Additional Higher Level 10.2 Inheritance Additional Higher Level Human karyotype

Nature of science: Developments in research follow improvements in techniques—autoradiography was used to establish the length of DNA molecules in chromosomes. (1.8)

Application: Cairns’ technique for measuring the length of DNA molecules by autoradiography Summary: Cells are grown in radioactive thymidine (thymine with deoxyribose) Cell DNA is added to membrane with photographic film Radioactivity of DNA exposes film. The length of the DNA can then be measured in micrometers Read details on p. 150-151

Nature of science: Making careful observations—careful observation and record keeping turned up anomalous data that Mendel’s law of independent assortment could not account for. Thomas Hunt Morgan developed the notion of linked genes to account for the anomalies. (1.8) Looking for patterns, trends and discrepancies—Mendel used observations of the natural world to find and explain patterns and trends. Since then, scientists have looked for discrepancies and asked questions based on further observations to show exceptions to the rules. For example, Morgan discovered non-Mendelian ratios in his experiments with Drosophila. (3.1)

Essential idea: Genes may be linked or unlinked and are inherited accordingly.

Unlinked genes (on different chromosomes) segregate independently as a result of meiosis.

Gene loci are said to be linked if on the same chromosome. Linked genes: genes located on the same chromosome that tend to be inherited together

Application: Morgan’s discovery of non-Mendelian ratios in Drosophila. Sex-linkage: genes located on a sex chromosome F:\OneDrive\Desktop\animations from bio powerpoints\Chapter 12 BDOL IC Thomas Hunt Morgan observed linked genes in Drosophila melanogaster (fruit flies)

Theory of knowledge The law of independent assortment was soon found to have exceptions when looking at linked genes. What is the difference between a law and a theory in science? Read the following: http://www.livescience.com/21457-what-is-a-law-in-science-definition-of-scientific-law.html

Crossing over

Recombinant offspring Recombinants are offspring that have different genotypes than their parents! Practice: For the cross TtGG x ttGg, identify the recombinant offspring.

Explain an example of a cross between two linked genes. Alleles are usually shown side by side in dihybrid crosses, for example, TtBb. In representing crosses involving linkage, it is more common to show them as vertical pairs, for example:

Identify which of the offspring are recombinants in a dihybrid cross involving linked genes. (See example of a TEST CROSS below) TtBb x ttbb (remember test cross) Possible gametes= (TB or tb) x tb With recombination all gametes are possible for the first parent. (TB, Tb, tB, tb x tb) The recombinants are those combinations that are different than the original parent chromosomes (because of crossing over).

Chi-squared tests are used to determine whether the difference between an observed and expected frequency distribution is statistically significant.

Skill:Use of a chi-squared test on data from dihybrid crosses.

Chi Square Steps: Step 1: State the Hypotheses Null hypothesis: Variable A and Variable B are independent (independent assortment) Alternative Hypothesis: Variable A and Variable B are not independent (linked genes) Note: do each step in your notes for the example that follows

Calculated as follows: Chi Square Steps: Step 2: Make a contingency table with observed and expected values   Round Yellow Round Green Wrinkled Yellow Green Total Observed (o) 315 108 101 32 556 Expected (e) 312.75 Calculated as follows: (9/16) x 556 104.25 (3/16) x 556 34.75 (1/16) x 556    Note: Expected values are determined by doing your cross, assuming independent assortment

Chi Square Steps: Step 3: Calculate chi-squared Round Yellow   Round Yellow Round Green Wrinkled Yellow Green Total Observed (o) 315 108 101 32 556 Expected (e) 312.75 104.25 34.75

Chi Square Steps: Step 4: Determine the number of degrees of freedom df = the number of classes -1 Ex. if there are 4 possible phenotypes, df would be 3 (i.e. 4-1)

Chi Square Steps: Step 5: Compare X2 to Critical Value for significance level (p) of 0.05 and the appropriate df. X2 > critical value = there is evidence the genes are linked (accept alternate hypothesis) X2 < critical value = the genes assort independently (accept null hypothesis)

Critical Values Table for X2

Chi-Squared assignment from book. Page 454 https://docs.google.com/forms/d/1ALkMjJGyW7Nc9yomUOhzBxOQiGqht2d8FcRSM5K5tPM/viewform?usp=send_form Enter your responses in the Google form above

Performing a Chi-Square test with the TI-83/84 Press [2nd MATRIX] Select [EDIT - > 1:A] Copy the data by typing in each number and then pressing ENTER Now press STAT. Under the TESTS sub-menu, scroll down and select C:X2 TEST.  Press ENTER. Move the cursor down to DRAW and press ENTER.

Utilization: An understanding of inheritance allowed farmers to selectively breed their livestock for specific characteristics.

End of IB

Genetic recombination Crossing over Allows recombination of linked genes. Linkage Group: A pair or set of genes on a chromosome that tend to be transmitted together. Genetic map - Ordered list of gene loci on chromosome The further apart 2 genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency 1 map unit= 1% recombination frequency A recombination frequency of 50% is expected for unlinked genes (on different chromosomes).

**Problem: Three genes are linked on the same chromosome in fruit flies: body color (b), wing size (vg) and cinnabar eyes (cn). The recombination frequency between cn and b is 9%, the recombination frequency between cn and vg is 9.5%, and the crossover frequency between b and vg is 17%. Draw a genetic map showing the relative locations of these genes.

Answer to question

The Chromosomal Theory of Inheritance Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment

Chromosomal errors, II Alterations of chromosomal structure: Deletion: removal of a chromosomal segment Duplication: repeats a chromosomal segment Inversion: segment reversal in a chromosome Translocation: movement of a chromosomal segment to another

Barr Bodies X-inactivation: 2nd X chromosome in females condenses into an Barr body (inactive X chromosome) (e.g., patches in calico/tortoiseshell pattern cats) inactivation occurs during early embryo development Which X chromosome is inactivated is random.

Human sex-linkage SRY gene: gene on Y chromosome that triggers the development of testes Fathers= pass X-linked alleles to all daughters only (but not to sons) Mothers= pass X-linked alleles to both sons & daughters Sex-Linked Disorders: Color-blindness; Duchenne muscular dystrophy (MD); hemophilia

Extranuclear (cytoplasmic) genes Found in mitochondria (also in plant plastids)– Remember these organelles have their own circles of DNA! Inherited from mother (why do all mitochondria come from mom?)

Genomic imprinting (Optional) Def: a parental effect on gene expression Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm. Fragile X syndrome: higher prevalence of disorder and retardation in males