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5 Why not an exact 3:1 ratio?

Before we leave the maize breeding experiments we will look more closely at some actual values obtained for the F2 generation and how closely they fit the expected phenotypic ratio of 3:1. Table 2 gives the ratios of purple to white grains for eight cobs of the F2 generation, counted by members of the S103 Course Team.

Notice the variability of the results between cobs in Table 2. One of the most striking features of biological results is their variability. First,
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1.3.3 A breeding experiment: stage two

We now turn to the second stage of the breeding experiment, but this time we will follow the phenotypes and genotypes simultaneously. The purple (Gg) grains of the F1 generation are planted and when these have developed into mature F1 plants they produce male and female flowers. These F1 plants are crossed with each other, as shown in Figure 8. The fertilised ovules develop into grains borne on cobs, and these grains are the beginning of the second f
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1.3.2 A breeding experiment: stage one

In the first stage of the breeding experiment, shown in Figure 5, plants from the pure-breeding purple-grained variety are crossed with (fertilised by) plants from the pure-breeding white-grained variety. This can be done by artificially dabbing pollen grains from one plant onto the female flowers of another plant. These plants are the parental generation (abbreviated to P), and the cobs resulting from the cross are the first offspring generation or first filial (pronounc
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1.3.1 Inheritance of colour in maize

We can trace the inheritance of characters in animals and plants by following the phenotype from generation to generation, in breeding experiments. We will describe work with maize (Zea mays), alternatively called corn (sweetcorn, or corn on the cob), which occurs throughout the world as an extremely important commercial crop plant, and which is used extensively in genetic research. We can also study the inheritance of characters at the level of the genotype. In this section we will ju
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1.2 Like begets like

It is possible to follow a character, such as eye colour or hair colour in humans, that is handed down from generation to generation. Such characters are said to be inherited characters (or heritable characters) and are determined by genes. A gene can be considered as a unit of inheritance, which determines a particular character and which is passed on from parent to offspring.

Genes maintain the differences between species, such as oak and human, but they also contribute
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1 Unit introduction

Living organisms use the components of the world around themselves and convert these into their own living material. An acorn grows into an oak tree using only water, oxygen, carbon dioxide, some inorganic materials from the soil, and light energy. Similarly a human baby grows into an adult by digesting and metabolising food and drink. The parents in each case pass to their progeny, or offspring, the information and specification for building cells from materials around them. This information
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Learning outcomes

After studying this unit you should know:

  • the number of chromosomes is characteristic of each species and can vary enormously between species

  • genetics is based on the concept of the gene as the unit of inheritance

  • that sexual reproduction always includes two distinctive processes: the production of gametes, which involves meiosis, and fertilisation. The two processes are accompanied by changes in the chromosome number, from diploid to haploid and fr
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Introduction

This unit looks at how units if inheritance are transmitted from one generation to the next. First you will look at what happens to the chromosomes of animals and plants during the process of sexual reproduction. Then you will examine how genes are transmitted in particular patterns from generation to generation. These two approaches combine to illustrate how the patterns of inheritance can be explained by the behaviour of chromosomes during sexual reproduction.

This unit is from our ar
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Acknowledgements

The content acknowledged below is Proprietary (see terms and conditions). This content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Licence

Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit:

The content acknowledged be
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7 Summary

This unit has introduced the subject of diabetes and how it is diagnosed. It has discussed the structures and processes in the body that are important for controlling blood glucose levels and described what goes wrong when diabetes develops. The differences between Type 1 and Type 2 diabetes and other forms of diabetes have been highlighted. The concepts of genetics, risk and risk management have been introduced.

Now try to answer the following questions to test your knowledge.


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6.3 Risk

Risk is a difficult concept. Most of what we do in life involves making choices and taking risks. Sometimes the risks are small, and sometimes they are large. It can be difficult sometimes to know what the risk of doing something is. Past experience can also influence the way we think about risk. If one was knocked over by a car crossing the road, then even though the risk of it happening again is small we may remain worried and concerned about crossing the road.

How you explain risk is
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6.2 Genes

You may have heard of the term ‘gene’ or ‘genetic’. This term refers to the way the body inherits information from each parent. When we are born we receive half our genetic information from one parent and the other half from the other parent. Unless they are identical twins, children from the same parents will have slightly different genetic information giving each of them their own unique character. The important genetic information is packaged in chromosomes. We usually have 46 chro
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5.3 Type 2 diabetes

Type 2 diabetes was previously called non insulin-dependent diabetes. People with Type 2 diabetes produce insulin but it may be in insufficient amounts and/or their cells are resistant to the action of insulin (Figure 7). Hyperglycaemic symptoms, such as thirst and passing large amounts of urine, may be absent. Ketoacidosis does
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5.2 Type 1 diabetes

Type 1 diabetes was previously called insulin-dependent diabetes. This is because in people with Type 1 diabetes their pancreas fails to produce insulin and they are dependent on taking insulin for their treatment. It would be useful to look back at Figure 5 to remind yourself of the actions of insulin. As we have already discuss
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3.8 Glucagon

Glucagon is another hormone produced by the pancreas.

Question: Can you recall which cells make glucagon?

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3.7 Insulin

Insulin is a hormone produced by the pancreas. It has many actions, but is particularly important in keeping the blood glucose level normal.

Question: How does
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3.5 Muscle

There are different sorts of muscle in the body and they have different functions. Skeletal muscles are the muscles that, for example, are used for movement in your arms and legs.

Skeletal muscles store glucose as glycogen (Figure 4) and are able to use glucose as a fuel. Insulin stimulates muscles to take up glucose, and w
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3.2 Pancreas

The pancreas is a structure (an organ) that lies towards the back of the abdomen, the part of the body between the chest and the pelvis (hips). The abdomen contains the stomach, liver, spleen, pancreas, intestines and other structures. The pancreas is near the liver and the spleen (Figure 1) and opens into the small intestine.
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3.1 Introduction

The main role of glucose within the body is as a fuel but it also contributes to the fabric (tissue) by attaching to proteins. In people without diabetes, the blood glucose levels are kept within very narrow limits. The body does not allow them to become too high or too low. Several parts of the body are involved in this process. Some are large, for example the liver, and some are very small, such as the cells within the pancreas. Cells are small building blocks of the body and cannot
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2 What is diabetes?

Diabetes mellitus is a condition in which the glucose level in the blood is higher than it should be. The word ‘diabetes’ comes from the Greek word for ‘siphon’. A siphon is a way of removing liquid, and diabetes is used to describe disorders that remove liquid from the body, resulting in excessive thirst and the production of large amounts of urine. There are two forms of diabetes, diabetes mellitus and diabetes insipidus, of which diabetes mellitus is the more common. The wor
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