Principles of Biology

By

Frank H. Osborne, Ph. D.

Genetics

IntroductionLiving organisms resemble their parents.

•This is due to the transmission of traits or genetic characters from one generation to the next.

•With sexual reproduction, the offspring receives genetic material from each parent.

•In humans, half of your genes come from your mother and half from your father.

IntroductionWe know that the genetic material is DNA.

•DNA is organized into genes that are located on chromosomes in the nucleus of the cell.

•Historically, the understanding of the transmission of traits came before the understanding of the mechanism of transmission.

Introduction

Gregor Mendel worked on traits with his peas in the mid-19th century.

•The chromosome was not discovered until the 1890s. Once biologists put the traits and the chromosomes together, we began to understand the science of genetics.

Classical Mendelian InheritanceGregor Mendel was the abbot of a Catholic monastery in central Europe in the mid-19th century.

•One of his duties in the monastery involved tending the garden where he became interested in pea plants.

Characters of Mendel's PeasCharacter AppearanceFlowers-color Red or whiteFlowers-location Axial or terminalPods-color Green or yellowPods-structure Inflated or constrictedSeeds-appearance Round or wrinkledSeeds-color Green or yellowPlants-height Tall or short

Mendelian InheritanceRed and white flower color.

Mendel noted that some peas always had red flowers while others always had white flowers.

Those with red flowers came from plants with red flowers and produced plants with red flowers. These were pure-breeding red-flowered plants. Similarly, there were pure-breeding white-flowered plants.

Reproduction in Flowering Plants

The flower is the reproductive structure of the plant.

The female component of the flower is called the pistil.

The male parts are called the stamens.

Reproduction in Flowering Plants•The male stamens produce the pollen. The pollen grains are the male sexual units of the plant. They are produced in the anther of the flower that is supported by a filament.

Reproduction in Flowering Plants•In the female pistil is an ovary which contains ovules. The ovules are the female sexual units of the plant. Each ovule contains an egg that will become fertilized by the pollen. After fertilization, each ovule becomes a seed while the ovary becomes a fruit.

Reproduction in Flowering Plants•When plants reproduce, pollen from the anther of one flower is transferred to the stigma of another flower. The pollen grains digest their way through the style to the ovary.

Reproduction in Flowering Plants•In the ovary, chromosomes from one of the pollen grains fertilize each ovule. Sometimes, flowers can self-pollinate by transferring pollen from the anthers to the stigma in the same flower.

Crossing Plants•A cross involves transfer of pollen from the stamens of one flower to the pistil of another.•For example, pollen from a plant producing red flowers could be placed on the stigma of a plant with white flowers.•Or, pollen from a white-flowered plant could be used to inoculate the stigma of a red-flowered plant. •In either case the cross is the same.

Observing Results•Once the cross has been performed you must wait for the peas to develop in their pods.

•Then you must harvest the peas and put them away in storage to plant next Spring.

•Next Spring, you plant the seeds and see what comes up.

•So, in the case of Mendel's experiments, each cross took a year to complete.

Sample Mating

•Mendel took pure-breeding red-flowered plants and crossed them with pure-breeding white-flowered plants. These plants were the parental generation, represented by P.

Sample Mating

•The following year, the plants that came up all had red flowers. None of the plants had white flowers even though one of the parents had white flowers. This generation is known as the F1 generation of offspring.

Sample Mating•P generation plants are pure breeding .

•F1 generation has all red flowers. Mendel called the red color dominant.

Sample Mating•The following year, Mendel crossed F1 red-flowered plants from the previous year.

•The result was the F2 generation. In the F2 generation, the white trait returned. Mendel noticed that there were about three times as many red-flowered F2 plants as there were white-flowered F2 plants. Mendel called the hidden trait recessive.

Sample Mating•F1 cross and F2 results. After not being expressed in the F1 generation, white was expressed in the F2 generation.

Pod Color•Mendel crossed pure-breeding plants with green pods and pure-breeding plants with yellow pods.

•The F1 generation had all green pods.

Pod Color•The following year, Mendel continued by crossing F1 plants having green pods. In the F2 generation yellow returned.

Phenotypic Ratio

•Mendel noted in each case that there were about three times as many dominant plants as there were recessive plants.

•The ratio each time was about 3:1.

Phenotypic Ratio

•Each time, one trait was not expressed in the F1 generation. Mendel explained that the color was hiding in a recess somewhere in the plant. He termed them recessive. The traits expressed in the F1 generation he termed dominant.

Definitions

•An allele is a contrasting form of a gene. It is found on a chromosome. In the case of flower color, the alleles are red and white. An organism receives two alleles, one from the female parent and one from the male parent.

Definitions

•The genotype is the combination of alleles that the organism has in its cells. In the case of flower color, there are three genotypes. A plant may have two genes for red color, two genes for white color, or one gene for each color.

Definitions•The phenotype is the appearance of the organism when the genotype is expressed. A plant with red flowers is displaying the red phenotype, while a plant with white flowers is displaying the white phenotype.

•Human characters following simple Mendelian inheritance include: right-left handedness, curly- straight hair, light-dark eyes, widow's peak.

Genetic Symbols•Genetics problems are expressed using symbols. We use letters to represent the various genes and alleles. To reduce confusion, we generally use upper case letters for dominant alleles and lower case letters for the recessive alleles. For flower color:

•C - dominant color gene (red) - red flowers

•c - recessive color gene (white) - white flowers

Each Organism is Diploid•Because the plant receives one gene from the female parent and one gene from the male parent, every cell in the plant has two of each gene. The exception is the sex cells because they are haploid.

•Therefore, there are three possible combinations of flower alleles. Homozygous means both alleles are the same; heterozygous means that the alleles are different.

Genotypes of Flower Alleles•CC - homozygous dominant - gives red flower color. The plant received a dominant gene from each parent.

•Cc - heterozygous - gives red flower color. The plant received a dominant gene from one parent and a recessive gene from the other.

•cc - homozygous recessive - gives white flower color. The plant received a recessive gene from each parent.

Using Genetic Symbols•The first example of red and white flowers is repeated using genetic symbols.

Using Genetic Symbols•Each parent in a cross contributes one gene.

•With a pure-breeding red plant, it can contribute only a red gene. A white plant contributes a white gene.

•The F1 plant is heterozygous. It got a red gene from its red parent and a white gene from its white parent. The red is dominant so all F1 plants are red.

Using Genetic Symbols•When the F1 plants are crossed, a 3:1 phenotypic ratio results.

Using Genetic Symbols•In the F1 cross each parent can contribute either gene. When these plants are crossed, the genes separate and can produce any of four combinations.

•This separation of genes is known as Mendel's Law of Segregation (also called Mendel's First Law.

Using Genetic Symbols•One way to determine the possible combinations of alleles is the FOIL method that is used for multiplying binomials in algebra.

•From the cross Cc X Cc you take the first allele from each (CC), the outer alleles (Cc), the inner alleles (cC), and the last alleles (cc).

•The third (cC) can also be written as (Cc) because in the cell the sequence does not matter.

Phenotypic and Genotypic Ratios•We have already seen that the phenotypic ratio of plants with red flowers to plants with white flowers is 3:1.

•These two phenotypes account for all of the F2 offspring. But there are three genotypes.

•Homozygous dominant (CC) - ¼ of offspring

•Heterozygous (Cc) - ½ of offspring

•Homozygous recessive (cc) - ¼ of offspring

Phenotypic and Genotypic Ratios•Mendel's results were not exactly 3:1. When he did crosses, the results were a little higher or a little lower than 3:1.

•This is due to random fluctuations in the way that that the genes combined with each other. The 3:1 ratio is a theoretical prediction that is based on probability.

Test Cross•The test cross determines whether a dominant plant is homozygous or heterozygous.

•With red flowers you cannot tell just by looking at the plant. The test cross can determine this information.

•In a test cross, the plant with the dominant character is crossed with a homozygous recessive. In this case one with white flowers.

Test Cross•In a test cross of a red-flowered plant with a plant having white flowers, two outcomes are possible.

•A) the red parent is homozygous.–Result: all F1 progeny will be red.

•B) the red parent is heterozygous.–Result: half of the F1 progeny will be red and the other half will be white.

Test Cross

•In Possibility A, the red flowered plant contributes its red allele [C] while the white parent contributes the white allele [c]. The result is that all offspring will be heterozygous and display the red color.

Test Cross

•In Possibility B, the red parent is heterozygous. It has two different alleles. Each allele has a 50% change of being transmitted to the progeny. The result is red and white offspring in a 1:1 ratio.

Punnett Square•Crosses can be diagrammed using the Punnett square, named after Reginald Punnett (1875-1967), an early English geneticist.

•A Punnett square consists of rows and columns. The alleles (gametes) of one parent are written across the top, and the alleles of the other parent are written down the side.

•Then the letters are placed down or across to fill the square.

Punnett Square•In the case of Mendel's heterozygous F1 red plants, both parents were heterozygous with genotype Cc. The Punnett Square gives the possible outcomes.

Punnett Square•We predict that there will be a 3:1 ratio of phenotypes (red to white) with a 1:2:1 genotypic ratio (1 homozygous dominant, 2 heterozygous, 1 homozygous recessive). The same holds true for green and yellow pods.

Punnett Square•The Punnett square predicts possible outcomes.

•The genotype of the individual is determined by random chance.

•With the Punnett square, we know that the outcome for an individual will be found in one of the four cells. You will not get a different outcome that is not found in the diagram.

Human Genetic Diseases•Some human diseases follow simple Mendelian inheritance. These are as follows.

•Cystic Fibrosis•Gout•Sickle-Cell Anemia (caused by inheritance of a specific change in the DNA molecule)•Tay-Sachs Disease.

Codominance•When both genes are dominant, the flowers display a blended appearance when they are heterozygous.•As both genes are dominant, we would use C for red and C' for white. The results would be•CC - red•CC' - pink•C'C' - white

Dihybrid Inheritance

•Dihybrid inheritance is where two different genes are inherited simultaneously.

•We demonstrate this using flower color and pod color when they are inherited simultaneously.

Dihybrid Inheritance

•A dihybrid is heterozygous for two genes. To produce a dihybrid we would begin with parents, one of which is homozygous dominant for both genes and the other which is homozygous recessive for both genes.

Dihybrid Inheritance

Dihybrid Inheritance•The Punnett square is used to find the F2 generation.

•When F1 dihybrids are crossed, each makes four types of gametes, each with a unique combination of allelles.

•In the case of CcGg, the combinations are CG, Cg, cG, and cg. (You can find this by doing FOIL on CcGg.)

Dihybrid Inheritance•We see that the most predominant phenotype is red flowers with green pods.

•A plant with red flowers and green pods may be CCGG, CcGG, CCGg or CcGg. We can shorten this to C-G- where the dash indicates that the second allele does not matter.

•This permits us to summarize the results of the dihybrid cross as follows.

Dihybrid Inheritance

•The phenotypic ratio in a dihybrid cross is always 9:3:3:1. This ratio is a theoretical prediction of the results. Mendel's results came very close to this except that he had the usual variation associated with randomness.

Dihybrid Inheritance

•The alleles for flower color and pod color sort independently. This called the Law of Independent Assortment (or Mendel's Second Law). This is because the genes are on different chromosomes.

Polygenic Inheritance

•Polygenic inheritance involves the effects of multiple genes.

•An example is the inheritance of skin color in humans.

A Cross Involving Three Alleles•Sometimes it is necessary to determine outcome of crosses with more than two alleles.

•Consider this cross.

A Cross Involving Three AllelesSolving a problem like this involves a series of steps.

•Write down the genotypes of the parents.

•Determine all of the possible gametes each parent can produce.

•Use the Punnett technique to determine all of the possible combinations.

A Cross Involving Three Alleles

Step 1. Write the genotypes.

•Female parent - CCGgTt

•Male parent - CCGgtt

A Cross Involving Three AllelesStep 2. Determine the possible gametes. With the female parent we have the following:•One allele for flower color (C)•Two alleles for pod color (G and g)•Two alleles for height (T and t)•The total number of combinations in the product of these, 1 x 2 x 2 = 4. The combinations are CGT, CGt, CgT, and Cgt.

A Cross Involving Three AllelesStep 2. Determine the possible gametes. With the male parent we have the following:•One allele for flower color (C)•Two alleles for pod color (G and g)•One allele for height (t)•The total number of combinations in the product of these, 1 x 2 x 1 = 2. The combinations are CGt, and Cgt.

Step 3. Make a diagram for the cross.

Sex Linkage•All genes on a single chromosome are said to be linked. Sex linkage refers to the genes that are found on the X chromosome. An example is the gene for hemophilia.

•Hemophilia results from a sex-linked recessive gene that results in a lack of clotting factor VII. The gene is carried on the X chromosome.

Sex Linkage•Females have two X chromosomes, so they generally have a normal gene on one of them.

•They do not express hemophilia but can be carrying the recessive allele.

•In males, there is only one X chromosome. If a male carries the defective X chromosome, he will express the hemophilia trait. The y chromosome is considered to be genetically inert.

Diagram of Sex Linkage•When making diagrams of crosses involving sex linkage, it is important to keep the sex of the individuals in mind. The distribution of the sex chromosomes is more important than the distribution of the individual alleles, especially since the male does not have these types of alleles on the y chromosome.

Diagram of Sex Linkage•In a diagram, the sex chromosomes of the parents are shown with the alleles represented as superscripts.

•Inheritance of the hemophilia gene (h) and the normal gene (H).

Human Blood Groups•There are four main groups of human blood. These are known as blood types. The major blood types are O, A, B, and AB. Within each blood type a person can be Rh positive or Rh negative.

•In an average of 1000 people, the distribution of major blood types will be as shown in the table.

Number Group Percentage

390 Rh positive, group O 39.0%

350 Rh positive, group A 35.0%

90 Rh positive, group B 9.0%

40 Rh positive, group AB 4.0%

60 Rh negative, group O 6.0%

50 Rh negative, group A 5.0%

15 Rh negative, group B 1.5%

5 Rh negative, group AB 0.5%

The End

Principles of Biology

Genetics