Gene Segregation and Interaction PDF

III. Gene Segregation and Interaction A. Law of Segregation Mendel’s Law of Segregation describes how alleles for a single trait separate during gamete formation Each gamete receives only one allele from each gene pair, ensuring that offspring inherit one allele from each parent A. Law of...

III. Gene Segregation and Interaction A. Law of Segregation Mendel’s Law of Segregation describes how alleles for a single trait separate during gamete formation Each gamete receives only one allele from each gene pair, ensuring that offspring inherit one allele from each parent A. Law of Segregation Key Observations from Mendel's Experiments: 1. Each organism carries two alleles for each trait 2. Alleles segregate during gamete formation (meiosis) 3. During fertilization, alleles recombine to form the genotype of the offspring Monohybrid Cross: - Parental generation: RR (round seeds) × rr (wrinkled seeds) - F1 generation: All Rr (round seeds) - F2 generation (self-fertilization): 3:1 phenotypic ratio (round: wrinkled) B. Law of Independent Assortment Mendel's Law of Independent Assortment states that the alleles of different genes segregate independently of one another during gamete formation Dihybrid Cross: - Parents: RRYY (round yellow) × rryy (wrinkled green) - F1 generation: All RrYy (round yellow) - F2 generation: 9:3:3:1 phenotypic ratio (round yellow: round green: wrinkled yellow: wrinkled green) C. Chromosomal Basis of Mendelian Laws Mendel’s laws were later explained by the behavior of chromosomes during meiosis Segregation: Homologous chromosomes (and their associated alleles) separate during meiosis I Independent Assortment: Chromosomes carrying different genes assort independently when forming gametes, leading to varied genetic combinations D. Dominance Relationships 1. Incomplete Dominance (No Dominance) 2. Overdominance 3. Co-Dominance D. Dominance Relationships 1. Incomplete Dominance (No Dominance) The heterozygous phenotype is an intermediate between the two homozygous phenotypes Example: In Mirabilis jalapa (four o'clock plant), crossing red (RR) and white (rr) flowers produces pink (Rr) flowers F2 ratio: 1 red : 2 pink : 1 white D. Dominance Relationships 2. Overdominance The heterozygote has a superior or more exaggerated phenotype than either homozygote Example: In Drosophila, the heterozygote (w*/w) produces more fluorescent pigments than the homozygous wild type (w*/w*) or mutant (w/w) D. Dominance Relationships 3. Co-Dominance Both alleles in a heterozygote are fully and equally expressed Example: In MN blood groups, a heterozygote (MN) expresses both M and N antigens equally on red blood cells E. Multiple Alleles When more than two alleles exist for a gene in a population Example: ABO blood group in humans, which has three alleles (A, B, O) E. Multiple Alleles Isoalleles: Alleles that have similar phenotypic effects Mutant Isoalleles: Produce a variation from the normal phenotype Normal Isoalleles: Produce the typical phenotype F. Lethal Genes 1. Recessive Lethals Lethal when in the homozygous recessive condition Example: Tay-Sachs disease in humans (homozygous individuals die in infancy) F. Lethal Genes 2. Dominant Lethals Lethal in both the homozygous and heterozygous state Example: Huntington’s disease, where symptoms manifest later in life and cause death in heterozygous individuals F. Lethal Genes 3. Penetrance of Lethal Genes Describes the extent to which a lethal gene is expressed Some lethal genes are semi-lethal, meaning a portion of affected individuals survive 4. Environmental Influence on Lethal Genes Conditional Lethal: Lethal only under specific environmental conditions Example: Temperature-sensitive lethal mutations in Drosophila that are lethal at restrictive temperatures but survivable at permissive temperatures G. Modifier Genes Modifiers: Genes that change the phenotypic effects of other genes in a quantitative fashion Enhancement: Increases the effect of the main gene Dilution: Reduces the effect of the main gene Suppressors: Modifiers that mask or completely suppress the expression of mutant alleles  Dilution: Reduces G. Modifier Genes the effect of the main gene H. Gene Interactions Gene interactions often result in phenotypic ratios that deviate from Mendelian expectations These interactions occur when two or more genes influence a single trait 1. Novel Phenotypes 2. Recessive Epistasis 3. Dominant Epistasis 4. Complementary Genes 5. Duplicate Genes  1. Novel Phenotypes Interaction between alleles produces new H. Gene Interactions phenotypes Example: Comb shape in chickens, where the interaction of two genes results in four phenotypes (walnut, rose, pea, single) H. Gene Interactions 2. Recessive Epistasis A recessive allele at one gene locus masks the effects of another gene Example: Coat color in mice (9:3:4 ratio), where the cc genotype results in albinism regardless of the other gene’s allele  3. Dominant Epistasis H. Gene Interactions a. In summer squash, a dominant allele (W) masks the expression of another gene (Y) for color (12:3:1 ratio) H. Gene Interactions 3. Dominant Epistasis b. In fowl feather color, dominant inhibitors prevent color expression, producing a 13:3 phenotypic ratio H. Gene Interactions 4. Complementary Genes Two genes must both have dominant alleles for a particular phenotype to be expressed Example: Flower color in peas, where both genes are necessary to produce purple flowers (9:7 ratio) H. Gene Interactions 5. Duplicate Genes Either of two genes can produce the same phenotype Example: Seed capsule shape in Shepherd’s purse (15:1 ratio) I. Pseudoalleles Example: The Star-asteroid case in Drosophila, where recombination between closely linked genes produces new phenotypes This phenomenon, known as the Lewis Effect or Position Effect, indicates that phenotype is influenced by both genotype and allele position on the chromosome J. Environmental Influence on Gene Expression The environment can significantly influence gene expression and the resulting phenotype 1. Penetrance 2. Expressivity 3. Pleiotropy 4. Phenocopy J. Environmental Influence on Gene Expression 1. Penetrance Penetrance refers to the proportion of individuals with a specific genotype that express the expected phenotype Example: Some individuals with a dominant allele for polydactyly (extra fingers) may NOT express the trait, a phenomenon called incomplete penetrance J. Environmental Influence on Gene Expression 2. Expressivity Expressivity refers to the degree to which a genotype is expressed in individuals with the same genotype Example: In people with polydactyly, the extra digit may vary in size J. Environmental Influence on Gene Expression 3. Pleiotropy Pleiotropy occurs when a single gene affects multiple traits Example: The gene for phenylketonuria (PKU) affects multiple body systems, including brain development and skin pigmentation J. Environmental Influence on Gene Expression 4. Phenocopy A phenocopy occurs when an environmental condition mimics a genetic phenotype Example: Exposure to thalidomide during pregnancy can cause limb malformations like that of genetic conditions J. Environmental Influence on Gene Expression External Environmental Factors Temperature: Coat color in Siamese cats and Himalayan rabbits is influenced by temperature, with cooler body parts producing darker fur J. Environmental Influence on Gene Expression External Environmental Factors Light: Plants need light for chlorophyll production, and the intensity of light can affect growth patterns J. Environmental Influence on Gene Expression External Environmental Factors Nutrition: Nutritional factors can influence the expression of certain genes J. Environmental Influence on Gene Expression External Environmental Factors Maternal Relations: Blood type incompatibilities between mother and fetus can affect fetal survival J. Environmental Influence on Gene Expression Internal Environmental Factors Age: Some genetic traits manifest only later in life, such as baldness or Huntington's disease J. Environmental Influence on Gene Expression Internal Environmental Factors Sex: Traits such as milk production are limited to one sex J. Environmental Influence on Gene Expression Internal Environmental Factors Substrates: The reactions in an organism largely depend on the substrates it synthesizes, which may be genetically controlled K. Twin Studies Twin studies help distinguish between the effects of genetics and the environment by comparing traits between identical (monozygotic) and fraternal (dizygotic) twins Concordant: Twins share the same trait Discordant: One twin expresses the trait, and the other does not  The extent of twin concordance can measure the roles of environment and heredity in expressing a phenotype L. Probability and Statistical Testing 1. Product Law The probability of two independent events occurring together is the product of their individual probabilities Example: In a dihybrid cross, the probability of getting round yellow seeds is 9/16 2. Sum Law The probability of one of two mutually exclusive events occurring is the sum of their probabilities Example: The probability of getting either a dominant or recessive allele for a trait is the sum of their probabilities L. Probability and Statistical Testing 3. Level of Significance The degree to which the observed data differ from the expected results can be assessed through statistical significance testing 4. The Chi-Square Test A statistical method used to compare observed genetic ratios with expected ratios Formula: χ² = Σ (O - E)² / E L. Probability and Statistical Testing 5. Binomial Distributions Used to calculate the probability of specific combinations of genotypes appearing in offspring 6. Normal Distribution As sample sizes increase, the distribution of genetic traits in a population follows a bell-shaped curve