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Grade 11 University Biology - Unit 2 Genetics
Inheritance of Linked Genes
Chapter 6.2 – Pages 251-259
Mendel’s Law of Independent Assortment states that the alleles for a gene segregate independently of
the alleles for the other genes during gamete formation. That is, the inheritance of one trait has no
influence on the inheritance of another trait.
Let’s review this idea...
Using Mendel’s work with peas with two characteristics: seed shape and seed colour.
 The dominant allele for seed shape is round (R) while the recessive allele is wrinkled (r)
 The dominant allele for seed colour is yellow (Y) while the recessive allele is green (y)
 Mendel crossed two homozygous parents for the two traits (RRYY and rryy) to produce two pairs
of heterozygous alleles (Rr and Yy). This is called a DIHYBRID CROSS. The Punnett square
below illustrates the cross.

RY
RY
ry
RrYy
RrYy
ry
RrYy
RrYy
A heterozygous individual for two characteristics (RrYy) produces four possible gametes: RY,
Ry, rY and ry. NOTE: There is no gamete labelled Rr or Yy....rather, the alleles (R and r; Y and
y) separate independently during the formation of the gametes. This is the Law of Independent
Assortment (e.g., alleles R and r separate). Given your understanding so far, why does this
make practical and logical sense?
Yet, this is not always so.
Walter Sutton showed that alleles on the same chromosome do NOT assort independently. In other
words, some genes are inherited together, and as such, some traits are often inherited together (i.e.,
some traits are “linked”).
Linked Genes
 Genes that are on the same chromosome and that tend to be inherited together
 When a pair or set of genes are on the
same chromosome, they are usually
inherited together or as a single unit. For
example, in fruit flies the genes for eye
color and the genes for wing length are
on the same chromosome of fruit flies.
As such, these traits are inherited
together...or linked.
Crossing Over and Inheritance of Linked
Genes
 A single chromosome may contain a few
thousand genes
 All the genes on one chromosome are
called a LINKAGE GROUP. Why? The
traits tend to be inherited together.
 Yet, linked genes do NOT always stay
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
together. Why? Crossing Over during Prophase 1 of meiosis whereby non-sister chromosomes
exchange pieces of chromosomes
See Figure 6.11 on Page 252 and illustration above
NOTE: Crossing Over is a random event and it occurs infrequently
Using Gene Linkage for Chromosome Mapping
 It is known that alleles for a given pair of linked genes separate with a predictable frequency AND
the frequency is different for different pairs of linked genes
 The frequency of crossing over depends on how close the alleles of the linked genes are
positioned on a chromosome. That is, crossing over occurs more frequently between alleles that
are FAR APART on a chromosome.
 Predicting the rate of crossing over frequency is called chromosome mapping
Sex-Linked Inheritance
 A sex-linked trait is a trait controlled by genes on the X or Y chromosome. Recall, female is XX
and male is XY
 Humans have 23 pairs of chromosomes. One set is the sex chromosomes, while the other 22
pairs are autosomes (non-sex chromosomes). If an allele is found on an autosome, it is called
autosomal inheritance (...inheritance of alleles located on non-sex chromosomes).
 Thomas Hunt Morgan crossed fruit flies (Drosophila melanogaster). The cross between whiteeyed males and red-eyed females produced fruit flies with only red eyes in Generation F 1; yet,
when two F1 parents were crossed, the Generation F2 ratio was 100% females with red eyes,
50% males with red eyes and 50% males with white eyes. Try to explain why using a Punnett
square. NOTE: Assume white eye is homozygous recessive and red eye is homozygous
dominant.
P Generation
R
R
r
Rr
Rr
r
Rr
Rr
Outcome is the F1 generation is all flies with dominant heterozygous red eyes
F1 Generation
R
r
R
RR
Rr
r
Rr
rr
Outcome is the F2 generation is the anticipated 1:2:1 ratio of phenotypes expressions.
Yet, this does not explain the SEX difference observed.

There must be a gender link. That is, the gene for eye colour must be present on the X sex
chromosome. This is called a SEX-LINKED TRAIT
 A trait controlled by genes on the X chromosome or Y chromosome, and trait is
influenced by different rates as they appear on males and females

The X and Y chromosomes had very little homologous DNA. The X chromosome contains over
2000 chromosomes while the Y chromosome has about 100 chromosomes. As well, all other
genes on the X chromosome in the female consist of two copies (Recall XX). This results in
alleles on the X and Y chromosomes not always being paired equally. As such, an allele for a
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

particular trait may be located on only one sex chromosome (...usually the X chromosome).
The difference in expression of traits on X chromosome is called X-Linked Genes. Likewise,
difference on Y chromosome is Y-Linked Genes.
The X chromosome carries hundreds of genes but few, if any, of these have anything to do
directly with sex. However, the inheritance of these genes follows special rules due to the
following facts:
 males have only a single X chromosome
 almost all the genes on the X have no counterpart on the Y
 any gene on the X, even if recessive in females, will be expressed in males.
Repeat the experiment above, but assign the genotype to the X or Y chromosome.
X RX R
Female, red eyes (homozygous dominant)
X RX r
Female, red eyes (heterozygous dominant)
X rX r
Female, white eyes (homozygous dominant)
X RY
Male, red eyes
X rY
Male, white eyes
P Generation
XR
XR
Xr
X RX r
X RX r
Y
X RY
X RY
XR
Xr
XR
X RX R
X RX r
Y
X RY
X rY
F1 Generation
Also see Figure 6.13 on Page 254. Outcome explains the observed sex differences with
100% red-eyed females, 50% red-eyed males and 50% white-eyed males in the F2
Generation.
Sex-Linked Traits in Humans
 Many disorders are sex-linked (see Table 6.2 on Page 255)
Disorder is X-linked dominant
Affected male passes allele to only females with 100% chance
of disorder
Disorder is X-linked dominant
Affected female passes allele to son or daughter with 100%
chance of inheritance
Male only need to inherit one allele to inherit the disorder. The
female needs to inherit both alleles to inherit the disorder. Thus,
more males will be affected.
Disorder is X-linked recessive
Male may inherit the disorder with 25% chance of inheritance
Female may inherit the disorder (25% chance), but she is a
carrier (i.e., no phenotype expression)


Also see Figure 6.14 on Page 255
Hemophilia is X-linked recessive. It is also called the Royal Disease.
Barr Bodies
 As you know, human females inherit two
copies of every gene on the X
chromosome, whereas males inherit only
one (...with, of course, some exceptions).
 You also know, that there is an
inactivation of one of the chromosomes
during fertilization.
 What if the inactivation chromosome
remains? In this case, the cell is called a
Barr body or sex chromatin. Females
usually have one Barr body; males usually have none. Why?
 A Barr body is an inactive, condensed X chromosome in the female cell.
 Inactivation is random.
 The expression of the Barr body produces
different phenotypes (see Figure 6.16 on Page
258).
 The human abnormalities called Kleinefelter's
syndrome and Turner's syndrome both result
from an unnatural presence or absence of a Barr
body. In the former, the male possesses a Barr
body that it would normally not have (XXY and,
as such, 47 total chromosomes), and in the
latter, the Barr body is absent (X or less than 46
total chromosomes). The chart opposite is
Turner’s syndrome...note the missing X
chromosome (circled)
Questions
 Conduct Internet research of the Royal Disease. That is, “Why did it pass through royal families
of Europe?” Read the story reported in Aronova-Tiuntseva, Y. and C.F. Herreid. 2003.
Hemophilia: The Royal Disease” and follow the pedigree to explore the inheritance of the
disorder online at http://www.sciencecases.org/hemo/hemo.asp
 Do Sample Problem on Pages 256-257
 Answer Questions 11-20 on Page 258
 Answer Questions 2, 4-5, 7-10 and 15 on Page 259
Test your understanding
In watermelons, the green colour gene (G) is dominant over the striped colour gene (g). As well, the
round shape (R) is dominant over the long shape (r). A heterozygous round green colour watermelon
(GgRr) is crossed with another heterozygous round green colour watermelon (GgRr). Calculate the
expected phenotype ratio of the F1 generation.
Try solving the problem prior to reading the solution,
Solution
1. Determine possible gametes from each parent
In this case – GR Gr gR gr
2. Draw a Punnett square of the dihybrid cross and execute the cross. NOTE: Since there are four
possible gametes from each parent, make a 4 X 4 grid as shown
GR
Gr
gR
gr
GR
GGRR
GGRr
GgRR
GgRr
Gr
GGRr
GGrr
GgRr
Ggrr
gR
GgRR
GgRr
ggRR
ggRr
gr
GgRr
Ggrr
ggRr
ggrr
3. Determine which plants have the same phenotype. That is, the plants that express the same
traits (e.g., GGRR, GrRR and GgRr all are green and round shaped).
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9 = green and round (G_R_)
3 = green and long (G_rr)
3- striped and round (ggR_)
1 = striped and long (ggrr)
Therefore, the phenotype ratio is 9:3:3:1