Chapter 14: Mendel and the Gene Idea PDF

**Chapter 14** ============== MENDEL AND THE GENE IDEA ======================== \>\>\>\>\>\>The father of genetics: Johann (Gregor) Mendel Mendel was a monk who was trained in science at the University of Vienna. The realization that both parents contributed to the characteristics of the offspri...

**Chapter 14** ============== MENDEL AND THE GENE IDEA ======================== \>\>\>\>\>\>The father of genetics: Johann (Gregor) Mendel Mendel was a monk who was trained in science at the University of Vienna. The realization that both parents contributed to the characteristics of the offspring preceded the work of Mendel. The favored explanation of how this occurred was the blending theory of heredity. Modern genetics began with Mendel, and lead to the replacement of the blending theory with the particulate theory of heredity. Among Mendel's teachers were: -a physicist named Doppler who taught Mendel to use a quantitative -a botanist named Unger who interested Mendel in the inheritable These influences helped lead Mendel to the study of the variation of garden peas. Why peas?? -Many different **characters** and **traits (see table 14.1)** \-\--**Characters** \-\--**Traits** For Mendel's pea plants the characters and traits he examined were 2\. Flower position (axial or terminal) 3\. Seed color (yellow or green) 4\. Seed shape (round or wrinkled) 5\. Pod shape (inflated or constricted) 6\. Pod color (green or yellow) 7\. Stem length (tall or dwarf) **Mendel's Findings: A First Look at Phenotypic Ratios** Mendel started his experiments with **true-breeding** plant varieties which he **hybridized** (cross-pollinated) (see fig 14.3) \-\--**True-breeding** Terminology for breeding (or genetics) experiments -**F~1~ generation** -**F~2~ generation** **Mendel's Conclusion: Alternate Alleles and Dominance** Mendel observed the transmission of the selected traits over these three generations and arrived at two principles of heredity: \>\>\>\>\>According to the law of segregation, two **alleles** for a character are packaged into separate gametes.(see fig 14.5) \-\--**Alleles** In a **monohybrid cross,** one of Mendel's typical experiments (see fig 14.5), he examined the way traits (purple flowers or white flowers) were expressed over three generations (P, F~1~, and F~2~) He found that in the F~1~ generation these character traits did not blend as predicted. Instead the white color was lost. Mendel hypothesized that the loss of the white color meant the plants had lost that allele and when he crossed the F~1~ plants he would get only purple-flowered plants. He performed the crosses and the F~2~ generation had purple-flowered plants and white-flowered plants in a 3:1 ratio. REJECT THE HYPOTHESIS!! **Mendel's Conclusions: Segregation and Recombination** The white allele was not lost but was masked by the presence of the purple allele. Mendel had found that the trait for purple flowers was a **dominant trait** and the trait for white flowers was a **recessive trait**. Mendel repeated these experiments with the six other characters and found similar 3:1 ratios in the F~2~ generations. This led him to the following four-part model: **1. Alternative forms of genes are responsible for variations in inherited** **2. For each character, an organism inherits two alleles, one from each** 3\. **If the two alleles differ, one is fully expressed (dominant allele); the** 4\. **The two alleles for each character trait segregate during gamete** \-\--**Dominant allele** \-\--**Recessive allele** \-\--**Mendel's law of segregation** -**Punnett square** \-\--**Homozygous** \-\--**Heterozygous** **Genotype** versus **Phenotype** \-\--**Phenotype** \-\--**Genotype** **Determining Heterozygosity: Test Crosses and Back Crosses** Because the genotype is not always apparent from the phenotype a **testcross** can be use to determine the genotype (see fig 14.7). **--- testcross** **Segregation and Independent Assortment** \>\>\>\>\>\>According to the **law of independent assortment**, each pair of alleles segregates into gametes independently The parental crosses that Mendel did in his earlier experiments involved parental varieties that differed in a single trait (i.e., **monohybrid crosses**) In later experiments Mendel crossed parental varieties that differed at two characters (i.e., **dihybrid crosses**) (see fig.14.8) By using plants that were true-breeding (homozygous) for two different characters, Mendel tested how the traits for these characters segregated. He hypothesized that the two characteristics would either segregate together (Null hypothesis), or they would segregate independently (alternative hypothesis). If they segregated together then the F~2~ will show a 3:1 phenotype If they segregated independently then the F~2~ will have a 9:3:3:1 phenotypic ratio. Results showed a 9:3:3:1 ratio THE NULL HYPOTHESIS IS REJECTED!! **Independent Assortment: An Explanation** This result led Mendel to the **law of independent assortment**. \-\--**Mendel's law of independent assortment** \>\>\>\>\>Mendelian inheritance reflects the rules of probability (see fig.14.9) \- Probability of an event occurring ranges from 0 to 1. -The probabilities of all outcomes for a single event must add up to 1 -Each successive event is independent of previous events. \>\>\>Two basic rules of probability are used in solving genetic problems: 1\. **The rule of multiplication** Example: Probability that 2 coin tosses will both produce heads is 2\. **The rule of addition** Example: The probability that 2 coin tosses will produce 1 head and 1 tail. The probability for each way is 1/4 (because there are four possible outcomes), thus the total probability is 1/4 + 1/4 = 1/2. Probability provides statistical framework against which results can be compared. \>\>\>\>\>Significance of Mendel\'s research: Inheritance of traits is due to transmission of discrete factors (genes) that are passed from generation to generation according to rules of probability. Same principle applies to all sexually reproducing organisms. Mendel worked with traits that have dominant and recessive alleles, and follow the rule of independent assortment - the simplest scenario by which genetic information is sexually transmitted. \>\>\>\>\>\>The relationship between genotype and phenotype is rarely simple INTERMEDIATE INHERITANCE or **incomplete dominance (see fig 14.10)** \-\--**Incomplete dominance** \>sounds like blending theory but it's not -recessive trait can still emerge in the F~2~ -genotype is apparent from phenotype \>\>**Complete dominance** to **codominance** The dominance/recessive relationships vary over a continuum. COMPLETE \ CODOMINANCE DOMINANCE DOMINANCE \-\--**Codominance** \>\>Multiple alleles A gene may have more than two allele forms. ABO blood group is an example of multiple alleles and codominance ---------------- ------------------------ ----------------------------- --------------------------------- **BLOOD TYPE** **POSSIBLE GENOTYPES** **ANTIGENS PRESENT ON RBC** **ANTIBODIES PRESENT IN SERUM** A I^A^ I^A^ or I^A^ i A anti-B B I^B^ I^B^ or I^B^ i B anti-A AB I^A^ I^B^ AB none O ii O anti-A and anti-B ---------------- ------------------------ ----------------------------- --------------------------------- **Epistasis: One Gene Affecting Another** \>\>\>**Epistasis** -one gene that changes the expression of another gene.(see fig 14.12) \-\--**Epistasis** \>\>\>**Polygenic inheritance** of quantitative characters (see fig 14.13) -trait varies in a continue \-\--**Polygenic inheritance** -contrasted with pleiotropy and epistasis -Skin pigmentation **Pleiotropy: Multiple Phenotypic Effects** \>\>\>**Pleiotropy** - genes that affect many traits \-\--**Pleiotropy** \>\>\>Environmental impact on phenotype A single genotype may product a range of phenotypes in response to environmental factors. The phenotypic range of the **norm of reaction** for the genotype (see fig. 14.14). \-\--**Norm of reaction**