Expressing the genome

Objective: Explain how phenotype is the result of an interaction between genotype and the environment.

Getting started…

1. Name the term used to describe differences between individuals of the same species.

2. List three characteristics that all people share.

3. Define phenotype.4. Besides genetics, what other factors can

affect a person’s appearance and growth?5. Describe a trait in humans in which there

is a wide range of appearances.

Genes and Environment Variation describe differences between

individuals of the same species. The phenotype (physical appearance) is a

product of the genotype and the environment.

For some genes the environment has minimal effect (e.g. blood group), but for the majority the environment plays a significant role.

Skin colour…

1. How would you describe your skin type?2. What do you do to protect your skin? Why is

that important?3. What other characteristics do members of

your group have in common?4. What factors influence skin colour?5. How does your skin compare with that of

other family members?6. What geographical regions are your

ancestors from?

Skin colour…

Skin colour is due primarily to the presence of a pigment called melanin , which is controlled by at least six genes (polygenic trait). It is a natural sunscreen, providing protection from the sun’s UV rays.

Why do different nationalities have different skin colour?

What environmental factor contributes to these differences?

Skin colour…

The chart below describes how different skin types react to the sun.

Generally, the more sun-sensitive your skin is, the higher your risk of skin cancer.

Skin Type

Sun History

1 always burns, never tans; extremely sun-sensitive

2 burns easily, then tans a little; very sun-sensitive

3 sometimes burns, then tans slowly; sun-sensitive

4 burns a little, always tans5 rarely burns, tans well; fairly insensitive to

sun6 never burns, deeply colored; insensitive to

sun

Skin colour…

Skin colour…

Modern humans most likely evolved in the tropics, where they were exposed to high UV levels. But as they moved into regions away from the equator, where UV levels are lower, humans became fairer so as to allow enough UV radiation to penetrate their skin and produce vitamin D, the "sunshine vitamin," (also obtained from eating fish and marine mammals.)

Vitamin D is essential for maintaining healthy blood levels of calcium and phosphorous, and thus promoting bone growth.

Skin colour is essentially a balancing act between the evolutionary demands of photo-protection and the need to create vitamin D in the skin.

Skin colour…

1. How would you describe your skin type?2. What do you do to protect your skin? Why is

that important?3. What other characteristics do members of

your group have in common?4. What factors influence skin colour?5. How does your skin compare with that of

other family members?6. What geographical regions are your

ancestors from?

Now, refer back to these questions – are your answer still the same? Which ones do you need to modify?

Variation…

In any population there are two types of variation – discontinuous and continuous.

What do each of these terms mean? Is skin colour an example of

discontinuous or continuous variation? Explain your answer.

Discontinuous and continuous variation Read the information on ps 190 -191. Answer questions 1 and 2 on p 191.

Twin studies… Read the information in the HSW box on

page 188. Answer questions 2 and 3 on page 189.

Animal fur…

Some animals have fur colour that is a product of the environment e.g. Siamese cats and Californian rabbits should have black fur all over as their genotype codes for the enzyme tyrosinase that converts tyrosine into melanin.

However, the enzyme is denatured by body heat, so only the cold parts of the animal are black (tail, ears etc.) and the rest is white.

Examples…

Read p186 on Siamese cats and answer question 1 a and b on p189.

Read p187 on the lac operon on p187 and answer question 3 on p187.