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Genetics- l.

posted Jan 27, 2015, 7:20 PM by Ranmini Perera   [ updated Aug 25, 2016, 12:14 PM by Upali Salpadoru ]

What is genetics?

In genetics, the biologists                                             study how the characteristics of an organism             is passed on to  their off springs                              and the causes for variations. 

Fig.1 Ninety nine (99%) percent of our genes are the same irrespective of nationality.

What are  genes?


Fig.2. Double Helix nature of DNA.

The unit, where a certain character is kept in a coded form is a gene

Thiis really is a particular location in a very complex and a long molecule of DNA. 

deoxyribenucleicacid )

These provide instructions for the synthesis of RNA ( Ribonucleic acid) These ynthesie  proteins that are responsible for different traits. (Characters)

The DNA molecule has two important properties.

  • It can make copies of itself. If you pull the two strands apart, each can be used to make the other one (and a new DNA molecule).
  • It can carry information. The order of the bases along a strand is a code - a code for making proteins. Such a section is a Gene.

    The DNA molecule is a long thread about 3 metres long. 

A human has   trillions of cells, and each and every nucleated cell  possess a complete set of  DNA of that individual.

Almost all the DNA is enclosed within the nucleus. 

(There are some DNA coming from the mother inside the mitochondria.) 

What is a chromosome?

  The long DNA molecules get coiled and wound around proteins called histones. When a cell is about to divide Each molecule condense and become visible as a structure called a chromosome . They exist as pairs and each part is known as a chromatid. Every live specimen has a definite number of chromosomes.  

Humans have 23 pairs.


Fig.3 A pair of chromosomes. 

Reduction Division.- Meiosis.

Passing of genes is not a straight forward task. Genes get thoroughly mixed due to a process called cross over when sex cells are formed during the cell division known as Reduction division or meiosis. For meiosis click this.

Although this is a carefully maneuvered process, mistakes do occur. They are known as mutations and may cause a gradual change in the offspring which may be for better or worse.

The 22 autosomes are numbered by size. The other two chromosomes, X and Y, are the sex chromosomes. This picture of the human chromosomes lined up in pairs is called a karyotype.

The last pair determines the sex. 

In females sex chromosomes  have two of the same kind (X and X). 

Males have two distinct sex chromosomes (X and Y).

During the formation of sex cells the chromosome number is halved

So each gamete cell (sperms and egg cells) can acquire only half the number of genes from a parent.

             Fig.4 A set of chromosomes from a male human cell.

More on Genes

The unit, where a certain character is chemically encoded has been termed a gene. A Human being has more than 20,000 genes. The largest chromosome, chromosome 1, contains about 8000 genes. The smallest chromosome, chromosome 21, contains about 300 genes.

A gene is really a particular location in the DNA molecule. Usually 3 base pairs carry the code for a   gene . They carry instructions to make necessary proteins for  our traits

A trait is one of our characteristic such as skin colour, voice, height etc.They not only contain the recipes for the production of body tissues and enzymes required for respiration and digestion etc.,  even the time and the duration for various actions such as growing a beard and producing milk after pregnancy  are precisely programmed.

Every person has two copies of each gene , one from father and the other from mother. In all human beings 99% of the genes are the same while less than 1 % may be slightly different. These genes with slight differences are called alleles.Such differences cause the variations in human beings.

What is the nature of DNA?

The DNA molecule is a polymer made up of several units called nucleotides.  A nucleotide consists of 3 parts.  

1. A phosphate group.            2. De oxyribose sugar.                  3. A base.

They only differ according to the bases present.

The nucleotides join together as shown in the diagram below.The two strands twist to form the double helix.  The order of bases in one strand contains the genetic code.

 Fig. 5 DNA Polymer.

Fig. 6. A nucleotide of the DNA polymer.

Gregor Mendel (1822–1884) first came across the idea of genes, in the 1860s. 

He studied inheritance in pea plants and hypothesized a factor that conveys traits from parent to offspring. Mendel was also the first to hypothesize independent assortment, the distinction between dominant and recessive traits, the distinction between a heterozygote (A person carrying two different forms of a gene inherited from father and the mother) and homozygote (A person having two similar genes inherited from the two parents), which is the genotype (the genetic material of an organism) and phenotype (the visible traits of that organism).

According to the theory of Mendelian inheritance, variations in phenotype are due to variations in genotype, or the organism's particular set of genes, each of which specifies a particular trait. Different forms of a gene, which may give rise to different phenotypes, are known as alleles. Organisms such as the pea plants Mendel worked on, along with many plants and animals, have two alleles for each trait, one inherited from each parent. Alleles may be dominant or recessive; dominant alleles give rise to their corresponding phenotypes when paired with any other allele for the same trait, whereas recessive alleles give rise to their corresponding phenotype only when paired with another copy of the same allele. But this is not always the case; in this situation the alleles are completely dominant. In other cases the alleles could be incompletely dominant or co-dominant. Incomplete dominance is when a characteristic in the heterozygote s intermediate in the degree between the two homozygotes. And co-dominance is when the heterozygote simultaneously expresses 2 alleles that have different kinds of effect.

 Chromosomes within cells are the carriers of genetic material, and they are made of DNA (deoxyribonucleic acid). DNA consists of a chain made from four types of nucleotide subunits: adenine, cytosine, guanine, and thymine. Each nucleotide subunit consists of three components: a phosphate group, a deoxyribose sugar ring, and a base. Adenine and guanine are purines, and cytosine and thymine are pyrimidines. The DNA in a cell has double helix structure, in which two individual DNA strands twist around each other in a right-handed spiral. In this structure, the base pairing rules specify that guanine pairs with cytosine and adenine pairs with thymine (each pair contains one purine and one pyrimidine). The base pairing between guanine and cytosine forms three hydrogen bonds, whereas the base pairing between adenine and thymine forms two hydrogen bonds. The two strands in a double helix are complementary.

One end of a DNA polymer contains an exposed hydroxyl group on the deoxyribose; this is known as the 3' end of the molecule. The other end contains an exposed phosphate group; this is the 5' end. So processes such as DNA replication occur in only one direction. All nucleic acid synthesis in a cell occurs in the 5' to 3' direction.

Q. 1.0 What is the number of ……….

1.Chromosome pairs in a man?

2.Chromosome pairs in a woman?

3.Chromatids in a single pair of chromosomes?

4.Chromatids in a single cell after mitosis?

5.Chromatids in a single cell after meiosis?

4x5 = 20 marks.


Give a single word to describe these phrases.

  1. Male sex cells in a flower.

  2. Female sex cells in a flower.

  3. What happens after pollination.

  4. The cell formed after fertilization.

  5. Male sex cells of an animal.

  6. Female sex cells of an animal.

  7. Instructions recorded in a cell for growth and development of an individual.

  8. A section in the DNA molecule That maintain the informations for a particular characteristic.

  9. Mistakes that may occur during meiosis.

  10. Scientific study of inheritance.

5x8 =40 marks


    Deoxyribose nucleic acid is a very long molecule called a 1.……………. A single repeating unit of it is called a 2…………….

These molecules are present in the 3. …………… of a cell and are 4. …………….with a normal microscope.

When a cell is about to undergo division, each molecule form into structures called 5………………….  

The two strands of the molecule are made of a 6…………………………

And are connected by the 7……………………….There are 8…………….

Types of bases in a DNA molecule.

4x8 = 32 Marks


Name the numbered units in this diagram of a DNA molecule.

DNA.jpg       1x8=8 marks.

For Answers click Answers -Bio.