Mendel and Heredity Chapter 12
12.1 Objectives: 1.) summarize the importance of Mendel s experiments 2.)Differentiate between genes and alleles. 3.) Explain that alleles determine what physical traits are expressed in an individual.
A Brief History In the past, people did not understand how traits were inherited. Two theories : Blending Theory-offspring are a straight mix Particulate Theory- traits are inherited as particles and offspring receive a piece Then Gregor Mendel studied pea plants and learned some things about genetics
Basic Terms to Know Genetics the study of heredity Heredity the passing of characteristics from parents to offspring Traits the characteristics that are inherited Genes segments of DNA (chemical factors) that determine traits
Who was Gregor Mendel? He discovered how traits are inherited Gregor Mendel Born 1822 Austria Lived in a Monastery in charge of garden. Worked with the pea plants and studied their traits and how they are passed from parent to offspring. Father of Genetics
Mendel chose pea plant traits that had two forms of each trait Trait Dominant Characteristic Recessive Characteristic Seed Shape Round Wrinkled Seed Color Yellow Green Seed Coat Color Gray White Pod Shape Smooth Constricted Pod Color Green Yellow Flower Position Axial Terminal Plant Height Short Tall Therefore, there are 2 alleles for every pea plant trait.
Basic Terms to Know Alleles alternative or different forms of a gene that produce different characteristics for each trait Each parent donates one set of instructions (genes) to the offspring. The fertilized egg would then contain 2 genes for every characteristic one from each parent. The two genes that code for the same characteristic are the alleles.
Plant Reproduction Pea plants can be self-fertilized or cross-fertilized
Plant Reproduction Pea plants reproduce through self-pollination or cross-pollination Pea plants Self-Pollinate (1 Parent) True-Breeding identical offspring every generation, plants that look like themsleves Pea plants can also Cross-Pollinate pollen from one plant is used to fertilize the egg cells in another plant (2 Parents) Hybrids-offspring of two true-breeding plants that are cross pollnated
Mendel s Experiments Mendel cross-pollinated two truebreeding plants (1 white, 1 purple) that had the contrasting trait of flower color He found that all the offspring were purple. He allowed the F1 offspring to self-pollinate- the offspring were mostly purple (75%) and some white (25%) P Generation F1 Generation F2 Generation
Mendel s Experiment
Mendel s Experiments When Mendel crossed plants with contrasting characteristics for the same trait (like flower color), the resulting offspring had only one of the characteristics. For example: Purple x White= All Purple From these experiments, Mendel drew 2 conclusions
Mendel s Conclusions 1. Inheritance is determined by factors that are passed from one generation to the next. (today we call them genes)
Mendel s Conclusions 1. Each gene has two alleles and the alleles are either Dominant or Recessive. (Principle of Dominance) The trait that is dominant will appear over the recessive trait. (The dominant allele always overpowers the recessive allele) An organism will have the recessive trait only when the dominant allele is NOT present
Meiosis Connection! Mendel did not understand how chromosomes separate during meiosis but he learned something about the process Chance decides which alleles will end up in each gamete through meiosis Law of segregation-when an organism produces gametes, each pair of alleles is separated and each gamete has an equal chance of receiving either one of the alleles.
12.2 Objectives 1. Differentiate between genotype and phenotype 2. Differentiate between dominant and recessive alleles Apply probability in predicting the outcomes of offspring inheriting certain traits Create a punnett square and use it to analyze the probability of offspring inheriting traits
12.2 Probability and Punnett Squares
Genetics and Probability Mendel realized that the principles of probability can be used to explain genetics Probability - is the mathematical chance or likelihood that a particular event will occur (p) *Can be written as a fraction or ratio p = 2/3 p = 2:3 p = 2 out of 3
Practice Problems 1. You flip a coin once. What is the probability of the coin showing tails? ½ or 1 out of 2 2. What is the probability of flipping two coins and having both show heads? ½ x ½ = ¼ or 1 out of 4 3. What is the probability of flipping two coins and having one show heads and the other show tails? ½ x ½ = ¼ or 1 out of 4 4. What is the probability of flipping heads three times in a row? ½ x ½ x ½ = 1/8 or 1 out of 8
How is all of this related to genetics? The same method is used to calculate the probability that an offspring will inherit a certain allele. The way in which alleles segregate is completely random. The principles of probability can be used to predict the outcomes of genetic crosses.which leads us to. PUNNETT SQUARES!!!
Genotype/Phenotype Genotype the genetic makeup of an organism. The inherited information an organism contains in the form of genes and alleles. Phenotype the physical characteristics of an organism. The way an organism looks. Codes For DNA/Chromosomes (Genotype) Red (Phenotype)
Representing the Different Alleles 1. Use the same letter for the different alleles of a gene. Plant Height = Tall or Short We would use the letter T for plant height 2. Use a capital letter for the dominant allele T = Tall 3. Use a lowercase letter for the recessive allele t = Short
You cannot always know an organism s Genotype by simply looking at its Phenotype Homozygous having 2 identical alleles for the same trait Heterozygous having 2 different alleles for the same trait Genotype Phenotype Homozygous Dominant TT Tall Homozygous Recessive tt Short Heterozygous Tt Tall
How to Make a Punnett Square 1. Draw a square that is divided into 4 sections 2. Write the genotype of the alleles of the father along the top of the box 3. Write the genotype of the alleles from the mother along the left hand side of the box 4. Fill in the inside of the box with one allele from the top of the box and one allele from the left hand side of the box.
Let s try one.bb x Bb B B B BB BB b Bb Bb
Mendel s first experiments were crosses between 2 parent plants that differed in 1 trait. Mendel chose this trait to be plant height (H). P-Generation: Phenotypes: Tall x Short Genotypes: TT x tt This produced the F 1 Generation: Phenotypes: All Tall! Possible Genotypes: TT or Tt Hybrid: offspring of crosses between parents of different traits have a heterozygous genotype Tt How do we know which one??? Mendel allowed the F 1 generation to self-pollinate and he produced the F 2 generation. Phenotypes: 3 Tall and 1 Short (How did the short reappear???)
We can use Punnett Squares to Figure it Out! P Generation = TT x tt T T t Tt Tt t Tt Tt The only possible Genotype for the F 1 Generation is Tt
Could we figure out the F 2 Generation? Yes! We would allow the F 1 generation to selfpollinate and produce the F 2 generation (Tt x Tt) T t T TT Tt t Tt tt Genotypes of F 2 : TT, Tt, and tt Phenotypes of F 2 : 3 Tall and 1 short!
The numbers in the genotypes and phenotypes can also be represented Genotypic Ratio: 1 TT : 2 Tt : 1 tt as ratios We say that the ratio is 1 : 2 : 1 (Homozygous Dominant : Heterozygous : Homozygous Recessive) Phenotypic Ratio: -3 Tall : 1 Short -We say that the ratio is 3 : 1 (Dominant Trait : Recessive Trait)
Stop & Think 1) What are dominant and recessive alleles? 2) Use the terms heredity, genetics, and traits in ONE sentence. 3) What is the difference between the genotype and phenotype of an organism? 4) What is the relationship between truebreeding organisms and a homozygous genotype?
Stop & Think 1) How do geneticists use the principles of probability? 2) What is the probability that a tossed coin will come up tails twice in a row? 3) Why do geneticists use Punnett Squares? 4) What do the boxes in a Punnett Square represent? 5) What is the probability of two heterozygous parents producing a heterozygous offspring?
12.3 Exceptions to Mendel s Objectives: Genetics 1. Explain codominance and use probability to predict this type of inherited trait 2. Explain incomplete dominance and use probability to predict this type of inherited trait 3.Explain multiple alleles and how it relates to heredity 4.Explain what polygenic traits are and how they relate to heredity
A Summary of Mendel s Principles 1. The inheritance of traits is determined by genes, which are passed from parents to offspring. 2. Law of Dominance When two or more alleles for a gene exist, some alleles may be dominant and others may be recessive. 3. Law of Segregation Organisms inherit two copies of each gene (one from each parent.) These genes are segregated (separated) from each other when gametes are formed during meiosis. 4. Law of Independent Assortment The alleles for different genes separate independently of one another. (This is why we have to FOIL when completing a dihybrid Punnett Square!)
Exceptions to Mendel s Rules Not all genes show simple patterns of dominant and recessive alleles. The majority of genes have more than two alleles. Traits are also sometimes controlled by more than one gene.
Exceptions to Mendel s Rules 1) Incomplete Dominance One allele is not completely dominant over another. There is an appearance of a third and different phenotype. The heterozygous phenotype is somewhere in between the two homozygous phenotypes. Both alleles of the gene combine to form a new trait. Example: Red, White, and Pink Snapdragons
Exceptions to Mendel s Rules 2) Codominance Both alleles contribute to the phenotype of the heterozygous organism. The offspring have a phenotype that expresses both alleles equally. Examples: Roan cattle red x white -> roan Black and white chickens
Exceptions to Mendel s Rules 3) Multiple Alleles Genes that have more than two alleles for a specific trait. Individuals still only inherit two alleles (one from each parent); there are just more possible alleles to inherit!
Exceptions to Mendel s Rules 3) Multiple Alleles Examples: Rabbit fur color has 4 different alleles Alleles: C, c ch, c h, c
Multiple Alleles (cont d) Human Blood Type ALLELE I A I B i CODES FOR Type "A" Blood Type "B" Blood Type "O" Blood GENOTYPES I A I A or I A i I B I B or I B i RESULTING PHENOTYPES Type A Type B I A I B ii Type AB Type O The alleles I A and I B are always expressed they are codominant. Both I A and I B are dominant to i.
Exceptions to Mendel s Rules 4) Polygenic Traits Traits controlled by two or more genes. The genes can be on the same chromosome or different chromosomes. All heterozygotes are intermediate in phenotype. Example: Skin color in humans
Stop & Think 1) What is the difference between the heterozygous phenotype in incomplete dominance versus codominance? 2) Which exceptions to Mendel s principles is demonstrated with blood type in humans? 3) Why does skin color in humans not follow Mendel s basic principles of genetics?
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