From Mendel to DNA

Learning Objectives1. What did Mendel’s experiments teach us

about inheritance?2. What are DNA fingerprints?

3. How are specific proteins made in the body?

Gregor Mendel

• Carried out breeding experiments with peas.

• Used pure strains of: round, wrinkled, green and yellow peas.

• Cross bred the peas and counted the different offspring.

• Found that characteristics were inherited in clear predictable patterns.

Mendel suggested that there were separate units of inherited material. He realised

some characteristics were dominant over others and that they never mixed together.

Inheritance - the story of life

Genes

A gene is a section of DNA coding for a particular feature.

e.g. eye colour, attached/unattached ear lobes, ability to roll tongue.

DNA is a chemical that stores your genetic material.

So what is DNA....

DNA

Structure of DNA

Sequence of bases forms the genetic code

G C

AT

CG

Each base is like one letter in a four-letter alphabet.

These letters make up 3 letter groups called codon triplets.

Each triplet is one piece of information.

Questions

1. How did Mendel’s experiments with peas convince him that there were distinct ‘units of inheritance’ which were not blended together in offspring?

2. Why didn’t people accept his ideas?3. The development of the microscope

played an important part in helping to convince people that Mendel was right. How?

4. Explain with reference to the structure of DNA the saying ‘One gene, one protein’.

DNA fingerprinting

‘DNA fingerprinting’ – a technique that uses the unique patterns in your

DNA to identify you.

DNA fingerprinting

• Certain areas of your DNA produces very variable patterns under the microscope.

• These patterns are more similar between people who are related than between total strangers.

• The patterns are known as DNA fingerprints.

• They can be produced from very tiny samples of DNA from body fluids such as blood, saliva, semen.

DNA fingerprinting – what are some of the

uses?

Questions

1. Two men claim to be the father of the same child.

Explain how DNA fingerprinting

could be used to find out which one is the real father.

Inheritance in Action

Learning Objectives1. How is sex determined in humans?2. Can you predict what features a

child might inherit?

We inherit characteristics from our parents....

Nucleus

chromosome

gene

DNA

Humans have 46 chromosomes – 23 from the mother and 23 from the father.

The complexity of an organism does not seem to be correlated with the

number of chromosomes it has.

Fruit fly – 8

Kangaroo – 12

Human – 46

Chicken – 78

Fern - 1200

In 22 cases each chromosome in the pair is a similar shape and has genes carrying information

about the same things. But one pair of chromosomes may be different – these are the sex

chromosomes.

Chromosomes from a female

Chromosomes from a male

• Sex chromosomes– Two X chromosomes

mean you are female– One X chromosome

and a Y chromosome mean you are male.

• Twins are born. Twin A is XY and twin B is XX. What sex are the two babies?

Pair 23 – the non-matching pair of chromosomes

Chromosomes

• The chromosomes we inherit carry our genetic information in the form of genes. A gene can be pictured as a position on a chromosome.

• Many of these genes have different forms.

• Alleles – are different versions of the same gene.– For example the gene for dimples may

have the dimple or no-dimple allele.

• An allele can be dominant or recessive.

• Individuals can be homozygous or heterozygous.– Individuals who are homozygous for a

certain gene carry two copies of the same allele.

– Individuals who are heterozygous for a certain gene carry two different alleles.

• A recessive characteristic will only be shown if an individual is homozygous for the recessive allele.

• A dominant characteristic will be shown even if an individual is heterozygous for the dominant allele.

Genetic diagrams are used to show possible outcomes of a particular cross. Dominant allele is shown by a capital letter, and a recessive allele by a lower case letter.

Cystic fibrosis

Huntington’s disease

Downs syndrome

Haemophilia

Malaria Measles

Meningitis

Tetanus

Inherited or not?