In this lesson we will learn how characteristics of living things are influenced by genetic and environmental factors.
By the end of this lesson you will be able to:
- Describe what traits are and explain how they are influenced by genetic and environmental factors.
- Describe what genes and alleles are and explain why individuals have two alleles for each gene.
- Distinguish between homozygous and heterozygous individuals.
- Distinguish between genotype and phenotype.
- The genetic information encoded in DNA influences physical, biochemical and behavioural characteristics of organisms.
- This genetic information is passed on to future generations during reproduction.
- The transfer of genetic factors from one generation to the next is known as heredity or biological inheritance and the study of heredity is called genetics.
- Heredity explains why you share more characteristics with your parents and siblings than you do with unrelated individuals.
(Image: Sponchia, Pixabay)
- A trait is a characteristic of an organism.
- Traits include physical attributes, such as height, hair colour and wing span, as well as non-physical attributes, such as temperament, sociability and intelligence.
- Traits can be divided into qualitative traits and quantitative traits.
- Qualitative traits exhibit discontinuous variation. That is, they fall into discrete categories.
- Examples of qualitative traits include blood type, eye colour and presence or absence of horns.
- Quantitative traits exhibit continuous variation. That is, they do not fall into discrete categories.
- Examples of quantitative traits include weight, skin colour and leaf size.
- Most traits are influenced by both genetic and environmental factors.
- Genetic factors refer to the biological information encoded in DNA.
- Environmental factors refer to non-genetic, external or internal influences, such as nutrition, exercise, education and social conditioning.
- Quantitative traits are more susceptible to environmental factors than qualitative traits.
- For example, a person’s weight is influenced by genetic factors, but diet and exercise make a significant contribution.
- Genetic components of traits are passed on from parents to offspring during reproduction, but environmental components are not passed on.
- For example, the natural hair colour of parents will contribute to the hair colour of offspring, not hair colour resulting from dying or bleaching.
(Image: skeeze, Pixabay)
- Genes are sequences of DNA that code for specific products.
- Since DNA is packaged into chromosomes in cell nuclei, an individual chromosome may contain hundreds or even thousands of genes.
- Genes usually code for proteins – a large group of molecules with a wide variety of function in living things.
- For example, proteins are the major structural material in your body (such as skin and muscle), they catalyse chemical reactions (as enzymes), they form part of your immune system (as antibodies) and they regulate many biological processes (as hormones).
- The synthesis of products by a cell, based on instructions encoded in DNA, is called gene expression.
- Traits are usually influenced by a combination of different genes, making the relationship between genes and traits very complex.
- This is especially so for quantitative traits, which can be influenced by many genes. Qualitative traits are usually influenced by either a single gene or a small number of genes.
- When first learning genetics, the relationship between gene and trait is often simplified to ‘one gene per trait’. For example, we might say “the gene for flower colour”, but keep in mind that this is just to simplify the study of heredity.
(Images: designua, Adobe Stock)
- An allele is a variant of a gene. In other words, different alleles (of the same gene) will code for different versions of that gene’s product.
- For example, a gene for flower colour may have two alleles – one that codes for red flowers and another that codes for white flowers.
(Images: designua, Adobe Stock; _aine_, Adobe Stock)
Homozygosity and Heterozygosity
- Individuals inherit two alleles for every gene – one allele comes from the father and one allele comes from the mother. (The only exceptions are genes on the X and Y chromosomes of males – we will learn more about these in the next lesson.)
- When an individual has two alleles that are the same, they are said to be homozygous for that gene.
- When an individual has two alleles that are different, they are said to be heterozygous for that gene.
- For example, the allele for red flower colour may be represented as ‘R’ and the allele for white flower colour may be represented as ‘r’. If an individual is ‘RR’ or ‘rr’, they are homozygous; if they are ‘Rr’, they are heterozygous.
Genotype and Phenotype
- The combination of alleles for a gene (eg RR, Rr or rr) is referred to as an individual’s genotype.
- Genotype, therefore, relates to genetic information stored in DNA.
- When describing an individual as homozygous or heterozygous, we are describing its genotype.
- The variant of a trait (eg red flower colour or white flower colour) is referred to as an individual’s phenotype.
- Phenotype, therefore, relates to an observable characteristic.
- When describing an individual’s appearance, we are describing its phenotype.
- As we will see in the next lesson, individuals with the same genotype will have the same phenotype, but not all individuals with the same phenotype will have the same genotype.
(Images: _aine_, Adobe Stock)
- Genetic information encoded in DNA influences physical, biochemical and behavioural characteristics of organisms.
- The transfer of this genetic information from parents to offspring is known as heredity or biological inheritance.
- The study of heredity is called genetics.
- A trait is a characteristic of an organism.
- Traits include physical and non-physical attributes.
- Quantitative traits exhibit continuous variation, whereas qualitative traits fall into discrete categories.
- Traits are influenced by both genetic factors and environmental factors.
- Genetic components of traits are passed on from parents to offspring, but environmental components are not passed on.
- Genes are sequences of DNA that code for specific products, usually proteins.
- The production of proteins by a cell, based on instructions encoded in DNA, is called gene expression.
- An allele is a variant of a gene that codes for a different version of that gene’s product.
- An individual that is homozygous for a particular gene has two alleles that are the same.
- An individual that is heterozygous for a particular gene has two alleles that are different.
- The combination of alleles for a gene is referred to as an individual’s genotype.
- The variant of a trait is referred to as an individual’s phenotype.
- Genotype relates to genetic information stored in DNA whereas phenotype relates to observable characteristics.
(Header image: Fahkamram, Adobe Stock)