×

Warning

Please Login to Read More...

DNA determines your appearance!- Why do we resemble our parents? Each of us receives traits – in the form of genes, from our mother and father. This passing of genes from parent to child is the basis of inheritance. This passing down of characteristics is called heredity.
Why do we resemble our parents?

Have people ever said to you, "It's in your genes?", "like father, like son", "she's got her mother's blue eyes" or dimples or freckles............ May be your father's curly hair or height? Do you have any traits from your grandparents or great-grandparents? They were probably talking about a physical characteristic, personality trait, or talent that you share with other members of your family.

Among the physical traits you inherit from your parents are – your general build, your skin color, your eye color and shape, your hair color and texture, straightness or curliness, plus the shape and size of your nose, ears, hands, and feet. Among the mental traits you inherit are – your ability to learn, plus talent in special fields such as art, music, mechanics, or science.

In addition, you inherit emotional traits, how you respond to certain situations or feelings, how easily you get upset or how calm you stay. All these traits, however, are greatly influenced by your environment, the world you live in, what your parents and teachers teach you, and what you learn from your community.

There are, however, certain physical traits that your parents may have now that you cannot inherit. For example, if your father has a scar that he got when he fell playing tennis, you would not inherit it. However, if he was born with a birthmark on his back, you might inherit that. If your mother had an operation to shorten or turn up her nose, you would inherit her old nose, not her new one. However, if she had freckles on that nose, you might inherit them.

DNA determines your appearance DNA determines your appearance Everything about our body, from eyes to the big toes, is the expression of genes.
So, how do you inherit traits?

However, parents do not, in any literal sense, give their children dimples, eyes, hair, or any other traits. What, then, is actually inherited? The answers to questions like these lie in your genes.

Genes influence the traits you inherited in predictable–and unpredictable–ways. We know that genes play an important role in shaping how we look and act and even whether we get sick. Why do we resemble our parents? Why do we look like our brothers and sisters? It's because of genes. Each of us receives traits– in the form of genes, from our mother and father. This passing of genes from parent to child is the basis of inheritance.

Genes control our lives The main function of genes is to control the production of the proteins in the organism's cell.
Inheritance of genes

Every parent, human, animal, or plant, passes down certain characteristics to their children, so that these offspring will resemble them, but not be exactly like them. This passing down of characteristics is called heredity.

Parents endow their offspring with coded information in the form of hereditary units called genes. The tens of thousands of genes we inherit from your mother and father constitute your genome. This genetic link to your parents accounts for family resemblance such as shared eye color or freckles. Our genes program the specific traits that emerge as we develop from fertilized eggs into adults.

The genes that you have in your body right now make up your genotype. This genotype then determines your physical appearance, which is called your phenotype.

We begin our study of genetics, by examining how chromosomes pass from parents to offspring in sexually reproducing organisms. The processes of meiosis (a special type of cell division) and fertilization (the fusion of sperm and egg) maintain a species' chromosome count during the sexual life cycle.

directions The Objective of Chromosomes The "directions" present on chromosomes made you grow into an individual. All these "directions" are contained in 46 tiny body cells called chromosomes.
How Life begins?

When you began life as an egg produced by your mother and fertilized by your father (fertilization is a process where egg and sperm fuses to form zygote), you came with a pre–packaged set of "directions" on how to grow and what to look like. These "directions" made you grow into an individual.

All these "directions" are contained in 46 tiny body cells called chromosomes. Of these 46, 23 come from your father and 23 from your mother. These chromosomes carry tiny particles from each of your parents with all the characteristics you could inherit from them. These particles, called genes, are the detailed "directions" that decide not only what you will look like, your physical traits, but also what you will be able to learn your mental traits.

The interesting things about your 23 pairs of chromosomes are that each pair of genes gives "directions" for only one particular trait. Your father may have given you a gene for blonde hair and your mother for brown hair.

In many cases, one gene is more powerful than the other. This is called the dominant gene. If your eyes are blue like your father’s and not brown like your mother’s, then your father’s gene for eye color was the dominant one and your mother’s gene for brown eyes was recessive.

Father's two sets of chromosomes. Father's two sets of chromosomes. In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. Twenty–two of these pairs, called autosomes, look same in both males and females. The 23rd pair, the sex chromosomes, differ between males and females. Females have XX chromosomes, while males have X Y chromosome.
Chromosome pairs in Humans

Every human being is defined by two complete sets of 23 chromosomes, which are the large packages of DNA that contain our genes. As a result, each of us has two complete sets of genes. When a mother and a father conceive a child, each parent contributes exactly one complete set of 23 chromosomes to the child.

So human cells have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes), giving a total of 46 chromosomes per cell.

However each one of the egg and sperm cells which are called gametes has 23 chromosomes and when your biological parents' egg and sperm combines a full set with 23 pairs is created. So, half your chromosomes come from your mother and half from your father. Genes always comes in pairs. The genes on your chromosomes come in pairs – like a pair of shoes but they aren't always identical. Thus, for every gene, you have two versions called alleles.

Random pattern of inheritance. Random pattern of inheritance. Humans have 22 pairs of autosomal chromosomes. We inherit autosomal chromosomes from both of our parents randomly. One chromosome from each pair comes from our mother and the other is from our father, which determines the gender of particular individual.
Pattern of inheritance

Although the anatomical and physiological differences between women and men are numerous, the chromosomal basis for determining sex is rather simple.

In humans and other mammals, there are two varieties of sex chromosomes, designated X and Y. A person who inherits two X chromosomes, one from one parent, usually develops as a female. A male develops from a zygote containing one X chromosome and one Y chromosome.

In both testes and ovaries, the two sex chromosomes segregate during meiosis (meiosis occurs in gametic cells only), and each gamete receives one. Each ovum contains one X chromosome. In contrast, sperm fall into two categories: Half sperm cells contain X chromosome, and half contain a Y chromosome.

We can trace the sex of each offspring to the moment of conception: If a sperm cell bearing an X chromosome happens to fertilize an ovum, the zygote is XX, a female; if a sperm cell containing a Y chromosome fertilizes an ovum, the zygote is XY, a male. Thus, sex determination is a matter of chance– a fifty–fifty chance.

Siblings inherit different combinations of genes Pattern of inheritance in case of siblings and in identical twins

Figure 1: Siblings inherit different combinations of genes from their parents, hence they look different.

Figure 2: Identical twins receive exactly the same combination of genes from their parents. That's why, they look exactly alike!
Why siblings look different?

Randomness in selection of chromosomes:

As we already discussed that, each parent contributes one complete set of chromosomes to each child, however, the set of chromosomes from each parent is through random selection of 23 chromosomes from the 46 chromosomes (23 pairs). The only rule is that the child must receive exactly one of each chromosome from both the parents.

So, our question is – "If we inherit our parents 'genes', then why don't we look exactly like them? ––––The answer to this question is......If the complete 23 pairs inherits, as it is to the child, he will look exactly like his father or mother, but what actually happens is– the genes randomly passes to the next generation. (see the figure in previous split to observe the pattern of inheritance). There, shown the set of chromosomes of father, in the similar way– the offspring inherits chromosomal sets from mother".

As a result, each brother and sister inherits a very different combination of chromosomes. From each parent, there are 223 possible combinations. This is the reason, why siblings look different. What about identical twins then? Identical twins receive exactly the same combination of genes from their parents. That's why, they look exactly alike!

The human life cycle The human life cycle: In each generation, the doubling of the number of chromosomes sets that result from fertilization is offset by the halving of the number of sets that result from meiosis. For humans, the number of chromosomes in a haploid cell is 23, consisting of one set (n= 23); the number of chromosomes in the diploid zygote and all somatic cells arising from it is 46, consisting of two sets (2n= 46).
What does haploid and diploid mean?

Most of the time, people talk about haploid and diploid conditions of humans and other multicellular organisms. Actually what does this mean? Let's try to resolve and find out, what actually haploid and diploid condition mean.

We inherit one chromosome of each pair from each parent. So the 46 chromosomes in our somatic cells (Any cell of a living organism other than the reproductive cells are called somatic cells) are actually two sets of 23 chromosomes– a maternal set (from our mother) and a paternal set (from our father).

The number of chromosomes in a single set is represented by ' n '. Any cell with two chromosome sets is called diploid cell and has a diploid number of chromosomes, abbreviated ' 2n '. For humans, the diploid number is 46 (2n= 46), the number of chromosomes in our somatic cells.

Unlike somatic cells, gametes (sperm and egg cells) contain a single chromosome set. Such cells are called haploid cells, and each has a haploid number of chromosomes (n). For humans, the haploid number is 23 (n=23), the number of chromosomes found in gametes. The set of 23 consists of the 22 autosomes plus a single sex chromosome.

An unfertilized egg cell (also called an ovum) contains an X chromosome, but a sperm cell may contain an X or Y chromosome. A person who inherits two X chromosomes, one from one parent, usually develops as a female. A male develops from a zygote containing one chromosome and one Y chromosome.

Human cells are “diploid”, which means that they have two complete sets of 23 chromosomes. Diploid condition of chromosomes Human cells are "diploid", which means that they have two complete sets of 23 chromosomes.
Extending the concepts of haploid and diploid conditions

The human life cycle begins when a haploid sperm cell from the father fuses with a haploid ovum from the mother. This union of gametes, culminating in fusion of their nuclei, is called fertilization. The resulting fertilized egg or zygote, is diploid because it contains two haploid sets of chromosomes bearing genes representing the maternal and paternal family lines.

As human develops from a zygote to a sexually mature adult, mitosis generates all the somatic cells of the body. Both chromosome sets in the zygote and all the genes they carry are passed with precision to our somatic cells. The only cells of the human body not produced by mitosis are the gametes, which develop in the gonads– ovaries in females and testes in males.

Imagine what would happen if human gametes were made by mitosis: They would be diploid like the somatic cells.

At the next round of fertilization, when two gametes fused, the normal chromosome number of 46 would double to 92, and each subsequent generation would double the number of chromosomes yet again.

This hypothetical situation of constantly increasing chromosome number in sexually reproducing organisms is avoided through the process of meiosis. This type of cell division reduces the number of sets of chromosomes from two to one in the gametes, compensating for the doubling that occurs at fertilization.

somatic cells Nature of Somatic cells Any cell of a living organism other than the reproductive cells are called somatic cells, which are diploid in nature.
All about reproduction

In animals, meiosis occurs only in the ovaries or testes. As a result, each human sperm and ovum is haploid (n= 23). Fertilization restores the diploid condition by combining two haploid sets of chromosomes, and the human life cycle is repeated, generation after generation. Indeed, the process of fertilization and meiosis are the unique trademarks of sexual reproduction.

Fertilization and meiosis alternate in sexual life cycles, offsetting each other's effects on the chromosome number and thus perpetuating a species' chromosome count. Thus, meiosis occurs during production of gametes, which undergo no further cell division prior to fertilization. The diploid zygote divides by mitosis, producing a multicellular organism that is diploid.

Now, let's start understanding what is mitosis and meiosis and how these two phenomena help one to grow in size, and show genetic variations in offspring.

Cells reproduce themselves by dividing. Cells reproduce themselves by dividing. Cells divide in order to produce new cells that replace damaged, dying or lost cells.
Mitosis

The human body is made up of about a hundred trillion cells. But, we didn't start out that way. We each began as one single cell. How does one cell become a trillion? Cells reproduce themselves by dividing. A "mother" cell divides into two "daughter" cells that are exactly like the mother cell.

After growing for a while, these two cells divide to make four cells. If cell division continues, eventually there will be a trillion cells! Cells don't just split in half, though. Before they can divide, cells must make some preparations. Let's think about chromosomes for a minute. Most human cells are "diploid", which means that they have two complete sets of 23 chromosomes.

Before it divides, the mother cell makes a copy of each chromosome. This means that, for a short time, the mother cell has four complete sets of 23 chromosomes! Why does the mother cell do this? – So that when it divides, each daughter cell receives two complete sets of chromosomes. This cell division is called “mitosis”. Thus, through mitosis, cells can keep on dividing– from one cell all the way to a trillion.

Growth Growth The number of cells within an organism increases by mitosis and this is the basis of growth in multicellular organisms.
Why Mitosis?

We need it for growth and repair. Think about the example at the start of this topic, starting off as a zygote to human being. In order to grow into a human being, we have to have enough cell division to make trillions of cells! The previous example was for growth, but mitosis is also important for repair.

When your skin is cut, you certainly want more skin cells to come in and repair the cut. So, the nearby skin cells undergo mitosis (there are also other things that happen that are beyond the scope of this short discussion) to repair the cut.

Cell replacement Cell replacement Red blood cells have short live spans of about 4 months and need to be replaced constantly by mitosis.
Mitosis also play a key role in cell replacement

Cells are constantly sloughed off, dying and being replaced by new ones in the skin and digestive tract. When damaged tissues are repaired, the new cells must be exact copies of the cells being replaced, so as to retain normal function of cells.

When red blood cells die, the hemoglobin within them is released and broken up. The iron in hemoglobin is retrieved, transported to the bone marrow by a protein called transferrin and used again in the production of new red blood cells; the remainder of the hemoglobin becomes a chemical called bilirubin that is excreted into the bile which is secreted into the intestine, where it gives the feces their characteristic yellow–brown color.

chromosomes after fertilization. Relationship between Meiosis and Fertilization Meiosis is a special type of cell division that produces gametes with half as many chromosomes. The opposite process would be syngamy or fertilization, which is the union of the egg and sperm to restore the 2n number. This results in a zygote.
Meiosis:

Almost all human cells have two complete set of 23 chromosomes. But two types of human cells have only one complete set of chromosomes. A cell with only one complete set of chromosomes is called "haploid" condition. Haploid cells are made through a different type of cell division, called as meiosis.

Meiosis begins with one diploid mother cell. This mother cell copies its chromosomes and divides, in a process similar to mitosis. The story doesn't end there, as the two daughter cells further divide producing four haploid cells. Why would we need haploid cells? Meiosis is used to produce haploid sperm and egg cells.

A child is conceived when a sperm cell from the child's father fuses with the mother's egg cell. The father's sperm cell brings one set of 23 chromosomes, and the mother's egg cell contains one set. The resulting cell, now diploid, is called as zygote.

Soon, the zygote begins to divide, ultimately forming a child. This is how we all start out as a single cell. Now, a zygote begins its journey to express in the form of a human being.

A zygote begins its journey to express in the form of a human being: A human starts life as a single fertilized egg cell. As it accumulates nutrients, that egg cell can grow by dividing to produce more cells found in an embryo. Eventually the cells develop into various tissues and organs comprising the organism.
Why meiosis?

Meiosis is the process that results in the formation of sperm cells and egg cells. The cells that will undergo meiosis are typically found in the testis and ovaries of males and females respectively. These germ–line cells (The sequence of cells which develop into eggs and sperm) are diploid, having two sets of chromosomes, but will undergo meiosis to produce haploid cells, having one set of chromosomes. During fertilization, these haploid cells fuse to form a diploid offspring. Thus, Meiosis is essential for sexual reproduction.

Meiosis produces gametes, or sex cells, needed for fertilization. Without meiosis, all sex cells would be diploid... meaning that they have the full number of chromosomes for the species. So, when two diploid gametes join together, they form a zygote with twice the species number of chromosomes. Meiosis generates genetic variation. Meiosis involves important mechanisms that automatically generate a wide variety of variation. They are one of the reasons why we all look different (except for identical twins) and why evolution occurs faster with more variation.

Meiosis is essential for sexual reproduction Meiosis is essential for sexual reproduction Meiosis is important because it ensures genetic diversity in cells during reproduction.
You are unique and yet resemble in many ways to your parents

It is due to combination of Meiosis and Mitosis:

Meiosis reduces the number of chromosome sets from two (diploid) to one (haploid), whereas mitosis conserves the number of chromosome sets. Therefore, mitosis produces daughter cells genetically identical to their parent cell and to each other, whereas meiosis produces cells that differ genetically from their parent cell and from each other.

While in mitosis, genes are generally transferred faithfully from one cellular generation to the next; in meiosis and subsequent sexual reproduction, genes get mixed up. Sexual reproduction actually expands the variety created by meiosis, because it combines the different varieties of parental genotypes. Thus, because of recombination and sexual reproduction, there are trillions of possible genotypes in the human species. Thus, both mitosis and meiosis are involved in completion of human life cycle.

References

  • Biology – Seventh edition: Campbell.Reece
  • http://genome.pfizer.com/station2-9.cfm
  • http://learn.genetics.utah.edu/content/begin/tour/mitosis.swf
  • http://www.bigsiteofamazingfacts.com/how-are-you-like-your-parents
  • http://bio1151.nicerweb.com/Locked/media/ch01/grow.html
  • http://faculty.stcc.edu/BIOL102/Lectures/lesson8/mitosis.htm
  • http://library.thinkquest.org/C0118084/Gene/Chromosomal_Inheritance/SignificanceMitosis.htm
  • http://wiki.answers.com/Q/What_happens_to_hemoglobin_when_red_blood_cells_die
  • http://www.nature.com/scitable/topicpage/mitosis-meiosis-and-inheritance-476
  • http://uk.answers.yahoo.com/question/index?qid=20080225154736AARfwHn
  • http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter11/how_meiosis_works.html