HL IB Biology Genetics – Part1

1. HL IB BiologyGenetics – Part1 4.3.1 Define genotype, phenotype, dominant allele, Recessive allele, Codominant alleles, Locus, homozygous, heterozygous, Carrier, Test Cross Genotype: the alleles of an organism. dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier and test cross. Phenotype: the characteristics of an organism. Dominant allele: an allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state. Recessive allele: an allele that only has an effect on the phenotype when present in the homozygous state. Codominant alleles: pairs of alleles that both affect the phenotype when present in a heterozygote. (The terms incomplete and partial dominance are no longer used.) Locus: the particular position on homologous chromosomes of a gene. Homozygous: having two identical alleles of a gene. Heterozygous: having two different alleles of a gene. Carrier: an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele. Test cross: testing a suspected heterozygote by crossing it with a known homozygous recessive. (The term backcross is no longer used.) 4.3.2 Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid. The grid should be labelled to include parental genotypes, gametes, and both offspring genotype and phenotype. Genetics simulation software is available. 4.3.3 State that some genes have more than two alleles (multiple alleles). 4.3.4 Describe ABO blood groups as an example of codominance and multiple alleles. Phenotype Genotype O ii A IAIA or IAi B IBIB or IBi AB IAIB 4.3.5 Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans. 4.3.6 State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans. 4.3.7 Define sex linkage. 4.3.8 Describe the inheritance of colour blindness and hemophilia as examples of sex linkage. Both colour blindness and hemophilia are produced by a recessive sex-linked allele on the X chromosome. Xb and Xh is the notation for the alleles concerned. The corresponding dominant alleles are XB and XH . 4.3.9 State that a human female can be homozygous or heterozygous with respect to sex-linked genes. 4.3.10 Explain that female carriers are heterozygous for X-linked recessive alleles. 4.3.11 Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance. Statisticians are convinced that Mendel�s results are too close to exact ratios to be genuine. We shall never know how this came about, but it offers an opportunity to discuss the need for scientists to be truthful about their results, whether it is right to discard results that do not fit a theory as Louis Pasteur is known to have done, and the danger of publishing results only when they show statistically significant differences. TOK: Reasons for Mendel�s theories not being accepted by the scientific community for a long time could be considered. Other cases of paradigm shifts taking a long time to be accepted could be considered. Ways in which individual scientists are most likely to be able to convince the scientific community could be considered, and also the need always to consider the evidence rather than the views of individual scientists, however distinguished. 4.3.12 Deduce the genotypes and phenotype For dominant and recessive alleles, upper-case and phenotypes of individuals in pedigree charts. lower-case letters, respectively, should be used. Letters representing alleles should be chosen with care to avoid confusion between upper and lower case. For codominance, the main letter should relate to the gene and the suffix to the allele, both upper case. For example, red and white codominant flower colours should be represented as CR and Cw , respectively. For sickle-cell anemia, HbA is normal and Hbs is sickle cell. There are many social issues in families in which there is a genetic disease, including decisions for carriers about whether to have children, personal feelings for those who have inherited or passed on alleles for the disease, and potential problems in finding partners, employment and health or life insurance. There are ethical questions about whether personal details about genes should be disclosed to insurance companies or employers. Decisions may have to be made about whether or not to have screening. These are particularly acute in the case of Huntington disease.

2. Genetics Vocabulary • P generation = parent generation • F1 generation = first offspring generation (F=filial, Latin for "son") • F2 generation = second generation of offspring • Genotype—the alleles possessed by an organism • Eg: PP, Pp, pp • Phenotype—the physical characteristics of an organism • Eg: Purple flower pea plant could be true breed or mix breed

3. Genetics Vocabulary • Homozygous—having two identical alleles of a gene Eg: • pea plant that is true breeding for purple flowers (PP) • Pea plant that is true breeding for white flowers (pp) • Heterozygous—having two different alleles of a gene • Eg: • pea plant that has purple flowers but are not true breeding for purple flowers (Pp) • Carrier—an individual that has a recessive allele of a gene that does not have an effect on their phenotype • Eg: • heterozygous pea plant with purple flowers but are not true breeding for purple flowers (Pp)

4. Genetics Vocabulary • Dominant allele—an allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state. • Recessive allele—an allele that only has an effect on the phenotype when present in the homozygous state. • Codominant alleles—pairs of alleles that both affect the phenotype when present in a heterozygote • Locus – The particular position of a gene on a chromosome

5. Mendel's Experiment Gregor Mendel – Austrian Monk • used pea plants in garden to study heredity b/c of • availability in many varieties (contain many traits) • can self-fertilize ( easy to artificially pollinate & easy to keep track of parentage of plants) • only tracked characters that varied in an "either-or" manner (instead of more or less bended manner) • eg: plants have either: • purple flowers or white flowers, no blended colours • smooth or winkled, no between smoothness • only used plants that were "pure" in certain characteristics • "Pure"= True-breeding = varieties that produce offspring that are of the same variety

6. Mendel's Experiment Hypothesis: • Particulate hypothesis – parents pass on discrete heritable units (genes) that retain their separate identities in offspring • genes can be sorted and passed along, generation after generation, undiluted • eg: deck of card • accounts for traits reappearing after skipping a generation • Blending hypothesis – genetic material contributed by the two parents blend • eg: blue and yellow paints blend to make green • cannot explain many observations & breeding results

7. Mendel's Experiment Mendel's Experiment • used very large sample sizes (used hundreds of plants) • cross-pollinated 2 true-breeding but contrasting pea varieties (P1 generation) • hybridization = crossing 2 true-breeding varieties • observed F1 generation • crossed plants from F1 generation • observed F2 generation  many observations were based on F2 generations

8. Mendel's Experiment Pure purple x Pure purple in P1 generation • First crossed P1: purple flowers + purple flowers • F1 Results: all purple flowers • Crossed 2 purple flowers from F1 generation • F2 Results: all purple flowers Pure white x Pure white in P1 generation • First crossed P1: white flowers + white flowers • F1 Results: all white flowers • Crossed 2 white flowers from F1 generation • F2 Results: all white flowers Pure purple x Pure white in P1 generation • First crossed P1: purple flowers + white flowers • F1 Results: all purple flowers • Crossed 2 purple flowers from F1 generation • F2 Results: purple flowers & white flowers (3:1 ratio)

9. Mendel's Experiment

10. Punnet Square Punnett square:a diagram to predict the allele composition of offspring from a cross between parents • uses capital letter for dominant allele, lowercase letter for recessive allele • eg: P = purple–flower allele, p = white–flower allele • lists all the possible female gametes along one side of square • lists all the possible male gametes along an adjacent side of square • the boxes represent the offspring resulting from all the possible unions of male and female gametes. Eg

11. Punnet Square

12. Punnet Square – Test Cross The Testcross • A test to determine whether an organism is homozygous or heterozygous for a dominant trait • Done by crossing the “unknown” organism with a known homozygous recessive For a single trait: • If unknown is homozygous for the dominant trait: • result will show 100% dominate phenotype

13. Punnet Square – Test Cross • If unknown is heterozygous for the trait: • result will show 50% dominate phenotype, 50% recessive phenotype

14. Punnet Square

15. More Genetics Vocabulary Complete dominance • Dominant trait is expressed over recessive trait in heterozygote • phenotypes of the heterozygote and dominant homozygote are the same) • eg: Heterozygote: dominant red and recessive white  red Codominance • Both dominant and recessive traits are expressed in heterozygote • phenotypes of both alleles are exhibited in the heterozygote • eg: Heterozygote: dominant red and recessive white  red+white • Slight difference when denoting alleles: • Use main letter(s) to specify the gene • Use letter to specify allele types after the gene letter • Eg: Codominant red and white: CR and Cw • (C=colour, R=red, w=white)

16. More Genetics Vocabulary Incomplete Dominance • A blend of dominant and recessive traits is expressed in heterozygote • phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele • eg: Heterozygote: dominant red and recessive white  pink Example: Sickle–Cell Disease • incomplete dominance in phenotype • homozygous recessive individual has the full disease • heterozygous individual - healthy but suffers some sickle-cell symptoms • codominant at the molecular level as both normal and abnormal hemoglobins are made in heterozygotes

17. Multiple Alleles & ABO Blood Groups Multiple Alleles • Mendel’s peas: 2 alleles per trait • Most genes contain many alleles  Multiple alleles Example: ABO blood groups in humans • 4 possible phenotypes for blood: A, B, AB, or O • letters A & B = carbohydrates A and B on the surface of red blood cells • carb A = type A • carb B = type B • carb A and B = type AB • lacking carb A and B = type O

18. Multiple Alleles & ABO Blood Groups • the four blood groups result from various combinations of three different alleles for the enzyme (I) that attaches the A or B carbohydrate to red blood cells • enzyme encoded by IA allele adds the A carbohydrate • enzyme encoded by IB allele adds the B carbohydrate • enzyme encoded by the i allele does not add A nor B • each person carries two alleles  six genotypes and 4 phenotypes are possible • IA and the IB alleles are dominant to the i allele. • IA IA and IA i  type A blood • IB IB and IB i  type B blood • ii (recessive homozygotes) type O blood (red blood cells do not have neither the A nor the B carbohydrate) • IA and IB alleles are codominant  both are expressed in the phenotype of IA IB heterozygotes

19. Multiple Alleles & ABO Blood Groups • Blood transfusion safety: • if type A receives type B or type AB blood, recipient′s immune system will attack “foreign” B substance on donated blood cells, causes donated blood cells to clump, can cause death • if type B receives type A or type AB blood, recipient′s immune system will attack “foreign” A substance on donated blood cells, causes donated blood cells to clump, can cause death • type O = universal donator b/c lack of carbohydrate, won’t cause immune system to react • type AB = universal acceptor b/c immune system recognizes and accepts carbohydrate A, carbohydrate B, won’t have problems with O which doesn’t have carbohydrates at all

20. Sex Chromosomes Sex Chromosomes • Female: XX Male: XY • Some genes are present on the X chromosome and absent from the shorter Y chromosome in humans. • Only short segments at either end of the Y chromosome are homologous with corresponding regions of the X • allow the X and Y chromosomes in males to pair and behave like homologous chromosomes during meiosis • If gamete from sperm carries Y chromosome  boy • If gamete from sperm carries X chromosome  girl • 50% chance of girl, 50% chance of boy

21. Sex Linkage Sex-Linkage • Sex linkage is the association of a characteristic with gender, because the gene controlling the characteristic is located on a sex chromosome • sex chromosomes carry genes for many characters in addition to sex characters • gene located on sex chromosome = sex-linked gene • sex-linked gene usually refer to a gene on the X chromosome • fathers pass sex–linked alleles to all daughters • since they only can pass X chromosomes, not y to daughters • mothers can pass sex–linked alleles to both sons and daughters • since both son and daughter receives X chromosome • If a sex–linked trait is due to a homozygous recessive allele: • a female will have phenotype if she receives alleles from mom and dad • a male will have phenotype if he receives 1 allele from mom (b/c of the 1 X chromosome) • more males have sex-linked recessive disorder than females • female carriers are heterozygous for X-linked recessive alleles • human female can be homozygous or heterozygous with respect to sex-linked genes • Examples of sex-linked characteristics: hemophilia & colour blindness

22. Sex Linkage - Hemophilia Hemophilia • a sex–linked recessive disorder • absence of one or more of the proteins required for blood clotting • when a hemophiliac is injured, bleeding is prolonged because a firm clot is slow to form • Small cuts in the skin are usually not a problem, but bleeding in the muscles or joints can be painful and can lead to serious damage. • Treatment: intravenous injections of the missing protein • A carrier (female) has a recessive allele of a gene but has normal phenotype b/c of normal dominant allele • XH = normal allele, Xh = hemophilia allele

23. Sex Linkage - Hemophilia Boys: • 50% boys will be hemophiliac, 50% normal Girls • 100% of girls will have normal phenotype • 50% of girls will have normal alleles • 50% of girls will be carriers • None of the girls will be hemophiliac

24. Sex Linkage – Colour Bindless Colour Blindess • a sex–linked recessive disorder • colour vision deficiency - certain colours cannot be distinguished from each other • red/green colour blindess is most common, cannot distinguish between red and green • caused by malfunction of the retina which converts light energy into electrical energy that is then transmitted to the brain. • A carrier (female) has a recessive allele of a gene but has normal phenotype b/c of normal dominant allele • XB= normal allele, Xb= colour blindess allele

25. Sex Linkage – Colour Bindless Colour Blindess Boys: • 50% boys will be colour blind, 50% normal Girls: • 100% of girls will have normal phenotype • 50% of girls will have normal alleles • 50% of girls will be carriers • None of the girls will be colour blind

26. Pedigree Analysis Pedigree Analysis Human traits: • many human traits follow Mendelian patterns of inheritance • but much more difficult to study human inheritance than plant inheritance • 1 human generation: 20 years vs 1 plant generation: months • human produce few offspring compared to plants • can’t cross human offsprings like in plants

27. Pedigree Analysis Pedigree Analysis • collect information from a family’s history for a trait & assemble info in a family tree • squares = males, circles = females • horizontal line connecting a male and female = mating • offspring listed below parents in their order of birth from left to right • shaded squares and circles = individuals who exhibit the trait

28. Pedigree Analysis – Autosomal pedigree Characteristics of Autosomal dominant pedigree • Every affected individual has at least one affected parent • Affected individuals who mate with unaffected individuals have a 50% chance of transmitting the trait to each child • Two affected individuals may have unaffected children. Characteristics of Autosomal recessive pedigree • Unaffected parents who have affected offspring • Equal numbers of affected males and females • All offspring are affected when both parents are affected • In rare traits, the unaffected parents of an affected individual may be related to each other

29. Pedigree Analysis – Sex-Linked pedigree Characteristics of Sex-Linked Recessive pedigree • traits occur more commonly in males than females • all sons of an affected mother are affected • affected fathers never transmit the trait to their sons • unaffected parents may have affected offspring Characteristics of Sex-Linked Dominant pedigree • each generation usually has an affected individual • all daughters of affected males are affected • both sons and daughters of an affected heterozygous female may be affected • twice as many females as males are affected