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Introduction to Inheritance Lecture 12 Fall 2008
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Mendel’s Work Gregor Mendel Austrian Monk Published work in 1866
Importance of Mendel’s work Determined rules of inheritance Heritable factors (genes) passed on from parents to offspring Heritable factors retain their individual identities generation after generation
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Mendel’s Work Why was Mendel successful?
2 Why was Mendel successful? Chose an appropriate study organism Pea plant Small, easy and quick to grow Many varieties available Character: heritable feature that varies among individuals Trait: variant of a character Able to control mating Short reproductive cycles with many offspring Mathematically analyzed the data he collected Quantitative experiments Luck Traits he studied were controlled by one gene each, with simple dominant, recessive patterns and no crossing over
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Flower structure Floral organs Sepals Petals Stamens Carpels 3
Outer whorl, often green Can be colored like petals Petals Attract pollinators Bright colors Stamens “Male” reproductive structure Produces pollen (sperm) Carpels “Female” reproductive structure Base of carpel is ovary (egg) Fig. 9.3
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Mendel’s Work Controlled mating Pea flowers typically self-fertilize
4 Controlled mating Pea flowers typically self-fertilize Egg and sperm from same parent Pollen from stamens land on carpel in same flower Prevent possibility of pollen from other flowers by enclosing flower in a bag Cross fertilization could be controlled by hand fertilization Egg from one parent and sperm from another parent Pollen removed by hand from one plant and placed on carpel of another plant See Fig. 14.2
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Mendel’s Work 5 Created true breeding lines of individuals for each character Allowed plants to self fertilize for many generations True breeding individuals carry hereditary determinants for only one form of trait i.e., alleles are the same Individuals produce only offspring with that same trait See Table 14.1
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Mendel’s Work What happens when true breeding lines are crossed?
6 What happens when true breeding lines are crossed? E.g., when a purple flowered plant is crossed with a white flowered plant Terms Hybrid: offspring of two different true-breeding varieties Genetic cross: cross fertilization between two different varieties P generation: parental generation F1 generation: offspring F2 generation: offspring of F1 generation
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Mendel’s Work Terms Homozygous (homozygote)
7 Terms Homozygous (homozygote) Individuals that carry identical alleles for a gene Heterozygous (heterozygote) Individuals that carry 2 different alleles for a gene Dominant allele The allele that is expressed in an organism Capital letter (e.g., P for purple flower) Recessive allele Allele that is only expressed if no dominant form Lower case letter (e.g., p for white flower)
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Monohybrid Cross & The Law of Segregation
8 Monohybrid cross: cross between parent plants that differ in only 1 characteristic Punnett Square See Fig 14.3 &14.5
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Mendel’s hypotheses 9 1. There are alternative forms of genes, called alleles 2. For each inherited characteristic, an organism inherits 2 alleles (1 from each parent) Homozygous: 2 identical alleles Heterozygous:2 different alleles 3. In a heterozygote, one allele is dominant (is expressed) while the other is recessive (is not expressed) Dominant allele: capital letter (e.g., P for purple flower) Recessive allele: lower case letter (e.g., p for white flower) 4. Law of segregation Two alleles for a heritable character segregate during gamete formation and end up in different gametes
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Alleles and Chromosomes
10 Genotype Sequence of nucleotide bases in DNA All the alleles of every gene present in a given individual Gene : a discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA Phenotype Any observable traits in an individual Physical, physiological & behavioral The allele that is expressed Loci (locus) Specific location of genes along a chromosome See Fig. 14.4
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Dihybrid Cross and the Law of Independent Assortment
11 What would happen in a dihybrid cross? Dihybrid – mating of parental varieties differing in two characteristics Two hypothesis The traits travel together (dependent) The traits travel independently Seed color Y = yellow y = green Seed shape R = round r = wrinkled Dominant Recessive
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Dihybrid Cross and the Law of Independent Assortment
12 See Fig. 14.8
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Mendel’s Laws Law of independent assortment:
13 Law of independent assortment: Each pair of alleles assorts independently of the other pairs of alleles during gamete formation. The inheritance of one characteristic has no effect on the inheritance of another characteristic Law of segregation: Two alleles for a heritable character segregate during gamete formation and end up in different gametes
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Testcross Dominant genes are expressed (phenotype)
14 Dominant genes are expressed (phenotype) How do you tell the genotype? Two possibilities Homozygous for dominant allele (e.g., BB) Heterozygous with dominant allele (e.g., Bb) Use a test cross Mate individual of dominant phenotype (but unknown genotype) with an individual of recessive phenotype (and therefore recessive genotype)
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15 Testcross Problems with test cross?
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Probability & the Punnett Square
16 Punnett Squares Allow for prediction of the outcome of a particular mating What is the probability that two independent events will occur together E.g., two heads from two coin tosses E.g., two B alleles (one from each gamete) Gametes fuse randomly, so independent events Each coin toss or gamete formation is independent of the other See Fig. 14.9
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Probability & the Punnett Square
17 Rule of Multiplication For independent events, the probability of a compound event is the product of the separate events E.g. probability that an F1 generation will have BB ½ X ½ = ¼ Rule of Addition The probability that any one of two or more mutually exclusive events will occur is calculated by adding their individual probabilities Multiplication rule provides individual probabilities that are added together E.g., probability that an F1 generation will have a Bb ¼ + ¼ = ½ See Fig. 14.9
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Variations on Mendel’s Laws
18 Mendel worked with characteristics that were controlled by simple dominant/recessive inheritance of one gene But many characteristics not that simple Incomplete dominance Multiple Alleles Codominance Pleiotrophy Polygenic Inheritance
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Variations on Mendel’s Laws
19 Variations on Mendel’s Laws Incomplete Dominance An inheritance pattern in which the heterozygote phenotype is a blend or combination of both homozygote phenotypes Example: snapdragons F1: all pink F2: 1:2:1 Fig
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Variations on Mendel’s Laws
20 Multiple Alleles More than two different forms of a gene Any individual can only have two alleles Example: human blood groups Three alleles: A, B, O Six genotypes Four phenotypes A, B, AB, O Codominance Both alleles are expressed in heterozygous individuals Example: AB Makes both A & B carbohydrate Fig
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Variations on Mendel’s Laws
21 Pleiotrophy A pattern of genetic expression in which one gene affects more than one phenotypic trait Example: sickle-cell anemia and malaria What is sickle cell anemia? Abnormal hemoglobin produces sickle shaped red blood cells Sickled RBC’s do not carry oxygen efficiently Homozygous – suffer from disease Heterozygous – normally healthy, But Allele for sickle cell and normal cell codominant Both normal cells and sickle cells in body Sickle cells get destroyed over time by body defenses
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Variations on Mendel’s Laws
22 Example: sickle-cell anemia and malaria Why is sickle-cell anemia so common in Africans and African-Americans? 1 in 400 African-American children have it 1 in 10 are heterozygous
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Variations on Mendel’s Laws
23 Why is sickle cell anemia so common in Africans and African-Americans? Individuals who are heterozygous for sickle-cell anemia are more resistant to malaria Malaria common in many parts of Africa Malaria is caused by a microorganism Microorganism spends part of its lifecycle in a blood cell In heterozygous individuals, invasion of a RBC by microorganism causes cell to sickle Sickled cell, and microorganism it contains, destroyed by body’s immune system Selective advantage to be heterozygous for sickle-cell anemia
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Variations on Mendel’s Laws
24 Variations on Mendel’s Laws Fig Epistasis A gene at one locus alters the phenotypic expression of a gene at a second locus E.g., hair color in mice Locus for gene color Locus for pigment deposition If no pigment is deposited (recessive trait), then it doesn’t matter what gene is dominant at the locus for gene color
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Variations on Mendel’s Laws
25 Polygenic Inheritance Additive effects of two or more genes on a single phenotypic character Quantitative characters Vary along a continuum within a population Example: skin color At least three genes involved Genes inherited separately Genes display incomplete dominance AABBCC=very dark aabbcc = very light Any other combination falls along the gradient Fig
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Environmental Factors
26 Environmental Factors Environmental factors can influence phenotype, BUT Environmental influences are not inherited Only genetic influences (genotype) are inherited Exception? Mutations from environment (physical, chemical) that occur in reproductive cells Fig
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Dominant & Recessive Disorders
27 Dominant phenotypes Genotype: AA, or Aa Recessive phenotype Genotype: aa Dominant does not mean a phenotype is “normal” or more common in a population Wild type Trait that is found most often in nature Can be recessive
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Dominant & Recessive Disorders
28 Mendel worked with characteristics that were controlled by simple dominant/recessive inheritance of one gene Many diseases controlled by a single gene Most genetic disorders recessive Most from 2 heterozygous parents The closer the parents are related, the more likely they are to carry the same recessive alleles Inbreeding: mating of close relatives
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Dominant & Recessive Disorders
29 Dominant & Recessive Disorders Deafness caused by recessive allele If disease is caused by a single gene loci with a dominant/recessive pattern, the probability of an offspring having that disease can be determined by a Punnett square Fig. 9.14
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Dominant & Recessive Disorders
30 Lethal recessive disorders more common than lethal dominant Individual can be a carrier (Ll) of a lethal recessive allele Allele remains in population If a dominant disorder is lethal: If it kills individual when young, then not passed on allele becomes less common in population If it kills individual when past reproductive age, then can be passed on allele remains within the population
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