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MENDEL’S LAWS Copyright © 2009 Pearson Education, Inc. Intro to Genetics Heredity is the transmission of traits from one generation to the next. Genetics is the scientific study of heredity. Gregor Mendel – began the field of genetics in the 1860s, – deduced the principles of genetics by breeding garden peas, and – relied upon a background of mathematics, physics, and chemistry. © 2012 Pearson Education, Inc. Intro to Genetics Gregor Mendel discovered principles of genetics in experiments with the garden pea – Mendel showed that parents pass heritable factors to offspring (heritable factors are now called genes) – Advantages of using pea plants – Controlled matings – Self-fertilization or cross-fertilization – Observable characteristics with two distinct forms – True-breeding strains Copyright © 2009 Pearson Education, Inc. Intro to Genetics True-breeding varieties result when self-fertilization produces offspring all identical to the parent. The offspring of two different varieties are hybrids. The cross-fertilization is a hybridization, or genetic cross. True-breeding parental plants are the P generation. Hybrid offspring are the F1 generation. A cross of F1 plants produces an F2 generation. © 2012 Pearson Education, Inc. Figure 9.2D Traits Character Dominant Recessive Flower color Purple White Axial Terminal Yellow Green Round Wrinkled Inflated Constricted Green Yellow Tall Dwarf Flower position Seed color Seed shape Pod shape Pod color Stem length Mendel’s law of segregation describes the inheritance of a single character Example of a monohybrid cross – Parental generation: purple flowers white flowers – F1 generation: all plants with purple flowers – F2 generation: 3/4 of plants with purple flowers 1/4 of plants with white flowers Mendel needed to explain – Why one trait seemed to disappear in the F1 generation – Why that trait reappeared in one quarter of the F2 offspring Copyright © 2009 Pearson Education, Inc. Figure 9.3B_s3 The Explanation P generation Genetic makeup (alleles) White flowers Purple flowers PP pp Gametes All P All p F1 generation (hybrids) All Pp Gametes 1 2 P Alleles segregate 1 2 p Fertilization Sperm from F1 plant F2 generation P Phenotypic ratio 3 purple : 1 white P Eggs from F1 plant Genotypic ratio p 1 PP : 2 Pp : 1 pp p PP Pp Pp pp Mendel’s law of segregation describes the inheritance of a single character Genes are found in alternative versions called alleles a genotype is the listing of alleles an individual carries for a specific gene For each characteristic, an organism inherits two alleles, one from each parent; the alleles can be the same or different – A homozygous genotype has identical alleles – A heterozygous genotype has two different alleles Copyright © 2009 Pearson Education, Inc. Gene loci Genotype: Dominant allele P a B P a b Recessive allele Bb PP aa Homozygous Heterozygous Homozygous for the for the dominant allele recessive allele Mendel’s law of segregation describes the inheritance of a single character If heterozygous, the dominant allele determines the organism’s appearance, and the recessive allele has no noticeable effect – The phenotype is the appearance or expression of a trait – The same phenotype may be determined by more than one genotype Law of segregation: Allele pairs separate (segregate) from each other during the production of gametes so that a sperm or egg carries only one allele for each gene Copyright © 2009 Pearson Education, Inc. The law of independent assortment is revealed by tracking two characters at once Example of a dihybrid cross – Parental generation: round yellow seeds wrinkled green seeds – F1 generation: all plants with round yellow seeds – F2 generation: 9/16 of 3/16 of 3/16 of 1/16 of plants plants plants plants with with with with round yellow seeds round green seeds wrinkled yellow seeds wrinkled green seeds Mendel needed to explain – Why nonparental combinations were observed – Why a 9:3:3:1 ratio was observed among the F2 offspring Copyright © 2009 Pearson Education, Inc. The law of independent assortment is revealed by tracking two characters at once Law of independent assortment – Each pair of alleles segregates independently of the other pairs of alleles during gamete formation – For genotype RrYy, four gamete types are possible: RY, Ry, rY, and ry Copyright © 2009 Pearson Education, Inc. Hypothesis: Independent assortment Hypothesis: Dependent assortment P generation rryy RRYY ry Gametes RY F1 generation rryy RRYY ry Gametes RY RrYy RrYy Sperm Sperm 1 – 2 F2 generation 1 – 2 RY 1 – 2 1 – 4 ry 1 – 4 RY Eggs 1 – 2 RY 1 – 4 ry Hypothesized (not actually seen) 1 – 4 rY 1 – 4 Ry 1 – 4 ry RY RRYY RrYY RRYy RrYy RrYY rrYY RrYy rrYy rY Eggs 1 – 4 1 – 4 9 –– 16 Ry RRYy RrYy RRyy Rryy RrYy rrYy Rryy rryy ry Actual results (support hypothesis) 3 –– 16 3 –– 16 1 –– 16 Yellow round Green round Yellow wrinkled Green wrinkled Chromosome separation during meiosis accounts for Mendel’s laws Mendel’s Laws correlate with chromosome separation in meiosis – The law of segregation depends on separation of homologous chromosomes in anaphase I – The law of independent assortment depends on alternative orientations of chromosomes in metaphase I Copyright © 2009 Pearson Education, Inc. F1 generation All round yellow seeds (RrYy) R r y Y r R y Y R Y y Y y R R Y y Anaphase I of meiosis r Y R r R Y Metaphase I of meiosis (alternative arrangements) r Metaphase II of meiosis r Y y r R Y y y Y Y r r r 1 – ry 4 1 – rY 4 Fertilization among the F1 plants F2 generation R Gametes y 1 – RY 4 r 9 :3 :3 :1 y y R R 1 – 4 Ry Geneticists use the testcross to determine unknown genotypes Testcross – Mating between an individual of unknown genotype and a homozygous recessive individual – Will show whether the unknown genotype includes a recessive allele – Used by Mendel to confirm true-breeding genotypes Copyright © 2009 Pearson Education, Inc. Testcross: B_ Genotypes bb Two possibilities for the black dog: BB B Gametes b Offspring Bb or Bb All black b B b Bb bb 1 black : 1 chocolate Mendel’s laws reflect the rules of probability The probability of a specific event is the number of ways that event can occur out of the total possible outcomes. Rule of multiplication – Multiply the probabilities of events that must occur together Rule of addition – Add probabilities of events that can happen in alternate ways Copyright © 2009 Pearson Education, Inc. F1 genotypes Bb male Formation of sperm Bb female Formation of eggs 1 – 2 1 – 2 1 – 2 B B B b B B 1 – 4 1 – 4 1 – 2 b b B 1 – 4 F2 genotypes b b b 1 – 4 VARIATIONS ON MENDEL’S LAWS Copyright © 2009 Pearson Education, Inc. Incomplete dominance results in intermediate phenotypes Incomplete dominance – Neither allele is dominant over the other – Expression of both alleles is observed as an intermediate phenotype in the heterozygous individual Codominance – Neither allele is dominant over the other – Expression of both alleles is observed as a distinct phenotype in the heterozygous individual – Observed for type AB blood Copyright © 2009 Pearson Education, Inc. P generation Red RR White rr r R Gametes F1 generation Incomplete Dominance in snapdragon flower color Pink Rr Gametes 1 – 2 R 1 – 2 r Sperm 1 – 2 F2 generation R 1 – 2 r 1 – 2 R RR rR 1 – 2 r Rr rr Eggs Blood Group (Phenotype) Genotypes Red Blood Cells O ii A IAIA or IAi Carbohydrate A B IBIB or IBi Carbohydrate B CoDominance of AB Blood Type AB IAIB Many genes have more than two alleles in the population Multiple alleles – More than two alleles are found in the population – A diploid individual can carry any two of these alleles – The ABO blood group has three alleles, leading to four phenotypes: type A, type B, type AB, and type O blood Copyright © 2009 Pearson Education, Inc. Blood Group (Phenotype) Genotypes Red Blood Cells Antibodies Present in Blood Anti-A Anti-B O ii A I AI A or I Ai Carbohydrate A Anti-B B IBIB or IBi Carbohydrate B Anti-A AB IAIB — Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left O A B AB A single character may be influenced by many genes Polygenic inheritance – Many genes influence one trait – Skin color is affected by at least three genes Copyright © 2009 Pearson Education, Inc. Figure 9.14 P generation aabbcc AABBCC (very light) (very dark) F1 generation AaBbCc AaBbCc Sperm 1 8 F2 generation 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 Fraction of population Eggs 1 8 1 8 1 8 1 8 1 8 1 64 6 64 15 64 20 64 15 64 6 64 1 64 Skin color Fraction of population 20 –– 64 15 –– 64 6 –– 64 1 –– 64 Skin color SEX CHROMOSOMES AND SEX-LINKED GENES Copyright © 2009 Pearson Education, Inc. 9.21 Sex-linked genes exhibit a unique pattern of inheritance Sex-linked genes are located on either of the sex chromosomes – Reciprocal crosses show different results – White-eyed female red-eyed male and white-eyed males red-eyed females – Red-eyed female white-eyed male and red-eyed males red-eyed females – X-linked genes are passed from mother to son and mother to daughter – X-linked genes are passed from father to daughter – Y-linked genes are passed from father to son Copyright © 2009 Pearson Education, Inc. Female Male Female Male XR Xr Xr Y Xr Y XR XR Sperm Xr Y Eggs XR XR Xr XR Y Sperm XR Xr Y XR XR XR Y Xr Xr Xr Y Eggs R = red-eye allele r = white-eye allele Xr Female Male XR Xr XR Y Sperm XR Y XR XR XR XR Y Xr Xr XR Xr Y Eggs Pedigree Analysis Copyright © 2009 Pearson Education, Inc. 9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees A pedigree – Shows the inheritance of a trait in a family through multiple generations – Demonstrates dominant or recessive inheritance – Can also be used to deduce genotypes of family members Copyright © 2009 Pearson Education, Inc. Dominant Traits Recessive Traits Freckles No freckles Widow’s peak Straight hairline Free earlobe Attached earlobe Figure 9.8B First generation (grandparents) Second generation (parents, aunts, and uncles)FF or Ff Third generation (two sisters) Female Male Attached Free Ff Ff ff ff ff Ff Ff Ff ff ff FF or Ff 9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene Inherited human disorders show – Recessive inheritance – Two recessive alleles are needed to show disease – Heterozygous parents are carriers of the disease-causing allele – Probability of inheritance increases with inbreeding, mating between close relatives – Dominant inheritance – One dominant allele is needed to show disease – Dominant lethal alleles are usually eliminated from the population Copyright © 2009 Pearson Education, Inc. Sex-linked disorders affect mostly males Males express X-linked disorders such as the following when recessive alleles are present in one copy – Hemophilia – Colorblindness – Duchenne muscular dystrophy Copyright © 2009 Pearson Education, Inc. Queen Victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis Gene Linkage Copyright © 2009 Pearson Education, Inc. 9.17 Genes on the same chromosome tend to be inherited together Linked Genes – Are located close together on the same chromosome – Tend to be inherited together Copyright © 2009 Pearson Education, Inc. Figure 9.18C The Explanation The Experiment Gray body, long wings (wild type) Black body, vestigial wings GgLl Female Male Female gl gl gl ggll Male Crossing over ggll GgLl GL GL Gl g l gl gL Eggs Sperm Offspring Gray long Black vestigial Gray vestigial Black long Offspring GL g l G l g L g l g l g l g l Parental 965 944 Parental phenotypes 206 185 Recombinant phenotypes Recombination frequency 391 recombinants 2,300 total offspring 0.17 or 17% Recombinant 9.18 Crossing over produces new combinations of alleles Linked alleles can be separated by crossing over – Recombinant chromosomes are formed – Thomas Hunt Morgan demonstrated this in early experiments – Geneticists measure genetic distance by recombination frequency Copyright © 2009 Pearson Education, Inc. Figure 9.18A p L p l PL Parental gametes pl p L Tetrad (pair of homologous chromosomes) P l Crossing over Recombinant gametes 9.19 Geneticists use crossover data to map genes Genetic maps – Show the order of genes on chromosomes – Arrange genes into linkage groups representing individual chromosomes Copyright © 2009 Pearson Education, Inc. Figure 9.19A Section of chromosome carrying linked genes g c l 17% 9% 9.5% Recombination frequencies