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Kelso High School Biology Inheritance Name……………………..... Class……….. Keep this booklet safe – you will need it for revision. Inheritance Use this table as a checklist for your revision. For this topic you might also find it useful to look back over the ‘Reproduction’ topic notes. = core = challenge Understood and learnt Level Need to do more work Need to get help - see my teacher. You need to be able to…… State that a species is a group of interbreeding organisms whose offspring are fertile. State that variation can occur within a species. Give examples of continuous and discrete variation. Explain what is meant by continuous and discrete variation. State the sex cells are called gametes. State that each gamete carries one set of chromosomes. State that each body cell has two matching sets of chromosomes. State that the reduction of the number of chromosomes to a single set occurs during gamete formation. Describe how a double set of chromosomes is achieved at fertilisation. -1- State that genes are parts of chromosomes and there are two forms of each gene. State that certain characteristics are determined by genetic information received from the parents and give examples from plants and animals. Identify examples of phenotypes of the same characteristic. State the meaning of the word genotype Identify examples of homozygous, dominant and recessive characteristics from the numbers and phenotypes of given crosses. Identify generations as P, F1 and F2 from given examples of crosses. State that the phenotypes of the F1 in a homozygous cross are uniform. Explain monohybrid crosses in terms of genotypes Predict the phenotype ratios of the F2 offspring of a monohybrid cross. State that the sex of a child is determined by specific chromosomes called X and Y chromosomes. State that in humans, each male gamete may have an X or a Y chromosome while ach female gamete has an X chromosome. Explain how the sex of a child is determined with reference to X and Y chromosomes. Date of ‘Inheritance ’ End of Unit Assessment: …………………………………. Remember, in addition to testing your knowledge the assessment will include problem solving questions, eg graph drawing, calculations, etc. -2- Variation Species A species is a group of plants or animals that can breed together to produce __FERTILE OFFSPRING________. Factors Affecting Ecosystems Many things can affect ecosystems. The dogs above may all look different but they are the same species because they can interbreed. Variation The differences between members of the same species is called variation. There are two types of variation: ____________DISCRETE________________ ____________CONTINUOUS________________ Discrete Variation If the variation in a characteristic allows individuals to be divided into ___2___ or more ______DISTINCT__________ groups, the characteristic shows discrete variation. Examples include: -3- Continuous Variation If the variation in a characteristic can be measured (eg height in m) and it shows a wide _______RANGE______ occurring between two extremes (eg between the shortest and the tallest), then it is an example of continuous variation. Examples include: _HEIGHT__________ __PULSE RATE _____ SHELL DIAMETER__ _____SEED LENGTH__________ -4- Inheritance Before starting this section it would be a good idea to go over your ‘Reproduction’ notes again. Chromosomes and Genes The sex cells (gametes) contain only one set of chromosomes. When fertilisation takes place, the chromosome set of the sperm joins with the chromosome set of the egg. The nucleus of the fertilised egg (zygote) now contains two matching sets of chromosomes. The zygote divides to produce all the other cells of the body. This means that every cell has the same two sets of chromosomes. When the two sets of chromosomes are examined, they can be arranged in pairs – one of the pair will have come from the mother and the other from the father. (___set of chromosomes) (___set of chromosomes) (___sets of chromosomes) Each chromosome carries information on tiny units called genes. It is these genes that determine the characteristics of an organism. All characteristics are determined by a pair of genes. The genes for each characteristic exist in two forms. One form is usually dominant and the other is recessive. One example in people is tongue rolling. There are two tongue rolling genes. One gene allows you to roll your tongue, the other gene does not. Depending on which combination of these genes your cells contain, you either can or cannot roll your tongue If you get a tongue rolling gene from both your parents you will also be able to roll your tongue. If you get a non-tongue rolling gene from both your parents you will also be unable to roll your tongue. -5- If you get a tongue rolling gene from one parent and a non-tongue rolling gene from the other parent the two genes will be in competition. In this case the person will be a tongue roller because the gene for tongue rolling is more powerful than the gene for non-tongue rolling. Genes which are more powerful in a pair are said to be the dominant gene. Genes which are the least powerful in a pair are said to be the recessive gene. The Monohybrid Cross The two forms of a gene can be represented by letters. The dominant gene is always represented by a capital letter and the recessive gene by a small letter. Tongue rolling is dominant to non-tongue rolling. These genes can be represented as follows: T = tongue roller t = non-tongue roller Each characteristic of an organism is determined by two forms of the gene. The two forms of the gene present in an organism is known as its genotype. A tongue roller has the genotype TT or Tt A non-tongue roller has the genotype tt. If the two genes are the same (TT or tt) for a characteristic the genotype is said to be homozygous. If the two genes are different (Tt) for a characteristic the genotype is said to be heterozygous. -6- The simplest genetic cross involves one characteristic and is called a monohybrid cross. In this example colour in green coloured pea pods is dominant to yellow colour. G = Green pod g = Yellow pod X Parents phenotype Green pea pod Yellow pea pod Parents genotype GG gg G g Gametes Fertilisation F1 genotype Gg F1 Phenotype All green pod The next step involves crossing two individuals from the F1 generation: X F1 phenotype Green pea pod Green pea pod F1 genotype Gg Gg G or g G or g Gametes A punnet square is then used to show the possible ways in which these two sets of gametes could combine during fertilisation: Female gametes G g G F2 genotypes and phenotypes F2 phenotype ratio Male gametes GG Green pod Gg Green pod g Gg gg Green pod Yellow pod 3 Green pod : 1 Yellow pod -7- Sex Determination A person’s sex is determined by a pair of chromosomes, the sex chromosomes. They are called the X and Y chromosomes. A female has the genotype XX A male has the genotype XY The diagram below shows how sex is inherited: Parents phenotype Male Parents genotype XY XX X or Y X Gametes X Female Fertilisation F1 genotype F1 Phenotype F1 Phenotype Ratio XX XY Female 1 Male : -8- 1 Glossary You should know and be able to use all of the following terms: Variation Species Variation Offspring Fertile Continuous Variation Discrete Variation Term Definition Continuous variation A characteristic which varies from one extreme to another, eg from very small to very tall. Discrete variation A characteristic which allows individuals to be divided into two or more distinct groups, eg earlobes attached or earlobes unattached. fertile Able to produce offspring. offspring New individuals produced when parents reproduce. species A group of organisms that can interbreed to produce fertile offspring. variation Differences which occur between members of the same or of different species. -9- Inheritance Genes F1 Homozygous Dominant Chromosomes Heterozygous Genetic Information Monohybrid Cross Inherited Characteristics P Fertilisation Term Recessive Gametes Phenotype F2 Genotype Definition chromosomes Thread-like structures made up of genes and found in the nucleus of a cell. dominant The form of a gene which always shows itself in the phenotype; usually given a capital letter as its symbol. F1 The first generation of a cross (first filial generation) F2 The second generation of a cross (second filial generation) fertilisation Occurs when two different gametes fuse to form a zygote. gametes Sex cells, eg sperm, egg, pollen, ovules. gene Small parts of a chromosome which control the inherited characteristics. Genetic information Information which determines hair colour, tongue rolling and many other characteristics and is passed on from one generation to the next in the genes. genotype The set of genes an organism possesses; usually written as a formula, eg Tt homozygous An individual that possesses two identical forms of a gene, eg TT or tt. -1- heterozygous An individual that possesses two different forms of a gene, eg Tt Inherited characteristics Characteristics which are passed on from one generation to the next. monohybrid A cross between two parents which have only one difference, eg flower colour. P Parents phenotype The outward appearance of an organism Recessive The form of a gene which is masked by a dominant gene or is only expressed when the dominant gene is absent, eg tt. -2-