Mendelian Genetics In honor of Mendels 189 th
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Mendelian Genetics In honor of Mendel’s 189 th birthday Google used this as their header on July 20, 2011 1
Gregor Mendel (1822 -1884) Responsible for the Laws governing Inheritance of Traits 2
§§ § § Gregor Johann Mendel Austrian monk Studied the inheritance of traits in pea plants Developed the laws of inheritance Mendel's work was not recognized until the turn of the 20 th century 3
§ Gregor Johann Mendel Between 1856 and 1863, Mendel cultivated and tested some 28, 000 pea plants He found that the plants' offspring retained traits of the parents Called the “Father of Genetics" § § 4
Mendel’s Pea Plant Experiments 5
Site of Gregor Mendel’s experimental garden in the Czech Republic 6
Why peas, Pisum sativum? § § Can be grown in a small area Produce lots of offspring Produce pure plants when allowed to self -pollinate several generations Can be artificially cross-pollinated 7
Reproduction in Flowering Plants Pollen contains sperm Produced by the stamen Ovary contains eggs Found inside the flower Pollen carries sperm to the eggs for fertilization Self-fertilization can occur in the same flower Cross-fertilization can occur between flowers 8
Mendel’s Experimental Methods Mendel hand-pollinated flowers using a paintbrush He could snip the stamens to prevent self-pollination He traced traits through the several generations 9
How Mendel Began Mendel produced pure strains by allowing the plants to selfpollinate for several generations 10
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Crossing pea plants 1 Removed stamens from purple flower 2 Transferred sperm- bearing pollen from stamens of white flower to eggbearing carpel of purple flower By crossing (mating) two true-breeding varieties of an organism, scientists can study patterns of inheritance. In this example, Mendel crossed pea plants that varied in flower color. APPLICATION TECHNIQUE Parental generation (P) 3 Pollinated carpel Stamens Carpel (male) (female) matured into pod 4 Planted seeds from pod RESULTS When pollen from a white flower fertilizes eggs of a purple flower, the first-generation hybrids all have purple flowers. The result is the same for the reciprocal cross, the transfer of pollen from purple flowers to white flowers. Figure 14. 2 5 Examined First generation offspring (F 1) offspring: all purple flowers
Eight Pea Plant Traits Seed shape --- Round (R) or Wrinkled (r) Seed Color ---- Yellow (Y) or Green (y) Pod Shape --- Smooth (S) or wrinkled (s) Pod Color --- Green (G) or Yellow (g) Seed Coat Color ---Gray (G) or White (g) Flower position---Axial (A) or Terminal (a) Plant Height --- Tall (T) or Short (t) Flower color --- Purple (P) or white (p) 13
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What Do the Peas Look Like? 16
Punnett Square Used to help solve genetics problems 17
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How to make a Punnett Square: Rr x rr 19
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Genetic Practice Problems 21
Breed the P 1 generation tall (TT) x dwarf (tt) pea plants T T t t 22
Solution: tall (TT) vs. dwarf (tt) pea plants T T t Tt Tt produces the F 1 generation t Tt Tt All Tt = tall (heterozygous tall) 23
Breed the F 1 generation tall (Tt) vs. tall (Tt) pea plants T t 24
Solution: tall (Tt) x tall (Tt) pea plants T t T TT Tt tt produces the F 2 generation 1/4 (25%) = TT 1/2 (50%) = Tt 1/4 (25%) = tt 1: 2: 1 genotype 3: 1 phenotype 25
§ § § Genetic Terminology Trait - any characteristic that can be passed from parent to offspring Heredity - passing of traits from parent to offspring Genetics - study of heredity 26
Following the Generations Cross 2 Results in Cross 2 Hybrids Pure all get Plants Hybrids 3 Tall & 1 Short TT x tt Tt TT, Tt, tt 27
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Did the observed ratio match theoretical ratio? The theoretical or expected ratio of plants producing round or wrinkled seeds is 3 round : 1 wrinkled Mendel’s observed ratio was 2. 96: 1 The discrepancy is due to statistical error The larger the sample the more nearly the results approximate to theoretical ratio 29
Generation “Gap” Parental P 1 Generation = the parental generation in a breeding experiment. F 1 generation = the first-generation offspring in a breeding experiment. (1 st filial generation) From breeding individuals from the P 1 generation F 2 generation = the second-generation offspring in a breeding experiment. (2 nd filial generation) From breeding individuals from the F 1 generation 30
Particulate Inheritance § § Mendel stated that physical traits are inherited as “particles” Mendel did not know that the “particles” were actually Chromosomes & DNA 31
§ § § Designer “Genes” Alleles - two forms of a gene (dominant & recessive) Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R) Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r) 32
§ § More Terminology Genotype - gene combination for a trait (e. g. RR, Rr, rr) Phenotype - the physical feature resulting from a genotype (e. g. red, white) 33
Genotype & Phenotype in Flowers Genotype of alleles: R = red flower r = yellow flower All genes occur in pairs, so 2 alleles affect a characteristic Possible combinations are: Genotypes RR Rr rr Phenotypes RED YELLOW 34
§ Genotypes Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (e. g. RR or rr); also called purebred Heterozygous genotype - gene combination of one dominant & one recessive allele (e. g. Rr); also called hybrid § 35
Phenotype vs. Genotype ure 14. 6 Phenotype Purple 3 Purple Genotype PP (homozygous) 1 Pp (heterozygous) 2 Pp (heterozygous) Purple 1 White pp (homozygous) Ratio 3: 1 Ratio 1: 2: 1 1 36
Genes and Environment Determine Characteristics 37
Types of Genetic Crosses § § Monohybrid cross - cross involving a single trait e. g. flower color Dihybrid cross - cross involving two traits e. g. flower color & plant height 38
Monohybrid Crosses 39
P 1 Monohybrid Cross Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Wrinkled seeds RR x rr r r R Rr Rr Genotype: Rr Phenotype: Phenotype Round Genotypic Ratio: All alike Phenotypic Ratio: All alike 40
P 1 Monohybrid Cross Review § §§ § Homozygous dominant x Homozygous recessive Offspring all Heterozygous (hybrids) Offspring called F 1 generation Genotypic & Phenotypic ratio is ALL ALIKE 41
F 1 Monohybrid Cross Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Round seeds Rr x Rr R RR Rr rr Genotype: RR, Rr, rr Phenotype: Phenotype Round & wrinkled G. Ratio: 1: 2: 1 P. Ratio: 3: 1 42
Results of Monohybrid Crosses Inheritable factors or genes are responsible for all heritable characteristics Phenotype is based on Genotype Each trait is based on two genes, one from the mother and the other from the father True-breeding individuals are homozygous ( both alleles) are the same 43
F 1 Monohybrid Cross Review §§ §§ § Heterozygous x heterozygous Offspring: 25% Homozygous dominant RR 50% Heterozygous Rr 25% Homozygous Recessive rr Offspring called F 2 generation Genotypic ratio is 1: 2: 1 Phenotypic Ratio is 3: 1 44
…And Now the Test Cross Mendel then crossed a pure & a hybrid from his F 1 generation This is known as an F 2 or test cross There are two possible testcrosses: Homozygous dominant x Hybrid Homozygous recessive x Hybrid 45
F 2 Monohybrid Cross st (1 ) Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Round seeds RR x Rr R RR Rr Genotype: RR, Rr Phenotype: Phenotype Round Genotypic Ratio: 1: 1 Phenotypic Ratio: All alike 46
F 2 Monohybrid Cross (2 nd) Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Wrinkled seeds x Round seeds rr x Rr R r r Rr Rr r rr rr Genotype: Rr, rr Phenotype: Phenotype Round & Wrinkled G. Ratio: 1: 1 P. Ratio: 1: 1 47
F 2 Monohybrid Cross Review §§ §§ Homozygous x heterozygous(hybrid) Offspring: 50% Homozygous RR or rr 50% Heterozygous Rr Phenotypic Ratio is 1: 1 Called Test Cross because the offspring have SAME genotype as parents 48
Mendel’s Laws 49
Law of Dominance If one copy of the dominant trait is present, the dominant trait will be expressed. 50
Law of Dominance 51
Law of Segregation During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring. 52
Applying the Law of Segregation 53
Law of Independent Assortment Alleles for different traits are distributed to sex cells (& offspring) independently of one another. This law can be illustrated using dihybrid crosses. 54
Answer: 1. Rr. Yy: 2 n = 22 = 4 gametes RY Ry r. Y ry 2. Aa. Bb. CCDd: 2 n ABCD ABCd a. BCD a. BCd = 23 = Ab. CD ab. CD 8 gametes Ab. Cd ab. CD 3. Mm. Nn. Oo. PPQQRrss. Tt. Qq: 2 n = 26 = 64 gametes 55
Dihybrid Cross A breeding experiment that tracks the inheritance of two traits. Mendel’s “Law of Independent Assortment” a. Each pair of independently b. Formula: 2 n alleles segregates during gamete formation (n = # of heterozygotes) 56
Dihybrid Cross Traits: Seed shape & Seed color Alleles: R round r wrinkled Y yellow y green Rr. Yy RY Ry r. Y ry x Rr. Yy RY Ry r. Y ry All possible gamete combinations 57
Dihybrid Cross RY Ry r. Y ry 58
Dihybrid Cross RY RY RRYY Ry RRYy r. Y Rr. YY ry Rr. Yy Ry r. Y ry RRYy Rr. YY Rr. Yy RRyy Rr. Yy Rryy Rr. Yy rr. YY rr. Yy Rryy rr. Yy rryy Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9: 3: 3: 1 phenotypic ratio 59
Dihybrid Cross Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9: 3: 3: 1 60
A dihybrid cross Illustrates the inheritance of two characters Produces four phenotypes in the F 2 generation EXPERIMENT Two true-breeding pea plants— one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F 1 plants. Self-pollination of the F 1 dihybrids, which are heterozygous for both characters, produced the F 2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant. P Generation YYRR yyrr Gametes F 1 Generation YR yr Yy. Rr Hypothesis of independent assortment Hypothesis of dependent assortment Sperm 1⁄ Sperm RESULTS CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. 1⁄ 2 YR 1⁄ 2 3⁄ 4 1⁄ 1⁄ 4 Yr 1⁄ 4 y. R 1⁄ 4 yr Eggs 1⁄ 2 YR F 2 Generation YYRR Yy. Rr (predicted offspring) 1 ⁄ yr 2 Yy. Rr yyrr 4 YR 1⁄ 1⁄ 4 YR 4 Yr 4 y. R 4 yr 1⁄ YYRR YYRr Yy. RR Yy. Rr YYrr Yy. Rr Yyrr Yy. RR Yy. Rr yy. RR yy. Rr 4 Phenotypic ratio 3: 1 1⁄ 9⁄ 16 Yy. Rr 3⁄ 16 Yyrr yy. Rr 3⁄ 16 yyrr 1⁄ 16 Phenotypic ratio 9: 3: 3: 1 315 108 101 32 Phenotypic ratio approximately 9: 3: 3: 1 Figure 14. 8 61
Question: How many gametes will be produced for the following allele arrangements? Remember: 2 n (n = # of heterozygotes) 1. Rr. Yy 2. Aa. Bb. CCDd 3. Mm. Nn. Oo. PPQQRrss. Tt. Qq 62
Test Cross A mating between an individual of unknown genotype and a homozygous recessive individual. Example: bb. C__ x bbcc BB Bb bb = = = brown eyes blue eyes CC = curly hair Cc = curly hair cc = straight hair b. C b___ bc 63
Test Cross Possible results: bc b. C b___ C bb. Cc or bc b. C b___ c bb. Cc bbcc 64
Summary of Mendel’s laws LAW DOMINANCE SEGREGATION INDEPENDENT ASSORTMENT PARENT CROSS OFFSPRING TT x tt tall x short 100% Tt tall x x Tt tall Rr. Gg x Rr. Gg round & green x round & green 75% tall 25% short 9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods 65
Beyond Dominant and Recessive Alleles What are some exceptions to Mendel’s principles?
Beyond Dominant and Recessive Alleles Some alleles are neither dominant nor recessive. Many genes exist in several different forms, and are therefore said to have multiple alleles. Many traits are produced by the interaction of several genes.
Beyond Dominant and Recessive Alleles Despite the importance of Mendel’s work, there are important exceptions to most of his principles. In most organisms, genetics is more complicated, because the majority of genes have more than two alleles. In addition, many important traits are controlled by more than one gene. Mendel’s principles alone cannot predict traits that are controlled by multiple alleles or multiple genes.
Incomplete Dominance and Codominance 69
THINK ABOUT IT Mendel’s principles offer a set of rules with which to predict various patterns of inheritance. There are exceptions to every rule, and exceptions to the exceptions. What happens if one allele is not completely dominant over another? What if a gene has several alleles?
Incomplete Dominance A cross between two four o’clock plants shows a common exception to Mendel’s principles. The F 1 generation produced by a cross between redflowered (RR) and whiteflowered (WW) plants consists of pink-colored flowers (RW), as shown.
Incomplete Dominance In this case, neither allele is dominant. Cases in which one allele is not completely dominant over another are called incomplete dominance. In incomplete dominance, the heterozygous phenotype lies somewhere between the two homozygous phenotypes.
Incomplete Dominance F 1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties. Example: snapdragons (flower) red (RR) x white (rr) R R RR = red flower rr = white flower r r 73
Incomplete Dominance R R r Rr Rr produces the F 1 generation All Rr = pink (heterozygous pink) 74
Incomplete Dominance 75
Codominance Cases in which the phenotypes produced by both alleles are clearly expressed are called codominance. For example, in certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens have a color described as “erminette, ” speckled with black and white feathers.
Codominance Two alleles are expressed (multiple alleles) in heterozygous individuals. Example: blood type 1. 2. 3. 4. type A B AB O = = IAIA or IAi IBIB or IBi I AI B ii 77
The ABO blood group in humans Is determined by multiple alleles Table 14. 2 78
Co-dominance and Multiple Alleles Rh Factor 79
Codominance Problem Example: homozygous male Type B (IBIB) x heterozygous female Type A (IAi) IB IB IA I AI B i I Bi 1/2 = IAIB 1/2 = IBi 80
Another Codominance Problem • Example: male Type O (ii) x female type AB (IAIB) IA IB i I Ai I Bi 1/2 = IAi 1/2 = IBi 81
Codominance Question: If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents? boy - type O (ii) AB (IAIB) X girl - type 82
Codominance Answer: IA IB i i I AI B ii Parents: genotypes = IAi and IBi phenotypes = A and B 83
Sex-linked Traits (genes) located on the sex chromosomes Sex chromosomes are X and Y XX genotype for females XY genotype for males Many sex-linked traits carried on X chromosome 84
Hemophilia is a sex-linked trait in humans. The disorder results in failure of the blood to clot. 85
Female Carriers 86
Hemophilia in the Royal Families of Europe 87
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Sex-linked Traits Example: Eye color in fruit flies Sex Chromosomes fruit fly eye color XX chromosome - female Xy chromosome - male 90
Sex-linked Trait Problem Example: Eye color in fruit flies (red-eyed male) x (white-eyed female) X RY x X r Remember: the Y chromosome in males does not carry traits. X y RR = red eyed Rr = red eyed X rr = white eyed Xy = male XX = female X- 91
Sex-linked Trait Solution: X X- X- X X- y 100% red eyed carriers female 50% white eyed male 92
Sex-linked Traits Calico cats are all female, unless there is a genetic mutation (XXY) male 93
Thomas Hunt Morgan discovered that Mendel’s principles apply to animals using fruit flies 94
Many human traits follow Mendelian patterns of inheritance Humans are not convenient subjects for genetic research However, the study of human genetics continues to advance Cloverleaf tongue folding Genetic or not? 95
Pedigree Analysis A pedigree Is a family tree that describes the interrelationships of parents and children across generations. Circles are females, squares are males. If it is filled in-the person has the trait 96
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Inheritance patterns of particular traits Can be traced and described using pedigrees Ww Ww ww ww ww Ww WW or Ww Widow’s peak ww Ww Ww ww First generation (grandparents) Second generation (parents plus aunts and uncles) Ff FF or Ff Ff Ff Third generation (two sisters) ww No Widow’s peak Attached earlobe ff ff Ff Ff ff FF or Ff Ff ff Free earlobe (a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe) 99
Normal male karyotype Normal female karyotype 100
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Turner Syndrome XO female 103
Klinefleter’s Syndrome XXY male Rate of occurrence: About 1 in 500 to 1 in 1000 males 104
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Recessively inherited disorders Show up only in individuals homozygous for the allele Carriers Are heterozygous individuals who carry the recessive allele but are phenotypically normal 107
Cystic Fibrosis Symptoms of cystic fibrosis include Mucus buildup in the some internal organs Abnormal absorption of nutrients in the small intestine 108
Sickle-Cell Disease Sickle-cell disease Affects one out of 400 African-Americans Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells Carriers (hybrids) are resistant to malaria Symptoms include Physical weakness, pain, organ damage, and even paralysis 109
Dominantly Inherited Disorders Some human disorders Are due to dominant alleles-polydactyly (extra digits) and achondroplasia A form of dwarfism that is lethal when homozygous for the dominant allele Figure 14. 15 110
Dominantly Inherited Disorders Huntington’s disease Is a degenerative disease of the nervous system Has no obvious phenotypic effects until about 35 to 40 years of age. The singer Arlo Guthrie died of Huntington’s. Figure 14. 16 111
Fetal testing (b) Chorionic villus sampling (CVS) (a) Amniocentesis Amniotic fluid withdrawn A sample of chorionic villus tissue can be taken as early as the 8 th to 10 th week of pregnancy. A sample of amniotic fluid can be taken starting at the 14 th to 16 th week of pregnancy. Fetus Suction tube Inserted through cervix Centrifugation Placenta Uterus Chorionic vi. IIi Cervix Fluid Fetal cells Biochemical tests can be Performed immediately on the amniotic fluid or later on the cultured cells. Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping. Biochemical tests Several weeks Several hours Karyotyping Figure 14. 17 A, B Karyotyping and biochemical tests can be performed on the fetal cells immediately, providing results within a day or so.
Newborn Screening Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States
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