The Work of Gregor Mendel:Monohybrid, Dihybrid, Incomplete, Codominance, Multiple Alleles, Polygenic Traits
Genetics: study of hereditary
Every living being- plant, animal, microbe or human being, has a set of characteristics inherited from its parent or parents. Genetics: study of hereditary
Gregor Mendels Peas:Austrian Monk in charge of the garden, Studied peas
What he knew:fertilization- during sexual reproduction a male sperm (pollen) and female egg (ova) joined which produced a new cell, which began to develop into a tiny embryo encased within a seed.When Mendel took control of the garden, he had a garden full of self-breeding garden peas which were true-breeding
True-breeding-pure genes, if allowed to self pollinate, these plants would produce identical copies of themselves.
What he had to work withOne stock of seeds: produced Tall plants,One stock of seeds: produced Short Plants, One line produced: green Seeds, Another line produced: yellow Seeds
cross pollinationBasis of Mendels Experiments: Tall, Short, green Seeds, yellow seedsHe wanted to cross breed these plants called cross pollination
Genes and Dominance:Mendel studied 7 different pea plant traitsTrait: a specific characteristic, such as seed color or height
1.Form of ripe seedSmoothWrinkled 2.Color of seed albumenYellowGreen 3.Color of seed coatGreyWhite 4.Form of ripe podsInflatedConstricted 5.Color of unripe podsGreenYellow 6.Position of flowersAxialTerminal 7.Length of stemTallDwarf
Mendel crossed these plants and studied their offspringHe called each of the original plants the P (Parental) generation
F1 generationHe called the offspring, F1 or first filial generationFilius and filia are the Latin words for son and daughterTherefore the child in the picture below is the F1 generation of those parents
HybridsThe offspring of crosses between parents with different traits are called hybridsEx: toyota prius
Hybrids in other areasMythology: Centaurs Biology: Zeedonk, Liger
So, what were the results? Did they have a mixture of all the traits?NO, all the hybrids had the characteristics of only ONE of the parents.In each cross, the character of the other parent seemed to disappear!
First Conclusion:biological inheritance is determined by factors that are passed from one generation to the nextToday, we called these GENESDifferent forms of a gene are called Allele
Second conclusion: Principle of DominanceP of D: states that some alleles are dominant and others are recessiveAn organism with a dominant form will ALWAYS show the trait
Mendels FindingsTall plant was Dominant and Short plant was recessive/ Yellow seeds Dominant and green seeds are recessive
Some common human dominant traits
Segregation:Mendel wanted to know what happened to the recessive traits. Did they disappear?He took the F1 generation and crossed them with one another and made the F2 generation
F2 generation:the recessive traits reappeared or 25% of all the plants had the recessive traits
He concluded theF1 plants produced gametes (sex cells), the 2 alleles segregated from one another so that each gamete carried a single copy of the geneIn the F1 generation, each gamete had 1 copy of the Tall gene and one copy of the short gene.
Genetics and Probability:Probability: the likelihood that an event will occurCoin toss: 2 possibilities: head or tailsThe probability or chances are equal, 1 in 2 chanceThat is or 50% chanceIf you flip a coin 3 times in a row what are the chances that you will get heads every time: x x = 1/8 1 in 8 chance of flipping heads 3 times in a row!
So what?The principles of probability are used to predict the outcomes of genetic crosses
Punnett Squares the gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett Square
Very important terms to know!Homozygous: 2 identical alleles (TT or tt) considered true-breedingHeterozygous: 2 different alleles (Tt) considered HybridsPhenotypes: physical characteristics, like tall or shortGenotypes: genetic characteristics like TT, Tt, or tt
Tutorial The figure represents a monohybrid cross of F1-hybrid plants. Both parent plants are heterozygous (Ss) for an allele that determines seed shape. Presence of the dominant allele (S) in homozygous (SS) or heterozygous (Ss) plants results in spherical seeds. Homozygous recessive (ss) plants have dented seeds.
Setting up a Punnett square 1. Set up a 2 by 2 Punnett square.
2. Write the alleles for parent 1 on the left side of the Punnett square. Each gamete will have one of the two alleles of the parent. In this particular cross, half of the gametes will have the dominant (S) allele, and half will have the recessive (s) allele.
3. Write the alleles from parent 2 above the Punnett square. For this heterozygous parent (Ss), half of the gametes will have the dominant (S) allele, and half will have the recessive (s) allele.
4. Fill the squares for parent 1. Fill each square with the allele from Parent 1 that lines up with the row.
5. Fill the squares for parent 2. Fill each square with the allele from Parent 2 that lines up with the column.
Interpreting the results of a Punnett square We now have the information for predicting the outcome of the cross. The genotypes in the four boxes of the Punnett square are each equally likely to occur among the offspring of this cross. We may now tabulate the results.
Genotypes that resulted from this monohybrid cross (Ss x Ss)
25% 50% 25% homozygous heterozygous homozygous dominant dominant recessive
Phenotypes that resulted from this monohybrid cross (Ss x Ss)
Independent Assortment: Dihybrid Crosses2 factor CrossMendel crossed true breeding plants that produced only round yellow peas (RRYY) with plants that produced wrinkled green peas (rryy)All the F1: round yellow peas
Showed that the allele for yellow and round peas are dominant Provided the hybrid plants for the F1 cross to produce the F2 generationF2: RrYyFound a grand mix of traits proving that the genes practiced independent assortment meaning that the seed shape and color are independent of one another
F2 results//9:3:3:19 yellow and round, 3 green and round, 3 yellow and wrinkled, 1 green and wrinkled
Tutorial: Dihybrid CrossesDetermine all possible combinations of alleles in the gametes for each parent. Half of the gametes get a dominant S and a dominant Y allele; the other half of the gametes get a recessive s and a recessive y allele. Both parents produce 25% each of SY, Sy, sY, and sy.
1. Punnett square. Since each Parent produces 4 different combinations of alleles in the gametes, draw a 4 square by 4 square punnett square.
2. Place in Gametes from Parent 1 List the gametes for Parent 1 along one edge of the punnett square.
3. Place Gametes from Parent 2 List the gametes for Parent 2 along one edge of the punnett square.
4. Fill in Alleles from Parent 1
Fill out the squares with the alleles of Parent 1.
5. Fill in Alleles from Parent 2 Fill out the squares with the alleles from Parent 2. The result is the prediction of all possible combinations of genotypes for the offspring of the dihybrid cross, SsYy x SsYy.
Predicting the phenotype of offspring There are 9 genotypes for spherical, yellow seeded plants. They are: SSYY (1/16) SSYy (2/16) SsYY (2/16) SsYy (4/16)
Predicting the phenotype of offspringTwo recessive alleles result in green seeded plants. There are 2 genotypes for spherical, green seeded plants. They are: SSyy (1/16) Ssyy (2/16)
Predicting the phenotype of offspringTwo recessive s alleles result in dented seeded plants. There are 2 genotypes for dented, yellow seeded plants. They are: ssYY (1/16) ssYy (2/16)
Predicting the phenotype of offspringA ssyy plant would be recessive for both traits. There is only 1 genotypes for dented, green seeded plants. It is: ssyy (1/16)
A phenotypic ratio of 9:3:3:1 A phenotypic ratio of 9:3:3:1 is predicted for the offspring of a SsYy x SsYy dihybrid cross.
Summary of Mendels Principles:Inheritance of biological characteristics is determined by genes passed from parents to offspringIn cases where 2 alleles exist: 1 is dominant, 1 is recessiveEach adult has 2 copies of each gene, one from each parentGenes for different things usually segregate independently
Beyond Dominant and Recessive alleles:some alleles are neither dominant nor recessivemany traits are controlled by multiple alleles or genes
If Mendel were given a mommy black mouse & a daddy white mouse & asked what their offspring would look like, he would've said that a certain percent would be black & the others would be white. He would never have even considered that a white mouse & a black mouse could produce a GREY mouse! For Mendel, the phenotype of the offspring from parents with different phenotypes always resembled the phenotype of at least one of the parents. In other words, Mendel was unaware of the phenomenon of INCOMPLETE DOMINANCE.
1. Incomplete Dominance:F1 Generation: red flower (RR) and white flower (rr)F2 Generation: pink (Rr) With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third