SBI3U

Mendel Demonstrated:
◦ Law of Segregation
◦ Law of Independent Assortment
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
The alleles for a gene segregate
independently during meiosis.
This applies to alleles located on
homologous chromosomes
This theory was proven to be incorrect.
See pg. 251- Does not show the 9:3:3:1 ratio
 Alleles
on the same chromosome do not
assort independently.
 Some genes that are on the same
chromosome are inherited together and
are called linked genes.
 An example of this additional complex
pattern is found in sweet pea plants.
Linked genes:
on the same
chromosome
and tend to
be inherited
together.
Linkage Group:
Genes on a chromosome that
are always inherited together
(unless crossing over occurs)
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All genes on any one chromosome are
called a linkage group b/c they tend to be
inherited together
Crossing over can prevent linked genes
from staying together.
◦ Ex. 2 genes are on the same chromosome
◦ The alleles for the linked genes switch
◦ Now they are unlinked and will migrate to
different gametes
UNIT 2 Chapter 6: Complex Patterns of Inheritance

Section 6.2
Recombinant alleles: linked genes that have been
separated during crossing over
In most of the gametes
formed, there is no crossing
over—they maintain the
linkage of the alleles. In a
small minority of gametes,
crossing over occurs and
alleles of previously linked
genes become unlinked.


Used to determine location of genes on a
chromosome
◦ Useful for organisms that reproduce
rapidly (e.g. fruit flies)
Frequency of crossing over is connected to
how close the pairs of linked genes are
◦ Closer together = low frequency
◦ Further apart = high frequency

Thomas Hunt Morgan studied the fruit fly
(Drosphila melanogaster)
◦ He crossed a pure red eyed female with a
white eyed male and this is what he
found…
Genotype
Phenotype
XR XR
Female with red eyes (homozygous
dominant)
XR Xr
Female with red eyes (heterozygous)
Xr Xr
Female with white eyes (homozygous
recessive)
XR Y
Male with red eyes
Xr Y
Male with white eyes
XR
XR
Xr
XR Xr
Y
XR Y
XR Xr
XR Y
In the F1 generation:
100% of offspring
had red eyes.
XR
Xr
In the F2
generation:
XR Xr
100% of the
females have red
eyes.
Xr Y
50% of the males
have white eyes
XR
XR XR
Y
XR Y
50% of the males
have red eyes.
Conclusion: The gene for eye colour is
connected to gender and located on
the X chromosome.
 Traits that are controlled by genes on either the
X or Y chromosome are called sex-linked
traits.
The X chromosome contains
2000 genes


The Y chromosome only contains 100 genes

Sex chromosomes contain some genes that
are unrelated to sex determination.

Each sex-linked gene can contain dominant
or recessive alleles,
X-Linked Dominant:
◦ Affected males pass on
only to daughters (100%
chance)
◦ Females pass to both
sons and daughters
(100% chance)
◦ Very rare
◦ Ex. Fragile X Syndrome
:
X-Linked Recessive
◦ Affect more males than
females because males
only need one allele to
be affected, while females
need 2 recessive alleles to
be affected
◦ Ex. hemophilia, redgreen colour vision
deficiency
X – linked recessive
X – linked dominant
An X-linked recessive trait like CVD will affect more males than females
in a family. These males will have difficuty distinguishing between
shades of red and green
Determine the probability that a woman who is
a carrier for hemophilia and a man without
hemophilia will have a child with hemophilia.
a)
b)
c)
What proportion of the female children with
have hemophilia?
What proportion of male children will have
hemophilia?
Is it possible for these parents to have a
female with hemophilia?
A human female with red-green color
deficiency (CVD) marries a normal male.
a)
b)
What proportion of their children will have
CVD?
Is it possible for the parents to have a female
with CVD? Why or why not?
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Read and make notes 6.2
Complete pg. 258 Q# 11-13, 16, 17