TOPIC INHERITANCE

Transcription

1 TOPIC INHERITANCE

2 INTRO IB BIO 10.2 Recall from Topic 3.4 that Punnett squares can be used to predict the genotypic/phenotypic ratios of monohybrid cross. However, this method can only be used to correctly analyze unlinked genes. 2 Y = yellow (dominant) y = green (recessive) Yy x yy Genotype: - 50% yy - 50% Yy Phenotype: - 50% green - 50% yellow

3 10.2 A Unlinked Genes

4 asdf IB BIO 10.2 Understandings U2: Unlinked genes segregate independently of meiosis. Unlinked genes are those that are located on different chromosomes. As a result, during meiosis they segregate independently of each other (Law of Independent Assortment). 4

5 IB BIO 10.2 Understandings U2: Unlinked genes segregate independently of meiosis. The Law of Independent Assortment states that alleles for different genes sort into gametes independently of each other. 5

6 IB BIO 10.2 Applications A2: Completion and analysis of Punnett squares for dihybrid traits. Dihybrid crosses are crosses involving two separate unlinked traits (monohybrid is crossing one trait). For example: G = green g = yellow R = round r = wrinkled 6 GgRr x GgRr Dihybrid Cross Heterozygous Green Heterozygous Round X Heterozygous Green Heterozygous Round Possible Outcomes: GGRR GGRr GGrr GgRr Ggrr ggrr ggrr ggrr

7 IB BIO 10.2 Applications Dihybrid crosses are set up similarly to monohybrid crosses and can be used to determine genotypic/phenotypic ratios. 7 A2: Completion and analysis of Punnett squares for dihybrid traits. Dihybrid Cross GR Gr gr gr GgRr x GgRr GR Gr gr gr

8 IB BIO 10.2 Applications Dihybrid crosses are set up similarly to monohybrid crosses and can be used to determine genotypic/phenotypic ratios. 8 A2: Completion and analysis of Punnett squares for dihybrid traits. Dihybrid Cross GR Gr gr gr GgRr x GgRr GR Gr gr gr

9 IB BIO 10.2 Applications Dihybrid crosses are set up similarly to monohybrid crosses and can be used to determine genotypic/phenotypic ratios. 9 A2: Completion and analysis of Punnett squares for dihybrid traits. Dihybrid Cross GR Gr gr gr GgRr x GgRr GR Gr gr gr

10 IB BIO 10.2 Applications Dihybrid crosses are set up similarly to monohybrid crosses and can be used to determine genotypic/phenotypic ratios. 10 A2: Completion and analysis of Punnett squares for dihybrid traits. Dihybrid Cross GR Gr gr gr GgRr x GgRr GR Gr gr gr GGRR GGRr GgRR GgRr GGRr GGrr GgRr Ggrr GgRR GgRr ggrr ggrr GgRr Ggrr ggrr ggrr

11 IB BIO 10.2 Dihybrid Cross 11 Applications A2: Completion and analysis of Punnett squares for dihybrid traits. GR Gr GR Gr gr gr GGRR GGRr GgRR GgRr GGRr GGrr GgRr Ggrr gr GgRR GgRr ggrr ggrr gr GgRr Ggrr ggrr ggrr Dihybrid Cross Phenotypic Ratio 9 Green/Round : 3 Green/Wrinkled : 3 Yellow/Round : 1 Yellow/Wrinkled

12 IB BIO 10.2 Dihybrid Cross Practice Problem 12 Applications S1: Calculation of predicted genotypic and phenotypic ratios of offspring of dihybrid crosses (unliked autosomal) In Drosophila (fruit flies) the allele for grey color is dominant over black. Straight wings are dominant over curly wings. A heterozygous grey-straight winged fly was crossed with a black-curly winged fly. G = grey S = straight g = black s = curly If these genes are not linked, which phenotypes would you expect in F 1? Give the expected ratio of the phenotypes.

13 IB BIO 10.2 Applications S1: Calculation of predicted genotypic and phenotypic ratios of offspring of dihybrid crosses (unliked autosomal) gs gs gs gs Dihybrid Cross Practice Problem GgSs x ggss GS Gs gs gs 13 _ Grey/Straight: _ Grey/Curly : _ Black/Straight : _ Grey/Straight

14 IB BIO 10.2 Applications S1: Calculation of predicted genotypic and phenotypic ratios of offspring of dihybrid crosses (unliked autosomal) Dihybrid Cross Practice Problem GgSs x ggss GS Gs gs gs gs GgSs Ggss ggss ggss gs GgSs Ggss ggss ggss gs GgSs Ggss ggss ggss gs GgSs Ggss ggss ggss 4 Grey/Straight: 4 Grey/Curly : 3 Black/Straight : 1 Grey/Straight 14

15 VIDEOS IB BIO 10.2 AK Lectures: Mendels Law of Segregation AK Lectures: Law of Independent Assortment Bozeman Science: Dihybrid Crosses 15

16 REVIEW IB BIO Define unlinked genes Define dihybrid cross. 3. Provide phenotypic and genotypic ratios for the following crosses. a) Flowers - DDww x ddww - D = tall, d = short, W = purple, w = white b) Guinea Pigs - BbRr x BbRr - B = black fur, b = white fur R = rough fur, r = smooth fur

17 10.2 B Linked Genes

18 IB BIO 10.2 Understandings U1: Gene loci are said to be linked if on the same chromosome. The Laws of Segregation and Independent Assortment can typically 18 only be applied to genes/alleles that are located on different chromosomes (aka unlinked).

19 IB BIO 10.2 Understandings U1: Gene loci are said to be linked if on the same chromosome. However, genes that are adjacent 19 and close together on the same chromosome tend to move as one. The closer two genes are, the more linked they are. This means they are less likely to separate as a result of crossing over during gamete formation. Linked Genes

20 IB BIO 10.2 Understandings U1: Gene loci are said to be linked if on the same chromosome. As a result of being linked, genes move together and so 20 recombinanation is typically not seen. The case below shows 100% linkage, meaning recombination has not separated pairs AB and ab. Linked Genes

21 IB BIO 10.2 Skills S2: Identification of recombinants in crosses involving two linked genes. Because linked genes do not move independently of each other, dihybrid crosses cannot be used to predict the outcomes of a cross. 21 However, data from dihybrid crosses can be used to conclude whether recombination has occurred and whether two genes are likely linked.

22 IB BIO 10.2 Skills S2: Identification of recombinants in crosses involving two linked genes. Identifying Recombinants Consider the following dihybrid cross involving flies with genes for grey/black bodies and long/short wings: G = grey body L = long wings GgLl x GgLl g = black body l = short wings 22

23 IB BIO 10.2 Skills S2: Identification of recombinants in crosses involving two linked genes. Identifying Recombinants In dihybrid crosses, alleles are usually shown side by side like in the example below: GgLl x GgLl G = grey g = black L = long l = short paternal chromosome maternal chromosome G g L l X G g L l 23 G L g l G l g L recombinant chromosomes

24 IB BIO 10.2 Skills By comparing the genotype of the offspring to that of the parent, you can see which are a product of recombination 24 S2: Identification of recombinants in crosses involving two linked genes. parent chromosomes G g offspring without G L g l G L recombination g l g l g l L l X G g L l G G L L Note that a copy each chromosome in the offspring can be found in one of the parents.

25 IB BIO 10.2 Skills By comparing the genotype of the offspring to that of the parent, you can see which are a product of recombination 25 S2: Identification of recombinants in crosses involving two linked genes. parent chromosomes G g offspring without G L g l G L recombination g l g l g l L l X G g L l G G L L offspring with recombination G G l L g g L l G G l l g g L L Notice that some of the recombinant genotypes are the same as the non-recombinant genotypes and thus cannot be differentiated. So, data analysis must be done to identify recombinants.

26 IB BIO 10.2 Skills Identifying Recombinants The Punnett square and expected ratios for this cross would be: 26 S2: Identification of recombinants in crosses involving two linked genes. GL Gl GL Gl gl gl gl gl grey body/long wings : black body/long wings : grey body/short wings: black body/short wings

27 IB BIO 10.2 Skills S2: Identification of recombinants in crosses involving two linked genes. Identifying Recombinants The Punnett square and expected ratios for this cross would be: GL Gl gl gl GL GGLL GGLl GgLL GgLl Gl GGLl GGll GgLl Ggll 27 gl GgLL GgLl ggll ggll gl GgLl Ggll ggll ggll 9 grey body/long wings : 3 black body/long wings : 3 grey body/short wings: 1 black body/short wings

28 IB BIO 10.2 Skills S2: Identification of recombinants in crosses involving two linked genes. Identifying Recombinants The following table shows expected and observed phenotypes in a population of 10,000 offspring: Grey body Long wings Black body Long wings Grey body Short wings Black body Short wings EXPECTED OBSERVED Frequency % Frequency % 5, , , , , Notice the difference between expected and observed outcomes. This is an indication that genes might be linked.

29 IB BIO 10.2 Skills S2: Identification of recombinants in crosses involving two linked genes. Identifying Recombinants Since the offspring with a grey body/long wings and black body/short wings, we can reasonably conclude that they are linked. Grey body Long wings Black body Long wings Grey body Short wings Black body Short wings EXPECTED OBSERVED Frequency % Frequency % 5, , , , , So, offspring with black body/long wings and grey body/short wings must be the result of recombination, but there is a way to be sure.

30 IB BIO 10.2 Understandings U5: Chi-squared tests are used to determine if the difference between observed and expected frequencies is significant. The chi-squared test is a method of determining whether the 30 difference between observed and expected frequences is statistically significant. This is important as reality rarely matches prediction. It can be used to determine if traits are unlinked (H 0 ) or linked (H a ) The formula is shown here: Chi-Squared Test X 2 = Chi-squared stastic Σ = summation of all values

31 IB BIO 10.2 Skills S3: Use of a chi-squared test on data from dihybrid crosses. Using Chi-Squared Test on Dihyrid Cross Data 1. Create a table similar to the following, where each class is an expected phenotype (e.g. red/long, white/short) Expected (E) Observed (O) (O-E) 2 /E Class 1 Class 2 Class 3 Class 4 TOTAL 2. Determine the expected frequencies of your dihybrid crosses, assuming independent assortment has occurred. 3. Record observed frequencies for each and then calculate the (O-E) 2 /E value. 31

32 IB BIO 10.2 Skills S3: Use of a chi-squared test on data from dihybrid crosses. Using Chi-Squared Test on Dihyrid Cross Data 4. Determine the X 2 value by summing all the values in the bottom row. Expected (E) Observed (O) (O-E) 2 /E Class 1 Class 2 Class 3 Class 4 TOTAL 5. Determine the degrees of freedom (df) using the following formula: 32 df = (# of classes 1)

33 IB BIO 10.2 Skills S3: Use of a chi-squared test on data from dihybrid crosses. Using Chi-Squared Test on Dihyrid Cross Data 6. Refer to a X 2 table to find the critical value using the df you calculated and a p value of This p value refers to a 5% chance of incorrectly concluding that the genes are linked, when they are actually unlinked.

34 IB BIO 10.2 Skills S3: Use of a chi-squared test on data from dihybrid crosses. Using Chi-Squared Test on Dihyrid Cross Data 7. Compare your calculated X 2 value to that shown in the table If your X 2 value is higher than that in the table, you can conclude the genes are linked and the difference is significant. - If your X 2 value is lower, you can conclude they are unlinked.

35 REVIEW IB BIO Define linked genes Consider the following cross: RrLl x rrll R = red, r = white, L = long, l = short - Calculate the genotypic and phenotypic ratios - Determine the expected frequency of phenotypes if 10,000 offspring were analyzed. 3. Explain the application of the X2 test in analyzing results from a dihybrid cross.

36 VIDEOS IB BIO 10.2 Bozeman: Linked Genes Khan Academy: Morgan & Fruit Flies Khan Academy: Chi Square Test AK Lectures: Polygenic Inheritance 36

37 10.2 C Variation

38 IB BIO 10.2 Understandings U3: Variation can be discrete or continuous. Members of the same species have slightly different sets of characteristics. Some of these differences are inherited from their parents and others are the result of the environment. 38 Variations The differences between individuals of the same species are called variations. Such differences can be categorized as one two types, discontinuous and continuous.

39 IB BIO 10.2 Understandings U3: Variation can be discrete or continuous. Discontinuous variation is variation that has distinct groups for organisms to belong to. Either an organism has a trait or it doesn't. 39 Variation Discontinuous Variation These traits tend to be qualitative and can be represented using a bar graph. Examples include eye color and blood groups.

40 IB BIO 10.2 Understandings U3: Variation can be discrete or continuous. Continuous variation is a variation that has no limit on the value that can occur within a population. They also tend to be polygenic. 40 Variation Continuous Variation Every organism in a species shows the characteristics, but to a different extent. Examples include height, weight and heart rate.

41 IB BIO 10.2 Understandings U4: The phenotypes of polygenic characteristics tend to show continuous variation. Polygenic traits are those that are influenced by two more more 41 traits. Because of this, continuous variation is observed. Patterns of inheritance for these traits statistically differ from predicted ratios.

42 IB BIO 10.2 Understandings U4: The phenotypes of polygenic characteristics tend to show continuous variation. For example, human skin color is determined by at least 3 independent genes (6 alleles), resulting in continuous variation. As shown here, there is a defined curve in skin color phenotyopes. 42

43 IB BIO 10.2 Understandings U4: The phenotypes of polygenic characteristics tend to show continuous variation. For example, human skin color is determined by at least 3 independent genes (6 alleles), resulting in continuous variation. As shown here, there is a defined curve in skin color phenotyopes. 43

44 IB BIO 10.2 Applications A3: Polygenic traits such as human height may also be influenced by environmental factors. Polygenic traits, such as human height, can be influenced by environmental factors such as diet and exercise. These influences result in a range of values in the species and so display continuity. 44

45 IB BIO 10.2 Applications A3: Polygenic traits such as human height may also be influenced by environmental factors. Even with the same genetic information, identical twins can display different variations of a trait based on the environments they are exposed to. 45

46 VIDEOS IB BIO 10.2 SnapRevise: Types of Variation AK Lectures: Polygenic Inheritance 46