Heredity – Chapter 4

1. Heredity – Chapter 4 Mendelian Genetics, Monohybrid and Dihybrid Crosses and Beyond Mendel’s Laws

2. Heredity • Physical characteristics • Plants and livestock • Only certain characteristics passed on and this remained a mystery • Hybrid  offspring that differ from their parents in one or more traits. • Parents and children???? • An Austrian monk in the late nineteenth century started to piece everything together

3. Heredity – Genetics of Inheritance • Trait – distinguishing characteristic that will make an individual unique • Through selective breeding with animals and plants it became clear that certain physical traits could be transmitted (inherited) by each generation from the preceding generation • However, the specifics were completely unknown

4. Heredity – Genetics of Inheritance • Genetics – branch of biology dealing with the principles of variation and inheritance in animals and plants • History of Genetics • Mendel – The Grandfather of Genetics • 1853 to 1861 in the Czech Republic • Why did he succeed? • Organism – distinctive traits, reproductive processes (cross pollination and self pollination) • Design • Analyze

5. Mendelian Genetics – AnIntroduction • Purebred organism – descended from ancestors of a distinct type or breed • Through true breeding he was able to achieve truly purebred organisms • He completed this process for seven distinctive phenotypic characteristics: • Seed shape, seed colour, flower colour, pod colour, pod shape, plant height, flower position

6. Mendel & Peas: The grandfather of genetics

8. Mendelian Genetics – Inheritance of One Trait • P generation – parent generation • A true breeding tall plant is crossed with a true breeding short plant • F1 generation – first filial generation • Refers to the offspring that are produced and are often referred to as hybrid plants because they are no longer purebreds • This cross is called a monohybrid because only one trait is being examined

9. Mendelian Genetics – Inheritance of One Trait • Blending theory stated that all offspring in the F1 generation should have been medium height however all were tall! • The allele for tall plants must have been dominant and the allele for short plants must have been recessive • Allele  two or more alternate forms of a gene. Located at the same position on homologous chromosomes. • Dominant trait  characteristic always expressed • Recessive trait  characteristic that is latent (present but inactive) • Principle of Dominance = when individuals with contrasting traits are crossed the offspring will express only the dominant trait

10. The flower structure

12. Mendelian Genetics – Inheritance of One Trait • F1 generation was allowed to self pollinate • F2 generation gave the Mendelian ratio • 75% of offspring resembled dominant parent from the P generation and 25% resembled recessive parent from the P generation (fig. 5, p. 132) • First Law of Heredity: • Law of Segregation – inherited traits are determined by pairs of alleles from each parent. These alleles separate during gamete formation giving each offspring only one allele from each parent • Homozygous vs. Heterozygous

13. Mendelian Genetics – Probability • When flipping a coin the probability of heads is equal to tails for each flip of the coin. • The chance therefore that two or more independent events will occur together is the product of their individual probabilities occurring alone. • ½ x ½  ¼

14. Mendelian Genetics – Punnett Squares • Punnett Square – a simple grid used to illustrate all possible combinations of gametes from a given set of parents. • Allows you to calculate the probability of inheriting a particular trait by creating all potential genotypes from a mating between two parents. • Genotype – genetic make up of an organism. • Phenotype – is the appearance of the trait in an organism determined by the genetic make up and the environment to which the organism is exposed.

15. Mendelian Genetics – Test Cross • Black sheep  brittle wool and difficult to dye. • How would you avoid getting a sheep like this? • Use a homozygous white ram • But, how do you know if the ram is homozygous or heterozygous? • Complete a test cross • Unknown crossed with a homozygous recessive • If any black sheep born, you know the ram was heterozygous

16. Mendelian Genetics – Selective Breeding & Pedigrees • Crosses can easily be done with plants and even some animals in controlled situations • Inbreeding breeding is limited to a number of desirable phenotypes. • Hybridization breeding of two different parents to produce offspring with desirable characteristics of both parents

17. Mendelian Genetics – Pedigrees • Pedigrees - a diagram that illustrates the genetic relationship among a group of individuals (fig. 4, p. 142) • To look at human crosses family records need to be examined (medical, historical, anecdotal). • Records extending across several generations can be arranged in the form of a family pedigree. • Analysis of different pedigrees has shown that some traits are inherited as simple dominant traits • There are only two possible alleles for each trait • Tongue rolling and widow’s peak are two examples of simple dominant traits in humans • Sex linked or Autosomal • Dominant or Recessive

18. Beyond Mendel’s Laws • Not all characteristics on earth can be explained following Mendel’s concepts • Multiple Alleles – many genes have more than two alleles and this occurs in human blood typing and the glycoproteins that are present on red blood cells • Fly eye colour (p.143 & 144) • Incomplete Dominance – a blending whereby the heterozygote exhibits an intermediate phenotype between two homologous phenotypes (fig. 1, p. 144) • Co-dominance – when both alleles for a trait are equally dominant (fig. 2, p. 145)

19. Mendelian Genetics – Inheritance of Two Traits • Mendel’s initial work involving investigating just one trait. We are aware however that organisms are composed of many different traits. • Mendel wanted to know if the inheritance of one characteristic influenced the inheritance of a different characteristic. • He needed to perform a dihybrid cross – crossing of two pea plants that differed in two traits (pea shape and pea colour)

20. Mendelian Genetics – Probability • When flipping a coin the probability of heads is equal to tails for each flip of the coin. • The chance therefore that two or more independent events will occur together is the product of their individual probabilities occurring alone. • ½ x ½  ¼ • However dominant traits are ¾ and if dihybrid then it is ¾ x ¾  9/16

21. Mendelian Genetics – Inheritance of Two Traits • He found that the results came out in a 9:3:3:1 ratio. • 9 round yellow, 3 round green, 3 wrinkled yellow and 1 wrinkled green (fig. 4, p. 152) • Second Law of Heredity: • Law of Independent Assortment – inheritance of alleles for one trait does not affect the inheritance of alleles for another trait • A test cross will also work to determine a genotype of an unknown individual with two traits the same as with one trait