Non-Mendelian Genetics Notes PDF

© Getting Down With Science  Non-Mendelian Genetics Many traits do not follow the ratios predicted by Mendel’s laws. Why? Varying degrees of dominance Many traits are produced through multiple genes acting together Some traits are determined by genes on the sex chromosomes Some genes are...

© Getting Down With Science  Non-Mendelian Genetics Many traits do not follow the ratios predicted by Mendel’s laws. Why? Varying degrees of dominance Many traits are produced through multiple genes acting together Some traits are determined by genes on the sex chromosomes Some genes are adjacent or close to one another on the same chromosome and will segregate as a unit Some traits are the result of non-nuclear inheritance (ie chloroplast and mitochondrial DNA) © Getting Down With Science Degrees of Dominance © Getting Down With Science  Degrees of Dominance Alleles can show varying degrees of dominance In Mendel’s experiments, he worked with traits that showed complete dominance ○ Homozygous dominant and heterozygous individuals are phenotypically the same Incomplete dominance: neither allele is fully dominant ○ F1 generation has a phenotype that is a mix of those of the parental generation Example: red flowers crossed with white flowers will produce pink offspring © Getting Down With Science  Degrees of Dominance Codominance: two alleles that affect phenotype are both expressed ○ Example: human blood group Type AB blood: A and B are both expressed Multiple Alleles: genes that exist in forms with more than two alleles ○ Example: human blood group Alleles: IA, IB, i © Getting Down With Science  Practice Problems 1. A black mouse (BB) is crossed with a white mouse (bb) and the resulting offspring are gray. What is the easiest explanation to explain this phenomenon? Answer: incomplete dominance © Getting Down With Science  Practice Problems 2. Cattle can be red (RR= all red hair), white (WW= all white hair), or roan (RW= red and white hair). What is the best explanation for this phenomenon? Answer: Codominance 3. A red cow is crossed with a roan cow. What would the phenotypic ratio of the offspring be? Cross: RR x RW R R Answer: R RR RR 1:1 red and roan 50% red W RW RW 50% roan © Getting Down With Science  Practice Problems 4. A woman with type A blood has a child with a man who has type B blood. With this limited information, what are possible genotypes for the woman? For the man? Answer: The woman could be either IAIA or IAi The man could be either IBIB or IBi © Getting Down With Science Multiple Genes © Getting Down With Science  Multiple Genes In many cases, two or more genes are responsible in determining phenotypes. © Getting Down With Science  Multiple Genes Epistasis: the phenotypic expression of a gene at one locus affects a gene at another locus Example: coat color in labs and some mice ○ One gene codes for pigment and a second gene determines whether or not that pigment will be deposited in the hair © Getting Down With Science  Multiple Genes Polygenic inheritance: the effect of two or more genes acting on a single phenotype ○ Example: height, human skin color © Getting Down With Science Sex Chromosomes © Getting Down With Science  Sex-Linked Genes Thomas Hunt Morgan experimented with fruit flies and determined that specific genes can be carried on sex chromosomes Sex-linked gene: a gene located on either the X or the Y chromosome ○ Y-linked genes: genes specifically found on the Y chromosome Very few Y-linked genes, so very few disorders ○ X-linked genes: genes found on the X chromosome © Getting Down With Science  Inheritance of X-Linked Genes Fathers can pass X-linked alleles to all of their daughters, but none of their sons Mothers can pass X-linked alleles to both daughters and sons Affected father Affected mother N = normal XNXN x XnY XNXn x XNY n = affected Xn Y XN Y XN XNXn XNY XN XNXN XNY XN XNXn XNY Xn XNXn XnY © Getting Down With Science  Inheritance of X-Linked Genes If an X-linked trait is due to a recessive allele: ○ Females will only express trait if they are homozygous ○ Because males only have one X chromosome, they will express the trait if they inherit it from their mother They are called hemizygous (since the term heterozygous does not apply) Due to this males are much more likely to have an X-linked disorder © Getting Down With Science  X-Linked Disorders Duchenne muscular dystrophy ○ Progressive weakening of muscles Hemophilia ○ Inability to properly clot blood Color blindness ○ Inability to correctly see colors © Getting Down With Science  X-Inactivation Females inherit two X chromosomes, which is double than males! During development, most of one of the X chromosome in each cell becomes inactive ○ The inactive X in each cell of a female condenses into a Barr body Helps to regulate gene dosage in females © Getting Down With Science Linked Genes © Getting Down With Science  Genetic Recombination Genetic recombination: production of offspring with a new combination of genes from parents Parental types: offspring with the parental phenotype Recombinants: offspring with phenotypes that are different from the parents © Getting Down With Science  Genetic Recombination Mendel also observed recombinants during his crosses Example: green wrinkled plant crossed with a yellow- round plant ○ yyrr x YyRr YR yr Yr yR yr YyRr yyrr Yyrr yyRr 50% Parental 50% Recombinant phenotypes phenotypes 50% recombination, however, indicates that genes are unlinked, or on different chromosomes © Getting Down With Science  Linked Genes Linked genes: genes located near each other on the same chromosome that tend to be inherited together Meiosis and random fertilization generate genetic variation in offspring due to: ○ Independent assortment of chromosomes ○ Crossing over in Meiosis I ○ Any sperm can fertilize any egg © Getting Down With Science  Linked Genes: Crossing Over Linked genes show parental phenotypes in offspring at higher than 50% During crossing over chromosomes from one paternal chromatid and one maternal chromatid exchange corresponding segments © Getting Down With Science  Linked Genes: Crossing Over Crossing over helps to explain why some linked genes become separated during meiosis The further apart two genes are on the same chromosome, the higher the probability that a crossing over event will occur between them and the higher the recombination frequency © Getting Down With Science  Mapping Distance Experiments performed by Sturtevant, allowed scientists to map genes and their locations on chromosomes Linkage map: genetic map that is based on recombination frequencies ○ The distance between genes are map units One map unit is equivalent to a 1% recombination frequency Express the relative distances along chromosomes 50% recombination means that the genes are far apart on the same chromosome or on two different chromosomes © Getting Down With Science  Mapping Distance Numbers represent their distance from the “aristae” trait © Getting Down With Science © Getting Down With Science Non-Nuclear DNA © Getting Down With Science  Non-Nuclear DNA Some traits are located on DNA found in the mitochondria or chloroplasts Both chloroplasts and mitochondria are randomly assorted to gametes and daughter cells ○ In animals, mitochondria are transmitted by the egg, NOT the sperm Therefore, ALL mitochondrial DNA is maternally inherited ○ In plants, mitochondria and chloroplasts are transmitted in the ovule, NOT the pollen Therefore, both mitochondrial and chloroplast determined traits are maternally inherited © Getting Down With Science Statistical Analysis: Chi Square © Getting Down With Science  Goodness of Fit Test Chi-square: a form of statistical analysis used to compare the actual results (observed) with the expected results Helps to: ○ Determine whether the data obtained experimentally provides a “good fit” to the expected data ○ Determine if any deviations from the expected results are due to random chance alone or to other circumstances (ie. linked genes, data collection error) Designed to analyze categorical data © Getting Down With Science  Chi Square (X ) 2 Use the equation to test the “null” hypothesis ○ The prediction that data from the experiment will match the expected results Formula: Symbol means “sum” © Getting Down With Science  Example You are interested in examining the trait that allows people to roll their tongue. Tongue rolling is dominant to non tongue rolling You survey 100 people and you find: ○ 90 people were able to roll their tongue ○ 10 people could not roll their tongue You also took genetic data from the parents of all 100 people ○ The parental genotypes for all 100 people was Rr Null: any difference between the observed and expected data is due to chance © Getting Down With Science  Example: How to Solve Step 1: Determine what your expected and observed values are. ○ Observed (Actual) values: the numbers that you get in your data Usually no calculations ○ Expected values: based on probability Need to do calculations In this example, you know that all parents are Rr for the trait. So, set up a Punnett square to determine the expected numbers of people who can and cannot roll their tongue © Getting Down With Science  Example: How to Solve Step 2: Make a table The table you make should always follow this general format: Total Observed- O Expected- E O-E (O-E)2 (O-E)2/E © Getting Down With Science  Example: How to Solve Fill in the table with the trait you are looking at Trait Tongue Roller Non Tongue Total Roller Observed- O Expected- E O-E (O-E)2 (O-E)2/E © Getting Down With Science  Example: How to Solve Step 3: Plug in your data to the table and solve Trait Tongue Roller Non Tongue Total Roller Observed--O Expected--E O-E (O-E)2 (O-E)2/E © Getting Down With Science  Example: How to Solve If you performed the calculations correctly, your table should look like this: Trait Tongue Roller Non Tongue Total Roller Observed--O 90 10 100 Expected--E 75 25 100 O-E (90-75) = 15 (10-25) = -15 0 (O-E)2 225 225 450 (O-E)2/E 225/75=3 225/25=9 12= X2 © Getting Down With Science  Example: How to Solve Step 4: Determine the degrees of freedom for your experiment ○ With X2, you must determine the probability that the difference between the observed and expected values occurred simply by chance ○ You need to compare your calculated value to the appropriate value in a “degrees of freedom” table © Getting Down With Science  Example: How to Solve Step 4 Continued: ○ To calculate the degrees of freedom: ○ Degrees of freedom = # of categories – 1 For this problem, there are two categories (tongue rollers and non-tongue rollers) Degrees of freedom = 2 – 1 ○ Degrees of freedom = 1 © Getting Down With Science  Example: How to Solve P represents the confidence 0.05 means that you are 95% confident that your observed data fits your expected data extremely well 0.01 means that you are 99% confident that your observed data fits your expected data extremely well As a general rule of thumb, you look at the p = 0.05 row, unless instructed otherwise © Getting Down With Science  Example: How to Solve These values are the critical values for the degrees of freedom © Getting Down With Science  Example: How to Solve Interpreting results and degrees of freedom chart: If Χ2 > critical value: there is a statistically significant difference between the actual and expected values. ○ REJECT null hypothesis If Χ2 < critical value: there is NOT a statistically significant difference between the actual and expected values ○ Fail to Reject (accept) null hypothesis © Getting Down With Science  Example: How to Solve For this example, Χ2 = 12 12 > 3.84 Therefore, there is a statistically significant difference between the observed and expected population ○ What does this mean? Reject the null hypothesis Differences are not due to chance so there must be another explanation © Getting Down With Science  Practice Problem 1. In peas, yellow seeds (A) are dominant over green (a) seeds. In a cross between two plants both heterozygous for seed color, the following was observed: Yellow= 4400 Green= 1624 Does the data fit the predicted phenotypic ratio? © Getting Down With Science  Practice Problem Seed color Green Yellow Total (phenotype) Observed--O Expected--E O-E (O-E)2 (O-E)2/E © Getting Down With Science  Seed color Green Yellow Total (phenotype) Observed--O 1624 4400 6024 Expected--E 6024 (.25) = 6024 (.75) = 6024 1506 4518 O-E 118 -118 0 (O-E)2 13924 13924 27848 (O-E)2/E 9.25 3.08 12.32=x2 Data does not fit the expected phenotypes, we reject the null hypothesis, results are not due to chance © Getting Down With Science Environmental Effects on Phenotype © Getting Down With Science  Environmental Factors Various environmental factors can influence gene expression and lead to phenotypic plasticity ○ Individuals with the same genotype exhibit different phenotypes in different environments Examples: Temperature can change coat color in rabbits and Siamese cats Soil pH can affect flower color UV exposure can increase melanin production in the skin © Getting Down With Science Chromosomal Inheritance and Disorders © Getting Down With Science  Genetic Disorders Some genetic disorders can be linked to affected or mutated alleles or chromosomal changes © Getting Down With Science  Mutated Alleles Tay-Sachs disease ○ Autosomal recessive disease Mutated HEXA gene Body fails to produce an enzyme that breaks down a particular lipid Affects central nervous system and results in blindness Sickle cell anemia ○ Autosomal recessive disease Mutated HBB gene Sickled cells contain abnormal hemoglobin molecules © Getting Down With Science  Chromosomal Changes Nondisjunction: chromosomes fail to separate properly in meiosis I or meiosis II Karyotyping can detect nondisjunction Example: Down Syndrome ○ Three copies of chromosome 21 © Getting Down With Science