Genetics

1. Genetics

2. What are the key components of chromosomes? • DNA -heterochromatin -euchromatin • Proteins • Found in nucleus • You should understand the relationship between DNA and proteins (chromatin packing and histones)

3. Key terms • Eukaryotic chromosomes-made of DNA and proteins (histones) • Gene-heritable factor that controls specific characteristics -made up of a length of DNA, found on a specific chromosome location (a locus) C. Allele-one specific form of a gene (all found at the same locus) -Example: Everyone has the gene for eye color. The possible alleles are blue, brown, green, etc.

4. More Key Terms D. Genome-total genetic material of an organism or species (Example: The Human Genome) E. Gene pool-total of all genes carried by individuals in a population

5. Mutations • Chromosome mutations-involve large sections of chromosomes (or the whole thing) -Ex: Down’s syndrome, Turner’s syndrome

6. Mutations B. Gene mutation-involves changes in single base pairs -Some mutations may not have any effect on the cell and may involve: 1. part of the sense strand of DNA which is not transcribed 2. part of the DNA that a cell does not use 3. changes in second or third bases of a codon (since the genetic code is degenerate the same base may still be coded for)

7. Mutations B. Gene mutation-involves changes in single base pairs Example: Insertion or deletion of single organic bases -changes the DNA sequence that willbe transcribed and translated original DNA sequence: ATG-TCG-AAG-CCC transcribed: UAC-AGC-UUC-GGG translated: tyr-ser-phe-gly addition of base A: ATA-GTC-GAA-GCC-C transcribed: UAU-CAG-CUU-CGG translated: thy-glu-leu-arg

8. Mutations: Base substitutions and sickle-cell anemia • Hemoglobin-protein that helps RBC carry oxygen • Hb is a gene that codes for hemoglobin -made of 146 amino acids C. In some cases one base is substituted for another normal: (HbA) base substitution: (HbS) CTC CAC GAG GUG -after transcription and translation HbA produces glutamic acid and HbS produces valine

9. Mutations: base substitutions and sickle-cell anemia D. The altered hemoglobin HbS is crystalline at low oxygen levels causing the RBC to become sickled and less efficient at oxygen transport E. Symptoms of sickle cell anemia -physical weakness -heart or/and kidney damage -death

10. Mutations: base substitutions and sickle-cell anemia F. In heterozygous people (one normal allele and one sickle cell allele) -the alleles are codominant, but the normal allele is expressed more strongly -in codominance both alleles are expressed (one is not dominant to the other) -some sickled cells present, but most are normal -some people show mild anemia (deficiency of the hemoglobin, often accompanied by a reduced number of red blood cells and causing paleness, weakness, and breathlessness)

11. Mutations: base substitutions and sickle-cell anemia G. Advantages of being heterozygous -In areas where malaria is infested: -Plasmodium cannot live in erythrocytes with HbS -Heterozygous individuals have a reduced chance of contracting the protist that is carried by mosquitoes

13. Karyotyping • Karyotyping-process of finding the chromosomal characteristics of a cell -chromosomes are stained to show banding and arranged in pairs according to size and structure -You should be able to look at a karyotype and determine the sex of the individual and if non-disjunction has occurred

14. Amniocentesis and karyotyping • Amniocentesis -performed at around 16 wks -sample of amniotic fluid is taken and cultured -when there are enough cells, a karyotype can be performed -chromosomes are arranged into pairs to detect any abnormalities -can be used to detect Down’s syndrome (a.k.a. trisomy 21) -can be used to recognize sex or non-disjunction

15. Amniocentesis

16. Karyotyping and Chorionic villus sampling Sampling is performed around 11 weeks of pregnancy Cells are gathered from chorionic villi (cells from the zygote) Cells are cultured, DNA is extracted and a karyotype is made

17. A Side by Side Comparison

18. Meiosis Key Terms • Diploid-having two sets of chromosomes • Homologous chromosomes-matching pairs of chromosomes -have the same genes -are not identical (one chromosome comes from each parent, thus alleles may be different) -found in diploid cells C. Haploid-having only one set of chromosomes

19. More Meiosis Terms D. Chromatids -two parts of a chromosome E. Centromere -part of a chromosome that connects the chromatids F. Reduction division-in organisms that reproduce sexually -reduction of the number of chromosomes by half (from a diploid nucleus to a haploid nucleus) -think of eggs and sperm; both are haploid (when they unite the diploid number is restored)

20. Meiosis B. Produces gametes (sperm and egg) C. Overview 1. homologous chromosomes pair (diploid) 2. two divisions (meiosis I and meiosis II) 3. result=4 haploid cells D. When gametes unite (to produce a diploid cell) a cell with homologous pairs is created -one set of chromosomes is from the mom and one set of chromosome is from the dad

21. chiasmata Meiosis (details) Tetrad • Interphase -DNA replication • Prophase I -chromosomes condense -nucleolus becomes visible -spindle formation -synapsis-homologous chromosomes are side by side (they become a tetrad and are intersected at the chiasmata) -nuclear membrane begins to disappear

22. Meiosis (details) C. Metaphase I: Bivalents move to equator D. Anaphase I: -Homologous pairs split -One chromosome from each pair moves to opposite pole E. Telophase I: -Chromosomes arrive at poles -Spindle disappears

23. Meiosis (details) F. Prophase II – new spindle is formed at right angles to the previous spindle G. Metaphase II – Chromosomes move to equator H. Anaphase II – -Chromosomes separate -Chromatids move to opposite poles

24. Meiosis (details) I. Telophase II -Chromosomes arrive at poles -Spindle disappears -Nuclear membrane reappears -Nucleolus becomes visible -Chromosomes become chromatin -Cytokinesis

25. Chiasmata formed during a synapsis Crossing over of homologous chromosomes • An important source of variation -creates new combinations B. Happens during prophase I C. Called a synapsis D. Recombination-reassortment of genes into different combinations from those of the parents

26. H E H E H E A B C D A B C D A B C D h e h e h e Crossover Start A genotype HE B genotype HE C genotype he D genotype he Crossover B and C become recombinants Results A genotype HE B genotype He C genotype hE D genotype he

27. Meiosis and genetic variation • The number of possible gametes produced by random orientation of chromosomes is 2n (where n is = to the haploid number of chromosomes) • In humans the production of 1 gamete has over 8 million possible combinations (223) C. Recombination (in prophase I) + Random orientation of chromosomes (in metaphase I) = an infinite number of variations

28. Meiosis and Non-disjunction • Disjunction - when the homologous chromosomes separate in anaphase I • Aneuploidy -happens when chromosomes do not separate (in anaphase I or II) -caused by non-disjunction -result: one cell missing a chromosome and one cell having an extra chromosome -Total number of chromosomes = 2n ±1 C. Polyploidy- having more than two complete sets of chromosomes (common in plants)

29. Karyotype of non-disjunction Normal karyotype (2n=46) Abnormal karyotype (aneuploidy) 2n + 1 = 47

30. Non-disjunction and Down’s syndrome • One of the parent gametes contains two copies of chromosome 21 • The zygote then ends up with 3 copies -2 from one parent -1 from the other C. Down’s syndrome = trisomy 21 D. Chances of non-disjunction of chromosomes increases with age in females (in males too, but less of an effect)

31. Non-disjunction and Down’s syndrome E. Female age has a greater effect because: -gametes are produced before birth -more exposure to chemicals, radiation, etc. F. Male age has less effect because they do not produce gametes until puberty G. Genetics may also increase the likelihood of having a child with Down’s syndrome

32. Theoretical Genetics Key Terms • Dominant allele-the allele that always shows in the heterozygous state (Example: Bb=brown) • Recessive allele-the allele that only shows in the homozygous recessive state (Example: bb=blue) • Codominant alleles-pairs of alleles where two differing alleles are shown in the phenotype in a heterozygote • Homozygous -having two identical alleles of a gene (Example: BB or bb) • Heterozygous -having two different alleles of a gene (Example: Bb)

33. More Vocabulary F. Carrier- a person who has a recessive allele, but does not express it (they are generally heterozygous, Bb) G. Genotype-alleles that a person has (the letters) Ex: Bb H. Phenotype- the physical characteristics the a person shows (caused by the genotype) Ex: brown hair or blue eyes I. Test cross- crossing two or more genotypes to find the possible genetic outcomes

34. Mendel’s Monohybrid Crosses • Punnett square-shows possible outcomes from a test cross • Mendel studied characteristics of pea plants Wrinkled and round peas (round peas are dominant)

35. Gregor Mendel’s Findings

36. Mendel’s Monohybrid Crosses C. Mendel found tall is dominant over short D. His procedures were: 1. Start with 2 pure breeding homozygous plants (This is the P generation.) -Plant 1=tall (TT) -Plant 2= short (tt) 2. Cross-breed the plants -Place pollen from the tall plant in the short plant and vice versa.

37. Mendel’s Monohybrid Crosses D. His procedures were: 3. The F1 generation is the 1st group of offspring. -All were tall, and had a heterozygous genotype. (Tt) -This is an application of the law of segregation. -All offspring had a ‘T’ from one parent and a ‘t’ from the other parent

38. Mendel’s Monohybrid Crosses D. His procedures were: 4. The F1 offspring were then crossed. (Tt x Tt) -Possible outcomes can be found using a Punnett square GENOTYPES -25% TT -50% Tt -25% tt PHENOTYPES -75% Tall -25% Short

39. Multiple alleles • When genes have more than two alleles • Example: Blood type has 4 phenotypes based on three alleles (IA,IB and i) • IA and IB are codominant and i is recessive • This is why parents can have kids with different blood types

40. Multiple alleles D. Perform a test cross for P: mother with O blood type and father with AB blood type. What are the possible phenotypes?

41. Multiple alleles Perform a test cross for P: mother with O blood type and father with AB blood type. What are the possible phenotypes for F1? P = ii x IAIB None of the children can have the same blood type as either of the parents.

42. Codominance • When neither allele for a gene is recessive • Example: Blood type • Alleles A and B are both dominant (both are expressed) • i is recessive to alleles A and B • One letter is chosen and the possible alleles are written in upper case letters to illustrate codominance

43. Sex chromosomes and gender Only possibility for P generation = XX and XY -The X chromosome is larger and carries more genes than the Y chromosome -Examples of genes on X, but not Y = color blindness and hemophilia -Many sex-linked traits are related to the X chromosome. The sex of all babies is determined by the chromosomes in the sperm from the man.

44. Sex linkage • Genes carried on sex chromosomes (usually X) • Example: Hemophilia-a blood disorder that prevents clotting -patients do not produce clotting factors that allow coagulation of blood, and thus torn blood vessels are prevented from closing -most common in boys (they get it from their mother’s X chromosome, as they only get one X, which means only one chance to get the dominant allele)

45. Sex linkage C. Two parents without hemophilia: XHXh and XHY *The XhXh does is very rare *Males cannot be heterozygous carriers because they only have one X. *Females can be carriers and pass on the trait to the next generation. They can be heterozyg. or homozyg. * XH -Normal and Xh –Hemophilia

46. Predict the genotypic and phenotypic ratios of monohybrid crosses for each of the following. 1. Sickle cell anemia: HbA=normal and HbS=sickle cell HbAHbS x HbAHbS 2. Colorblindness: XB=normal and Xb=colorblind XBXb x XbY 3. Hemophilia: XH=normal and Xh=hemophilia XhXh x XHY 4. Blood type: IAi x IBi **You should be able to determine if alleles are codominant because both alleles will be represented by capital letters. You should also know if the inherited traits are sex-linked.

47. Mendel’s Law of Segregation • States: The separation of the pair of parental factors, so that one factor is present in each gamete. (This is how it is written in the IB book.) • The two alleles for each characteristic segregate during gamete production. This means that each gamete will contain only one allele for each gene. This allows the maternal and paternal alleles to be combined in the offspring, ensuring variation. (This is from wikipedia.)

48. Mendel’s Law of Segregation and Meiosis • Mendel looked at genes (on chromosomes) • Found that each gene appeared twice (in homologous pairs) • Figured out that when a synapsis occurs in prophase I followed by a separation in anaphase I, homologous chromosomes move to opposite poles • One chromosome from each pair ends up in a gamete

49. Mendel’s Law of Independent Assortment • States that allele pairs separate independently during the formation of gametes • Any one pair of characteristics may combine with any one of another pair of characteristics • See p. 163 in Green Book

50. Independent assortment and meiosis • Any combination of chromosomes is possible in metaphase I (there is no ‘master plan’ for the order that they line up on the metaphase plate before separation) • Mendel thought all genes were inherited separately and had no relationship -Ex: Pea plants could be green or yellow and wrinkled or round. Shape and color had nothing to do with each other, because the genes are on separate chromosomes. Any combination could have been produced (wrinkled/green, wrinkled/yellow, round/green, round yellow) C. This is demonstrated in Dihybrid crosses. D. Today we realize that there are many exceptions to this law.