Variation, Inheritance and Evolution

Lesson Set 1: Growth

Lesson Set 2: Reproduction

Lesson Set 3: Heredity

Lesson Set 1: Growth

INTRODUCTION

Heredity is the process by which characteristics are passed from one generation to the next.

The branch of Biology which studies how heredity works is called genetics.

IN THE BEGINNING

Our life begins with a Zygote as a result of a male and a female gamete fusing together. This is sexual reproduction. A zygote is a single cell that grows and reproduces itself. This replication happens thousands upon thousands of times, forming tissues and organs.

The original single cell had all the information needed to make us. We will have inherited information from both our parents, and each of our cells will carry the original information.

QUESTION

How is all this information passed from cell to cell?

INFOMATION:

Growth:

Growth is a process involving cell enlargement, cell division and cell specialisation.

Mitosis:

Mitosis is the process occurring during growth and asexual reproduction, involving each chromosome making an exact copy of itself, resulting in the formation of two daughter cells each with the same number of chromosomes as the parent cell.

WHAT IS A CHROMOSOME?

Chromosomes are thread-like structures that show up clearly in the nucleus when cell division occurs.

They are otherwise not apparent, indeed the whole nucleus is indistinct until cell division occurs. Chromosomes replicate themselves exactly during division.

ACTIVITIES

I

Download the following GIF files and print them off. They contain the necessary information. Colour them and add to them as you learn more

Chromosome Facts

The Function of Chromosomes in the Nucleus of the Cell

II

Follow this link

http://library.advanced.org/16985/index.html

Lesson Set 2: Reproduction

INTRODUCTION

Mitosis is the process occurring during asexual reproduction and cloning. The offspring are genetically identical to the parent.

Sexual reproduction involves the production of male and female gametes or sex cells. This process is called Meiosis. The gametes or sex cells are different from normal body cells as the paired chromosomes of the parent are split and each half goes in to a different gamete.

Meiosis then involves the reassortment of genetic material.

Meiosis results in variation. The individuals formed are genetically different to either of their parents though they only have genetic material from each.

ACTIVITIES

I

Download the following GIF file and use it to make note as you read the middle section of the Chapter 21.

The Human Life Cycle in Terms of Chromosomes

Mutations: Things can go wrong

Lesson Set 3: Heredity

INTRODUCTION

Are we all different? The way we look on the outside is our PHENOTYPE. It is the expression of what our genes are building on the inside.

Think of the following phenotypes:

• eye colour: blue, green grey. hazel, brown or as one of my friends had – half brown and half blue
• hair colour: brown, black, blond, red or changing with age as mine did from blond to mousy
• stature: short, medium, tall, muscular, weedy, pear shaped, straight
• covering: fat, thin, evenly distributed, blobby
• blood group: AB,A,O,B
• ears: joined lobes, free lobes
• tongue: able to roll, not able to roll

Have fun!

Now try and decide which vary and which you are either one or another.

Continuous variations are the ones that vary across a spectrum and are very dependent on environmental factors such as availability of types of food and health, or practice.

Discontinuous variations are ones you either are or are not. For example a dwarf will never grow tall by eating more. Environmental factors can play a part however. Try and collect a few.

Remember, how we appear in any one characteristic is known as our phenotype.

GREGOR MENDEL

Gregor Mendel was a member of the Augustinian monastery in Brunn, Austria then but Czechoslovakia now. He became very interested in the problem of heredity: why are parents and offspring so similar and why do they so often differ in characteristics.

Mendel’s curiosity led him to carry out breeding experiments on plants. He chose the garden pea as the subject for his research. The garden pea has many discontinuous variations such as being tall or short, having wrinkled peas or smooth peas…It also grows and reproduces itself quickly and, being a flowering plant, the pollen source and target can be controlled through isolation.

Find out more by following these links

http://library.advanced.org/16985/history.htm

http://library.advanced.org/28599/index.htm

Especially the games in the interactive section!

One of experiments can be summarised

1. Observe that pea plants are always either tall or short. They are never in between and choose this characteristic to study.
2. Take a tall plant and self-pollinate it. Allow the seeds to germinate and grow. Select the plants that are tall and self pollinate them. Plant these seeds and do the same again until you only get tall plants. You now have a true breeding tall pea plant.
3. Do the same as 2 for a short plant.
4. Now cross the pure breeding tall pant with the pure breeding short plant.
5. The resulting seeds were sown. They were known as the first filial generation or F1 generation.
6. Now this was where Mendal showed his scientific strength. He then self pollinated the plants from the F1 generation and planted them. This was the F2 generation.

These were Mendal’s results

So the F2 generation showed that dwarfness could skip a generation.

Other F2 products were noted

Mendal noted that one character always dominated in the F1 generation and that in the F1 generation this character was always in a ratio near to 3:1 with the recessive character. Check this for yourself. Can you find any more of his results? Mendal called the characteristics expressed factors. These factors were the phenotype.

Mendal concluded:

• in the F1 generation parental factors are combined but only one expressed.
• in the F2 generation the swamped factor is expressed in the ratio

dominant factor : recessive character = 3:1

Find out how long it took for Gregor Mendel’s work to be recognised.

Can you find out the problems he had in convincing people of the validity of his research?

To what extent were these problems because of the historical context.

Maybe you could design a web page.

ANALYSING MENDEL’S RESULTS

Look at it this way;

Tallness is termed dominant and written T.

Dwarfness is termed recessive and written t as it is recessive to T.

Mendel said that each offspring received a characteristic from both its parents.

Þ a pure breeding dwarf plant would be tt and a pure breeding tall plant would be TT.

Þ all the F1 generation must be Tt having tallness from one parent and dwarfness from the other.

Þ The F2 generation could be TT, Tt or tt.

Arranging these in what is called a Punnet diagram you can see:

All the F1 offspring would be tall and have the factors Tt

Selfing the F1 generation:

Taking all the crosses above to be random, the probability would be that over a large sample, you would find I the F2 generation

¾ would be tall and ¼ would be dwarf

if Mendel’s swamping theory were true and his getting one factor from each parent idea was right.

Mendel’s results confirmed this.

MENDEL WAS RIGHT!

We now know that heredity depends on chromosomes in the nucleus of each cell.

The chromosomes are inherited from the parents. In sexual reproduction, the chromosomes come in pairs and each half of the pair comes, one from one parent and one from the other.

Mendel’s factors are actually genes on the chromosomes.

Chromosomes are made up of millions of genes manufacturing the enzymes and proteins that control the form and functions of the cells that make up an organism.

We now know:

1. Genes that control a given characteristic come in pairs.
2. Often one set of genes dominates the effect of the others.
3. The paired genes are on corresponding chromosomes.
4. The paired genes occupy corresponding positions.

Different forms of a gene are known as alleles. Alleles are either dominant or recessive.

TT, Tt and tt are examples of genotype. The genotype is the combination of gene pairs whereas the character expressed is the phenotype.

If the genes of the genotype are the same we say that the genotype is homozygous.

If the alleles are both dominant then we describe the genotype as being dominant homozygous.

When the genotype is mixed eg, Tt, then we would call it heterozygous.

What we have been describing above is termed Monohybrid inheritance since we only look at on set of alleles.

Activities

I

Copy and confirm the following punnet diagram on sex determination 

The offspring then have a 50% chance of being either male or female.

II

Familiarise yourself with the genetic diseases

cystic fibrosis

and

sickle cell anaemia.

You will only be expected to analyse data showing the phenotype.

III

Copy and learn to draw the diagram on the principles of genetic engineering in Chapter 21.

IV

Research selective breeding.

Essay

Designer Babies