BIOLOGY · Stage 9 · Chapter 11: Human Genetics and Development
Biology 11.4 Variation between individuals — Chromosomes, DNA and Genes
5 min read 6 parts Lesson 4 of 9
Lesson summary
11.4 Variation between individuals — Chromosomes, DNA and Genes — Alhussan International Academy — Biology (Stages 7–9) notes: Describe the relationship between chromosomes, DNA and genes and explain how genes and the environment cause variation between individuals.Nucleus — it h…
- Human numbers
- Double helix
- Homozygous
- Heterozygous
- Dominant allele
- Recessive allele
Section 1 of 6Inside the nucleus
- Nucleus
- it holds the genetic material, organised into chromosomes.
- Chromosome
- a very long thread-like molecule of DNA wound around proteins.
- (deoxyribonucleic acid)
- a very long thread-like molecule of DNA wound around proteins.
- Gene = a section of a DNA molecule that carries the instructions for making one protein and so controls a characteristic.
- Characteristics include eye colour, blood group and the production of an enzyme.
- Human numbers
- humans have around 20 000 genes arranged on 46 chromosomes, and each chromosome carries many genes.
Section 2 of 6Structure of DNA
- Double helix
- two strands twisted together like a spiral staircase.
- Bases
- each strand has a backbone and a sequence of four bases, A, T, C and G.
- The bases pair in a fixed way, A with T and C with G.
- Code
- the order of the bases is a code, and every three bases code for one amino acid.
- Different orders produce different proteins, which is how a gene determines a characteristic.
Section 3 of 6Alleles
- Allele = a different version of the same gene.
- The gene for ear lobe shape has an allele for attached lobes and one for free lobes.
- Pairs
- chromosomes come in pairs, one from each parent, so each person has two copies of every gene.
- Homozygous
- two identical alleles of a gene.
- Heterozygous
- two different alleles of a gene.
- Dominant allele
- it shows its effect even when only one copy is present.
- Recessive allele
- it shows its effect only when both copies are recessive.
Section 4 of 6Genetic variation
- Random inheritance
- you inherited a random selection of genes from each parent, and so did any brothers or sisters.
- Siblings may share many genes and look similar, or share few and look different.
- The characteristics you inherit make you different from everyone else, unless you have an identical twin.
- Meiosis and fertilisation
- meiosis shuffles chromosomes so each gamete is different, and fertilisation joins gametes at random.
- Mutation
- a random change in the base sequence of a gene, which is another source of new alleles.
- Inherited variation
- some characteristics, such as eye colour and blood group, are determined by genes alone and nothing changes them.
- Characteristics such as being very tall or having small ears can be seen in several generations of a family.
Section 5 of 6Environmental variation
- Environment
- everyone has a different environment, which causes environmental variation.
- What you eat affects your body mass, and exercise levels affect muscles and strength.
- Genes and environment together
- they determine characteristics such as height, body mass and intelligence.
- Height
- it is partly set by genes and partly by environment.
- Your maximum height is set by your genes, but without the right balance of food while growing you cannot reach your full potential.
- Average heights have risen between 1896 and 1996 in every area as diet and standard of living improved.
| Type | Cause | Example |
|---|---|---|
| Genetic | Different combinations of alleles from parents; mutations | Blood group, eye colour |
| Environmental | Surroundings and lifestyle | Scars, tanned skin, body mass |
| Both | Genes interacting with environment | Height, skin colour |
Section 6 of 6Identical twins
- Formation
- an embryo splits in two as it grows, forming two babies who are genetically exactly the same.
- Shared features
- characteristics completely determined by genes, such as blood group, are the same.
- Differences
- they become less identical as they grow older, because diet, health and exercise differ.
- Differences between identical twins therefore show environmental variation.
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