Interactions Genes and the Environment

Development of Organisms

What determines an organism’s phenotype?

An organism’s phenotype results from the interaction between its genotype and the environment.

How can the environment affect phenotype?

Environmental conditions can affect gene expression, enzyme activity, growth and development, changing the characteristics an organism displays.

Why can genetically identical organisms have different phenotypes?

They may experience different environmental conditions, causing differences in gene expression, growth or development.

Give examples of environmental factors that can affect phenotype

Temperature, light, nutrient availability, diet, smoking and other environmental conditions.

How does temperature affect the coat colour of Siamese cats?

The enzyme involved in melanin production works at lower temperatures but is inactive at normal body temperature, so cooler extremities develop darker fur.

Why are the ears, paws, nose and tail of a Siamese cat darker?

These areas have a lower temperature, so the enzyme involved in melanin production remains active and produces dark pigment.

Why is most of a Siamese cat’s body pale?

The higher body temperature causes the enzyme involved in melanin production to become inactive, so less dark pigment is produced.

How does the Siamese cat demonstrate an interaction between genotype and environment?

The cat’s genotype determines its potential coat colour, while temperature affects the activity of the enzyme responsible for pigment production.

What may happen if a patch of fur is removed from a rabbit and the skin is cooled while the fur grows back?

The new fur may grow darker because the lower temperature allows the pigment-producing enzyme to remain active.

What is an operon?

A unit consisting of linked genes involved in regulating the production of proteins.

What is the lac operon?

A group of genes in E. coli that controls the production of enzymes required to metabolise lactose.

What is lactose?

A disaccharide sugar found in milk that can be broken down into glucose and galactose.

Which enzyme breaks down lactose in E. coli?

β-galactosidase.

Why does E. coli not continuously produce large amounts of β-galactosidase?

Producing enzymes when they are not needed would waste energy and resources.

What happens to the lac operon when lactose is absent?

An active repressor binds to the DNA and prevents RNA polymerase from transcribing the genes for lactose-digesting enzymes.

What is the role of the repressor in the lac operon when lactose is absent?

It binds to the DNA and blocks RNA polymerase, preventing transcription of the genes.

Why is β-galactosidase not produced when lactose is absent?

The repressor prevents RNA polymerase from transcribing the gene, so no mRNA is produced for β-galactosidase.

What happens to the lac operon when lactose is present?

Lactose binds to the repressor and changes its shape, preventing it from binding to the DNA.

What happens after lactose inactivates the repressor?

RNA polymerase can bind to the DNA and transcribe the genes needed to produce lactose-digesting enzymes.

How does lactose affect gene expression in E. coli?

Lactose acts as an inducer by binding to and inactivating the repressor, allowing genes for lactose metabolism to be expressed.

Why is the lac operon an example of environmental control of gene expression?

The presence or absence of lactose in the environment determines whether the genes for lactose-digesting enzymes are expressed.

What is the advantage of operon systems to bacteria?

They allow bacteria to rapidly alter their metabolism in response to changes in the environment and avoid wasting energy producing unnecessary proteins.

How can environmental conditions before birth affect human development?

Conditions in the uterus can affect fetal growth and development.

How can maternal malnutrition affect a fetus?

The fetus may not receive enough nutrients to reach its full growth and developmental potential.

How can smoking during pregnancy affect the fetus?

It can reduce fetal growth and increase the likelihood of premature birth.

Why is it difficult to separate genetic and environmental effects on human phenotype?

Humans have different genotypes and experience different environments, making it difficult to control all variables.

What is meant by the nature versus nurture debate?

It considers the relative effects of genetic inheritance and environmental factors on phenotype.

Why are twin studies useful for investigating genetic and environmental effects?

They allow scientists to compare individuals with similar or identical genetic material who may have experienced different environments.

Why are identical twins particularly useful in genetic studies?

Identical twins have essentially the same genetic material, so phenotypic differences between them can provide evidence of environmental effects.

Why are identical twins raised apart especially useful?

They have essentially the same genotype but experience different environments, helping scientists separate genetic and environmental influences.

How can identical twins raised together be used in studies?

They provide individuals with essentially the same genotype and a similar environment for comparison.

Why are non-identical twins useful as a comparison group?

They are the same age and usually experience similar environments but are genetically no more similar than ordinary siblings.

What does greater similarity between identical twins than non-identical twins suggest?

The characteristic has a strong genetic component.

What does a large difference between identical twins raised apart suggest?

Environmental factors have an important influence on the characteristic.

What do twin studies suggest about human height?

Height has a strong genetic component but is also affected by environmental factors.

What environmental factors can affect body mass?

Factors such as diet, lifestyle and family eating habits.

Why is IQ difficult to classify as entirely genetic or environmental?

Both genetic and environmental factors appear to contribute to variation in IQ.

What did the study of 5,000 pairs of twins suggest about variation in BMI and waist circumference?

About 77% of the variation was associated with genetic factors and about 23% with the home environment.

What are epidemiological studies?

Studies of large populations used to investigate associations between environmental factors and characteristics or disease.

Why are large sample sizes useful when studying environmental effects on humans?

They reduce the influence of chance and make the results more representative and reliable.

What did studies comparing babies of smokers and non-smokers show?

Babies of mothers who smoked during pregnancy were more likely to have a low birth weight and to be born prematurely.

What percentage of babies were low birth weight in the smoking group shown in the study?

12.4%.

What percentage of babies were low birth weight in the non-smoking group shown in the study?

6.4%.

What percentage of babies were premature in the smoking group shown in the study?

13.4%.

What percentage of babies were premature in the non-smoking group shown in the study?

11.3%.

What conclusion can be drawn from the smoking study?

Smoking during pregnancy has an environmental effect on fetal phenotype, including growth and development.

What are the two main types of variation?

Discontinuous variation and continuous variation.

Define discontinuous variation

Variation in which phenotypes fall into distinct categories with no intermediate values.

Give examples of discontinuous variation

Blood group and biological sex.

What usually determines characteristics showing discontinuous variation?

One gene or a small number of genes.

How much influence does the environment usually have on discontinuous variation?

The environment usually has little or no effect.

Why is ABO blood group an example of discontinuous variation?

Individuals fall into distinct blood groups A, B, AB or O with no intermediate categories.

Define continuous variation

Variation in which a characteristic can take a wide range of values with many intermediate phenotypes.

Give examples of continuous variation

Height, body mass and the number of leaves on a plant.

What usually causes continuous variation?

The combined effects of multiple genes and environmental factors.

Why are characteristics showing continuous variation often polygenic?

Several genes at different loci contribute to the phenotype.

What does polygenic mean?

A characteristic is controlled by several different genes.

Why does polygenic inheritance produce continuous variation?

Different combinations of alleles from several genes produce many possible phenotypes.

How does the environment increase continuous variation?

Different environmental conditions alter how the genetic potential of individuals is expressed.

Why does human height show continuous variation?

Height is controlled by multiple genes and is also influenced by environmental factors such as nutrition.

What genetic factors can influence height?

Genes affecting factors such as bone length, growth hormone production, growth rate and sex differences.

How can diet affect human height?

A balanced diet provides nutrients needed for growth, allowing an individual to get closer to their genetically determined height potential.

How can malnutrition affect height?

A lack of nutrients such as protein and calcium during important growth periods can prevent an individual from reaching their full genetic height potential.

Can good nutrition always make a person tall?

No. Genetic factors set limits on height, so someone with alleles associated with short stature will not become very tall simply through good nutrition.

Why is a large sample needed when studying continuous variation?

Continuous characteristics have a wide range of values, so a large sample is needed to represent the population accurately.

Why should samples used to investigate continuous variation be selected randomly?

Random sampling reduces sampling bias and makes the sample more representative of the population.

Why should organisms be sampled from different parts of their habitat?

Environmental conditions may differ between areas, so sampling widely produces a more representative result.

How can continuous variation be displayed graphically?

Using a frequency distribution graph or histogram.

What type of distribution is often produced by continuous variation?

A normal distribution.

What does a normal distribution look like?

A symmetrical bell-shaped distribution in which most values occur close to the mean and fewer occur at the extremes.

What is the mean?

The average value of a set of measurements.

What is standard deviation?

A measure of how spread out values are around the mean.

What proportion of values in a normal distribution lie within one standard deviation of the mean?

Approximately 68%.

What proportion of values in a normal distribution lie within two standard deviations of the mean?

Approximately 95%.

What does a small standard deviation indicate?

Most values are clustered closely around the mean.

What does a large standard deviation indicate?

The values are more widely spread around the mean.

What is the key difference between discontinuous and continuous variation?

Discontinuous variation produces distinct categories, whereas continuous variation produces a range of values with many intermediates.

How do genetic factors differ between discontinuous and continuous variation?

Discontinuous characteristics are usually controlled by one or a few genes, while continuous characteristics are usually polygenic.

How do environmental effects differ between discontinuous and continuous variation?

Environmental factors usually have little effect on discontinuous characteristics but often have a substantial effect on continuous characteristics.

Why do characteristics influenced by both genotype and environment often show continuous variation?

Many genes can produce numerous genetic combinations, and environmental factors further modify the phenotype, creating a wide range of values.