYRR ) with green-wrinkled seeds (yyrr ) Dihybrid cross – F1 plants were dihybrids, heterozygous for both characters (YyRr) Law of Independent Assortment Alleles of different genes assort independently of each other during gamete formation Mendel’s Law of Segregation can be explained by the pairing and segregation of homologous chromosomes during meiosis I  The Law of Independent Assortment can also be explained by the behavior of chromosomes during meiosis Random alignment of chromosome pairs during Loading… meiosis I leads to the independent assortment of genes found on different chromosomes