Genetic inheritance
A short video explaining gene inheritance and demonstrating how to use a Punnett square
Genetic key terms
- Deoxyribonucleic acid. The material inside the nucleus of cells, carrying the genetic information of a living being. is a natural polymer made of nucleotides. It forms two strands in a double helix. It carries the genetic code, which controls how an organism develops and functions. With the exception of identical twins, each person's DNA is unique.
- The structure made of DNA that codes for all the characteristics of an organism. are contained inside the cell's The nucleus controls what happens inside the cell. Chromosomes are structures found in the nucleus of most cells. The plural of nucleus is nuclei.. These are long, thin, threadlike structures made from molecules of DNA that store genetic information. Chromosomes are found in the nucleus of body cells 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 variants.
- A The basic unit of genetic material inherited from our parents. A gene is a section of DNA which controls part of a cell's chemistry - particularly protein production. is a region of DNA containing the instructions for a cell that give the order of amino acids in a protein. Some characteristics are controlled by a single gene, such as earwax texture in humans or pea shape in peas. Each gene might have different forms, and these are called Different forms of the same gene..
The diagram shows the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes
- A Sex cell (sperm in males and ova/eggs in females). is a sex cell. Male and female gametes fuse during fertilisation. In humans, the male gamete is the sperm and the female gamete is the ovum (egg cell). Both the egg and the sperm have 23 chromosomes, which combine to make a fertilised egg (A fertilised egg cell.) of 46 chromosomes.
- A different version of a gene. They are small changes in the DNA created by mutations. Most changes do not affect the way the gene is expressed. are different versions of the same gene. For example, the type of earwax you have is determined by a single gene. One variant causes wet earwax and the other variant causes dry earwax.
- The The alleles that an organism has for a particular characteristic, usually written as letters. is the collection of alleles that determine an organism's characteristics. When these interact with the environment they are expressed as a The visible characteristics of an organism which occur as a result of its genes..
- Different forms of the same gene. are the two copies of a gene on a pair of chromosomes. They may contain exactly the same or different information.
The diagram shows the chromosomes of three different people. Individuals A and B both have brown eyes, even though A is heterozygous and B is homozygous (dominant). Only people with a homozygous recessive genotype can have blue eyes as shown by individual C.
Alleles may be either dominant or recessive:
- A An allele that always expresses itself whether it is partnered by a recessive allele or by another like itself.allele is always expressed, even if only one copy is present. Dominant alleles are represented by a capital letter, for example, A. The allele for wet earwax is dominant. You only need one copy of this allele to have wet earwax. Two copies will also give you wet earwax.
- A Describes the variant of a gene for a particular characteristic which is masked or suppressed in the presence of the dominant variant. A recessive gene will remain dormant unless it is paired with another recessive gene.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 dry earwax is recessive. You need two copies of this allele to have dry earwax.
- This describes a genotype in which the two alleles for the characteristic are identical. alleles are both identical for the same characteristic, for example AA or aa. A homozygote is an individual who has identical alleles for a particular gene.
- This describes a genotype in which the two alleles for a particular characteristic are different. alleles are both different for the same characteristic, for example Aa. A heterozygote is an individual who has different alleles for a particular gene.
Most characteristics are a result of multiple genes interacting, rather than a single gene.