World Builders™                                                                  Session Four  --  Microbiology



      How Characteristics are Inherited       

The plans for building each living being can be found in the nucleus of each one of its cells.


     Living organisms reproduce themselves. This is an amazing and mysterious power! Human beings have wondered how it was that cats produce kittens, dogs produce puppies, human beings produce human babies, and the young are faithful reproductions of the parents.

     Scientists have discovered that every cell contains a set of instructions about how to reproduce itself, function, repair itself, and grow. These instructions are in the chromosomes, which are made up of a chemical called DNA. Every cell in an organism's body contains the same set of chromosomes. If two organisms have the identical chromosomes (as identical twins do) they are very much alike. People have family resemblances because they share some of the same chromosomes, and therefore have characteristics in common.

     The earliest form of reproduction is asexual reproduction. Single cells make copies of their chromosomes and divide into two cells. Because both cells have identical sets of chromosomes, the cells are identical (unless there has been some sort of error in the copying process). They thrive in identical conditions, and are stressed by the same environmental changes.

     Chromosomes are made up of millions of short sequences of amino acids called genes. The genes influence, and sometimes determine absolutely, the characteristics that the organism will express.

     When sexual reproduction occurs, the new organism gets chromosomes from each of its parents. This makes each new organism unique.


The new organism may be better fitted to survive than its parents.

The new organism may be able to respond to environmental changes and to exploit new opportunities.

The species can adapt rapidly to a changing environment.


Some of the new organisms will be less fitted to survive.

Organisms need to find a suitable mate in order to reproduce.

Let's see how characteristics are passed from parents to children.

Example One: Flowers Have Different Colors

This little plant has pink flowers.

The color of the flowers is determined by a gene.

In this diagram, the chromosome that carries the gene for flower color is green.

The gene that controls the flower color makes the color pink, and is represented by a pink circle.

Notice that the plant has two chromosomes. (Chromosomes in cells come in pairs.)

Each chromosome in this pair has a gene for pink flowers.

This little plant is the same kind of plant as the one above, but it has blue flowers.

The color of the flower is determined by a gene in the same place as the one for pink flowers, but this gene says make blue!

So this plant has blue flowers.

Notice, however, that the blue is patterned.

The gene for blue is recessive.

If you cross a blue flowered plant with a pink flowered plant, the pink gene will be dominant over the blue one. The new flower will have pink blossoms.


This is how it works:

Notice that each new plant has a gene for pink flowers on one chromosome and a gene for blue flowers on the other one.

However, the color pink is dominant over blue, so all the flowers are pink.

This new generation is called the F1 generation.


Punnett Squares:

This is how scientists diagram what we have just done:


Here we see the genetic makeup of the parents for the flower color that we are studying.

The plant that we have named the mother has two genes for pink flowers.

If both genes are the same, the organism is homozygous for that quality.

Scientists put a symbol for that gene in the squares that are under the symbol for the mother's chromosomes. See the chromosome with a pink gene in each yellow box.

The plant that we have named the father is homozygous for blue flowers.

Scientists put the symbol for the blue flower gene in the boxes to the right of the symbol for the father's chromosomes. See the chromosome with a blue gene in each yellow box.

The yellow boxes represent the genetic makeup of the children.

None of these children are homozygous. They are all heterozygous. Each one has a pink flower gene from the mother and a blue flower gene from the father.


Here is a more formal way to present the same information.

We use a capital P to represent the dominant gene for pink flowers.

We use a lower case p to represent the recessive gene for blue flowers.

Then we bring the symbols down into the boxes just as we did in the first chart.

Again we see that the children are heterozygous. They have a gene from each parent.

Now, let's do the next step:


Now look at the new plants!

Three have pink flowers and one has blue flowers.

However, only one of the pink flowered ones will "breed true".

This time we got:

one plant that is homozygous for pink flowers, just like the pink grandparent.

one plant that is homozygous for blue flowers, just like the blue grandparent.

two plants that are heterozygous: they look pink, but also carry the recessive gene for blue.

This generation is called the F2 generation.


Let's use the Punnett Squares again. Do you remember how they work?

These parents are members of the F1 generation.

You can see their flower color genes represented in the diagrams.

The children are diagramed in the colored boxes again.

Can you figure out which gene came from which parent?

Follow the mother's genes down.

Follow the father's genes across.

See? This is not so hard!

Now let's do it with letters.

The capital P represents the dominant pink flower color.

The lower case p represents the recessive blue flower color gene.

Here we see the ration: 1 : 2 : 1

One homozygous pink flowering plant

Two heterozygous plants that carry copies of both genes

One homozygous blue flowering plant.

No matter how many plants are grown from this cross, the ratios will always be 1:2:1.

This was discovered by Gregor Mendel, a monk whose research in a monastery garden laid the foundation for the modern science of genetics.


Check Your Understanding at The Chromosome Kindergarten!

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© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at .