International A Level Biology / Development of organisms
Phenotype: genotype plus environment
Edexcel International A Level Biology (YBI11), chapter 9: Development of organisms, specification point 3.20. Free lesson.
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A phenotype is the observable characteristics of an organism, produced by an interaction between its genotype and its environment. Understanding both helps explain why organisms with the same alleles can still look different.
How genotype and environment shape phenotype
Your phenotype is what you can observe, such as height, colour or leaf shape. It results from an interaction between your genotype, the alleles you have, and your environment, such as diet, temperature or light. The genotype sets what is possible, while the environment affects how much of it is expressed. Use both genotype and environment when explaining a phenotype.
Why Himalayan rabbits have dark extremities
Every cell in a Himalayan rabbit has the same genotype, including an allele for an enzyme that makes melanin, the dark pigment. The enzyme works only in cool skin. This means temperature, rather than a difference in genotype, explains the rabbit's different fur colours. The ears, nose, feet and tail are cooler, so the enzyme works and melanin makes the fur dark. The body is warmer, so the enzyme is inactive. No melanin is made there, and the fur is white.
How the environment affects people and plants
The same interaction appears in human height and plant characteristics. Alleles set what an organism could develop, but environmental conditions can limit or change what is seen. For human height, alleles set the height a person could reach. A poor diet during childhood, with too little protein, can stop them reaching it. For hydrangeas, alleles provide a pigment in the petals. In acidic soil, the plant absorbs aluminium ions and its flowers are blue. In alkaline soil, they are pink. A white hydrangea has no pigment for acidic soil to turn blue. For chlorophyll, alleles code for its…
What identical twins show about genes and environment
Identical twins develop when one zygote splits into two embryos early in development. They have the same alleles, so differences between them must be environmental. Comparing twins raised together or apart can help show how strongly genes and environment affect a characteristic. If identical twins raised apart differ more than those raised together, the environment has a big effect on that characteristic. Non-identical twins share about half their alleles. If identical twins are much more alike, genes have a big effect. Even identical twins can differ in phenotype. Epigenetics…
How to explain differences when the genotype is the same
Plants grown from cuttings of the same plant are clones. They grow from one parent by mitosis, so they have the same genotype. If their heights differ, explain the difference using an environmental factor rather than claiming their alleles differ. Name what differed, such as the amount of light, water or mineral ions. Be specific about whether there was more or less of that factor.
Predicting fur colour when skin is cooled
If the back of a Himalayan rabbit is kept cool while its fur grows back, the melanin-making enzyme can work there. The genotype stays the same; only the temperature changes.