Inheritance
Inheritance is the transmission of genetic information from generation to generation.
Chromosomes and Genes
| Term | Definition |
|---|---|
| Chromosome | A thread-like structure of DNA that carries genetic information in the form of genes |
| Gene | A length of DNA that codes for a protein |
| Allele | A version of a gene |
| Haploid | When a nucleus contains a single set of unpaired chromosomes (gametes) |
| Diploid | When a nucleus contains 2 sets of paired chromosomes (body cells) |
Chromosomes
These bundles of DNA are found in the nucleus.
Human Cells
The chromosomes in all diploid cells are arranged in pairs. However, different species will have a different number of chromosomes.
- Human diploid cells have 23 pairs of chromosomes (46 in total)
- Human gamets are haploid and only have 23 unpaired chromosomes
Cell Division
Mitosis
This is a type of nuclear division that produces genetically identical cells.
- The duplication of chromosomes occurs before mitosis
- Mitosis plays a vital role in:
- growth
- repair of damaged tissue
- replacement of cells
- asexual reproduction
Meiosis
This type of nuclear division is also called reduction division because the chromosome number is halved from diploid to haploid, resulting in genetically different cells.
- Meiosis is involved in the production gametes such as:
- sperm (male)
- egg (female)
Monohybrid Inheritance
| Term | Definition |
|---|---|
| Genotype | The genetic structure of an organmism in terms of alleles present |
| Phenotype | The observable features of an organism |
| Homozygous | When the genotype has identical alleles of a particular gene |
| Heterozygous | When the genotype has different alleles of a particular gene |
| Dominant Allele | An allele that is expressed if it is present |
| Recessive Allele | An allele that is only expressed if a dominant allele is not present |
Punnett Squares
Punnet squares are used to show the possible genotypes of a single offspring after sexual reproduction.
- Dominant alleles are usually represented with an uppercase letter
- Recessive alleles are usually represented with a lowercase letter
There are a maximum of 3 possible genotypes in a simple Punnet square (ignoring co-dominance).
- Homozygous Dominant
- Homozygous Recessive
- Heterozygous
Only parents with the same pair of homozygous alleles can ensure pure-breeding.
Parents with heterozygous alleles will have the most variation.
Gender is controlled by a dominant gene in the Y chromosome and a recessive gene in the X chromosome. Only males have a Y chromosome.
Variation and Selection
| Term | Definition |
|---|---|
| Variation | Differences between individuals of the same species |
| Mutation | A change in a gene or a chromosome |
| Adaptation | The process by which populations become more suited to their environment over many generations due to natural selection |
Variation
There are 2 types of variation.
- Phenotypic
- Genes & Environment
- Continuous
- Genotypic
- Genes
- Discontinuous
The possible phenotypes resulting from variation can be grouped into 2 categories.
- Continuous
- When a range of phenotypes can be expressed
- Caused by both genes and the environment
- Continuous data like height in humans
- Discontinuous
- When only a limited number of phenotypes can be expressed
- Caused by genes alone
- Discrete data like blood group
Mutations
Ionising radiation and some chemicals can increase the rate of mutation. Mutations allow organisms that produce asexually to develop new traits. For example, antibiotic resistance is caused by mutations in bacteria.
Selection
Natural Selection
Natural selection takes place without human intervention.
- Lots of variation within populations
- Production of many offspring
- Increase in population increases competition for resources
- Many individuals struggle for survival
- Individuals that are better adapted to the environment reproduce
- Advantageous alleles are passed down to the next generation
The development of strains of antibiotic resistant bacteria is an example of evolution by natural selection.
- Antibiotic is given
- Mutated bacteria that are resistant to antibiotic survive
- Antibiotic kills non-resistant bacteria
- Resistant bacteria multiplies
- The genes for antibiotic resistance are passed down to the next generation
Artificial Selection
Traits and characteristics are selected by humans during artificial selection.
- Humans select individuals with desirable features
- These individuals get crossed to produce the next generation
- Offspring showing desirable features selected