11-1 The Work of Gregor Mendel photo credit: W. Perry Conway/CORBIS Copyright Pearson Prentice Hall

Genetics is the scientific study of heredity. Gregor Mendel’s Peas Gregor Mendel’s Peas Genetics is the scientific study of heredity. Gregor Mendel was an Austrian monk. His work was important to the understanding of heredity. Mendel carried out his work with garden peas. Copyright Pearson Prentice Hall

Gregor Mendel’s Peas During sexual reproduction, sperm and egg cells join in a process called fertilization. Fertilization produces a new cell called a zygote. Copyright Pearson Prentice Hall

Genes and Dominance A trait is a specific characteristic that varies from one individual to another. Mendel studied seven pea plant traits, each with two contrasting characteristics. He crossed plants with each of the seven contrasting characteristics and studied their offspring. Copyright Pearson Prentice Hall

Genes and Dominance The 7 traits of pea plants: Copyright Pearson Prentice Hall

Genes and Dominance Each original pair of plants is the P (parental) generation. The offspring are called the F1, or “first filial,” generation. The offspring of crosses between parents with different traits are called hybrids. Copyright Pearson Prentice Hall

Today, scientists call the “factors” that determine traits genes. Genes and Dominance Mendel’s 2 Laws 1. Law of Dominance Mendel's first conclusion was that biological inheritance is determined by “factors” that are passed from one generation to the next. Today, scientists call the “factors” that determine traits genes. Copyright Pearson Prentice Hall

The different forms of a gene are called alleles. Genes and Dominance Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms that produced different characteristics for each trait. The different forms of a gene are called alleles. Copyright Pearson Prentice Hall

Genes and Dominance The principle of dominance states that some alleles are dominant and others are recessive. An organism with a dominant allele for a trait will always exhibit that form of the trait. An organism with the recessive allele for a trait will exhibit that form only when the dominant allele for that trait is not present. Copyright Pearson Prentice Hall

1. In humans, hairy knuckles are dominant over hairless knuckles PRACTICE…… For each trait below, assign the symbols for the dominant and recessive alleles 1. In humans, hairy knuckles are dominant over hairless knuckles DOMINANT = _____ = _____________________ RECESSIVE = _____ = _____________________ 2. In tomatoes, fuzzy stems are dominant over smooth stems In watermelons, green skin is dominant over striped skin

Segregation 2. Mendel’s Law of Segregation When any organism produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene. Copyright Pearson Prentice Hall

Segregation Copyright Pearson Prentice Hall During gamete formation, alleles segregate from each other so that each gamete carries only a single copy of each gene. Each F1 plant produces two types of gametes—those with the allele for tallness and those with the allele for shortness. The alleles are paired up again when gametes fuse during fertilization. The TT and Tt allele combinations produce tall pea plants; tt is the only allele combination that produces a short pea plant. Copyright Pearson Prentice Hall

11-2 Probability and Punnett Squares photo credit: W. Perry Conway/CORBIS Copyright Pearson Prentice Hall

Genetics and Probability The likelihood that a particular event will occur is called probability. The principles of probability can be used to predict the outcomes of genetic crosses. Copyright Pearson Prentice Hall

Punnett Squares The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square. Punnett squares can be used to predict and compare the genetic variations that will result from a cross. Copyright Pearson Prentice Hall

Punnett Squares A capital letter represents the dominant allele for tall. A lowercase letter represents the recessive allele for short. In this example, T = tall t = short The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants. Copyright Pearson Prentice Hall

Punnett Squares Gametes produced by each F1 parent are shown along the top and left side. Possible gene combinations for the F2 offspring appear in the four boxes. The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants. Copyright Pearson Prentice Hall

Punnett Squares Organisms that have two identical alleles for a particular trait are said to be homozygous. Organisms that have two different alleles for the same trait are heterozygous. Homozygous organisms are true-breeding for a particular trait. Heterozygous organisms are hybrid for a particular trait. Copyright Pearson Prentice Hall

Punnett Squares All of the tall plants have the same phenotype, or physical characteristics. The tall plants do not have the same genotype, or genetic makeup. One third of the tall plants are TT, while two thirds of the tall plants are Tt. Copyright Pearson Prentice Hall

S = _____________ s = ______________ PRACTICE……. For the following gene and trait, fill in the table with the missing information below. In guinea pigs, the allele for smooth coat is dominant over rough coat. S = _____________ s = ______________ Genotype Description Genotype Phenotype Homozygous Dominant Homozygous Recessive Heterozygous

Punnett Squares The plants have different genotypes (TT and Tt), but they have the same phenotype (tall). Although these plants have different genotypes (TT and Tt), they have the same phenotype (tall). TT Homozygous Tt Heterozygous Copyright Pearson Prentice Hall

Probability and Segregation One fourth (1/4) of the F2 plants have two alleles for tallness (TT). 2/4 or 1/2 have one allele for tall (T), and one for short (t). One fourth (1/4) of the F2 have two alleles for short (tt). The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants. Copyright Pearson Prentice Hall

Probability and Segregation Because the allele for tallness (T) is dominant over the allele for shortness (t), 3/4 of the F2 plants should be tall. The ratio of tall plants (TT or Tt) to short (tt) plants is 3:1. The predicted ratio showed up in Mendel’s experiments indicating that segregation did occur. Copyright Pearson Prentice Hall

Probabilities predict the average outcome of a large number of events. Probabilities Predict Averages Probabilities predict the average outcome of a large number of events. Probability cannot predict the precise outcome of an individual event. In genetics, the larger the number of offspring, the closer the resulting numbers will get to expected values. Copyright Pearson Prentice Hall

11-3 Exploring Mendelian Genetics Copyright Pearson Prentice Hall

Independent Assortment To determine if the segregation of one pair of alleles affects the segregation of another pair of alleles, Mendel performed a two- factor cross. The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y). Copyright Pearson Prentice Hall

The Two-Factor Cross: F1 Mendel crossed true-breeding plants that produced round yellow peas (genotype RRYY) with true- breeding plants that produced wrinkled green peas (genotype rryy). RRYY x rryy  RrYy All of the offspring were RrYy. Next, he crossed 2 of these plants (RrYy X RrYy) Copyright Pearson Prentice Hall

Here are the results of that cross: When Mendel crossed plants that were heterozygous dominant for round yellow peas, he found that the alleles segregated independently to produce the F2 generation. Copyright Pearson Prentice Hall

The alleles for seed shape segregated independently of those for seed color. This principle is known as independent assortment. Genes that segregate independently do not influence each other's inheritance. Copyright Pearson Prentice Hall

Independent Assortment The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes. Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms. Copyright Pearson Prentice Hall

A Summary of Mendel's Principles Genes are passed from parents to their offspring. If two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive. In most sexually reproducing organisms, each adult has two copies of each gene. These genes are segregated from each other when gametes are formed. The alleles for different genes usually segregate independently of one another. Copyright Pearson Prentice Hall

Beyond Dominant and Recessive Alleles Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes. Copyright Pearson Prentice Hall

1. Incomplete Dominance  When one allele is not completely dominant over another it is called incomplete dominance. In incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes. Black x White  Gray Copyright Pearson Prentice Hall

RR A cross between red (RR) and white (WW) four o’clock plants produces pink flowers (RW). WW Some alleles are neither dominant nor recessive. In four o’clock plants, for example, the alleles for red and white flowers show incomplete dominance. Heterozygous (RW) plants have pink flowers—a mix of red and white coloring. Copyright Pearson Prentice Hall

Practice Problems Flower color in snapdragons shows incomplete dominance. A genotype of RR results in a red flower. A homozygous WW plant is white. The intermediate trait is pink flowers. Give the genotype and phenotype ratios for the following crosses: RR x WW RW x RW

2. Codominance  In codominance, both alleles contribute to the phenotype. Both are dominant, both are shown. In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens are speckled with both black and white feathers. The black and white colors do not blend to form a new color, but appear separately. Copyright Pearson Prentice Hall

A roan cow or horse has both red and white hairs, not pink ones: Codominance  A roan cow or horse has both red and white hairs, not pink ones: Copyright Pearson Prentice Hall

Labs can be yellow, black, or chocolate 3. Multiple Alleles  Genes that are controlled by more than two alleles are said to have multiple alleles. An individual can’t have more than two alleles. However, more than two possible alleles can exist in a population. Labs can be yellow, black, or chocolate Copyright Pearson Prentice Hall

Humans show multiple alleles in their blood type: A, B, O A rabbit's coat color is determined by a single gene that has at least four different alleles. Copyright Pearson Prentice Hall

Different combinations of alleles result in the colors shown here. KEY C = full color; dominant to all other alleles cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles ch = Himalayan; color in certain parts of the body; dominant to c allele c = albino; no color; recessive to all other alleles Coat color in rabbits is determined by a single gene that has at least four different alleles. Different combinations of alleles result in the four colors you see here. photo credits: 1. ©John Gerlach/Visuals Unlimited 2.Animals Animals/©Richard Kolar 3. ©Jane Burton/Bruce Coleman, Inc. 4. ©Hans Reinhard/Bruce Coleman, Inc. AIbino: cc Chinchilla: cchch, cchcch, or cchc Himalayan: chc, or chch Full color: CC, Ccch, Cch, or Cc Copyright Pearson Prentice Hall

4. Polygenic Traits   Traits controlled by two or more genes are said to be polygenic traits. Skin color in humans is a polygenic trait controlled by more than four different genes. Copyright Pearson Prentice Hall

5. Genetics and the Environment Characteristics of any organism are determined by the interaction between genes and the environment. we can alter the color of our skin by tanning Siamese cats have dark hairs on the colder parts of their bodies Copyright Pearson Prentice Hall