Genetics

Why do family members resemble one another?

Why are birds of the same species all alike?

How can brown-eyed parents have a blue-eyed child?

How is cloning possible?

Factors responsible for character traits among living organisms

•Members of the same species of animals look very similar. Within a bird species, the shape of their beaks, their feathers and the position of their eyes are very much alike.

•Even humans have many characteristics in common, for example: the position of our eyes, nose, ears and mouth.

•We also differ in many ways, making us unique (eye and hair colour). The similarities and differences are called character traits.

Chromosomes

•Most eukaryotic cells contain a nucleus holding a person’s basic genetic information.•The main component of the nucleus is chromatin, made up of a molecule of DNA, combined with proteins. •When the cells are about to divide, chromatin contracts forming visible rods called chromosomes.

Humans have 46 chromosomes in the cells; 23 chromosomes inthe sex cells (eggs and sperm).

Other species have differing numbers of chromosomes; the number of chromosomes does not determine the complexity of the organism.

If you classify the chromosomes according to size and distinctive features, each chromosome is observed to have a partner of the same size and shape. The pair is called homologous chromosomes.

Humans have 23 pairs of homologous chromosomes.

An ordered representation of these pairs (an individual’s chromosomes) is known as a karyotype.

Normally, only one pair can contain chromosomes of different sizes, the pair of sex chromosomes.

As with most animals, two sex chromosomes are the same for females (XX pair), while in the male, one of the two chromosomes is smaller (XY pair).

DNA and Genes

Each chromosome is made up of DNA.

DNA is shaped like a twisted ladder (double helix). It consists of a series of chemical units called nucleotides.

Nucleotides consist of:•A sugar: deoxyribose (sides of the ladder)

•A phosphate group (sides of the ladder)

•One of 4 nitrogenous bases: adenine(A), thymine(T), guanine (G) or cytosine (C) (rungs of ladder)

Nitrogenous bases pair accordingly:

Adenine with thymine: A-T or T-A

Cytosine with guanine: C-G or G-C

A gene (a segment of DNA) is a particular sequence of bases. They differ from one another in their sequence of nitrogenous bases. Genes contain the information for protein making.

Proteins determine character traits and body functioning.

A genome consists of all the genes we receive from our fathers and mothers.

The difference between a human and a chimpanzee lies in only one percent of their genes. Nearly 99% of the two genomes are nearly identical.

The human body contains more than 100 000 proteins.

The sequence of amino acids in a chain determines the function of the protein.

Protein structure

Amino acid: A molecule that can combine with other amino acids to form proteins.

Some amino acid chains are made of 600 amino acids. Most, however, contain between 100 and 200 amino acids.

Protein Synthesis

Usually carried out by the ribosomes, it is the creation of proteins by the cells.

Since DNA cannot leave the nucleus, a messenger must carry the information from the DNA to the ribosomes: RNA

An RNA molecule is very similar to a DNA molecule with a few important differences:

RNA Single strandDNA Double strand

RNA Uracil base pairDNA Thymine base pair

RNA Sugar is riboseDNA Sugar is deoyxribose

Process of Protein synthesis

The are two major types of RNA:

Messenger RNA (mRNA): It acts as a messenger for carrying instructions about the gene to the ribosomes.

Transfer RNA (tRNA): It transfers the amino acids in the cell’s cytoplasm to the ribosomes in order to make proteins.

Principles of heredity

Heredity: The transmission of parent’s character traits to their offspring.

When a trait is said to be hereditary, this means it can be passed on from parents to offspring. Ex. Eye colour in humans, or the colour of pea blossoms are hereditary traits.

A scientist names Gregor Mendel was the first to study and establish the fundamental laws of heredity (Mendel’s Laws). The founding father of genetics published these results in 1865 but only gained significance by 1900.

Mendel’s experiments

Mendel created a Pure Line through self-pollination of the pea plant. This means a flower from a pea plant can be pollinated with pollen from the same flower. The results are particular traits that are passed on from generation to generation without variation.

He chose purple flowered pea plants and white flowered pea plants. To prevent self pollination he removed the stamens (male organs which produce pollen). He then used a paintbrush to deposit pollen from the white flowers to the carpal (female organ containing ovules). This is Crossbreeding.

The offspring of this cross were allowed to reproduce again.

Mendel did not know of genes or chromosomes. He knew there was some type of information unit present (he called Hereditary factor).

He also knew that there must be several alternatives for each of these traits. We call them alleles of a gene. Today we know that the various alleles are due to differences in the nucleotide sequences.

Homozygote and Heterozygote

Homozygote: An individual with two identical alleles for a given character trait.

Heterozygote: An individual with two different alleles for a given character trait.

Dominate and Recessive Alleles

Dominant allele: An allele that is expressed when an individual carries two different alleles for a given gene. It can be heterozygous dominant or homozygous dominant.

Recessive: An allele that is not expressed when an individual carries two different alleles for a given gene.

It is considered homozygous recessive

Genotypes and Phenotypes

Genotype: An individual’s genetic inheritance. It describes all of an individual’s alleles for specific genes.

Phenotype: The way in which a genotype expresses itself. It describes the appearance or state of the individual for one or more character traits.

To describe a genotype:•Each allele is represented by a letter.•The dominant from of the allele is a capitol letter while the recessive allele gets a lower case letter.

The Law Of Segregation Of Alleles

Gametes are formed through a process called meiosis. This type of cell division, unlike mitosis, results in cells with half the number of chromosomes normally present in the cells. Instead of carrying pairs of chromosomes, gametes contain only one chromosome from each pair. Thus, only one allele is present instead of two.

Law of Segregation of Alleles:The two alleles for a particular character trait separate when gametes are formed. Half of the gametes receive one of the two alleles and the other half receive the second allele.

Punnett Squares

It is a tool used to predict the genotypes and phenotypes of offspring.

Four steps must be followed to use this tool properly:

1. Determine the genotypes of the two parents.2. Find the possible genotypes for the gametes and place

them in circles.3. Place the possible gametes of one parent at the top of

the Punnett square and the other gametes on the left side.

4. Indicate all of the possible gamete combinations. This provides all of the genotypes and phenotypes.

The law of independent assortment of character traits.

The different pairs of alleles separate into gametes, independently of each other.

There are two natural methods of reproduction:

Sexual reproduction: It involves the combination of genes of two parents which result in genetically unique offspring. (except identical twins).

Asexual reproduction: It involves only one parent resulting in offspring genetically identical the parent. (clones)

Cloning

•Natural cloning•Artificial plant cloning•Animal cloning•Human cloning•Molecular cloning

It is the reproduction of an individual, part of that individual or one of its genes in order to obtain an exact copy.

We will look at:

Natural cloning: It produces genetically identical individuals through asexual reproduction.

Artificial plant cloning

Plant cuttings or in vitro methods. People often take a cutting form a plant and place it in water unit it grows roots. It is then transferred to a pot. In vitro methods are shown below. Since both methods involve human participation, that are called artificial plant cloning.

Animal cloning

1. Remove a cell from the individual to be cloned.2. Take an ovum from another individual and remove the

nucleus.3. Combining the cell and the ovum (no nucleus) results in an

embryo with the same genetic material as the individual.4. Implant the embryo into the uterus of a surrogate mother.

Human cloning

Human cloning has two forms:

1. Reproductive cloning: The application of cloning techniques to

obtain a new individual genetically identical to the one being cloned.

2. Therapeutic cloning: The application of cloning techniques to obtain tissues or organs genetically identical to those of a person in need of a transplant or medical grafting.

Molecular cloning: The production of multiple copies of the same gene.

Many illnesses have genetic causes, which means they are due to defective genes. Scientists may want to make multiple copies of these defective genes to study them more closely.