 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
Unlinked Genes
Linked Genes
 Found on different chromosomes
 Found on same chromosome
 Segregate independently of other
 Do not segregate independently
genes
 Inherited together (unless crossing-
over occurs)
 An exception to Mendel’s law of IA
 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
APPLICATION:
 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.
Skill:
 Complete p 447 Skills
Nature of Science: Looking for patterns, trends & discrepancies:
 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
 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:
 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
Application:
 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
Skill:
 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)
A.
B.
C.
D.
Aabb, AaBb
AaBb, aabb
aabb, Aabb
Aabb, aaBb
dihybrid test cross
recombinants
JQ
linked genes
jq
JQ
and
JJ
A.
jq
Jq
Qq
and
Qq
B.
Qq
Jq
JJ
and
jQ
C.
jq
JQ
jq
and
Jq
D.
jq
jQ
SKILL:
 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.
(http://www.statpac.com/surveys/statistical-significance.htm)
 Chi squared test
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.