After watching the following videos you should be to

1. Describe terms used in the study
2. Describe the variations in human beings.
3. Observe variations in flowering plants.
4. Distinguish between continuous and discontinuous variations.
5. Describe the factors that cause variations among plant and animals of the same species.

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Genetic key terms

• A gamete is a sex cell. In humans, gametes are sperm and eggs (ovums). DNA is a large and complex polymer, which is made up of two strands forming a double helix. DNA determines the characteristics of a living organism. With the exception of identical twins, each person’s DNA is unique.
• Chromosomes are contained inside the cell’s nucleus. These are long threads of DNA, which are made up of many genes.
• A gene is a small section of DNA on a chromosome, that code for a particular sequence of amino acids, to make a specific protein. It is the unit of heredity, and may be copied and passed on to the next generation.

Some characteristics are controlled by a single gene, such as fur in animals and red-green colour blindness in humans. Each gene might have different forms, and these are called alleles. The diagram shows the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes.

• Chromosomes are found in the nucleus of a body cell in pairs. One chromosome is inherited from the mother and one is inherited from the father. The chromosome in each pair carries the same gene in the same location. These genes could be the same, or different versions.
• Alleles are different versions of the same gene. For example, the gene for eye colour has an allele for blue eye colour and an allele for brown eye colour. For any gene, a person may have the same two alleles, known as homozygous or two different ones, known as heterozygous.
• The genotype is the collection of alleles that determine characteristics and can be expressed as a phenotype.

Alleles may be either dominant or recessive:

• A dominant allele is always expressed, even if one copy is present. Dominant alleles are represented by a capital letter, for example, A. The allele for brown eyes is dominant. You only need one copy of this allele to have brown eyes. Two copies will still give you brown eyes.
• A recessive allele is only expressed if the individual has two copies and does not have the dominant allele of that gene. Recessive alleles are represented by a lower case letter, for example, a. The allele for blue eyes is recessive. You need two copies of this allele to have blue eyes.
• Homozygous alleles are both identical for the same characteristic, for example AA or aa.
• Heterozygous alleles are both different for the same characteristic, for example Aa.

Most characteristics are a result of multiple genes interacting, rather than a single gene.

Variation

Individuals in a population are usually similar to each other, but not identical. Some of the variation within a species is genetic, some is environmental – the conditions in which they have developed and some is a combination of both.

Genetic causes of variation

Children generally look a little like their mother and their father, but are not identical to either. They inherit their features from each parent’s DNA.

Every sperm and egg cell contains half of the genetic information needed for an individual. Each sex cell is known as haploid, which has half the normal number of chromosomes. When the chromosomes fuse during fertilisation, a new cell is formed, which is known as a zygote. It has all the genetic information needed for an individual, which is known as diploid and has the full number of chromosomes.

Examples of genetic variation in humans include blood group, skin colour and natural eye colour.

Whether you have lobed or lobeless ears is due to genetic causes.

Biological sex is also an inherited variation – whether you are male or female is a result of genes you inherited from your parents.

Environmental causes of variation

Characteristics of animal and plant species can be affected by factors such as climate, diet, accidents, culture and lifestyle. For example, if you eat too much you will become heavier, and if you eat too little you will become lighter. A plant in the shade of a big tree will grow taller to reach more light.

Other examples of features that show environmental variation include:

• scars
• language and accent
• flower colour in hydrangeas as these plants produce blue flowers in acidic soil and pink flowers in alkaline soil

Genetic and environmental causes together

Some features vary because of a combination of genetic and environmental causes. For example, tall parents will pass genes to their children for height. Their children have the genetic potential to also be tall. However, if their diet is poor then they will not grow very well: their environment also has an impact on their height.