1. Topic 10.2 Inheritance

2. Understanding: Unlinked genes segregate independently as a result of meiosis
Segregation = separation of the 2 alleles of a gene during meiosis
Independent assortment = alleles of one gene segregate independently of the alleles of other genes (due to random orientation of chromosomes during metaphase I)
Unlinked genes = genes found on different chromosomes
Segregate independently
Linked genes = genes found on same chromosome & always inherited together
Do not segregate independently

3. Unlinked vs linked genes
Unlinked Genes
Linked Genes
Found on different chromosomes
Segregate independently of other genes
Found on same chromosome
Do not segregate independently
Inherited together (unless crossing- over occurs)
An exception to Mendel’s law of IA

4. Understanding: Gene loci are said to be linked if they are on the same chromosome
Thomas Hunt Morgan: the Fruit Fly scientist (Drosophila melanogaster)
Born 1866; college at 16; PhD from Johns Hopkins at 24; Nobel Prize at 67
He & his student observed that yellow body and white eyes are inherited together, and so established idea of “gene linkage”
He & his student, Alfred Sturtevant, created the first ever genetic map (identifying the locus of genes on a chromosome)
2 types of gene linkage:
Autosomal gene linkage = genes on same autosome
Sex linkage = genes on the X chromosome

5. Drosophila genetic map

6. Autosomes vs sex chromosomes

7. Application: Completion and analysis of Punnett squares for dihybrid traits.
Dihybrid cross = shows inheritance of 2 traits
Dihybrid cross = good way to show independent assortment of genes
The chance of a gamete containing S or s will not affect its chance of containing Y or y.
The chance of a gamete containing S or s = ½
The chance of a gamete containing Y or y = ½
The chance of a gamete containing two specific alleles (SY or Sy or sY or sy) = ½ * ½ = ¼
S and Y alleles segregate into gametes independently of each other.

8. Skill: Calculation of the predicted genotypic & phenotypic ratio of offspring of dihybrid crosses involved unlinked autosomal genes
Complete p 447 Skills

9. Nature of Science: Looking for patterns, trends & discrepancies: Mendel used observations of natural world to find & explain patterns & trends. Since then, scientists have looked for discrepancies & asked questions based on further observations to show exceptions to the rules. E.g. Morgan discovered non-Mendelian ratios in his experiments with Drosophila.
Thomas Hunt Morgan – discovered non-Mendelian ratios in Drosophila melanogaster (fruit fly)
Observed thousands and thousands of fruit flies by hand
Critical of Mendel’s theory of inheritance
Believed variation was better explained by environmental influence, then reconsidered this
Morgan’s studies reinforced some of Mendel’s conclusions AND identified exceptions to principle of independent assortment

10. Understanding: Variation can be discrete or continuous
Variation = differences between individual organisms
Discrete (discontinuous) variation = specific number of distinct categories
E.g. blood types A, B, AB or O; purple or white flowers; free or attached earlobes
Frequency of blood types in US:

11. Understanding: Phenotypes of polygenic characteristics tend to show continuous variation
Continuous variation = variation within a population in which a graded series of intermediate phenotypes falls between the extremes (a complete range of variation)
E.g. human height, human intelligence, hair, eye & skin color, bean mass
Shows normal distribution
Means that more than one gene is involved = polygenic inheritance

12. Application: Polygenic traits such as human height may also be influenced by environmental factors
Continuous variation due to the differences between categories is subtle and the effects of the environment blur these differences so much that they are undetectable
E.g. skin color – due to polygenetics and environment (exposure to UV)
E.g. identical twin athletes with different diets & exercise routines
E.g. identical mice clones fed different diets

13. Skill: Identification of recombinants in crosses involving 2 linked genes
Linked genes are inherited together, unless crossing over occurs
Recombination = new combination of alleles that is different from parents
Recombinants = individuals that have the new combination of alleles (different from parents)

14. Practice: A cross is performed between two organisms with the genotypes AaBb and aabb. What genotypes in the offspring are the result of recombination?
A. Aabb, AaBb
B. AaBb, aabb
C. aabb, Aabb
D. Aabb, aaBb

15. Practice: Which are the possible recombinants in a dihybrid test cross involving the linked genes ?

16. Skill: Use of a chi-squared test on data from dihybrid crosses.
The expected ratio of a dihybrid cross of 2 heterozygous individuals is 9:3:3:1.
What if the observed ratio is different?
Is there a significant difference?
Are the differences between observed & expected due to human error (e.g. sampling error) or due to chance alone?
“Statistical significance" = When a statistic is significant, it simply means that you are very sure that the statistic is reliable. It doesn't mean the finding is important or that it has any decision-making utility. This means that you are very sure that the difference is real (i.e., it didn't happen by fluke). It doesn't mean that the difference is large or important. Significance is a statistical term that tells how sure you are that a difference or relationship exists. For the statistician, it may be wise to adopt a policy of always referring to "statistical significance" rather than simply "significance" when communicating with the public. (
Chi squared test

17. 1.In the Sailfin Molly fish, gold colour (g) is recessive to normal colour (G). When a gold coloured fish was crossed with a normal colour fish, 55 of the offspring were normal colour and 45 were gold colour. One hypothesis is that the normal parent was heterozygous. The Chi-squared value is 1. The table shows the probability values.
Probability
Degrees of freedom
0.99
0.950
0.05
0.01 1
0.000
0.004
3.84
6.64 2
0.020
0.103
5.99
9.21 3
0.115
0.352
7.82
11.35
Which of the following is the correct response?
A. Accept the hypothesis because the Chi-squared value is less than 3.84.
B. Reject the hypothesis because the probability is less than 0.05.
C. Accept the hypothesis because the Chi-squared value is less than 5.99.
D. Reject the hypothesis because there is not enough evidence.