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Understanding Mendelian Genetics: Principles of Inheritance and Allele Interaction

This section explores the foundations of Mendelian genetics, detailing how traits are inherited from one generation to the next through the principles of heredity. Key concepts include the distinctions between dominant and recessive alleles, homozygosity and heterozygosity, and how traits segregate independently during gamete formation. Mendel's groundbreaking work with pea plants is highlighted, explaining his laws of segregation and independent assortment, along with various genetic crosses, including monohybrid and dihybrid crosses, as well as patterns of inheritance like incomplete dominance and codominance.

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Understanding Mendelian Genetics: Principles of Inheritance and Allele Interaction

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  1. Sexual Reproduction and Genetics Section 2 Mendelian Genetics How Genetics Began • The passing of traits to the next generation is called inheritance, or heredity. • Mendel performed cross-pollination in pea plants. • Mendel followed various traits in the pea plants he bred.

  2. Sexual Reproduction and Genetics Section 2 Mendelian Genetics How Genetics Began

  3. Sexual Reproduction and Genetics Section 2 Mendelian Genetics How Genetics Began

  4. Sexual Reproduction and Genetics Section 2 Mendelian Genetics • The parent generation is also known as the P generation. • The offspring of this P cross are called the first filial (F1) generation. • The second filial (F2) generation is the offspring from the F1 cross.

  5. Sexual Reproduction and Genetics Section 2

  6. Sexual Reproduction and Genetics Section 2 Mendelian Genetics

  7. Sexual Reproduction and Genetics Section 2 Mendelian Genetics Genes in Pairs • Allele • An alternative form of a single gene passed from generation to generation • Dominant (Purple flower color) • Expressed form of a trait represented by a capital letter and first letter of the trait (P). • Recessive (White flower color) • Only appears when both alleles are recessive. Represented by a lower case letter of the dominant trait (p)

  8. Sexual Reproduction and Genetics Section 2 Mendelian Genetics Dominance • An organism with two of the same alleles for a particular trait is homozygous. • PP or pp • An organism with two different alleles for a particular trait is heterozygous. • Pp

  9. Sexual Reproduction and Genetics Section 2 Mendelian Genetics Genotype and Phenotype • An organism’s allele pairs are called its genotype. • Example would be Pp • The observable characteristic or outward expression of an allele pair is called the phenotype. • Example would be purple flower color

  10. Sexual Reproduction and Genetics Section 2 Mendelian Genetics Mendel’sLaw of Segregation • The law of segregation, states that the two alleles for a trait segregate (separate) when gametes are formed. • Two alleles for each trait separate during meiosis. • During fertilization, two alleles for that trait unite. • Each offspring receives one allele from each parent resulting in two alleles for each trait.

  11. Sexual Reproduction and Genetics Section 2 Mendelian Genetics Mendel’sLaw of Segregation

  12. Sexual Reproduction and Genetics Section 2 Mendelian Genetics Mendel’sLaw of Independent Assortment • Mendel found that the inheritance of one trait, such as plant height, did not influence the inheritance of any other trait, such as flower color. • The law of independent assortment states that the alleles of different genes separate independently of one another during gamete formation. • This occurs during meiosis I when the homologous chromosomes line up along the metaphase plate

  13. Sexual Reproduction and Genetics Section 2 Mendelian Genetics Mendel’sLaw of Independent Assortment

  14. Sexual Reproduction and Genetics Section 2 Mendelian Genetics Mendel’sLaws

  15. Sexual Reproduction and Genetics Section 2 Mendelian Genetics Monohybrid Cross • A cross that involves hybrids for a single trait is called a monohybrid cross.

  16. Sexual Reproduction and Genetics Section 2 Mendelian Genetics Punnett Squares • Predict the possible offspring of a cross between two known genotypes

  17. Sexual Reproduction and Genetics Section 2 Mendelian Genetics

  18. Sexual Reproduction and Genetics Section 2 Mendelian Genetics

  19. Sexual Reproduction and Genetics Section 2 Mendelian Genetics Dihybrid Cross • The simultaneous inheritance of two or more traits in the same plant is a dihybrid cross. • For example, if you crossed a yellow round pea (YYRR) with a green wrinkled pea (yyrr) • What would be the predicted offspring genotypes and phenotypes?

  20. Sexual Reproduction and Genetics Section 2 Mendelian Genetics Punnett Square—Dihybrid Cross • 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.

  21. Sexual Reproduction and Genetics Section 3 Complex Patterns of Inheritance Incomplete Dominance • In some organisms, however, an individual displays a trait that is intermediate between the two parents, a condition known asincomplete dominance. • For example, when a snapdragon with red flowers is crossed with a snapdragon with white flowers, a snapdragon with pink flowers is produced.

  22. Sexual Reproduction and Genetics Section 3 Complex Patterns of Inheritance Incomplete Dominance

  23. Sexual Reproduction and Genetics Section 3 Complex Patterns of Inheritance Codominance • For some traits, two dominant alleles are expressed at the same time. • In this case, both forms of the trait are displayed, a phenomenon called codominance. • Codominance is different from incomplete dominance because both traits are displayed. • Instead of pink you would get a red and white flower • Comparing Dominances

  24. Sexual Reproduction and Genetics Section 3 Complex Patterns of Inheritance Codominance

  25. Sexual Reproduction and Genetics Section 3 Complex Patterns of Inheritance Polygenic Inheritance • When several genes influence a trait, the trait is said to be a polygenic trait. • The genes for a polygenic trait may be scattered along the same chromosome or located on different chromosomes. • Familiar examples of polygenic traits in humans include eye color, height, weight, and hair and skin color.

  26. Sexual Reproduction and Genetics Section 3 Complex Patterns of Inheritance Polygenic Inheritance

  27. Sexual Reproduction and Genetics Section 3 Complex Patterns of Inheritance Multiple Alleles • Genes with three or more alleles are said to have multiple alleles. • Example: Blood types A, B, AB, and O • Multiple Alleles

  28. Sexual Reproduction and Genetics Section 3 Complex Patterns of Inheritance Multiple Alleles

  29. Sexual Reproduction and Genetics Section 3 Complex Patterns of Inheritance Pedigree • Geneticists often prepare a pedigree, a family history that shows how a trait is inherited over several generations. • Pedigrees are particularly helpful if the trait is a genetic disorder and the family members want to know if they are carriers or if their children might get the disorder • Pedigree

  30. Sexual Reproduction and Genetics Section 3 Complex Patterns of Inheritance Pedigree

  31. Sexual Reproduction and Genetics Section 3 Complex Patterns of Inheritance Pedigree • Scientists can determine several pieces of genetic information from a pedigree: • Autosomal or Sex-Linked? If a trait is autosomal, it will appear in both sexes equally. If a trait is sex-linked, it is usually seen only in males. A sex-linked trait is a trait whose allele is located on the X chromosome. • Dominant or Recessive? If the trait is autosomal dominant, every individual with the trait will have a parent with the trait. If the trait is recessive, an individual with the trait can have one, two, or neither parent exhibit the trait.

  32. Sexual Reproduction and Genetics Section 3 Complex Patterns of Inheritance Sex-linked - Hemophilia • Sex-linked traits occur on the X chromosomes • Females have 2 Xs so they must have both defective alleles to have the genetic disorder • Males only have 1 X making sex-linked disorders much more common in males • Sex-linked with Flies • Hemophilia is a sex-linked trait a condition that impairs the blood’s ability to clot. • Hemophilia

  33. Sexual Reproduction and Genetics Section 3 Complex Patterns of Inheritance Sex-linked

  34. Sexual Reproduction and Genetics Section 3 Complex Patterns of Inheritance Sex-linked - Hemophilia

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