Inheritance - Genes and Inheritance (GCSE Biology)
Genes and Inheritance
Before we begin to describe genetic inheritance, we must understand some simple definitions.
- Gamete. A gamete is a sex cell. These cells contain half the normal number of chromosomes (23 in humans). This means that they are haploid cells.
- Chromosome. A chromosome is a structure in the nucleus of a cell that is made up of one condensed molecule of DNA.
- Gene. A gene is a section of DNA that codes for a certain protein or characteristic.
- Allele. An allele is a certain version of a gene.
- Dominant. When a dominant allele is present, it is always expressed in an organism.
- Recessive. Only the receive allele can be present if its characteristic is to be expressed.
- Codominant. Neither allele is recessive so characteristics of both alleles are expressed.
- Homozygous. Both alleles are the same, either dominant or recessive. Two identical homozygous individuals that breed together will be pure-breeding. This is because they can only pass down one allele (one characteristic).
- Heterozygous. One dominant and one recessive is present. A heterozygous individual will not be pure breeding as two different alleys can be passed down.
- Genotype. The genotype is the genetic sequence of an organism in terms of the alleles present.
- Phenotype. The phenotype is the physical characteristics expressed by an organism based on the environment and genotype. They are the observable features.
Genetic Inheritance
Inheritance is the transmission of genetic information from generation to generation.
The majority of phenotypic features (characteristics) require a few multiple genes in order to be expressed instead of a single gene inheritance. Some specific characteristics on the other hand however are controlled by only one gene. An example of this is the red-green colourblindness gene in humans and fur colour in mice. Red-green colour blindness is more complicated, due to other circumstances, but it is important to remember these two examples.
Example: Hair Colour
A good example to understand the terminology used above is hair colour. The allele for dark hair is dominant, whereas the allele for light hair is recessive.
If both alleles provided by the parents (the genotype) are dominant, the person will have dark hair (the phenotype). This is regarded to be homozygous dominant.
If both alleles are recessive, the person will have light hair. This is considered to be homozygous recessive.
If one dominant allele and one recessive allele is present, the person will have dark hair. This is considered to be heterozygous. This works as the dark hair is the dominant allele. As this allele is present, it must be expressed.
Inheritance refers to the passing of traits from parents to their offspring. These traits can include physical characteristics, such as eye color and height, as well as behavioral traits and diseases.
Genes are the units of heredity that carry information about a particular trait from one generation to the next. They are made up of DNA, which is a chemical code that contains instructions for making proteins in the body.
Dominant and recessive inheritance refer to how genes interact to determine a trait in an individual. A dominant gene is one that is always expressed, even if just one copy is present, while a recessive gene will only be expressed if two copies are present.
Alleles are different forms of the same gene. For example, one allele may code for blue eyes, while another allele may code for brown eyes. When an offspring inherits a gene from each parent, they have two alleles for that gene.
A gene mutation is a change in the DNA code that makes up a gene. This can result in a different trait being expressed, or a disease being caused. Some mutations are beneficial, while others are harmful.
Genetic variation refers to differences in the DNA code between individuals within a population. This variation can result from mutations, or from different combinations of alleles from each parent. Genetic variation is important because it allows populations to evolve and adapt to changing environments.
Polygenic inheritance refers to traits that are determined by multiple genes. For example, height is a polygenic trait that is determined by many different genes, each with a small effect on the final height of an individual.
Co-dominance refers to a situation where both alleles for a particular gene are expressed in an offspring, rather than one being dominant and the other being recessive. This results in a unique combination of traits being expressed.
Sex-linked inheritance refers to traits that are carried on the X or Y chromosome, which determine the sex of an individual. Because females have two X chromosomes and males have one X and one Y chromosome, certain traits are more commonly found in either males or females.
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