Summary From Last Class
 Blood type is controlled for by one gene and the
interactions between 3 alleles, referred to as IA IB Ii
alleles
 Blood type refers to the antigen present on the
membrane of red blood cells.
 Your body produces antibodies to antigens, not
present inside of you. These antibodies clot red blood
cells.
 Therefore, AB individuals (both antigens present) are
universal recipients and type O are universal donors
(no antigens present)
Revisiting Incomplete Dominance
 Traits exhibiting incomplete dominance are an excellent
way to prove genes do not mix.
 Despite exhibiting a blending of alleles, the F2
generation still produces white and red phenotypes
Looking at Incomplete Dominance
on the Cellular Level
 The individual alleles coding for
plant colour, still must sort
independently
 Or else the red and white
phenotypes would not reappear
in the F2 generation of plants
 Using your knowledge of dihybrid crosses, what do you
think the expected phenotype for the F1 generation
would be if these two plants were crossed?
 Assume, purple flower and long pollen are dominant
alleles and both plants have heterozygous genotypes.
 The F1 generation would be PpLl
 Now what if you cross-fertilized the F1 generation?
 What would your phenotype ratios be?
 Recall 9:3:3:1
 The results of this particular experiment are:
 This evidence indicates some alleles do not sort
themselves independently. How is this possible?
Linked Genes
 One chromosome can code
for thousands of genes.
 Therefore genes on the same
chromosome, tend to go
together in pairs and are
inherited together.
 This is referred to as a linked
gene
Crossing Over and Linked Genes
 Linked genes, do not always have to be inherited
together.
 Recall crossing over in prophase I of meiosis, the
exchange of genetic material
 Due to crossing over, genes
which were previously linked
become unlinked. Happens
very rarely
 If the linked genes were close
together on the chromosome,
would they be more or less
likely to become unlinked?
Why Does This Happen?
What is Hemophilia?
 The individual is
lacking the ability to
produce proteins in
their blood to help it
clot, when a cut
occurs.
 The trait coding for
these proteins are on
the X chromosome.
The trait coding for no
proteins is recessive.
Why Are Males Affected More?
 Genes on your sex chromosomes can also be linked
 Traits controlled or coded for be genes on your sex
chromosomes are referred to as sex-linked
 The human x chromosome is estimated to contain
about 2000 genes
 The Y chromosome contains fewer than 100! Most of
these genes are responsible for sex determination
 Males only have one copy
of the X chromosome
(XY)
 Females have two copies
of the X chromosome
(XX)
 Due to this, any X-linked genetic conditions, males
will be much more susceptible to because they only
have one copy of the X chromosome
 While females are ‘protected’ from genetic disorders
on the X chromosome because they have two copies of
X chromosome
Practice Problem
 Imagine you are a mother, who is eight months pregnant.
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You and your husband have had genetic screening done
and you find out, that you are a carrier for hemophilia
(heterozygous), while your husband does not have the
allele for hemophilia.
What are the chances you will have a son with hemophilia?
What are the chances you will daughter with hemophilia?
Let X and Y represent the sex chromosomes
Let H represent normal allele
Let h represent the hemophilia allel
 Red-Green Color Vision is
another trait located on
the X chromosome.
 Red-green color
deficiency is a controlled
for by a recessive gene.
Much like the hemophilia
gene.
Summary
 One chromosomes contains many genes. Traits/genes
located on one chromosome tend to go together and
do not follow the law of independent assortment.
 Similarly, the X chromosome codes for many traits,
including hemophilia and red-green colour deficiency
 Hemophilia and red-green colour deficiency are both
X-linked recessive conditions
 Males are more susceptible to X-linked conditions
because they only contain one copy of the X
chromosome
Quiz on Tuesday November 23
 Blood Type
 Monohybrid cross problems
 Dihybrid cross problems
 Sex linked problems
 Incomplete dominance
 Around 8 questions