Genetics Sec 2 PDF

# Genetics ## In this chapter, you will learn: * about how individuals within the same species can vary * about the different types of variation * about Gregor Mendel, Hugo de Vries and their work on genes * that chromosomes are found in the nucleus of a cell * that special cells called gametes comb...

# Genetics ## In this chapter, you will learn: * about how individuals within the same species can vary * about the different types of variation * about Gregor Mendel, Hugo de Vries and their work on genes * that chromosomes are found in the nucleus of a cell * that special cells called gametes combine to produce a fertilized egg * about the *difference between XX and XY chromosomes* * how genes, made of DNA, are responsible for individual variation * about DNA * about ideas before the theory of natural selection * about the theory of natural selection. ## Do you remember? * What do you call animals with a backbone and animals without a backbone? * How can you tell a fish from a mammal? * How is an amphibian different from a bird? ## Variation in living things Many living things have certain features in common as well as features which are different. For example, a cat, a rabbit and a monkey all have ears and a tail. However, these features vary from one kind of animal to the next. * For example, in the *species* shown in Figure 3.1, the external parts of the rabbit's ears are longer than the ears of the cat. * The external parts of the monkey's ears are on the side of its head, while the other two animals have them on the top. * The cat and the monkey both have long tails, but this monkey's tail is different because it is *prehensile,* which means the monkey can wrap it around a branch for support while it hangs from a tree to collect fruit. * Only monkeys that come from Central and South America have prehensile tails. * The rabbit's tail is much shorter than both the cat's tail and the monkey's tail. ## Variation within a species The individuals within a species are not identical. Each one differs from all the others in many small ways. * For example, one person may be tall, have small fingers and ears with free lobes while another person may be small, have long fingers and ears with attached lobes. * Another person may have different combinations of these features. ## Science extra: Types of variation There are two kinds of variation that occur in a species. They are called *continuous variation* and *discontinuous variation:* * **Continuous variation:** * A feature that shows continuous variation may vary in only a small amount from one individual to the next, but when the variations of a number of individuals are compared, they form a wide range. * Examples include the range of values seen in heights or body masses of humans. * **Discontinuous variation:** * A feature that shows discontinuous variation shows a small number of distinct conditions, such as being male or female, and having ear lobes or not having ear lobes. * There is not a range of values between the two, as there is between a short person and a tall person, for example. * Another feature showing discontinuous variation is 'hitchhiker's thumb', which is when the thumb in the thumbs-up position bends backwards towards the wrist. ## Inherited characteristics The features of living things, including humans, are also called *characteristics*. When scientists grasped the idea that characteristics could be passed from one generation to the next, the study of *inheritance* began which lead to the discovery of *genes* and the study of genetics. ## Gregor Mendel, Hugo de Vries and genes Gregor Mendel (1822-84) was an Austrian monk who studied mathematics and natural history. He set up experiments to investigate how features in one generation of pea plants were passed on to the next. * A pea plant will pollinate itself - it does not need bees to bring pollen grains from other flowers, although it can receive them. * In most flowers of flowering plants, there is a structure that produces and releases pollen called the anther and there is a structure that receives pollen grains called a stigma. When a pollen grain lands on a stigma, *pollination* is said to have occurred and then the process of *fertilisation and seed production* follows. * Mendel wanted to control the way the flowers pollinated, so he cut off the anthers of one flower, collected pollen from another flower and brushed it on to the stigma of the first. He completed his task by tying a muslin bag around the first flower to prevent any other pollen from reaching it. * Mendel performed thousands of experiments and used his mathematical knowledge to set out his results and to look for patterns in the way that the plant features were inherited. He suggested that each feature was controlled by an inherited factor. He also suggested that each factor had two sets of instructions and that parents pass on one set of instructions each to their offspring. Many years later it was discovered that Mendel's 'factors' were genes. ## Challenge yourself Make a model of a cell with chromosomes as shown in Figure 3.4. Select your materials and, if your teacher approves, make your model. What are the model's strengths and weaknesses? You may need it later for a presentation. ## Did you know? Chickens have 78 chromosomes while elephants have 56, a starfish has 36, rice has 24 and kangaroos have 16! ## Chromosomes As living things are made from cells, scientists began looking at cells for evidence that they may be involved in reproduction. * All cells have a cell membrane, cytoplasm and a nucleus. * When scientists examined living cells under the microscope, they saw that changes took place in the nucleus that are linked to reproduction. * Just before a cell divides, long strands of material appear in the nucleus. These strands are called *chromosomes*. ## chromosomes and genes Chromosomes are like threads of chemical messages. These messages are strung along each chromosome like the carriages of a very long train. Each message is called a *gene*. The genes provide all the information for how the cell grows, develops and behaves and how the body grows and develops too. Each pair of chromosomes has pairs of genes that carry information for a particular characteristic, such as eye colour or hair colour. These genes are situated at the same point on each chromosome, as Figure 3.5 shows. ## Natural selection and genetic changes Darwin didn't know about chromosomes, genes and DNA. All that knowledge came much later, but it supports his theory. We can think of natural selection genes in the following way: * Natural selection and certain combination of genes. They help the body develop so that it can survive in its environment. * If the environment changes, the combination of genes will threaten its survival. If one of the genes produces a feature which is particularly suited to the change in the environment, it will help the organism survie This will increase the organism's chances of breeding and then passing on the beneficial gene. If this happens, more offspring are produced with this beneficial gene, and they survive too. * Some of the offspring may also have genes that are favourable to the change in the environment, and these are also passed on to future generations. In time, there may be organisms with such a different combination of genes to the original parent population that they forma new species. ## Let's talk Imagine there was an inland where there were populations of the two types of bird in the enquiry and only one type of food, such as seeds. Now imagine that predators such as hawks arrive on the island and they can catch and feed on both types of bird. 1. How does the presence of the predator affect the time that the birds have to feed? Explain your answer. 2. Will both types of birds be well-fed with the predator on the island? Explain your answer. 3. Will the presence of the predator on the island affect the populations of the two types of bird? Explain your answer. ## Did you know? A group of baby cats (kittens), from one mother, is called a litter. A group of baby pigs (piglets), from one mother, is also called a litter. In fact, we use the word 'litter' to refer to a group of babies for many mammals. ## Let's talk The collective noun for a group of birds is a flock. We also say a flock of sheep and a flock of goats. What other collective nouns for groups of animals do you know? Do you know any other animals that are found in flocks? Share ideas in a group. ## End of chapter questions 1. What does the word 'inherited' mean? 2. What is a characteristic? 3. How may a litter of kittens show variation? ## Let's talk The collective noun for a group of birds is a flock. We also say a flock of sheep and a flock of goats. What other collective nouns for groups of animals do you know? Do you know any other animals that are found in flocks? Share ideas in a group. 4. What does the word 'vary' mean? 5. What is the difference between a chromosome and gene? 6. Why is DNA important in explaining how living things inherit characteristics from their parents? 7. A flock of birds with beaks 6 cm long settle on an island. They use their beaks to search deep within the sandy shore for worms and molluscs to eat. These invertebrates live 5 cm below the surface of the sand. The birds breed and produce offspring with beak lengths from 4-9 cm long. * As the offspring grow up and begin to feed on the shore, what do you think will happen to them? Explain your answer. * In time, the worms and molluscs change their behaviour and burrow down to 8 cm to live. Many years later, a scientist is the first to visit the island since the original flock of birds arrived. She knew from a previous scientist's notes about the characteristics of the birds. Why may she be surprised at how she finds the birds now? Explain your answer. ## Summary * There is variation between individuals within a species. * The nucleus of a cell contains chromosomes. * The characteristics that living things possess are inherited. * The inheritance of sex is determined by the combination of the X and Y chromosomes - females are XX and males are XY. * Chromosomes separate in gamete formation and the gametes fuse to form a fertilised egg. * Genes are located in chromosomes and are made from DNA. * Knowledge about the role of genes in inherited characteristics and the structure of DNA involved a wide range of people, working over a long period of time. * Our current knowledge about genetics can be used to evaluate some early ideas about how characteristics are inherited. * Living things evolve due to natural selection. ## Did you know? A group of baby cats (kittens), from one mother, is called a litter. A group of baby pigs (piglets), from one mother, is also called a litter. In fact, we use the word 'litter' to refer to a group of babies for many mammals. ## Let's talk The collective noun for a group of birds is a flock. We also say a flock of sheep and a flock of goats. What other collective nouns for groups of animals do you know? Do you know any other animals that are found in flocks? Share ideas in a group.