Edexcel Biology Gcse - Genetic Inheritance Part 2

A mutation rarely creates a new phenotype, but if the phenotype is changed as a result of a mutation and the new phenotype is suited to a particular environment, it can lead to a change in a species over time.

For example, if a mutation leads to a change in phenotype, such as feather colouring in birds, this change may allow those individuals to reproduce more frequently, due to them being more attractive and seen as a more desirable mate. This would result in the mutated gene being passed on more frequently than the original gene and would result in an increase in the proportion of birds with the new feather colour compared to the original feather colour. This is the basis of natural selection.

Natural selection describes how organisms that are better adapted to an environment are more likely to survive long enough to reproduce and pass on their genes. This process is called ‘survival of the fittest’ and is fundamental to the process of evolution.

Mutation occurs continuously and can be spontaneous. It can also happen because of:

ionising radiation

chemical mutagen, such as tar from cigarette smoke

inherited

The greater the dose of radiation a cell gets, the greater the chance of a mutation.

Ionising radiation includes gamma rays, X-rays and ultraviolet rays.

Mutations could cause different genes to be switched on or off, and this could create a different or faulty protein to be synthesised. For example, if the protein is an important enzyme, the specific substrate might not fit into the substrate binding site. If it is a structural protein such as collagen, it might lose its strength.

However, most DNA mutations do not significantly alter a protein, they only alter it slightly, or not at all, so its appearance or function is not changed.

Mutations can be positive and give an organism an advantage or negative and give a disadvantage but mutations that have a significant effect are rare. Some mutations may have a small effect but most mutations have no effect on the organism. They do not change to the organism's phenotype.

These mutations may change the activity of a protein if they occur within a gene. This might result in a change in phenotype or it might appear hidden, and be unnoticed. Alternatively, they might result in a serious consequence, such as genetic disease like cystic fibrosis.

In the mid-19th century Gregor Mendel (1822-1884) studied the inheritance of different characteristics in pea plants. He found that when he bred red-flowered plants with white-flowered plants, all the offspring produced red flowers. This went against the prediction that the colours would blend to produce pink flowers. If he bred these plants with each other, most of the offspring had red flowers, but some had white. This was because the allele for red flowers is dominant and the allele for white flowers is recessive.

One of Mendel's observations was that the inheritance of each characteristic is determined by 'units' that are passed on to descendants unchanged. We now know these as genes.

Mendel's work expanded the knowledge of genetic inheritance before DNA had even been discovered.

when he presented his work to other scientists he did not communicate it well so they did not really understand it

it was published in a scientific journal that was not well known so not many people read it

he could not explain the science behind why characteristics were inherited

The idea that genes were located on chromosomes emerged in the late 19th century when better microscopes and staining techniques allowed the visualisation and behaviour of chromosomes during cell division.

In the early 20th century, it was observed that chromosomes and Mendel's 'units' behaved in similar ways. This led to the theory that the 'units', now called genes, were located on chromosomes.

In the mid-20th century two scientists, James Watson and Francis Crick worked out the structure of DNA. By using data from other scientists Rosalind Franklin and Maurice Wilkins, they were able to build a model of DNA. They showed that bases occurred in pairs, and X-ray data showed that there were two strands coiled into a double helix. This model was used to work out how genes code for proteins.

Chromosomes are contained inside the cell's nucleus. These are long strands of DNA, which are made up of many genes.

A gene is a small section of DNA on a chromosome, that codes 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.

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.

DNA is made from base pairs which always come in the following combinations A-T, T-A, C-G and G-C. The order of these letters makes up an organism's genetic code.

An organism's genome is one copy of all of their DNA. With the exception of identical twins, no two people's genomes are the same.

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.

Chromosomes are found in the nucleus of a body cell in pairs.

chromosomes come in pairs.

one inherited from the mother.

one 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.

A dominant allele is always expressed, even if one copy is present. Dominant alleles are represented by a capital letter, for example you could use a B. The allele for brown eyes, B, 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, b. The allele for blue eyes, b, is recessive. You need two copies of this allele to have blue eyes.

Genetic crosses of single gene combinations (monohybrid inheritance) can be shown and examined using Punnett squares. These show the possible offspring combinations that could be produced, and the probability of these combinations can be calculated.

1) Determine the parental genotypes. You can use any letter you like but select one that has a clearly different lower case, for example: Aa, Bb, Dd.

2) Split the alleles for each parent and add them into your Punnett square around the edges.

3) Work out the new possible genetic combinations inside the Punnett square.

You may be asked to comment on the proportion of different allele combinations in the offspring, calculate a probability, ratio or just determine the phenotypes of the offspring.

Doctors can use a pedigree analysis chart to show how genetic disorders are inherited in a family. They can use this to work out the probability that someone in a family will inherit a condition. A pedigree analysis is usually undertaken if families are referred to a genetic counsellor following the birth of an affected child.

You can express the outcome of a genetic cross using probability (percentages), direct proportion or ratios. It is important to remember during the process of fertilisation, the allele combinations created are a random process, and that is why probability is used, as nothing is guaranteed. Each of the four possible offspring combinations is as likely to happen during every fertilisation event.

25%

50%