2. Genetics

3. All human cells have 46 chromosomes, 23 pairs. Different living things will have a different amount. The chromosomes are held in the nucleus of our cells.

4. The chromosomes are made up of long coiled molecules of DNA. This chemical is divided into regions called genes. The DNA molecule contains four different molecules called bases. There are four different bases. Each gene is made up of a pattern of these four bases.

5. The four bases are: cytosine, adenine, thymine, guanine (A, T, G and C) A and T always pair. C and G always pair.

6. FERTILISATION

7. Gametes are the sex cells – the sperm and the egg. These cells only have 23 chromosomes.

8. Because each egg and sperm have a random combination of genes this causes every offspring to have a different mixture of genes. This causes VARIATION

9. Gene control of characteristics All characteristics are controlled by a minimum of 2 genes. (one from mum one from dad) Different genes that control the same feature are called ALLELES Alleles can be dominant or recessive. Individuals can be heterozygous or homozygous.

10. THE ALLELES FOR SEX Gender (in mammals) is determined by sex chromosomes: XX (female) and XY (male). cross

11. THE ALLELES FOR EYE COLOUR B is the gene for brown eyes b is the gene for blue eyes Alleles: BB Bb bb

12. Dominant and recessive Some alleles for a feature are able to mask the influence of the other (weaker) gene when they are paired Stronger gene is DOMINANT and is represented by a capital letter e.g. H The masked gene is RECESSIVE shown as lower case of the same letter e.g. h

13. Inheritance

14. GENETIC DISEASES

15. Inherited diseases Cystic fibrosis Sufferers of this disease produce a thick, sticky mucus which coats their airways and lungs. If it is not cleared daily the person can get serious chest infections. The disease is caused by a recessive allele.

16. If a person is heterozygous (Cc) then they are a carrier but have a normal phenotype, they don’t develop cystic fibrosis but can pass it on to their children. What happens if a carrier (Cc) and a ‘normal’ person (CC) want to have children? What are the chances of the child having the disease, being a carrier or not a carrier?

17. But what happens if another two carriers wanted to have children?

18. What happens if a carrier (Cc) and another carrier (Cc) want to have children? What are the chances of the child having the disease, being a carrier or not a carrier?

20. Questions T tall allelet dwarf allele Predict the ratios of offspring from the following crosses. 1.TTxtt 2.TTxTt 3.TTxtt 4. TtxTt 5. Ttxtt 6. ttxtt

21. Sickle cell anaemia This inherited disease causes the red blood cells to change from their usual round shape to become pointed like a sickle. This shape change means that they get stuck in blood vessels and cannot pick up oxygen properly from the lungs. The allele responsible for it is a recessive. What happens if two carriers were to have children?

24. Where are the chromosomes found?

25. What are the sections of DNA?

26. What do genes code for?

27. How many pairs of chromosomes are found in a human nucleus?

28. Explain how sex is inherited.

29. Genetic variation is caused by: Mutation Gametes Fertilisation Mutations are usually harmful, but may be beneficial. Mutations can be caused by radiation or chemicals. Gene mutations occur because the DNA base sequence changes – altering or preventing the gene that is normally coded for.

30. Some mutations are useful, some are harmful. Cystic fibrosis Downs syndrome

31. But what causes intelligence, sporting ability or good health. Nature or nurture?

32. Which are inherited, which are environment and which are both? Body mass Red-green colour blindness Cystic fibrosis. Nose shape Sickle cell anaemia Earlobe shape Intelligence Height Eye colour