B1,B2 & B3

B1 – You and your genes

B1.1 What are genes and how do they affect the way that organisms develop?

A GENE is a short section of DNA. Genes carry instructions that control how you develop and function – they are long molecules of a molecule called DNA. Each gene codes for a specific protein by specifying the order in which AMINO ACIDS must be joined together.

These proteins can be:              STRUCTURAL PROTEIN: Gives the body structure, rigidity and strength JJJJJJJJJ                                          E.g. Skin, Hair, Muscles etc

FUNCTIONAL PROTEIN: Enables the body to function E.g. Enzymes,       ffffff                                             Antibodies etc.

The differences between individuals of the same species are described as VARIATIONS.

Variations may be due to:

• GENOTYPE – The genetic makeup of an organism. The different characteristics that an individual inherits, E.g. whether you have dimples or not.

• PHENOTYPE – The observable characteristics the organism has. How the environment changes an individual, E.g. cutting the skin may cause a scar.

IDENTICAL TWINS have the same set of genotype however any differences between them is because of environment.

CONTINOUS VARIATION shows when some characteristics are controlled by several genes working together e.g. eye colours and height. For instance it was originally believed that eye colour was due to a single gene. It is now known that there are a number of genes coding for different pigments in the iris, mainly on chromosome 15 in humans. This means that there is enormous variation in eye colour.

B1.2 Why can people look like their parents, brothers and sisters but not identical to them?

A human has 23 PAIRS OF CHROMOSOMES

Parents pass on their genes to their offspring in their sex cells.

A pair of chromosomes carries the same genes in the same place, on each chromosome within the pair. However, there are different versions of a gene called ALLELES. These alleles may be the same (HOMOZYGOUS) on each pair of chromosome, or different (HETROZYGOUS) – For example to give blue eyes or brown eyes.

Sex cells only contain one chromosome from each pair. When an egg cell and sperm cell join together, the fertilised egg cell contains 23 pairs of chromosomes. One chromosome in each pair comes from the mother, the other from the father.

WHICH CHROMOSOME WE GET FROM EACH PAIR IS COMPLETELY RANDOM. THIS MEANS DIFFERENT CHILDREN IN THE SAME FAMILY WILL EACH GET A DIFFERENT COMBINATION. THIS IS WHY CHILDREN IN THE SAME FAMILY LOOK A LITTLE LIKE EACH OTHER AND A LITTLE LIKE EACH PARENT, BUT ARE NOT IDENTICAL TO THEM. THE CHILD WILL SHARE SIMILARITIES WITH ITS PARENTS DEPENDING ON WHICH CHARACTERISTICS HAVE COME FROM THE FATHER AND WHICH HAVE COME FROM THE MOTHER AND WHICH ONES ARE DOMINANT AND RECESSIVE.

An allele can be DOMINANT or RECESSIVE

1. An individual with one or both DOMINANT alleles (in a pair of alleles) will show the associated DOMINANT…