Other forms of Genetic Inheritance

1. Other forms of Genetic Inheritance

2. Genes • Genes are parts of chromosomes that carry information • Most organisms get half their genetic information from one parent and half from the other so different forms of the same gene are called alleles

3. Genotype and Phenotype • Genotype describes the alleles present for a characteristic • Genotypes are always described using letters e.g. Bb, bb, BB • Phenotypes describe the outward physical appearance of a genotype and are always described using words e.g. Brown or blue eyes

4. Alleles • Alleles can be dominant, recessive or co-dominant • Dominant alleles show up in the physical appearance of an organism • Recessive alleles do not show up in the physical appearance of an organism unless there are two of them present • Co-dominant alleles both show up in the physical appearance

5. Alleles Two alleles that are the same is known as a homozygous genotype • Further description can be added to see if it dominant or recessive • AA would be homozygous dominant • aa would be homozygous recessive

6. Allele Two alleles that are different are known as a heterozygous genotype • No further description is needed here because in a heterozygous genotype there must be two forms of the same gene e.g. Aa

7. True Breeding Organisms which pass on characteristics in a predictable way are said to be true breeding • True breeding organisms are homozygous

8. Word Test • Different forms of the same gene. • Describes the alleles present for a characteristic. • Describe the outward physical appearance of a genotype and are always described using words e.g. Brown or blue eyes. • Alleles show up in the physical appearance of an organism. • Alleles do not show up in the physical appearance of an organism unless there are two of them present. • Alleles both show up in the physical appearance. • Two alleles that are the same. • Two alleles that are different. • Organisms which pass on characteristics in a predictable way.

9. True Breeding Monohybrid Cross

10. True Breeding Monohybrid Cross

11. Predictions Vs Reality • With true breeding parents, the expected ratio in the F2 generation is normally 3:1 • In reality, the expected numbers differ from the actual numbers • This is because fertilisation is a random process

12. Pupil Activity – example question (a)In peas the height of the plant is controlled by one gene which has two alleles. T represents the dominant allele for tall stems. t represents the allele for short stems.   True breeding, tall-stemmed pea plants were crossed with short-stemmed pea plants to produce the F1 generation.   (i) State the genotypes of the parents. (ii) State the phenotype of the F1 plants.  Plants from the F1 generation were crossed to produce the F2 generation of plants.   (iii) State the phenotypes and their expected ratio in the F2 generation.

13. Cystic Fibrosis • Cystic Fibrosis is caused by a mutation to a gene on one of the autosomes. • Mutation • Changes the shape of the transmembrane chloride ion channels (CFTR protein) • The CFTR gene is found on Chromosome 7 • The faulty gene is recessive

14. Inheritance of cystic fibrosis • Three possible genotypes • FF unaffected • Ff unaffected • ff cystic fibrosis • Remember gametes can only contain one allele for the CFTR gene • At fertilisation, any gamete from the father can fertilise any gamete from the mother • This can be shown in a genetic diagram

15. Genetic diagram showing the chances of a heterozygous man and a heterozygous woman having a child with cystic fibrosis.

16. Phenotype ratio of offspring • Genotype ratio 1FF: 2Ff: 1ff • Phenotype ratio 3 unaffected:1cystic fibrosis • Can also be expressed as • 25% chance of the child having cystic fibrosis • Probability of 0.25 that a child will inherit the disease • Probability that 1 in 4 that a child from these parents will have this disease.

17. Mini Plenary 1. In tomato plants the allele for red fruit is dominant to the allele for yellow fruit. If a heterozygous tomato plant is crossed with a plant which produces yellow fruit, the expected phenotype ratio of the offspring would be A 3 red : 1 yellow B 1 red : 3 yellow C 1 red : 2 yellow D 1 red : 1 yellow

18. Mini Plenary 2. Achoo syndrome is a dominant characteristic in humans which causes the sufferer to sneeze in response to bright light. A woman who is homozygous for the syndrome and a man who is unaffected have children. What proportion of their children would be expected to have Achoo syndrome? A 0% B 25% C 50% D 100%

19. Mini Plenary 3. Which term refers to a description of a characteristic of an organism? A genotype B phenotype C allele D natural selection

20. Mini Plenary 4. Which term refers to forms of a gene controlling the same characteristic? A genotypes B phenotypes C alleles D dominant

21. Co-dominance Sure, why not Red Cow RR White Cow WW Roan Cow RW

22. Co-dominance • Two alleles of a gene can be co-dominant when neither is dominant nor recessive. • Both alleles can be expressed equally • E.g. red cows crossed with white will generate roan cows. Roan refers to cows that have red coats with white blotches. • When the F1 roan are self crossed, the F2 have a phenotypic ratio of : 1 red:2 roan:1 white. • This type of inheritance is called co-dominance.

23. Codominance • Codominance describes a pair of alleles, neither of which is dominant over the other. • This means both have an effect on the phenotype when present together in the genotype

24. Revision Question • Coat colour in Galloway cattle is controlled by a gene with two alleles. The CR allele produces red hairs and therefore a red coat colour. The Cw allele produces white hairs. • A farmer crossed a true-breeding, red-coated cow with a true-breeding white-coated bull. The calf produced had roan coat colouring (made up of an equal number of red and white hairs). • Explain the result and draw a genetic diagram to predict the outcome of crossing two roan coloured animals.

25. Incomplete dominance • Incomplete dominance is where heterozygous individuals express a phenotype that is intermediate between the corresponding homozygous individuals

26. Flower colour in plants CR red Cw white Genotypes CRCR red flowers CRCW pink flowers CWCW white flowers Write out a genetic cross between a pure breeding red plant and a pure breeding white plant. Carry out the cross to the F2 generation. Write out the genotype and phenotype ratio for the F2 generation Incomplete dominance example

27. Sex Linkage At the end of the lesson you should be able to: • Represent the sex chromosomes of a female using the appropriate symbols • Represent the sex chromosomes of a male using the appropriate symbols • Give examples of conditions that are determined by the sex chromosomes • Carry out a sex linked cross

28. Male and Female Karyotypes A human karyotype consist of 46 chromosomes arranged as 23 pairs 22 pairs of chromosomes are homologous and contain corresponding genes. These chromosomes are called AUTOSOMES Pair 23 determines the SEX of an organism Pair 23 are exceptions. In humans chromosome 23 comes in 2 versions: X and Y. These are called HETEROSOMES

29. Sex Chromosomes • Females have 2 X chromosomes • They can only produce X eggs • They are therefore referred to as HOMOGAMETIC • Males have an X and a Y chromosome • They can produce either X sperm or Y sperm • They are therefore referred to as HETEROGAMETIC

30. Sex Chromosomes in closer detail Q What do you notice about the male’s sex chromosomes? A The Y chromosome is smaller than the X chromosome Q Which chromosome will therefore carry less information? A The Y chromosome

32. Female Male Non homologous portion of the X chromosome Missing area X X X Y • The female sex chromosome will act in the same way as a normal homologous chromosome • The female sex chromosomes have 2 alleles • Therefore any dominant allele will mask a recessive allele • However, in males, some of the genes on the X chromosome will have only one allele • This is because these genes will not be present on the Y chromosome • Where this allele is recessive, the recessive trait will be expressed as there is no corresponding allele on the Y chromosome to offer dominance

33. Sex Linked Genes • Genes that are found on the X chromosome but have no homologous allele on the Y chromosome are called SEX LINKED genes

34. Sex Linked disorders • Some X-linked recessive diseases: • Duchenne muscular dystrophy characterized by progressive weakening of muscles and loss of coordination. • Haemophilia A defined by lack of a protein involved in blood clotting. • Red/green colour blindness

35. Colour blindness • The gene for colour blindness is carried on the non homologous portion of the X chromosome • Normal sight (N) is dominant to Colour blindness (n) • It is important when you are writing out sex linked crosses that the sex of the individual is clearly indicated

36. Cross 1 normal female x colour blind man XNXN XnY P Gametes XN orXN Xn orY F1 genotype • Carrier girl: Carrier girl: Normal boy: normal boy F1 phenotype

37. Cross 2 carrier female x normal male XNY XNXn P Gamete genotype XN orY XN orXn F1 genotype F1 phenotype Normal girl : Carrier Girl : Normal Boy : Colour Blind boy

47. Family Trees P F1 F2

48. Polygenic Inheritance • Poly = many • Genic = genes • “many gene” inheritance • Two or more genes control a characteristic • This means there are lots of different phenotypes • Examples include: hand span, height, weight, foot length, leaf length

49. Continuous Variation • Continuous variation is when there are no clear cut distinctions between groups • E.g. Weight: some people are 8st 1lb, some are 9st 12lb • There are NO distinct groups e.g. 8st or 9st etc CONTINUOUS VARATION = POLYGENIC INHERITANCE

50. Normal distribution graphs • Continuous variation can be measured and plotted on a line graph or histogram: