Mendelian Genetics Lecture Notes PDF

Mendelian Genetics Learning objectives 1.1.1 Discuss the contribution of Gregor Mendel to the study of genetics 1.1.2 Explain why Gregor Mendel used true-breeding plants in his experiments 1.1.3 Differentiate between self-fertilization and cross-fertilization 1.1.4 Explain why Gregor Mendel used...

Mendelian Genetics Learning objectives 1.1.1 Discuss the contribution of Gregor Mendel to the study of genetics 1.1.2 Explain why Gregor Mendel used true-breeding plants in his experiments 1.1.3 Differentiate between self-fertilization and cross-fertilization 1.1.4 Explain why Gregor Mendel used cross-pollination to study inheritance 1.1.5 Differentiate between F1 and F2 generations 1.1.6 Distinguish between alleles and genes 1.1.7 Differentiate the dominant and the recessive alleles by using examples 1.1.8 Differentiate between the terms homozygous, heterozygous and hybrid, by using examples Learning objectives 1.1.9 Differentiate between the terms genotype and phenotype, by using examples 1.1.10 Describe what happens during segregation and independent assortment 1.1.11 Explain the inheritance of genes by applying Mendel's Law of Segregation and Law of Independent Assortment 1.1.12 Explain how rules of probability can be used to analyze the passage of one gene from parent to offspring 1.1.13 Predict the ratio of allele distribution and offspring genotypes and phenotypes in a monohybrid cross using a Punnett square and probability 1.1.14 Predict the ratio of allele distribution and offspring genotypes and phenotypes in a dihybrid cross using a Punnett square and probability 1.1.1 Discuss the contribution of Gregor Mendel to the study of genetics  Gregor Mendel formed hypotheses concerning how the traits were inherited.  Mendel did his research on pea plants and came out with the basics of genetics.  The passing of traits to the next generation is called inheritance, or heredity.  Gregor Mendel is regarded as the father of genetics.  Genetics is the science/study of heredity. 1.1.2 Explain why Gregor Mendel used true-breeding plants in his experiments  Pea plants are true-breeding, meaning that they always produce offspring with only one form of a trait.  Gregor Mendel used true-breeding plants in his experiments to easily separate the genes passed from generation to generation. 1.1.3 Differentiate between self-fertilization and cross- fertilization  Pea plants usually reproduce by self-fertilization.  Self-fertilization occurs when a male gamete within a flower combines with a female gamete in the same flower.  Cross-fertilization occurs when a male gamete within a flower combines with a female gamete from different flowers of the same species. Reproductive Structures of a Flower 1.1.4 Explain why Gregor Mendel used cross-pollination to study inheritance  Mendel noticed that some garden pea plants produced specific forms of a trait, generation after generation.  For example, he noticed that some varieties always produced green seeds and others always produced yellow seeds.  In order to understand how these traits are inherited, he performed cross pollination.  How did Mendel perform cross-pollination? Mendel performed cross-pollination by transferring a male gamete from the flower of one pea plant to the female reproductive organ in a flower of another pea plant.  What did Mendel do to prevent self-fertilization? To prevent self-fertilization, Mendel removed the male organs from the flower of the yellow-seed plant.  1.1.5 Differentiate between F1 and F2 generations  When Mendel grew the seeds from the cross between the green-seed and yellow-seed plants, all of the resulting offspring had yellow seeds.  The offspring of this P cross are called the first filial (F1) generation.  The green-seed trait seemed to have disappeared in the F1 generation, and Mendel decided to investigate whether the trait was no longer present or whether it was hidden, or masked.  Mendel planted the F1 generation of yellow seeds, allowed the plants to grow and self-fertilize, and then examined the seeds from this cross.  The results of the second filial (F2) generation—the offspring from the F1 cross.  The F2 generation plants a 3:1 ratio of yellow to 1.1.6 Distinguish between alleles and genes  A gene is a segment of DNA that controls a trait such as seed color, height, eye color, …….  A gene is represented by two letters/factors, each passed from one parent.  Example: A gene for Brown eye color can be represented by Bb. “B is the factor passed from one parent; b is the other factor passed from the other parent”.  An allele is defined as an alternative form of a single gene passed from generation to generation.  In the example above: B is an allele “Factor passed from one parent”; b is another allele. B b 1.1.7 Differentiate the dominant and the recessive alleles by using examples  The form of the trait that always appears in the F1 generation is a dominant trait.  The form of the trait that is masked in the F1 generation is a recessive trait.  In the cross between yellow-seed plants and green-seed plants, the yellow seed is the dominant trait while the green seed is the recessive form of the trait.  The dominant allele is represented by a capital letter.  The recessive allele is represented by a lowercase letter. 1.1.8 Differentiate between the terms homozygous, heterozygous (hybrid), by using examples  An organism with two of the same alleles for a particular trait is homozygous (pure) for that trait.  A Homozygous organism can show the Dominant or Recessive traits.  A homozygous dominant yellow-seed plants have YY alleles.  A homozygous recessive green-seed plants have yy alleles.  An organism with two different alleles for a particular trait is heterozygous (hybrid) for that trait.  A heterozygous organism will always show the dominant trait.  Example: Yy shows yellow-seed color. 1.1.9 Differentiate between the terms genotype and phenotype, by using examples  Genotype is the organism’s allele pairs.  In the case of plants with yellow seeds, their genotypes could be YY or Yy.  Phenotype is the observable characteristic or outward expression of an allele pair.  The phenotype of pea plants with the genotype yy will be green seeds. 1.1.10 Describe what happens during segregation and independent assortment  Mendel used homozygous yellow-seed and green-seed plants in his P cross.  During gamete formation “Meiosis”, the chromosome number is divided in half.  As a result, each gamete from the yellow-seed plant contains one Y.  Each gamete from the green-seed plant contains one y allele.  The resulting gametes contain only one of the pair of seed-color alleles. 1.1.10 Describe what happens during segregation and independent assortment  Mendel’s law of segregation states that the two alleles for each trait separate during meiosis “Figure A”.  During fertilization, two alleles for that trait unite.  Figure B shows the alleles uniting to produce the genotype Yy during fertilization.  All resulting F1 generation plants will have the genotype Yy and will have yellow seeds because yellow is dominant to green. These heterozygous organisms are called hybrids. 1.1.13 Predict the ratio of allele distribution and offspring genotypes and phenotypes in a monohybrid cross using a Punnett square and probability  Monohybrid cross is a cross that involves hybrids for a single trait.  Example: A cross between two plants that involves one trait “height” {Tall x short; Tall x Tall; Short x Short}  In order to predict the genotypes and phenotypes of offspring that result from a cross between organisms, a PUNNETT SQUARE is used.  In this example, two heterozygous Tall plants are crossed together.  A heterozygous Tall plant has the genotype Tt.  Check the next slide for detailed instructions 1.1.13 Predict the ratio of allele distribution and offspring genotypes and phenotypes in a monohybrid cross using a Punnett square and probability Offsprin g First step, write down the alleles of the Second step, cross each allele from the male and female parent in the Punnett male gamete with the other female square. gamete. Always write the Upper-case letter first. Third step, analyze the results of the offspring in the Punnett square. 1.1.13 Predict the ratio of allele distribution and offspring genotypes and phenotypes in a monohybrid cross using a Punnett square and probability  Can you roll your tongue like the person in the picture?  Tongue-rolling ability is a dominant trait, which can be represented by T over Non-Rolling tongue t.  Suppose both parents can roll their tongues and are heterozygous (Tt) for the trait.  What possible phenotypes could their children have? How many different genotypes are found in the Punnett Examine the Punnett square. square? Notice that the male gametes are written across the horizontal side One square and has T T , two squares have Tt, and one the female gametes are written on the vertical side of the Punnett square. square has tt. The possible combinations of each male and female gamete are Therefore, written onthe genotypic ratio the inside of each corresponding square. of the possible offspring is 1:2:1. 1.1.11 Explain the inheritance of genes by applying Mendel's Law of Independent Assortment Mendel concluded the Law of independent assortment while studying a cross between two plants that differ in two characteristics such as height and flower color. If the tall plant with purple flowers is heterozygous for both traits, the genotype would be written as: TtPp Tall (T) is dominant over short (t) Purple (P) is dominant over white (p) 1.1.11 Explain the inheritance of genes by applying Mendel's Law of Independent Assortment Law of independent assortment states that a random distribution of alleles occurs during gamete formation. The gametes (alleles) obtained from TtPp genotype would be: Tt Pp TP Tp tP tp Another example is shown in the figure. 1.1.14 Predict the ratio of allele distribution and offspring genotypes and phenotypes in a dihybrid cross using a Punnett square and probability  Examine the Punnett square in the Figure.  When the F1 generation is crossed (both heterozygous)—four types of alleles from the male gametes and four types of alleles from the female gametes can be produced.  The resulting phenotypic ratio is 9:3:3:1  9 yellow, round (Both Dominant traits) 3 green, round (One dominant, one recessive) 3 yellow, wrinkled (One dominant, one recessive) 1 green wrinkled (Both recessive traits)  Important Notes  In monohybrid cross, the phenotypic  In dihybrid cross, the ratio that results from a cross phenotypic ratio that results between two hybrid organisms (Tt x Tt) from a cross between two is always 3:1 dihybrid organisms (YyRr x YyRr) is always 9:3:3:1 End of Thank Slides you!