1. Variation and Inheritance National 5

2. Learning Outcomes Give examples of variation within species Describe how sexual reproduction maintains variation Explain the difference between discrete and continuous variation Explain the difference between single gene traits and polygenic traits and give examples of each Identify examples of dominant and recessive traits Explain what is meant by the term “phenotype” Give examples of different type phenotypes of a characteristic Explain what is meant by the term “genotype” Assign a genotype to an individual Explain what is meant by the terms “homozygous” and “heterozygous” Use Punnett squares to predict the inheritance of genes Understand the use of pedigree charts to investigate the inheritance of a characteristic Explain the importance of genetic counselling

3. Species Definition – Group of organisms that are able to – interbreed – produce fertile offspring Differences between organisms of the same species is variation.

4. Variation in A Species List the features which differ within these dog breeds.

5. Types of Variation Some variation is CONTINUOUS. can have any one of a large range of values. wide range from one extreme to the other e.g. height. This type of variation can be represented as a line graph or histogram

6. Types of Variation Some variation is discrete. can only have one of a fixed number of options there are distinct differences e.g. eye colour This type of variation can be represented as a bar graph Discrete variation

7. Continuous and Discrete Variation Fill in the table to indicate whether the variation is continuous or discrete. FeatureVariation Eye colour Weight Shoe size Height Skin colour Tongue rolling

8. Family resemblance Members of the same family often look similar. If the son and daughter each have children of their own one day, will they also look like their parents? Which parent do these children look more like? Humans, like all organisms, inherit characteristics from their parents. How are characteristics passed on? Why do members of the same family often look similar?

9. Genetic material When cells divide, it is essential that genes are copied into new cells. This ensures that new cell has a complete set of information. Genes are the basic unit of inheritance and are responsible for characteristics of an organism. Genes are located on chromosomes, each of which is made from a very long, tightly coiled molecule of DNA.

10. Different versions of genes Chromosomes in a homologous pair contain the same type of genes that code for the same characteristics, such as eye colour. Each different version of a gene is called an allele. Each chromosome in the pair, however, may have a different version of the gene. For example, the version of a gene on one chromosome may code for brown eyes, whereas the version of the gene on the other chromosome may code for blue eyes. allele for brown eyes allele for blue eyes

11. Genotype and Phenotype Definition – Genotype – combination of alleles organism has for particular characteristic - usually written as letters Phenotype – the physical appearance of these characteristics

12. Homozygous alleles If the alleles for a characteristic in a homologous pair are the same, the organism is said to be homozygous for that characteristic. What colour eyes will these homozygous pairs of alleles produce? allele for brown eyes allele for blue eyes

13. Heterozygous alleles The characteristic shown in the individual who has heterozygous alleles will depend on which allele is dominant and which allele is recessive. If the alleles for a characteristic in a homologous pair are different, the organism is said to be heterozygous for that characteristic. What colour eyes will this heterozygous pair of alleles produce? allele for brown eyes allele for blue eyes ?

14. Dominant or recessive? Dominant alleles are always expressed in a cell’s phenotype. Only one copy of the dominant allele needs to be inherited in order for it to be expressed. Dominant alleles (e.g. brown eyes) are represented by an upper case letter (e.g. ‘B’). The phenotype for a particular characteristic depends on which allele is dominant and which allele is recessive. Recessive alleles are only expressed in a cell’s phenotype if two copies of it are present. If only one copy is present, its effect is ‘masked’ by the dominant allele. Recessive alleles (e.g. blue eyes) are represented by a lower case letter (e.g. ‘b’).

15. What eye colour? The allele for brown eyes is dominant over the allele for blue eyes. The individual will have brown eyes, because the allele for brown eyes masks the allele for blue eyes. allele for brown eyes allele for blue eyes So, what colour will the eyes be of an individual who is heterozygous for eye colour?

16. Inheritance terms

17. Homozygous cross

18. Heterozygous cross

19. The life and work of Gregor Mendel

20. Mendel’s experiments Over seven years, Mendel experimented on more than 28,000 pea plants! Why were his experiments so successful? Pea plants grow quickly. Pea plants are available in pure-breeding (homozygous) strains. Many pea plant characteristics show discontinuous variation; they are either one form or another, with no intermediates. This means that their phenotypes are easily distinguishable.

21. Mendel’s early experiments

22. Monohybrid crosses There are two alleles controlling pea shape. This means there are three possible genotypes that the F2 generation of plants could inherit, leading to two possible phenotypes. SS ww Sw smooth wrinkly smooth Genotype heterozygous homozygous dominant homozygous recessive Phenotype The type of experiment that Mendel carried out, investigating just a single characteristic, is called a monohybrid cross. The likelihood of a trait being produced during a monohybrid cross can be mapped out using a Punnett Square.

23. What are Punnett Squares?

24. Mendel’s laws of inheritance After his research, Mendel proposed two laws of inheritance. Mendel’s first law: the law of segregation Mendel’s second law: the law of independent assortment Alternate versions of genes (alleles) cause variation in inherited characteristics. An organism inherits two alleles for each characteristic – one from each parent. Dominant alleles will always mask recessive alleles. The two alleles for each characteristic separate during gamete production. Genes for different characteristics are sorted independently during gamete production.

25. Finding the Genotype For some characteristics, the genotype of a homozygous recessive individual can be determined from their phenotype. A test cross can be used to determine whether an individual is homozygous or heterozygous for a dominant trait. But what about individuals that have brown fur? Is their genotype BB or Bb? For example, the allele for brown fur (B) in mice is dominant over the allele for white fur (w ). This means that all white mice must therefore have the genotype.

26. What is a test cross? During a test cross, an individual with an unknown genotype is crossed with a homozygous recessive individual. The phenotype of the offspring will reveal the unknown genotype. If all the offspring display the dominant phenotype, then the parent of unknown genotype must be homozygous for the characteristic. If half the offspring show the dominant phenotype, and half show the recessive phenotype, then the parent must be heterozygous for the characteristic.

27. Using test crosses to find genotype

28. Family Tree Example 1 Family trees can be used to show characteristics over generations This family tree shows inheritance of eye colour. Brown eyes is dominant to blue eyes.

29. Answers Phenotype of person 2 – blue eyed Phenotype of person 3 – brown eyed Genotype of person 1 -Bb Genotype of person 4 – Bb Person 7 likely to be homozygous dominant because all the offspring show dominant characteristic Genotype of person 8 – bb Geno type of person 9 not certain because they have dominant phenotypes but both parents must be heterozygous as they have blue eyed child

30. Family Tree Example 2

32. Glossary allele – One version of a gene, found at a specific location along a chromosome. dominant – An allele that is always expressed, even if the cell only contains one copy. gene – The unit of inheritance. genotype – The full set of genes of an organism. heterozygous – Having two different alleles of a specific gene. homologous chromosomes – A matched pair of chromosomes that carry genes for the same characteristics. homozygous – Having two identical alleles of a specific gene. monohybrid cross – A cross in which one pair of characteristics is studied.

33. Glossary phenotype – All the observable characteristics of an organism. recessive – An allele that is only expressed if two versions of it are present in a cell. variation – The difference between individuals within a population.

34. Anagrams

35. Multiple-choice quiz

36. TermDescription F1F1 The first generation of offspring of a particular cross F2F2 The “grandchildren” of a particular pair of individuals F This stands for filial – literal translation means “son of a daughter” Homozygous The organism has two identical versions of the same allele. This organism is said to be true-breeding Heterozygous The organism has two different versions of the allele. This organism is not true breeding Polygenic A characteristic showing continuous variation which is controlled by the alleles of more than one gene

37. TermDescription AlleleA form of gene occurring at the same position on each of a pair of homologous chromosomes Phenotype The appearance or features of an organism, usually the effect of the genotype plus the effect of the environment Genotype The genetic make up of an organism. Different combinations of alleles produce different genotypes Pure(true) breeding A group of individuals which is homozygous for the gene studied. Dominant A form of a characteristic which masks another form. Often used to describe an allele Recessive A form of a characteristic which is masked by another (dominant) form. Often used to describe an allele P Generation The parents in a particular cross