Heredity Chapter 4 Mendelian Genetics Monohybrid and Dihybrid

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

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 An and children? ? Austrian monk in the late nineteenth century started to piece everything together

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, unknown the specifics were completely

Heredity – Genetics of Inheritance Genetics – branch of biology dealing with the principles of variation and inheritance in animals and plants Mendel – The Grandfather of Genetics 1853 to 1861 in the Czech Republic Why did he succeed? 1. Organism – distinctive traits, reproductive processes (cross pollination and self pollination) 2. Design 3. Analyze

Mendelian Genetics – An Introduction 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 shape, plant height, flower position

Mendel & Peas: The grandfather of genetics

Mendelian Genetics – Inheritance of One Trait P generation – parent generation A true breeding tall plant is crossed with a true breeding short plant F 1 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

The flower structure

Mendelian Genetics – Inheritance of One Trait Blending theory stated that all offspring in the F 1 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

Mendelian Genetics – Inheritance of One Trait F 1 generation was allowed to self pollinate F 2 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

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 x½ ¼

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.

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

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

Mendelian Genetics – 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

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)

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)

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 x½ ¼ However dominant traits are ¾ and if dihybrid then it is ¾ x ¾ 9/16

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