Genes

1. Genes The inherited “instructions”

2. What is a gene? • A section of DNA that codes for a particular protein • Can be seen on dyed chromosomes as bands. The place it occupies (like its address) is called a LOCUS • An inherited instruction, coded in DNA

3. Human Genes • Humans have about 20 000 – 25 000 genes in their GENOME • Those that have been identified are documented on OMIM • Genes are usually named after the characteristic or function of their code. Often they are shortened to a group of letters. • Eg. BRCA1 = gene linked to breast cancer • ABO = controls ABO blood type • PKU1 = controls production of an enzyme. The faulty version of the gene causes a disease called phenylketonuria

4. Gene linkage • If genes are close to one another on a chromosome, they will most likely be inherited together (unless crossing over occurs) In this picture, it shows that most children have EITHER inherited BOTH M and HD, or neither. This is because they are found on the same arm of the chromosome. The third child is called a RECOMBINANT because a crossing over event has occurred, and they are not like either parental type http://www.ornl.gov/sci/techresources/Human_Genome/publicat/primer/linkage.gif

5. Alleles • Alleles are the different possibilities at each gene locus. Example 1. On the X chromosome, there is a gene called DMD, which controls muscle protein production. There are two possible forms of this gene: either you produce normal proteins, or you produce abnormal proteins. These are the two ALLELES for this gene. The alleles have been named: M= normal proteins m = abnormal proteins Example 2. On chromosome 9, there is a gene that controls your blood group, called ABO. In this case, there are THREE alleles (possible forms of the gene): IA = produce antigen A IB = produce antigen B i = produce neither antigen

6. How do alleles work? • We each have TWO copies of each gene, as we have paired chromosomes: one from your mother and one from your father • Therefore, you have two alleles for each gene. They interact with each other to cause different effects

7. When don’t we have two copies? • In all our diploid cells, there are two copies of each chromosome… or is there…? • In males, there is only one X chromosome, and one Y chromosome • This means that if there is a faulty gene on either male sex chromosome, it cannot be overridden by any healthy matching gene!

8. Genotype and phenotype • These are two VERY important words: • Genotype: the alleles that are present within an individual. For example, someone with two copies of the normal gene for colour vision (CBD gene) would have a genotype of VV. • Phenotype: a description of what the effect of the genotype is (ie. The characteristic produced in the individual). The phenotype for the person above would be “has normal full colour vision”

9. Your turn… • Someone has two copies of the IA ABO allele. Write their genotype AND phenotype. • Genotype: IAIA • Phenotype: Type A blood group

10. Interaction between alleles • Using our colour vision example from before (V and v are our alleles). There are three different combinations of alleles possible: • VV (homozygous) • Vv (heterozygous) • vv (homozygous)

11. Interaction continued • The allele that has the capital letter is DOMINANT to the one with a lower case letter (which is called the RECESSIVE). • This means, that as long as there is at least one copy of the dominant allele, the phenotype of the individual will reflect the dominant characteristic. VV (homozygous dominant) = full colour vision Vv (heterozygous) = full colour vision vv (homozygous recessive) = no full colour vision

12. Example: dominance • The rhesus gene controls the presence or absence of the rhesus protein on red blood cells. • Possible genotypes: D = rhesus positive, d = rhesus negative. • Questions: • Which is the recessive allele, and which is the dominant? • What are the possible combinations (genotypes) of this gene? • What are the possible characteristics produced (phenotype)? • Show all the genotypes and their corresponding phenotypes.

13. Summary: dominance and recessiveness Polydactyly: gene on chromosome 7. The dominant allele causes extra fingers and/or toes to be grown. Alleles: P =abnormal number of fingers and toes p = normal number of fingers and toes Possible genotypes: PP Pp pp Phenotypes

14. Being a “carrier” • Often deleterious (negative) alleles are the recessive ones. That means they can “hide” in a heterozygous individual. • If heterozygote mates with another heterozygote for the disorder, it is possible their child will be a homozygous recessive individual and have the disorder.

15. Proof of the “carrier” • We can show the possible allelic combinations of a mating event using a tool called a Punnett Square Adult 2 In this case, the r allele codes for a disorder (for example, Huntington’s Disease). Both adults do not show symptoms of the disorder, as the R is dominant. However, there is a one in four chance their offspring will be homozygous recessive and have the disorder. Adult 1 http://pad.wikihow.com/images/thumb/9/90/Complete_punnett_square_524.jpg/180px-Complete_punnett_square_524.jpg

16. Other allele interactions • Sometimes, full dominance does not occur. • There are a few different reasons why this may be. • This time we’re going to use a gene that controls colour in flowers as our example. There are two alleles: CR = red CW = white

17. Co-dominance • This occurs when there is not full dominance with either allele. • Phenotype of heterozygote is a characteristic that is an intermediate between each homozygote CRCR CWCW CRCW

18. Partial dominance • Similar to co-dominance, except the phenotype of the heterozygote has patches of each phenotype CRCR CWCW CRCW

19. Sex linked genes • As mentioned earlier, males only have one copy of each type of sex chromosome, so only one copy of each gene on those chromosomes. • Whether it is the dominant or recessive allele, that is the one that will show the phenotype

20. Turning genes on and off • Age related: developmental (eg. Genes to promote breast budding in females around the ages of 10-14) • Environmental triggers (eg. The cooler areas of a siamese kitten will turn a darker shade than warmer areas, hydrangeas produce different coloured flowers dependent on the pH of the soil they are grown in) Pale pink flowers = alkaline. Blue flowers = acidic

21. The importance of environment • Genetics is not only a study of the code of DNA, which is largely set in stone. • Environment plays a big role in the phenotype of many characteristics, while only playing a small role in others Phenotype = genotype + environment

22. Predicting offspring genotype • If we know the genotype of the parents, we can predict the genotype of the offspring, using test crosses (punnett squares) • Alternatively, parental genotype may be able to be determined if the offspring genotype is known

23. Example John Phenotypes: Blood group B Normal skin pigment Genotypes: IBi ? Tracey Phenotypes: Blood group B Normal skin pigment Genotypes: IBi ? Samson Phenotypes: Blood group O Albino Genotypes: ?

24. Working out the possibilities • Step 1: work out the possible genes within the sperm and egg cells • Step 2: perform monohybrid cross

26. What is the probability of heterozygotes producing different phenotypes? Genotype ratio: Phenotype ratio:

27. What if we want to work with combinations? • We want to find out all the combinations of skin pigment and blood types that Tracey and John’s children could have. • Step 1: work out all the genotype possibilities within the egg and sperm cells • Step 2: perform dihybrid test cross

28. Dihybrid cross Genotype ratio: Phenotype ratio:

29. On Friday: • Linked genes – predicting the next generation • Pedigrees – mapping previous generations

30. HOLIDAY HOMEWORK Worksheets (booklet) Practise exam questions Complete Chapter 9, and do chapter 10 Quick Check questions.