Section 5 - Inheritance

1. Section 5 - Inheritance National 4 & 5 – Multicellular Organisms

2. Why are we so similar yet different? • We all belong to the same species • We are similar in many ways • But, we show a great deal of variation • - continuous (e.g. height, weight) • - discrete (e.g. eye colour, blood type) • These similarities and differences are mainly determined by our genes

3. Learning Outcomes • By the end of this section I will be able to • - identify how genes determine characteristics • - map out patterns of inheritance, including family trees • - identify phenotypes and genotypes using punnet squares • - define dominant and recessive characteristics, and identify homozygous and heterozygous individuals

4. Inherited Characteristics • Our characteristics are determined by genetic information • E.g. hair colour, eye colour, tongue-rolling • Each parent passes on 1 piece of information for a certain characteristic • The pieces of information from each parent may be the same or different • A family tree can show how characteristics pass on through several generations

5. Phenotype • An organisms appearance resulting from genetic information received from parents • E.g • - wing shape – wild-type, weak, strong • - flower colour – red, white, purple • - eye colour – green, blue, brown

6. phenotype = genotype + environmental effects Phenotype and genotype The overall appearance of an organism depends on two things: 1. its genes (inherited characteristics) 2. the effects of the environment in which it lives. All the observable characteristics of an organism are called its phenotype. The full set of genes of an organism is called its genotype. An organism’s phenotype therefore depends on its genotype plus environmental effects.

7. Genes & Alleles • Each cell has two sets of chromosomes • One set from each parent • Each chromosome is made up of units called genes • Each gene contains information for a particular characteristic • Each gene normally has at least 2 different forms • e.g. flower colour could be purple or white • These different forms are called alleles

8. Inheritance studies • Show the inheritance patterns of certain characteristics • Usually done with an easily-bred species • e.g pea plants • Specific characteristics chosen for study • e.g. flower colour • When only 1 characteristic is examined, it is said to be a monohybrid cross • Parents (P) are bred to produce the first filial generation of offspring (F1) • The F1 generation are then interbred to produce the second filial generation (F2) • Phenotypes are observed to show how characteristics are passed on

9. Dominant/Recessive • F1 generation often shows only one characteristic coming through • This characteristic is said to be dominant • E.g. purple flower • The other characteristic is often hidden • It is said to be recessive • e.g white flower • BUT, in the F2 both characteristics often appear • Why?

10. Homozygous alleles allele forbrown eyes allele forbrown eyes allele forblue eyes allele forblue eyes If the alleles for a characteristic are the same, the organism is said to be homozygousfor that characteristic. What colour eyes will these homozygous pairs of alleles produce?

11. Heterozygous alleles ? allele forbrown eyes allele forblue eyes If the alleles for a characteristic are different, the organism is said to be heterozygousfor that characteristic. What colour eyes will this heterozygous pair of alleles produce? The characteristic expressed by heterozygous alleles will depend on which allele is dominant and which allele is recessive.

12. What eye colour? allele forbrown eyes allele forblue eyes The allele for brown eyes is dominant over the allele for blue eyes. So, what colour will the eyes be of an individual who has both alleles for eye colour? The individual will have browneyes, because the allele for brown eyes masks the allele for blueeyes.

13. Inheritance terms

14. B is the gene for brown eyes b is the gene for blue eyes

15. B is the gene for brown eyes b is the gene for blue eyes Parents Body cell in father with a pair of genes for brown eyes Body cell in mother with a pair of genes for blue eyes BB bb

16. B is the gene for brown eyes b is the gene for blue eyes Parents Body cell in father with a pair of genes for brown eyes Body cell in mother with a pair of genes for blue eyes BB bb Gametes

17. B is the gene for brown eyes b is the gene for blue eyes Parents body cell in father with a pair of genes for brown eyes body cell in mother with a pair of genes for blue eyes BB bb Gametes each sperm has a gene for brown eyes B B

18. B is the gene for brown eyes b is the gene for blue eyes Parents body cell in father with a pair of genes for brown eyes body cell in mother with a pair of genes for blue eyes BB bb Gametes each sperm has a gene for brown eyes each egg has a gene for blue eyes b b B B

19. B is the gene for brown eyes b is the gene for blue eyes Parents body cell in father with a pair of genes for brown eyes body cell in mother with a pair of genes for blue eyes BB bb Gametes each sperm has a gene for brown eyes each egg has a gene for blue eyes b b B B At fertilization There are 4 possible combinations of sperm and egg

20. B is the gene for brown eyes b is the gene for blue eyes Parents body cell in father with a pair of genes for brown eyes body cell in mother with a pair of genes for blue eyes BB bb Gametes each sperm has a gene for brown eyes each egg has a gene for blue eyes b b B B At fertilization There are 4 possible combinations of sperm and egg B B

21. B is the gene for brown eyes b is the gene for blue eyes Parents body cell in father with a pair of genes for brown eyes body cell in mother with a pair of genes for blue eyes BB bb Gametes each sperm has a gene for brown eyes each egg has a gene for blue eyes b b B B At fertilization There are 4 possible combinations of sperm and egg b b B B All the children of this F1 generation have genotype Bb and phenotype brown eyes

22. Parents (F1) father with brown eyes mother with brown eyes Gametes At fertilization

23. Parents (F1) father with brown eyes mother with brown eyes Bb Bb Gametes At fertilization

24. Parents (F1) father with brown eyes mother with brown eyes Bb Bb Gametes B b B b At fertilization

25. Parents (F1) father with brown eyes mother with brown eyes Bb Bb Gametes B b B b B b At fertilization B b

26. Parents (F1) father with brown eyes mother with brown eyes Bb Bb Gametes B b B b B b At fertilization B b

27. Parents (F1) father with brown eyes mother with brown eyes Bb Bb Gametes B b B b B b At fertilization B b A child who inherits the genes BB will have brown eyes A child who inherits the genes Bb will have brown eyes A child who inherits the genes bb will have blue eyes

28. Monohybrid Cross • In this example purple is dominant to white • P = purple • p = white • Let’s assume that both parents are homozygous (true breeding) • One is PP (purple), the other pp (white) • The F1 offspring would gain a purple allele (P) and a white allele (p) • - therefore only purple flowers in the F1generation (Pp) • When the F1 interbreed they could put forward either a purple (P) allele • - or a white (p) allele • A punnet square allows us to map out the possible combinations in the F2 • We discover that the numbers produced are 3 purple:1white • This is called the phenotypic ratio

29. Green flower (G) is dominant • Yellow (g) is recessive • All the F1 generation have the Gg genotype • They are all therefore green • Then the F1 plants are crossed • The results of the F2 are: • 1GG:2Gg:1gg • - this is called the genotypic ratio • - 3 green:1 yellow • - this is the phenotypic ratio 1 GG = green 2 Gg = green 1 gg = yellow

30. Expected vs Observed ratio • With a monohybrid cross, a 3:1 ratio would always be expected in the F2 generation • However, there is often a difference between the expected and the observed results This is because fertilisation is a random process, involving an element of chance A punnet square only shows the likely outcomes, not what will actually occur In real life, if you toss a coin 20 times, you would expect 10 tails:10 heads – rarely occurs Using the previous example the 200 offspring from the F2 generation doesn’t exactly match a 3:1 ratio

31. Using a test-cross On occasion, an organisms genotype may be uncertain, but needs to be identified. In this example purple (P) is dominant to white (p) We have a flower that is purple, but don’t know if it is homozygous (PP) or heterozygous (Pp) To help identify it’s genotype, it is crossed with a white flower (a white flower can only have a pp genotype) The offspring produced should prove what the unknown genotype is A test-cross is used to prove an unknown genotype

32. What is incomplete dominance? For example, when a red snapdragon plant is crossed with a white snapdragon plant, all the offspring flowers are pink. white Sometimes two different alleles are neither fully dominant or recessive to each other. In heterozygous individuals, this creates a phenotype that is a mix of the other two. This is called incomplete dominance. - because both the red and white alleles are expressed.

33. What is co-dominance? The human blood group system is controlled by three alleles: A, B and o. A and B are dominant while o is recessive. In heterozygous individuals who have A and B alleles, both are fully expressed - creating an extra phenotype (AB) This is called co-dominance.

34. Co-dominance in humans

35. The life and work of Gregor Mendel

36. 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 discrete variation; they are either one form or another. • This means that their phenotypes are easily distinguishable.

37. Y chromosome X chromosome What are sex chromosomes? Humans cells contain one pair of sex chromosomes,which control gender. • Males have one X andone Y chromosome (XY). • Females have two X chromosomes (XX). Y chromosomes are small and contain 78 genesX chromosomes are larger and contain 900–1,200 genes. Because females can only produce X gametes, it is the sperm that determine the sex of the offspring.

38. Boy or girl?

39. Multiple-choice quiz

40. Anagrams

41. Glossary (1/4) • acquired – A characteristic of an organism that depends on environmental factors. • allele – One version of a gene, found at a specific location along a chromosome. • carrier – An individual with a recessive allele, whose effect is masked by a dominant allele. • characteristic – A specific feature of an organism, such as eye colour. • co-dominance – A situation where two alleles are equally dominant. • continuous – Variation represented by a continuous range of values and which can be measured.

42. Glossary (2/4) • discontinuous – Variation represented by discrete categories. • 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.

43. Glossary (3/4) • incomplete dominance – A situation where two alleles are both partially expressed, producing an intermediate phenotype. • inherited –A characteristic of an organism that depends on its genes. • monohybrid cross –A cross in which one pair of characteristics is studied. • 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.

44. Glossary (4/4) • test cross –A situation where an individual with an unknown genotype is bred with a homozygous recessive individual to reveal the unknown genotype. • variation – The difference between individuals within a population.