Module Two 12.8 Which human characteristics show a simple pattern of inheritance?

Cell division

Chromosomes are made from DNA. Genes are short sections of DNA. Genetically identical cells are produced by a type of cell division called mitosis. In sexual reproduction, a male gamete fuses with a female gamete to produce a new cell. This is called fertilisation. Gametes are produced by a type of cell division called meiosis. They contain a single set of chromosomes, whereas body cells contain two sets of chromosomes.

DNA, genes and chromosomes

DNA

DNA (deoxyribose nucleic acid) molecules are large and complex. They carry the genetic code that determines the characteristics of a living thing.

Except for identical twins, each person's DNA is unique. This is why people can be identified using DNA fingerprinting. DNA can be cut up and separated, forming a sort of "bar code" that is different from one person to the next.

Genes

A gene is a short section of DNA. Each one codes for a specific protein by specifying the order in which amino acids must be joined together.

Chromosomes

The cell's nucleus contains chromosomes. These are made from long DNA molecules.

Mitosis

Mitosis is the type of cell division that leads to growth or repair. When a cell divides by mitosis:

• two new cells form; and
• each cell is identical to the other one, and also to the cell they were formed from.

Gametes and fertilisation

Gender
Human body cells have 23 pairs of chromosomes in the nucleus. One of these pairs controls the inheritance of gender (whether offspring are male or female):

• In males, the two sex chromosomes are different. They are XY.
• In females, the two sex chromosomes are the same. They are XX.
  

Gametes
Gametes are sex cells. The male gametes are the sperm, and the female gametes are the eggs.

Gametes contain one set of genetic information, but body cells contain two sets of genetic information.

Fertilisation
Fertilisation is the joining or fusion of a male gamete and a female gamete. The new cell that is formed divides over and over again by mitosis. This creates the many cells that eventually form a new individual.

Meiosis

Gametes are formed from cells in the reproductive organs by a type of cell division called meiosis.

	male	female
reproductive organ	testes	ovaries
gametes	sperm	eggs

The cells formed by meiosis have half the number of chromosomes as the cell that formed them. Human body cells contain 23 pairs of chromosomes, and human gametes contain 23 single chromosomes.

The main features of meiosis are:

• the chromosomes are copied; then
• the cell divides twice, forming four gametes.
  

Genetics and DNA

DNA - An Introduction

BBC - Science and Nature (genes)

The Genetic Learning Centre - the Basics of DNA

The Gene Scene - What's the Big Idea?

GlaxoSmithKline - Kid's Genetics

Stem Cell Therapy

Therapeutic stem cell cloning

Therapeutic stem cell cloning uses two methods to try and produce healthy tissue which will not be rejected for people who have diseases ranging from diabetes to Alzheimer's.

The first step is to produce cloned cells from the affected person. The nucleus is removed from a normal body cell of the patient. It is transferred to a unfertilised human ovum which has had its original nucleus removed. After a mild electric shock the new pre-embryo cell starts to develop, producing a collection of embryonic cells with the same genetic information as the patient. This is a cloned human embryo, but it has been formed with no intention to clone a whole human being. The embryo is not allowed to grow beyond 14 days (after which the nervous system begins to develop). Scientists working with embryos have to be licensedby the Human Fertilisation and Embryology Authority.

The embryo produced in this way is simply a source of stem cells with DNA which matches the patient perfectly. The idea is that stem cells will be harvested from the embryo, hence destroying it. The embryonic stem cells will then be cultured and directed to form the needed tissue, whether that is new brain cells, heart muscle, insulin producing pancreas cells or whatever. These potentially healing cells will then be transferred to the patient, where they can do their job without any fear of rejection.

Just like the work on embryonic stem cells and adult stem cells, therapeutic stem cell cloning is still in the very early stages of development. There are many technical barriers still to be overcome – getting the cloned embryo to develop, successfully culturing the cells and finding ways to direct the differentiation of the stem cells into the required adult tissue. There are many ethical considerations too, from producing cloned embryos to using them purely as a source of stem cells. The technique of therapeutic stem cell cloning still has a long way to go before it is a widely used and accepted technique in hospitals everywhere – but its medical potential is enormous

Ethics - embryonic stem cells

Many people think the new work on embryonic stem cells is a major breakthrough with the same potential to change health care as the discovery of antibiotics more than seventy years ago. They feel that it is acceptable to work with the very early human foetus for the potential benefit to millions of people.

The vast majority of human foetuses never make it beyond the early stages – in both natural conception and in infertility treatment, far more embryos fail to develop than go on to form living babies, so using a small number of early embryos is acceptable in this context.

People who agree with stem cell research suggest that once tissue lines from a relatively small number of willingly donated embryos are established, the need to use further embryos would be reduced. The medical advances which are within our grasp would change medicine as it has been known for many years and give hope of a cure to millions of people for whom there is, at the moment, no hope.

What is more, many supporters of stem cell research feel that adult stem cells do not offer a good alternative as they are much more limited in their scope for forming new and different tissues. They want research funding to be directed mainly at embryonic stem cell work.

There are others who feel that the use of embryonic tissue is wrong and an abuse of human rights.

Some people feel that every early human embryo has the potential to become a living human being and so should be afforded the same human rights as a fully grown adult. Others have strong religious convictions which make them feel that using embryos is killing and therefore wrong. These people feel that no medical advances are worth the moral evil of using embryonic tissue as a source of stem cells. They also feel that the use of adult stem cells offers an exciting and acceptable possible alternative and campaign for research funding to be directed to projects using these ethically less sensitive cells.

Stem Cells – the law

 

Work on embryos in the UK is overseen by the Human Fertilisation and Embryology Authority (HFEA). Set up when IVF treatment was first developed, it is widely respected and its decisions are often used as a model in other countries. Guidelines drawn up in 1990 are still the basis of all work using embryos, which until very recently had largely been for research into infertility treatments.

In 2002 new laws were passed which made it legal to use spare human embryos from infertility treatments as a source of embryonic stem cells for research into possible therapeutic uses. This can only done under special licences granted by the Government. Therapeutic stem cell cloning is also subject licensing by the Government.

There is an absolute ban on implanting cloned human embryos into a woman – in other words the cloning of whole human beings is completely illegal.

Stem Cells at work

Inheritance

Alleles are different forms of a gene. They can be dominant or recessive. Genetic diagrams help us understand the possible outcomes of when parents produce offspring. Huntington's disease is a disorder of the nervous system, caused by a dominant allele. Cystic fibrosis is a disorder of the cell membranes, caused by a recessive allele.

Alleles

Some characteristics, such as eye colour and the shape of the earlobe, are controlled by a single gene. These genes may have different forms.

Different forms of the same gene are called alleles (pronounced al-eels). The gene for eye colour has an allele for blue eye colour and an allele for brown eye colour.

Alleles are dominant or recessive:

• the characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair;
• the characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair.

For example, the allele for brown eyes is dominant and the allele for blue eyes is recessive. An individual who inherits one or two alleles for brown eyes will have brown eyes. An individual will only have blue eyes if they inherit two copies of the allele for blue eyes.

Genetic diagrams

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. If he bred these plants with each other, most of the offspring had red flowers but some had white flowers. This worked because the allele for red flowers is dominant, and the allele for white flowers is recessive. Genetic diagrams help you to understand how this works.

In a genetic diagram, you show all the possible alleles for a particular characteristic. There will be two alleles from one parent, and two from the other parent, making four altogether. You then draw lines to show all the possible ways that these alleles could be paired in the offspring. There will be four possible ways, but some or all of them could be repeated.

In genetic diagrams, the dominant allele is shown as a capital letter and the recessive allele is shown as a lowercase letter.

The Story of Mendel - some Interactive Exercises

Huntington's disease

Huntington's disease is an inherited disorder that affects the nervous system. It is caused by a dominant allele. This means that it can be passed on by just one parent if they have the disorder.

You are expected to be able to draw genetic diagrams for any combination of dominant and recessive alleles.

Cystic fibrosis

Cystic fibrosis is an inherited disorder that affects the cell membranes, causing the production of thick and sticky mucus. It is caused by a recessive allele. This means that it must be inherited from both parents.

Notice that the offspring with a combination of alleles are labelled as "carriers". A carrier has one copy of the faulty allele but does not have the disorder themselves. If both parents are carriers themselves they may not know that they are carriers but there is a 1 in 4 chance of them producing a child who has cystic fibrosis. It is possible to screen embryos to see if they carry alleles for genetic disorders.

If one parent is a carrier and one parent does not carry the allele for cystic fibrosis. They cannot produce a child with the disorder, but they can produce children who are carriers themselves.

Genetic Diagrams

Genetic Pedigress - 'Family Trees'

Genetic Inheritance Diagrams - information on how to set the diagrams out