Multicellular Organisms Key Area 2.5 Variation and Inheritance Variation Learning Intentions: •To learn about variation Success criteria: • Define the term ‘continuous variation’ with examples • Define the term ‘discontinuous variation’ with examples • Define the term ‘allele’ Variation • Differences that occur between individual members of a species are called variation. • Variation can be seen in animals, for example shell colour in clams. • Variation also exists in plants species, for example petal colour or carrot length. Variation • During fertilisation, genes from each parent are passed onto their offspring. • This process creates an organism that is genetically different from all other members of the species. • Sexual reproduction contributes to variation within a species. Types of variation • There are two different types of variation: • Discrete or discontinuous variation – is where a characteristic fall into distinct groups. • An example is the ABO human blood group system. • Every human can be placed into one of the blood groups A, B, AB or O, not in between. Types of variation • The second type of variation is called continuous variation. • This is where a characteristic can have any value in a range, for example height in humans. • Humans can have a range in height from around 60cm to 250cm once fully grown. Variation task From the list below, categorise the characteristics into discrete or continuous variation. seed shape in peas (round or wrinkled) body mass in humans milk yield in cattle wing length in fruit flies (long or short) length of index finger in humans ear lobe type in humans (attached or unattached) eye colour in fruit flies (red or white) body length in trout tongue rolling ability in humans resting heart rate in humans Discrete versus continuous variation Discrete variation Continuous variation Discrete versus continuous variation Discrete variation Continuous variation seed shape in peas body mass in humans wing length in fruit flies milk yield in cattle ear lobe type in humans length of index finger in humans eye colour in fruit flies body length in trout tongue rolling ability in humans resting heart rate in humans Examples of discreet variation tongue rolling ability ear lobes attached or unattached presence or absence of a widow's peak Presentation of data • The data obtained from a survey of a characteristics that show discrete variation is normally presented as a bar graph. • Each distinct group is represented by a bar. • There is a space between each bar. Presentation of data • A graph of human height usually takes on the form of a histogram. • The entire range of the characteristic is divided into groups (subsets). • Most people have a height close to the centre of the range with fewer very short or very tall people. • There are no spaces between the bars. Insert graph drawing task Polygenic Inheritance • Most characteristics shown by plants and animals show polygenic inheritance. • This means they are controlled by many genes that act together. • Polygenic characteristics show continuous variation. • Examples of this include height, weight, skin colour and hand span in humans. Key Terms • Discreet variation – variation that is clear-cut and observable as categories. eg. blood groups in humans. • Discontinuous – alternative term for discrete variation, which is variation that is clear cut and observable as categories. • Continuous variation – when a characteristic can have any value in a range. eg. height or weight in humans. • Polygenic – inheritance determined by the interaction of several genes acting together. Polygenic inheritance shows continuous variation. Genes • A gene is a single piece of genetic information made from a short section of DNA. • A gene carries the information to code for a particular characteristic. • Genes exist in different forms, called alleles. • Alleles are different versions of the same characteristic. • Alleles are given symbols, usually letters of the alphabet. Key Words • Gene – a short section of DNA that codes for a particular characteristic. • Alleles – different forms of the same gene. Allele symbols Organism Gene Human Tonguerolling Fruit Fly Pea Plant Allele Symbol What the allele codes for in organism R Allows tongue to be rolled up from sides r Tongue cannot be rolled up Wing Length W Normal wing length w Short wings – cannot fly Seed Colour Y Yellow pea seeds y Green pea seeds Variation in pea plants • Pea plants have many characteristics that can vary between individual plants. • Dominant characteristics are shown above the recessive characteristics in the table. Variation Learning Intentions: •To learn about variation Success criteria: • Define the term ‘continuous variation’ with examples • Define the term ‘discontinuous variation’ with examples • Define the term ‘allele’ Genetics Learning Intentions: •To learn about genetics Success criteria: • Define the term phenotype with examples • Define the term genotype with examples • Define the terms homozygous and heterozygous. Starter Starter Polygenic Gregor Mendel • Genetics is the study of variation and inheritance and has its own language. • In the early 19th century and Austrian monk called Gregor Mendel studied inheritance in garden pea plants. • He noted that pea plants produced yellow or green seeds and started experiments to study how pea seed colour was inherited. Research • Your teacher will show you some film clips about Gregor Mendel. • You will be asked to complete a research task on his life and work. • This may be completed in class or as homework. Introduction to Mendel (24 mins) Twig: Mendel and Inheritance Phenotype and genotype • Phenotype describes the outward appearance of the organism and is described in words. • For example in humans hair colour can be blonde, brown, black or red. Remember – the phenotype is the physical appearance of the organism! • An organism’s genotype describes the set of genes it possesses. • This is represented as different letters that refer to the different alleles for the gene. • Dominant alleles are given a capital letter and recessive alleles are given a lower-case letter. • The same letter is used for the alleles of one characteristic. Dominant and recessive traits • Diploid organisms have two copies of every gene. • Alleles can be dominant or recessive. • Dominant alleles always show up in the appearance of an organisms, even if only one copy is present. • Recessive alleles only show up in the appearance of an organism if there are two copies present in the genotype. Homozygous and heterozygous • The words ‘homozygous’ and ‘heterozygous’ are used to describe the set of alleles an organism possesses for one gene – its genotype. • If the alleles are the same, the organism is homozygous. • If the alleles are different the organism is heterozygous. Homozygous and heterozygous • One gene controls seed shape in pea plants. • The allele for round seeds (R) is dominant to the allele for wrinkled seeds (r). • A plant with two round alleles (RR) is described as homozygous. • A plant with two wrinkled alleles (rr) is also described as homozygous. • A plant with one round and one wrinkled allele (Rr) is described as being heterozygous. Key Words • Dominant – form of a gene that always shows up in the phenotype of the organism. • Genotype – the alleles that an organism has for a particular characteristic, usually written as letters. • Heterozygous – describes a genotype in which the two alleles for the characteristic are different. • Homozygous – describes a genotype in which the two alleles for the characteristic are the same. • Phenotype – the physical appearance of an organism. • Polygenic – inheritance determined by the interaction of several genes acting together. • Recessive – allele of a gene that only shows in the phenotype if there are two copies of the allele in the genotype. Monohybrid cross • Geneticists can investigate patterns of inheritance in plants and animals by studying one characteristic at a time. • Crosses are set up between two true breeding organisms that differ in only one way. eg. smooth and wrinkled seeds in pea plants. • The offspring produced are observed and the different numbers of each phenotype are counted. • This information can be used to determine which alleles are dominant and which are recessive. Monohybrid Cross • A monohybrid cross is a genetic cross between two organisms that differ in one way only. eg. sooth versus wrinkled pea seeds Genetics Learning Intentions: •To learn about genetics Success criteria: • Define the term phenotype with examples • Define the term genotype with examples • Define the terms homozygous and heterozygous. Genetics Learning Intentions: •To learn about genetics crosses. Success criteria: • Define the use of fruit flies in genetics experiments. • Define family trees. Starter Starter Heterozygous Recessive Monohybrid Fruit flies • Geneticists use a variety of plant and animal species to study inheritance. • An example of an animal used for genetic crosses is the fruit fly – Drosophila melanogaster • This “model species” has been used by scientists for over 100 years • Fruit flies are usually found in warmer regions of the world and feed on decaying fruit • Why do you think fruit flies are used for genetic crosses? Twig: Fruit flies Why are fruit flies used for genetic crosses? • • • • Flies are small, cheap to feed and keep Males and females can be told apart easily Short life cycle, produce many offspring at once Range of characteristics that can easily be identified eg. red versus white eyes • Have only 8 chromosomes so limited number of genes Viewing Drosophila • Your teacher will show you some breeding tubes containing fruit flies • Flies can be anaesthetised so they can be viewed under a microscope • Try to identify the differences between males and females • Return the flies safely before they wake up! female – pointed abdomen male – rounded abdomen Human inheritance • Unlike pea plants, humans do not breed to suit geneticists. • They also produce too few offspring to allow reliable conclusions to be drawn about phenotypic ratios. • However, the laws of genetics still apply and particular traits can be traced through several generations of a family by constructing a family tree. • A family tree is a diagram that shows how members of human family are related to one another. Family trees • Males and females are given different symbols. eg. males may be represented by a square and females by a circle. • Symbols are then shaded or left unshaded depending on the phenotype of the individual. • Individuals that produce offspring together are joined with a horizontal line. • A vertical line joins offspring with their parents. • It is possible to work out the genotypes of certain individuals in a family tree by analysing those of their parents and siblings. Say hello……to the Reebops Making Reebops • In this activity you will make “Reebops” to help you understand how the inheritance of different alleles leads to variation in offspring • Reebops are imaginary animals, made from play dough, pom poms and pipe cleaners • They have 16 chromosomes (in 8 pairs) in their body cells • Follow the instructions to find out what happens when these organisms breed! Genotype decoder key Characteristic antennae Genotype / phenotype code AA = 2 antennae Aa = 2 antennae aa = no antennae BB = 3 body segments Bb = 3 body segments bb = 2 body segments tail TT = curly tail Tt = curly tail tt = straight tail nose NN = red nose Nn = red nose nn = yellow nose legs LL = green legs Ll = green legs ll = yellow legs sex XX = female XY = male eyes EE = 2 eyes Ee = 2 eyes ee = one eye humps HH = 1 hump Hh = 1 hump hh = 3 humps body segments A a B b E e H h N n T t L l X X A a B b E e H h N n T t L l X Y Method 1 : Making gametes 1) Find your envelope containing Mum Reebop’s chromosomes and your Dad Reebop’s chromosomes. 2) Open each envelope, take out the cards, keeping Mum’s (pink) and Dad’s (blue) separate. 3) Sort each set of chromosomes into pairs of the same length. Dad will have two unmatched chromosomes! 4) Now turn the cards over and randomly take one chromosome from each pair of Mum’s chromosomes and place in a pile called ‘female gamete’. 5) Randomly take one chromosome from each pair of Dad’s chromosomes and place in a pile called ‘male gamete’. Making the gametes like this is a model of inheritance – which halves the number of chromosomes in the gamete (haploid number), so that when gametes combine at fertilisation, the new individual has the correct number (diploid number). Method 2 : Fertilisation 6) Fertilise the female gamete with the male gamete by mixing together the female and male gamete piles. This is now your selection of ‘baby genes’ (or your zygote chromosome set). 7) Put the chromosomes you haven’t used back into their original envelopes. 8) Sort out the chromosomes of your new individual into pairs. You have now mixed a random selection of half the chromosomes from one parent with a random selection of half the chromosomes from the other parent to make a new combination. Each parent donated the haploid number of chromosomes (8 chromosomes) to make the diploid number (16 – now in 8 pairs again). Method 3 : Development – from genotype to phenotype 11) Write down the letters you have obtained in the ‘genotype and phenotype table’ for your ‘baby Reebop’. For example, if you have one card with the letter A and another card with the letter a, your genotype is Aa. 12) Use the ‘decoder key’ to decide what the characteristics (phenotype) of your baby Reebop will be based on your genotype description. For example, if is genotype is BB, it will have 3 body segments. 13) Collect all the materials you need for your baby Reebop and build it with the characteristics that its genes determine. 14) Put your baby in the Reebop nursery with the other newborns! Materials • • • • • • Green straws – Antennae Plasticine – Body segments Pipe cleaners – Tail Large pompoms – Nose Straws – Legs Small pompoms - Humps Discussion questions • What do you notice about the features of the babies? • Are there any babies that are identical? • How much genetic material does each parent provide? • Where is this genetic material in the parent? • Explain how sexual reproduction introduces genetic variation into offspring. Genetics Learning Intentions: •To learn about genetics crosses. Success criteria: • Define the use of fruit flies in genetics experiments. • Define family trees.