SCIENCE: November 2011

law of refraction:

Bending Light

If you have ever half-submerged a straight stick into water, you have probably noticed that the stick appears bent at the point it enters the water (see Figure 1.) This optical effect is due to refraction. As light passes from one transparent medium to another, it changes speed, and bends. How much this happens depends on the refractive indexof the mediums and the angle between the light ray and the line perpendicular (normal) to the surface separating the two mediums (medium/medium interface) (See Figures 2a and 2b.) Each medium has a different refractive index (see list below.) The angle between the light ray and the normal as it leaves a medium is called theangle of incidence. The angle between the light ray and the normal as it enters a medium is called the angle of refraction.

Snell's Law

In 1621, a Dutch physicist named Willebrord Snell (1591-1626), derived the relationship between the different angles of light as it passes from one transperent medium to another. When light passes from one transparent medium to another, it bends according to Snell's law which states:Ni * Sin(Ai) = Nr * Sin(Ar),
where:
Ni is the refractive index of the medium the light is leaving,
Ai is the incident angle between the light ray and the normal to the meduim to medium interface,
Nr is the refractive index of the medium the light is entering,
Ar is the refractive angle between the light ray and the normal to the meduim to medium interface.

Critical Angle

Using the refraction simulator, notice how the light bends toward the normal when the light enters a medium of greater refractive index, and away from the normal when entering a medium of lesser refractive index. Then notice what happens when you move the flashlight to an angle close to 90 or -90 degrees in the medium with a higher refractive index. As you approach the critical angle the refracted light approaches 90 or -90 degrees and, at the critical angle, the angle of refractions becomes 90 or -90 and the light is no longer transmitted across the medium/medium interface. For angles greater in absolute value than the critical angle, all the light is reflected. This is calledtotal internal reflection.

Inheritance:
Gene and allel

Gene and allele are basically what make us who we are. They are genetic sequences of our DNA, although gene is a more general term than allele. To make an example: humans have facial hair. It can be thick or patchy. The first statement is a gene, the latter an allele.

Gene

Genes are the basic instructions for all life forms. All living things are dependent on genes for survival, as genes specify all proteins and functional RNA chains. Genes also contain the information and instructions to build and maintain our cells, and pass them on to our offspring. Genes not only tell us what we’ll look like, but also determine what kind of diseases we will be more vulnerable to.

Allele

Alleles are variants of a gene. When someone says this person got good genes, they are referring to an allele. They occur in pairs and produce opposite phenotypes that, by nature, are contrasting. If an allele has homogenous genes, then they are called homozygous. If they are different, they are called heterozygous. And heterozygous alleles will have a dominant and a recessive allele.

Difference between Gene and Allele

Genes are the parts of the DNA that determine what traits a person will have, while alleles are the different sequences of that DNA and they determine what kind of characteristics those traits will have. Alleles also occur in pairs, having a recessive and a dominant part. Genes don’t have any pairing at all. Also alleles can be either homozygous or heterozygous while genes don’t have such differentiation. Basically, an allele is just different types of the same gene. If there is a gene for hair color, one allele will be for black hair, the other for brown hair.

Alleles and genes are equally important in the development of all forms of life, and their differences can be seen in every living thing. The best example of how they manifest is you.

Complete Dominance: If two alleles have a complete dominance relationship, the phenotype of the heterozygote will be indistinguishable from the phenotype of the homozygous dominant. For example, for one of the gerbil fur color genes, that wild type agouti/brown allele (B) is completely dominant to the black (b) allele of the same gene. BB gerbils are brown; bb gerbils are black; Bb gerbils are brown. And you can't tell by looking at a brown gerbil whether it is BB or Bb, no matter how closely or carefully you look.
Incomplete Dominance: If two alleles have an incomplete dominance relationship, the phenotype of the heterozygote will be intermediate between the phenotypes of the two homozygotes. This is often described as "blending," though the alleles themselves do not blend. The phenotype of looks like the two traits have blended together. For example, in snapdragons, one of the various genes which control flower color has two alleles, one for red flowers and one for white flowers. The two homozygous plants will produce red and white flowers, respectively. But the heterozygote will produce pink flowers--as if the two homozygous conditions were blended together like paint. In this case, the actual flower color (phenotype) probably results from varying amounts of production of the red pigment. The homozygous red plant produces a lot of the pigment, the homozygous white plant produces none of the pigment, and the heterozygote produces half as much as the homozygous red. Note that there is no dominant allele here.

Every living thing—plant or animal, microbe or human being—has a set of characteristics inherited from its parent or parents. Since the beginning of recorded history, people have wanted to understand how that inheritance is passed from generation to generation. More recently, however, scientists have begun to appreciate that heredity holds the key to understanding what makes each species unique. As a result, genetics, the scientific study of heredity, is now at the core of a revolution in understanding biology.

INSTRUCTION

Gregor Mendel's Peas

The work of an Austrian monk named Gregor Mendel was particularly important to understanding biological inheritance. Gregor Mendel was born in 1822 in what is now the

Czech Republic

. After becoming a priest, Mendel spent several years studying science and mathematics at the University of Vienna. He spent the next 14 years working in the monastery and teaching at the high school. In addition to his teaching duties, Mendel was in charge of the monastery garden. In this ordinary garden, he was to do the work that changed biology forever.

Mendel carried out his work with ordinary garden peas. He knew that part of each flower produces pollen, which contains the plant's male reproductive cells, or sperm. Similarly, the female portion of the flower produces egg cells. During sexual reproduction, male and female reproductive cells join, a process known as fertilization. Fertilization produces a new cell, which develops into a tiny embryo encased within a seed. Pea flowers are normally self-pollinating, which means that sperm cells in pollen fertilize the egg cells in the same flower. The seeds that are produced by self-pollination inherit all of their characteristics from the single plant that bore them. In effect, they have a single parent.

When Mendel took charge of the monastery garden, he had several stocks of pea plants. These peas were true-breeding, meaning that if they were allowed to self-pollinate, they would produce offspring identical to themselves. One stock of seeds would produce only tall plants, another only short ones. One line produced only green seeds, another only yellow seeds. These true-breeding plants were the basis of Mendel's experiments.

Mendel wanted to produce seeds by joining male and female reproductive cells from two different plants. To do this, he had to prevent self-pollination. He accomplished this by cutting away the pollen-bearing male parts as shown in the figure at right and then dusting pollen from another plant onto the flower. This process, which is known as cross-pollination, produced seeds that had two different plants as parents. This made it possible for Mendel to cross-breed plants with different characteristics, and then to study the results.

Genes and Dominance

Mendel studied seven different pea plant traits. A trait is a specific characteristic, such as seed color or plant height, that varies from one individual to another. Each of the seven traits Mendel studied had two contrasting characters, for example, green seed color and yellow seed color. Mendel crossed plants with each of the seven contrasting characters and studied their offspring. We call each original pair of plants the P (parental) generation. The offspring are called the F₁, or “first filial,” generation. Filius and filia are the Latin words for “son” and “daughter.” The offspring of crosses between parents with different traits are called hybrids.

What were those F₁ hybrid plants like? Did the characters of the parent plants blend in the offspring? Not at all. To Mendel's surprise, all of the offspring had the character of only one of the parents, as shown at right. In each cross, the character of the other parent seemed to have disappeared.

From this set of experiments, Mendel drew two conclusions. 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 chemical factors that determine traits genes. Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms. These contrasting forms produced the different characters of each trait. For example, the gene for plant height occurs in one form that produces tall plants and in another form that produces short plants. The different forms of a gene are called alleles (uh-LEELZ).

Mendel's second conclusion is called the principle of dominance. The principle of dominance states that some alleles are dominant and others are recessive. An organism with a dominant allele for a particular form of a trait will always exhibit that form of the trait. An organism with a recessive allele for a particular form of a trait will exhibit that form only when the dominant allele for the trait is not present. In Mendel's experiments, the allele for tall plants was dominant and the allele for short plants was recessive. The allele for yellow seeds was dominant, while the allele for green seeds was recessive.

Segregation

Mendel wanted the answer to another question: Had the recessive alleles disappeared, or were they still present in the F₁ plants? To answer this question, he allowed all seven kinds of F₁ hybrid plants to produce an F₂ (second filial) generation by self-pollination. In effect, he crossed the F₁generation with itself to produce the F₂ offspring, as shown in the figure at right.

The F₁ Cross The results of the F₁ cross were remarkable. When Mendel compared the F₂plants, he discovered that the traits controlled by the recessive alleles had reappeared! Roughly one fourth of the F₂ plants showed the trait controlled by the recessive allele. Why did the recessive alleles seem to disappear in the F₁ generation and then reappear in the F₂generation? To answer this question, let's take a closer look at one of Mendel's crosses.

Explaining the F₁ Cross To begin with, Mendel assumed that a dominant allele had masked the corresponding recessive allele in the F₁ generation. However, the trait controlled by the recessive allele showed up in some of the F₂ plants. This reappearance indicated that at some point the allele for shortness had been separated from the allele for tallness. How did this separation, or segregation, of alleles occur? Mendel suggested that the alleles for tallness and shortness in the F₁ plants segregated from each other during the formation of the sex cells, or gametes (GAM-eetz). Did that suggestion make sense?

Let's assume, as perhaps Mendel did, that the F₁ plants inherited an allele for tallness from the tall parent and an allele for shortness from the short parent. Because the allele for tallness is dominant, all the F₁ plants are tall. When each F₁ plant flowers and produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene. Therefore, each F₁ plant produces two types of gametes—those with the allele for tallness and those with the allele for shortness.

Look at the figure below to see how alleles separated during gamete formation and then paired up again in the F₂ generation. A capital letter T represents a dominant allele. A lowercase letter t represents a recessive allele. The result of this process is an F₂generation with new combinations of alleles

Segregation of Alleles

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.

Mendel's Law of Independent Assortment

To this point we have followed the expression of only one gene. Mendel also performed crosses in which he followed the segregation of two genes. These experiments formed the basis of his discovery of his second law, the law of independent assortment. First, a few terms are presented.
Dihybrid cross - a cross between two parents that differ by two pairs of alleles (AABB x aabb)
Dihybrid- an individual heterozygous for two pairs of alleles (AaBb)
Again a dihybrid cross is not a cross between two dihybrids. Now, let's look at a dihybrid cross that Mendel performed.
Parental Cross: Yellow, Round Seed x Green, Wrinkled Seed
F₁ Generation: All yellow, round
F₂ Generation: 9 Yellow, Round, 3 Yellow, Wrinkled, 3 Green, Round, 1 Green, Wrinkled
At this point, let's diagram the cross using specific gene symbols.

Choose Symbol	Seed Color: Yellow = G; Green = g
	Seed Shape: Round = W; Wrinkled = w

The dominance relationship between alleles for each trait was already known to Mendel when he made this cross. The purpose of the dihybrid cross was to determine if any relationship existed between different allelic pairs.
Let's now look at the cross using our gene symbols.

Now set up the Punnett Square for the F₂ cross.

		Female Gametes
		GW	Gw	gW	gw
	GW	GGWW (Yellow, round)	GGWw (Yellow, round)	GgWW (Yellow, round)	GgWw (Yellow, round)
Male	Gw	GGWw (Yellow, round)	GGww (Yellow, wrinkled)	GgWw (Yellow, round)	Ggww (Yellow, wrinkled)
Gametes	gW	GgWW (Yellow, round)	GgWw (Yellow, round)	ggWW (Green, round)	ggWw (Green,<br)round)< td=""></br)round)<>
	gw	GgWw (Yellow, round)	Ggww (Yellow, wrinkled)	ggWw (Green, round)	ggww (Green, wrinkled)

The phenotypes and general genotypes from this cross can be represented in the following manner:

Phenotype	General Genotype
9 Yellow, Round Seed	G_W_
3 Yellow, Wrinkled Seed	G_ww
3 Green, Round Seed	ggW_
1 Green, Wrinkled Seed	ggww

The results of this experiment led Mendel to formulate his second law.
Mendel's Second Law - the law of independent assortment; during gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair
As with the monohybrid crosses, Mendel confirmed the results of his second law by performing a backcross - F₁dihybrid x recessive parent.
Let's use the example of the yellow, round seeded F₁.
Punnett Square for the Backcross

		Female Gametes
		GW	Gw	gW	gw
Male Gametes	gw	GgWw (Yellow, round)	Ggww (Yellow, wrinkled)	ggWw (Green, round)	ggww (Green, wrinkled)

The phenotypic ratio of the test cross is:

1 Yellow, Round Seed
1 Yellow, Wrinkled Seed
1 Green, Round Seed
1 Green, Wrinkled Seed

Friday 25 November 2011

know about lenses

law of refraction

Thursday 24 November 2011

law of refraction:

Bending Light

Snell's Law

Critical Angle

Wednesday 23 November 2011

Mendel's Law of Independent Assortment

Tuesday 22 November 2011

composition of urea

Urea & Chemical Fertilizers

Urea