Mendelian Genetics Notes

Genetics HEREDITY is the transmission of traits from parents to their offspring GENETICS is the study of heredity GREGORY MENDEL is the father of genetics

Gregory Mendel An Austrian monk (during the 1820’s) Given the task of tending the garden Studied the hereditary patterns of pea plants

Mendel’s Experiments Studied the 7 characteristics of pea plants Round vs. wrinkled peas, yellow vs. green peas, white vs. grey seed coats, inflated vs. constricted pods, yellow vs. green pod color, tall vs. short plants, flowers on sides vs. flowers on the ends

Mendel’s Experiments He let the plants pollinate themselves to produce pure strains Called TRUE BREEDING (made clones) Then he cross pollinated plants to see what would happen Called HYBRIDS (since parents had different traits) Results: patterns emerged in the offspring of the pea plants

Concepts from Mendel’s Work P GENERATION is the Parental Generation This is the original pure strain parents F1 GENERATION is the First Filial Generation This is the offspring from parents in the P generation (children) F2 GENERATION is the Second Filial Generation This is the offspring from two parents from the F1 generation (grandchildren)

Concepts from Mendel’s Work CHROMOSOMES are the rod shaped form DNA takes during meiosis that contains genes Offspring get one chromosome of each homologous pair from each parent at fertilization

Concepts from Mendel’s Work GENE is the segment of DNA that controls a particular hereditary trait

Concepts from Mendel’s Work ALLELES are the alternate forms of genes Dominant allele – Capital letter (T or tall) Recessive allele – small letter (t or not tall, or short) A pair of alleles would be TT, Tt, or tt Each allele is on a different homologous chromosome Meaning you get one allele from MOM and the other allele from DAD

Concepts from Mendel’s Work GENOTYPE is the genetic makeup of an organism; what genes it has (TT, Tt, or tt) PHENOTYPE is the physical appearance of an organism; how the genes are expressed (tall or short)

Concepts from Mendel’s Work HOMOZYGOUS is an organism with two of the same kind of alleles Homozygous dominant: TT Homozygous recessive: tt HETEROZYGOUS is an organism with different alleles for a trait Heterozygous: Tt

Mendel’s Three Principles PRINCIPLE OF DOMINANCE traits are controlled by pairs of genes One allele is DOMINANT meaning it masks or dominates the other trait One allele is RECESSIVE meaning it is masked in the presence of the dominant trait Ex. T is the dominant allele A plant with the genotype TT or Tt = DOMINANT A plant with the phenotype tt = RECESSIVE

HOW DID A TRAIT DISAPPEAR AND THEN REAPPEAR?

Mendel’s Three Principles PRINCIPLE OF SEGREGATION the two genes for a pair of traits segregate during meiosis so each sperm and egg only gets one gene from a pair

Mendel’s Three Principles PRINCIPLE OF INDEPENDENT ASSORTMENT genes for some characteristics are not connected to each other, so they are assorted into sex cells independently from each other This is why all tall people do not have brown eyes and all short people have blue eyes, etc.

Genetic Crosses PROBABILITY is the likelihood that a specific event will occur = number of one kind of event number of all events Ex. Flipping a coin and the probability of getting heads? ½ Probability of getting tails? ½ What is the probability if I flip a coin 3 times in a row that it will land heads up every time? If I flip a coin once and get heads, does that mean with 100% certainty that on the next flip I will get tails?

Probability What is the probability if I flip a coin 3 times in a row that it will land heads up every time? ½ X ½ X ½ = 1/8 If I flip a coin once and get heads, does that mean with 100% certainty that on the next flip I will get tails? NO!!!!!!!!!!!!!!!!!!!!!!! This means PAST OUTCOMES DO NOT AFFECT FUTURE OUTCOMES

Genetics uses Probability Used to predict the genotypic and phenotypic ratios of offspring produced by genetic crosses If two organisms produce offspring, what is the expected result of genotypes and phenotypes in their offspring?

Punnett Square PUNNETT SQUARE is a diagram used to aid (help) biologists in predicting probabilities in offspring

PARENT’S ALLELES IN GAMETES Punnett Square PARENT’S ALLELES IN GAMETES Note this DOES NOT SHOW ACTUAL results, just PREDICTIONS POSSIBLE OFFSPRING

Monohybrid Crosses MONOHYBRID CROSS is a cross between individuals that involves one pair of contrasting traits

Homozygous Dominant X Homozygous Dominant Key: T= Tall t = Short Cross: TT x TT If 100 plants 100 tall T T T TT TT TT TT

Homozygous Dominant X Homozygous Dominant Predicted genotypes of offspring: TT Genotypic ratio: 4 TT / 4 Total Predicted phenotypes of offspring: TALL Phenotypic ratio: 4 Tall / 4 Total 4 / 4 or 100% Probability of tall offspring

Homozygous Recessive X Homozygous Recessive Key: T= Tall t = Short Cross: tt x tt If 100 plants 100 short t t t tt tt tt tt

Homozygous Recessive X Homozygous Recessive Predicted genotypes of offspring: tt Genotypic ratio: 4 tt / 4 Total Predicted phenotypes of offspring: SHORT Phenotypic ratio: 4 Short / 4 Total 4 / 4 or 100% Probability of short offspring

Heterozygous X Heterozygous Key: T= Tall t = Short Cross: Tt x Tt If 100 plants 75 tall 25 short T t T t TT Tt Tt tt

Heterozygous X Heterozygous Predicted genotypes of offspring: TT, Tt, and tt Genotypic ratio: 1 TT/ 2 Tt / 1 tt Predicted phenotypes of offspring: TALL, SHORT Phenotypic ratio: 3 TALL / 1 SHORT 3 / 4 or 75% Probability of tall offspring 1 / 4 or 25% Probability of short offspring

Mendel’s Crosses T T t P  2 pure strains Key: T= Tall t = Short Tall x short Cross: TT x tt - F1 GENERATION = ALL Tt (Tall) Genotypic ratio: 4 Tt / 4 Total - Phenotypic ratio: 4 Tall / 4 Total T T t Tt Tt Tt Tt

Mendel’s Crosses T t T t Two F1 individuals crossed Key: T= Tall t = Short Tall x Tall Cross: Tt x Tt - F2 GENERATION = 75% Tall and 25% Short Genotypic ratio: 1 TT / 2 Tt / 1 tt Phenotypic ratio: 3 Tall / 1 Short If you have 100 offspring, you would expect 75 to be tall and 25 to be short T t T t TT Tt Tt tt

How can the Environment affect our phenotypes?

Incomplete Dominance INCOMPLETE DOMINANCE is the intermediate phenotype that results when one allele is not always totally dominant over the other

Incomplete Dominance R R I RI RI RI RI Ex. Snapdragons (flowers) Key: RR = Red II = Ivory RI = Pink What would we get if we crossed a pure Red flower with a pure Ivory flower? Red x Ivory Cross: RR x II Genotypic ratio: 4RI / 4 Phenotypic ratio: 4 Pink / 4 R R I RI RI RI RI

Incomplete Dominance R I R I RR RI RI II What would we get if we now allowed the F1 generation to self pollinate? Pink x Pink Cross: RI x RI Genotypic ratio: 1RR / 2RI/ 1 II Phenotypic ratio: 1 RED / 2 PINK / 1 IVORY Therefore, the genotypic and phenotypic ratios for incomplete dominance are the EXACT SAME R I R I RR RI RI II

Codominance CODOMINANCE is when both alleles from a gene are expressed in the heterozygous individual, at the SAME TIME

Codominance W W Y YW YW YW YW Ex. Flowers Key: YY = Yellow WW = White YW = Roan (Yellow with White Spots) What would we get if we crossed a pure Yellow flower with a pure White flower? Yellow x White Cross: YY x WW Genotypic ratio: 4YW / 4 Phenotypic ratio: 4 Roan / 4 W W Y YW YW YW YW

Multiple Allele Traits MULTIPLE ALLELE TRAITS occur when three or more contrasting alleles control a trait Example: Blood types ABO Gene for type A blood: IA Gene for type B blood: IB Gene for type O blood: i Both A and B are dominant, O is recessive A blood cells carry the A antigen (causes the body to make antibodies  clotting) B blood cells carry the B antigen O blood cells carry no antigen

Multiple Allele Traits Type A blood: IAIA or IAi Type B blood: IBIB or IBi Type AB blood: IAIB Type O blood: ii AB A B O

Polygenic Traits POLYGENIC TRAITS are traits controlled by more than one pair of genes Example: skin color, eye color, and height There are at least 4-7 pairs of genes that control each one of these traits These genes can be on the same chromosomes or different chromosomes There is a wide variety of phenotypes shown There is no necessary dominant/recessive, instead genes are turned on/off (if on will add some affect on trait)

Dihybrid Crosses DIHYBRID CROSS is a cross between individuals that involves the pairing of two contrasting traits There are more possible combinations of alleles There are 16 squares in the Punnett square used There are four combinations of alleles possible in gametes from each parent

FOIL Method Suppose: Parent: TtGg First TG Outside Tg Inside tG T = tall t = short G = green g = yellow Use the FOIL method to determine the possible gametes from each parent Parent: TtGg First TG Outside Tg Inside tG Last tg T t G g

Homozygous Dominant x Homozygous Recessive TTGG x ttgg Genotypic ratio: 16 TtGg / 16 Total Phenotypic ratio: 16 Tall, Green / Total All Plants = Tall and Green tg tg tg tg TtGg TG TG TG TG

Heterozygous x Heterozygous TtGg x TtGg TG Tg tG tg TTGG TTGg TtGG TtGg TTgg Ttgg ttGG ttGg ttgg TG Tg tG tg

Heterozygous x Heterozygous Genotypic ratio: TTGG 1 TTGg 2 TTgg 1 TtGG 2 TtGg 4 Ttgg 2 ttGG 1 ttGg 2 ttgg 1 Phenotypic ratio: Tall, green 9 Tall, yellow 3 Short, green 3 Short yellow 1

Summary of heterozygous vs. heterozygous cross Monohybrid Genotypic ratio: 1 : 2 : 1 Phenotypic ratio: 3 : 1 Dihybrid Genotypic ratio:1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 Phenotypic ratio: 9 : 3 : 3 : 1