The Work of Gregor Mendel Gregor Mendels Peas

The Work of Gregor Mendel

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 ordinary garden peas.

Mendel knew that • the male part of each flower produces pollen, (containing sperm). • the female part of the flower produces egg cells.

During sexual reproduction, sperm and egg cells join in a process called fertilization. Fertilization produces a new cell. Pea flowers are self-pollinating.

Mendel had true-breeding pea plants that, if allowed to self-pollinate, would produce offspring identical to themselves. Cross-pollination Mendel was able to produce seeds that had two different parents.

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 characters. He crossed plants with each of the seven contrasting characters and studied their offspring. Mendelian Genetics Video

Each original pair of plants is the P (parental) generation. The offspring are called the F 1, or “first filial, ” generation. The offspring of crosses between parents with different traits are called hybrids.

Mendel’s F 1 Crosses on Pea Plants

Mendel’s Seven F 1 Crosses on Pea Plants Mendel’s F 1 Crosses on Pea Plants

Mendel's first conclusion • biological inheritance is determined by factors that are passed from one generation to the next. Today, scientists call the factors that determine traits genes.

Each trait is controlled by one gene with two contrasting forms that produced different characters for each trait. The different forms of a gene are called alleles.

Mendel’s second conclusion is called the principle of dominance. The principle of dominance states that some alleles are dominant and others are recessive.

Mendel’s F 1 Crosses on Pea Plants

Segregation Mendel crossed the F 1 generation with itself to produce the F 2 (second filial) generation. The traits controlled by recessive alleles reappeared in one fourth of the F 2 plants.

Mendel's F 2 Generation P Generation Tall Short F 2 Generation F 1 Generation Tall Tall Short

The reappearance of the trait controlled by the recessive allele indicated that at some point the allele for shortness had been separated, or segregated, from the allele for tallness.

Mendel suggested that the alleles for tallness and shortness in the F 1 plants segregated from each other during the formation of the sex cells, or gametes.

Alleles separate during gamete formation.

Quiz 11 -1

Gametes are also known as a. genes. b. hybrids. c. alleles. d. sex cells.

The offspring of crosses between parents with different traits are called a. alleles. b. hybrids. c. gametes. d. dominant.

In Mendel’s pea experiments, the male gametes are the a. pollen. b. seeds. c. eggs. d. sperm.

In a cross of a true-breeding tall pea plant with a true-breeding short pea plant, the F 1 generation consists of a. all short plants. b. all tall plants. c. half tall plants and half short plants. d. all plants of intermediate height.

If a particular form of a trait is always present when the allele controlling it is present, then the allele must be a. mixed. b. recessive. c. dominant. d. hybrid.

Probability and Punnett Squares

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.

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

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

Gametes produced by each F 1 parent are shown along the top and left side.

Organisms that have two identical alleles for a particular trait are said to be homozygous. ex. TT or tt Organisms that have two different alleles for the same trait are heterozygous. ex. Tt Homozygous organisms are truebreeding for a particular trait. Heterozygous organisms are hybrid for a particular trait.

Physical characteristics are called the phenotype. The alleles are called the genotype.

The plants have different genotypes (TT and Tt), but they have the same phenotype (tall). TT Homozygous Tt Heterozygous

Probability & Segregation One fourth (1/4) of the F 2 plants have two alleles for tallness (TT). 1/2 have one allele for tall (T), and one for short (t). One fourth (1/4) of the F 2 have two alleles for short (tt). Phenotype Ratio 3: 1 Genotype Ratio 1: 2: 1

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.

Quiz 11 -2

Probability can be used to predict a. precise outcome of any event. b. average outcome of many events. c. how many offspring a cross will produce. d. which organisms will mate with each other.

Compared to 4 flips of a coin, 400 flips of the coin is a. more likely to produce about 50% heads and 50% tails. b. less likely to produce about 50% heads and 50% tails. c. guaranteed to produce exactly 50% heads and 50% tails. d. equally likely to produce about 50% heads and 50% tails.

Organisms that have two different alleles for a particular trait are said to be a. hybrid. b. homozygous. c. heterozygous. d. recessive.

Two F 1 plants that are homozygous for shortness are crossed. What percentage of the offspring will be tall? a. 0% b. 25% c. 50% d. 100%

The Punnett square allows you to predict a. only the phenotypes of the offspring from a cross. b. only the genotypes of the offspring from a cross. c. both the genotypes and the phenotypes from a cross. d. neither the genotypes nor the phenotypes from a cross.

Exploring Mendelian Genetics

Independent Assortment To determine if the segregation of one pair of alleles affects the segregation of another pair of alleles, Mendel performed a twofactor cross.

The Two-Factor Cross: F 1 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 All of the F 1 offspring produced round yellow peas (Rr. Yy).

The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y).

The Two-Factor Cross: F 2 Mendel crossed the heterozygous F 1 plants (Rr. Yy) with each other to determine if the alleles would segregate from each other in the F 2 generation. Rr. Yy × Rr. Yy

The Punnett square predicts a 9 : 3 : 1 ratio in the F 2 generation. Represents: Independent Assortment

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.

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.

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.

a. In most sexually reproducing organisms, each adult has two copies of each gene. These genes are segregated from each other when gametes are formed. b. The alleles for different genes usually segregate independently of one another.

Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.

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.

A cross between red (RR) and white (WW) four o’clock plants produces pinkcolored flowers (RW). RR WW

Codominance In codominance, both alleles contribute to the phenotype. 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.

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. A rabbit's coat color is determined by a single gene that has at least four different alleles.

Different combinations of alleles result in KEY the colors shown here. 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 chhc h, cch ch hh chc AIbino: cc Chinchilla: c Himalayan: c c, or c c, or c Full color: CC, Cc , or Cc c = albino; no color; recessive to all other alleles

Polygenic 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.

Aa. Bb. Cc aabbcc 20/64 Fraction of progeny 15/64 6/64 1/64 Aabbcc Aa. Bb. Cc Aa. Bbcc Aa. Bb. Cc AABBCc AABBCC

Quiz 11 -3

In a cross involving two pea plant traits, observation of a 9 : 3 : 1 ratio in the F 2 generation is evidence for a. the two traits being inherited independently of each other. b. an outcome that depends on the sex of the parent plants. c. the two traits being inherited together. d. multiple genes being responsible for each trait.

Traits controlled by two or more genes are called a. polygenic traits. b. multiple-allele traits. c. codominant traits. d. hybrid traits.

In four o'clock flowers, the alleles for red flowers and white flowers show incomplete dominance. Heterozygous four o'clock plants have a. pink flowers. b. white flowers. c. half white flowers and half red flowers. d. red flowers.

A white male horse and a tan female horse produce an offspring that has large areas of white coat and large areas of tan coat. This is an example of a. codominance. b. multiple alleles. c. incomplete dominance. d. a polygenic trait.

Mendel's principles apply to a. all organisms. b. fruit flies only. c. pea plants only. d. only plants and animals.

Meiosis

Each organism must inherit a single copy of every gene from each of its “parents. ” Gametes are formed by a process that separates the two sets of genes so that each gamete ends up with just one set.

Chromosome Number All organisms have different numbers of chromosomes. A body cell in an adult fruit fly has 8 chromosomes: 4 from the fruit fly's male parent, and 4 from its female parent.

These sets of chromosomes are homologous (Single Structure). Each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parent.

A cell that contains both sets of homologous chromosomes is said to be diploid. The number of chromosomes in a diploid cell is sometimes represented by the symbol 2 N. For Drosophila, the diploid number is 8, which can be written as 2 N=8.

The gametes of sexually reproducing organisms contain only a single set of chromosomes, and therefore only a single set of genes. These cells are haploid. Haploid cells are represented by the symbol N. For Drosophila, the haploid number is 4, which can be written as N=4.

Phases of Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. Meiosis Video

Meiosis involves two divisions, meiosis I and meiosis II. By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.

Meiosis I Interphase I Meiosis I Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis

Cells undergo a round of DNA replication, forming duplicate chromosomes. Interphase

Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. There are 4 chromatids in a tetrad. Prophase I

When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over. Crossing-over produces new combinations of alleles.

Spindle fibers attach to the chromosomes. Metaphase I

The fibers pull the homologous chromosomes toward opposite ends of the cell. Anaphase I

Nuclear membranes form. The cell separates into two cells. The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I. Telophase I and Cytokinesis

Meiosis II The two cells produced by meiosis I now enter a second meiotic division. Unlike meiosis I, neither cell goes through chromosome replication. Each of the cell’s chromosomes has 2 chromatids.

Meiosis II Telophase I and Cytokinesis I Meiosis II Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell. Prophase II

The chromosomes line up in the center of cell. Metaphase II

The sister chromatids separate and move toward opposite ends of the cell. Anaphase II

Meiosis II results in four haploid (N) daughter cells. Telophase II and Cytokinesis

Gamete Formation In male animals, meiosis results in four equal-sized gametes called sperm.

In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction.

Comparing Mitosis and Meiosis Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells.

Mitosis Meiosis a. Cells produced by mitosis a. Cells produced by meiosis have half the have the same number of chromosomes and alleles chromosomes as the original cell. parent cell. b. Mitosis allows an b. These cells are organism to grow and genetically different replace cells. from the diploid cell and from each other. c. Some organisms reproduce asexually by c. Meiosis is how sexually-reproducing mitosis. organisms produce gametes.

Quiz 11 -4

If the body cells of humans contain 46 chromosomes, a single sperm cell should have a. 46 chromosomes. b. 23 chromosomes. c. 92 chromosomes. d. between 23 and 46 chromosomes.

During meiosis, the number of chromosomes per cell is cut in half through the separation of a. daughter cells. b. homologous chromosomes. c. gametes. d. chromatids.

The formation of a tetrad occurs during a. anaphase I. b. metaphase II. c. prophase I. d. prophase II.

In many female animals, meiosis results in the production of a. only 1 egg. b. 1 egg and 3 polar bodies. c. 4 eggs. d. 1 egg and 2 polar bodies.

Compared to egg cells formed during meiosis, daughter cells formed during mitosis are a. genetically different, while eggs are genetically identical. b. genetically different, just as egg cells are. c. genetically identical, just as egg cells are. d. genetically identical, while egg cells are genetically different.