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Mendel’s Laws of Heredity Section 10.1 p. 253 - 262 Chapter 10 Mendel and Meiosis • Mendel chose which pea plants he would allow to fertilize or pollinate one another and then kept records of the offspring produced. Before we can go any further… • There are a bunch of vocab words we need to start using. – – – – – Alleles Dominant Recessive Homozygous Heterozygous Mendel’s ~ The Rule of Unit Factors • Each organism has two factors that control each of its traits. • These factors are now known as genes. • Genes come in different forms. • Alternative forms of a gene for each variation of a trait is an allele. alleles • We get two copies of each allele or directions for a trait. • One copy comes from each parent. • We may get a matching set or they may be different with the dominant one hiding or masking the recessive version. dominant • Observed trait of an organism that masks the recessive form of a trait. • If present, this is the trait we see. • “Dominant” is not always “good”. recessive • Trait of an organism that can be masked by the dominant form of a trait. • You need two copies, one from each parent, for a recessive trait to be expressed. • Recessive traits are not always “bad”. Mendel’s ~ The Rule of Dominance • Even though an organism carries two alleles for the same trait, only one trait is observed. • The dominant trait will be expressed. Homozygous • Having two of the same alleles for a gene • When there are two identical alleles for a trait. Heterozygous • Having two different alleles for the same gene. • Back to the peas, each pea has two height genes. • T = tall, t = short • TT and tt are homozygous. • Tt is heterozygous. Phenotype • Outward appearance of an organism, regardless of its genes. • An organism's traits (like green vs. yellow seeds, or pea plant height) • What you look like! Genotype • Combination of genes in an organism. • Can not tell from outward appearance • Set of directions on the inside. Genotype • If it is a recessive trait, the genotype is obvious. • Dominant traits are expressed whether the organism is heterozygous (2 different alleles) or homozygous (2 of same alleles) so you do not know for sure just from looking. hybrid • Offspring formed by parents having different forms of a trait. • Mendel’s crossbreed strains were made breeding know true-breeding plants to different true-breeding plants. • Working with his peas, Mendel used 355 cross-bred strains and 12,980 resultant hybrids. True-breeding or homozygous • True-breeding means that they only have one version of a trait to pass on or homozygous. • Sometimes we refer to homozygous organisms as being a “pure bred”. • If a pea plant came from parents that were tall, it is tall and all of its offspring are tall when crossed with other pea plants that are “true-breeding”, we can be fairly certain that it is homozygous. Poodles Wolves • What would their puppies look like? • In 1976 a person named C. Schleifenbaum lead a study to find out. poodle-wolf hybrids • They bred wolves to poodles. • Then looked at the fur marks and coat color in wolves and poodle-wolf hybrids. • Here are some pictures of their results. 1st generation ~ poodle-wolf hybrid 2nd generation ~ poodle-wolf hybrid Notice how the coats range from wolf-like to poodle-like Monohybrid cross • How can we explain the poodle-wolf hybrid results? • Poodles are “true-breeding” for one version of traits. • Wolves have a different version of these same traits. Monohybrid cross • In a monohybrid cross, the parents selected are true-breeding for the trait we will be studying. • The parents are known as the P1 generation. • Two P1 parents will be breed or crossed and the resulting offspring are the 1st or F1 generation. 1st generation • First generation is the F1 generation • “F” stands for filial ~ son or daughter • In the F1 generation, all of the offspring should be similar, showing all the dominant traits that were passed on by the P1 generation. st 1 generation • All of the poodle-wolf hybrids looked the same. Mendel’s ~ The Law of Segregation • Each plant has two different alleles, it can produce two different types of gametes. • During fertilization, male and female gametes randomly pair to produce four combinations of alleles. The Law of Segregation Monohybrid Crosses Homozygous P1 = All offspring will have the same genotype They will all have the same phenotypes & look like the dominant trait. Heterozygous P1 = The offspring will have a 1:2:1 ration in genotypes 3:1 ration in phenotypes Homozygous crosses • Mendel choose plants that were breeding true. • His peas must have been homozygous… • Only able to provide one type of allele or variety of that particular trait. Homozygous crosses • Mendel’s first pea plant crosses were between tall pea plants and short pea plants. • One parent has two tall alleles and the other parent has two short alleles. • They are both homozygous for their trait. How we represent the parents & offspring in a cross • A letter is selected to represent a trait. • The dominant version is shown with the capital. • The recessive version is shown with the lower case . How we represent the parents in a cross • With the height of pea plants a capital T is used to represent the tall version • A lower case t is used to represent the short version Genotypes in the cross • The tall pea parents will be = – TT, it can only give T alleles • The short pea parents will be = – tt, it can only give t alleles Punnett Squares • A punnett square is a way to predict the possible outcome of a breeding cross • Each box represents a reproductive opportunity or offspring. Short parent’s alleles t Tall parent’s alleles T T t Punnett Squares • The offspring gets one allele from each parent so they end up with two copies of alleles for each trait. • Divide each parent’s two alleles so that each box has one letter next to or above it Short parent’s alleles t Tall parent’s alleles T T t Monohybrid cross - Homozygous • If one parent is homozygous dominant for their trait and the other is homozygous recessive, the offspring will all have the same phenotype and genotype. Short parent’s alleles t Tall parent’s alleles T T t Monohybrid cross - Homozygous • The offspring’s genotype (what the alleles say) will be = • Tt • The offspring’s phenotype (what we see) will be = • The plants will be tall. • The ________ is the dominant trait. Short parent’s alleles t Tall parent’s alleles T T t Monohybrid cross t t Genotype Genotype Both Homozygous/ 1 –Recessive 1 -Dominant T T t Phenotype = T Phenotype = Dominant Genotype T t T t Phenotype = Dominant Genotype T t Phenotype = Dominant Dominant nd 2 generation • Second generation is the F2 generation • The second generation all has two different copies (heterozygous) for each trait. • Their offspring will show more variation because the alleles will regroup in many ways. Monohybrid cross - Heterozygous • If two organisms are selected out of the F1 generation to be parents for the next generation, they will each be heterozygous. • Sticking with our offspring of a tall pea plant & and a short pea plant, this group will have a genotype of Tt. • They will have a phenotype of tall. Monohybrid cross - Heterozygous • Remember, the offspring get one allele from each parent so they end up with two copies of alleles for each trait. Heterozygous parent’s alleles T Heterozygous parent’s alleles • Both parents are heterozygous so they have a genotype of Tt. T t t Monohybrid cross - Heterozygous • How many of the offspring will be tall? • How many of the offspring will be short? Heterozygous parent’s alleles T Heterozygous parent’s alleles T t t Monohybrid cross - Heterozygous • What are the possible genotypes for the offspring? • What is the ratio of tall (dominant) to short (recessive) plants in the F2 generation? Heterozygous parent’s alleles T Heterozygous parent’s alleles T t t Monohybrid cross T t Genotype Genotype Both Heterozygous T T T Phenotype = T Phenotype = Dominant Dominant Genotype t t t T Phenotype = Genotype t t Phenotype = Dominant Recessive 2nd generation • The second generation of poodle-wolf hybrids looks very different from each other! Crossing Homozygous Pea Plants Crossing Heterozygous Pea Plants