Chapter 6: Complex Patterns of Inheritance
Incomplete Dominance
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INCOMPLETE DOMINANCE is a condition in which neither allele for a gene completely conceals the
presence of the other. It results in intermediate expression of a trait. A heterozygote exhibits a phenotype
that is somewhere between a dominant phenotype and a recessive phenotype.
For example) The colour of snapdragons. A cross between true-breeding red-flowered plants and truebreeding white-coloured plants creates offspring with pink flowers in the F1 generation.
In the F2 generation, it is predicted that all three coloured flowers can occur at a ratio of 1:2:1
(red:pink:white).
When representing an incomplete dominance, upper-case and lower-case letters are not usually used to
represent the alleles, since neither allele is dominant over the other. One way to represent incomplete
dominance is by using superscripts. Example) CR (red-coloured flowers); and CW (white-coloured flowers).
Incomplete Dominance and Human Disease
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For example) A genetic disorder called familial hyper-cholesterolemia. It prevents tissues from removing
low-density lipoproteins (LDL) from the blood, causing very high levels of cholesterol in the bloodstream.
People who are homozygous dominant for the trait have 6X the normal amount of LDL in their blood and
may have a heart attack by the age of 2. Heterozygotes have twice as much LDL in their blood and may
have a heart attack by the age of 35.
Co-dominance
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CO-DOMINANCE is a condition in which both alleles for a trait are equally expressed in a heterozygote.
Both alleles are dominant.
For example) Roan animals (ie: cows) is heterozygote in which both base colour and white are both fully
expressed. You will see a mixture of red hairs and white hairs.
For example) Sickle Cell Anemia.
o It is often described as being the result of autosomal recessive inheritance, but it is actually an
example of co-dominance.
o It is caused by a specific form of the gene that directs the synthesis of hemoglobin. Hemoglobin
carries oxygen in the blood.
o The hemoglobin molecule that is made in individuals with the sickle cell allele leads to a C-shaped
(or sickled) red blood cell. These misshaped red blood cells do not transport oxygen effectively
because they cannot pass through small blood vessels. This leads to blockages and tissue damage.
o The allele for normal hemoglobin is represented as HbA, and the allele for sickle cell hemoglobin is
represented as HbS.
o Individuals who are homozygous (HbSHbS) have sickle cell anemia.
o Individuals who are heterozygous (HbAHbS) have some normal and some sickle cell red blood cells.
These individuals are said to have sickle cell trait, but they rarely experience any symptoms.
However, one advantage to being heterozygous is that you are more resistant to malaria. Since the
parasites for malaria infects the liver and eventually the red blood cells – the sickling of red blood
cells is thought to prevent the parasites from infecting the cells.
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The sickle cell trait is an example of the principle of HETEROZYGOUS ADVANTAGE which
describes a situation in which heterozygous individuals have an advantage over both homozygous
dominant and homozygous recessive individuals.
If both a man and woman have the sickle cell trait – their offspring have a 25% chance of being
“normal”, a 50% chance of having the sickle cell trait; and a 25% chance of having sickle cell
anemia.
Multiple Alleles
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A gene with more than two alleles is said to have multiple alleles.
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Human Blood Groups:
o In humans, a single gene determines a person’s ABO blood type. It determines what type of antigen
protein, if any, is attached to the cell membrane of red blood cells. (An antigen protein is a molecule
that stimulates the body’s immune system).
o The gene is designated I, and it has three common alleles: IA, IB, i.
o The combination of the three alleles produces four different phenotypes:
o Blood type A (IAi heterozygote, or IAIA homozygote)
o Blood type B (IBi heterozygote, or IBIB homozygote )
o Blood type AB (IAIB heterozygote)
o Blood type O (ii homozygotes).
o The allele i is recessive, and the other two are co-dominant.
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Other examples of traits determined by multiple alleles are: Rabbit coat colour, and Clover leaf patterns.
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Environmental Effects on Complex Patterns of Inheritance
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For example) Some genes are influenced by temperature. The dark colour of Himalayan rabbits is on the
cooler parts of their bodies (face, ears, tails, and feet). The colour change is due to the lower body
temperatures in these areas.
Polygenetic Inheritance
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There are traits that exhibit CONTINUOUS VARIATION. This is a range of variation in one trait resulting
from the activity of many genes. These are traits for which the phenotypes vary gradually from one
extreme to another.
Examples of traits that show continuous variation include height and skin colour in humans, ear length in
corn, and kernel colour in wheat.
Traits that exhibit continuous variation are usually controlled by more than one gene. Traits that are
controlled by many genes are called POLYGENETIC TRAITS. A group of genes that all contribute to the
same trait is called a polygene. Each dominant allele contributes to the trait. Recessive alleles do not
contribute to the trait. Example) skin colour – the more dominant alleles a person has, the darker the skin.
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Inheritance of Linked Genes
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Alleles on the same chromosome do not assort independently. Therefore, they do not follow Mendelian
inheritance patterns. Genes are inherited together. And, some traits are often inherited together or are
“linked”.
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In 1905, William Bateson and Reginald Punnett carried out a study to show the movement of alleles that
are on the same chromosome. Using sweet peas, they tracked the inheritance pattern of two traits: flower
colour and pollen shape.
o They knew that purple flowers were dominant to white flowers, and that long pollen shape was
dominant to round pollen shape.
o All four phenotypes that are predicted using a Punnett square were represented in the F2
generation. However, there were more phenotypes from the parental generation. This suggested
that the gametes produced by the parental generation, PL and pl, tended to assort together rather
than independently when producing the F2 offspring.
o Genes that do not assort independently are often called LINKED GENES. Linked genes are genes
that are on the same chromosome and that tend to be inherited together.
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Crossing Over and the Inheritance of Linked Genes
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A chromosome may contain up to a few thousand genes.
All of the genes on any one chromosome are called a linkage group because they tend to be
inherited together.
However, linked genes do not always stay linked. This is due to crossing over, which occurs during
prophase I of meiosis, when non-sister chromatids exchange pieces of chromosomes.
Crossing over occurs more often between alleles that are far apart on a chromosome than between
alleles that are close together. Therefore, a given pair of linked genes will separate more frequently
than the alleles for another given pair of linked genes if their alleles are farther apart on the
chromosome.
The process of determining the relative locations of genes on chromosomes is called chromosome
mapping. Chromosome mapping is useful for studying species that reproduce rapidly and
produce many offspring (such as plants and insects). It is not useful in human chromosomes.
Sex-Linked Inheritance:
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Thomas Hunt Morgan studied fruit flies. He found an unusual white-eyed male among his fly
population. He crossed the white-eyed male with a normal red-eyed female. All of the F1
generation has red eyes. This indicated that normal red eyes are dominant to the white-eyed
mutation.
However, when he crossed a male and female from the F1 generation, all the female offspring in the
F2 generation had red eyes. However, half the F2 generation males had red eyes, and half the F2
generation males had white eyes.
He concluded that the gene for eye colour was connected to gender. Therefore, the gene for eye
colour was located on the X chromosome.
Traits that are controlled by genes on either the X or Y chromosome are called SEX-LINKED
TRAITS, because they are linked to the genes that determine sex.
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Sex-Linked Genes
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The human X chromosome contains about 2000 genes, while the Y chromosome contains fewer than 100
genes.
The most important genes are the sex-determination genes. For all other genes on the X chromosome,
females have two copies, while males only have one copy. This allows for the difference in the expression
of traits for genes that are found on the X chromosome, and are referred to as X-linked genes.
Only a few genes are known to be Y-linked, because there are significantly fewer genes on the Y
chromosome.
The red and white eyes of fruit flies are a good example of X-linked genes. XR indicates red eyes, which is
the dominant phenotype; and Xr indicates white eyes, which is the recessive phenotype. Female flies may
be a carrier for the white-eye phenotype. However, if the allele for white eyes is present in males, it will
always be expressed. This means that X-linked traits are expressed more often in males.
See Page 254, Figure 6.13: Inheritance of Sex-Linked Traits.
Sex-Linked Traits in Humans
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Many sex-linked traits in humans are genetic disorders.
If a disorder is X-linked dominant, affected males pass the allele only to daughters, who have a 100%
chance of having the disorder.
Females can pass an X-linked dominant allele to either sons or daughters, all of whom will have the
disorder.
Most sex-linked inherited traits in humans are X-linked recessive traits. Therefore, while the male only
needs to inherit one allele to be affected; the female must inherit both alleles to be affected. Therefore, Xlinked recessive traits affect more males than females.
Examples of X-linked recessive conditions are: Hemophilia, and red-green colour vision deficiency.
Sex-linked Traits in Humans:
Condition
Inheritance Pattern
Red-green vision
deficiency (CVD)
Duchenne muscular
dystrophy
Hemophilia
X-linked recessive
Adrenoleukodystrophy
X-linked recessive
X-linked severe combined
immunodeficiency (SCID)
X-linked
hypophosphatemia
Hairy ears
X-linked recessive
Description
X-linked dominant
Cannot distinguish between certain shades of
red and green.
Progressive weakening of muscles and loss of
coordination.
Cannot produce a necessary blood clotting
factor.
A build-up of fatty acids that causes progressive
brain damage and death.
Decreased immune response due to low white
blood cell counts.
Softening of bone that leads to bone deformity.
Y-linked
Hair grows on the outside of the ears.
X-linked recessive
X-linked recessive
o Colour Vision Deficiency (CVD): An X-linked Recessive Trait:
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Individuals affected by CVD have varying degrees of difficulty distinguishing between different
colours or shades of colours.
One form, called red-green CVD, is an X-linked recessive disorder. Individuals with this disorder
have difficulty distinguishing between shades of red and green.
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o Hemophilia: A Common Sex-linked Trait in Humans:
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Hemophilia is a condition that affects the body’s ability to produce proteins involved in blood
clotting.
People with hemophilia can suffer serious blood loss form simple cuts and bruises.
Hemophilia is an X-linked recessive trait that affects more than 3000 Canadians.
Hemophilia is often referred to as the royal disease because it spread among the royal families of
Europe, through the descendants of Queen Victoria.
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Barr Bodies: Inactive X Chromosomes
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Since females carry two X chromosomes and males only have one X chromosome, why is there no
difference in the expression of X-linked genes between males and females?
o Every cell has only 1 functioning X chromosome. In every female cell, one of the X chromosomes is
inactive.
o The inactive chromosome is condensed tightly into a structure known as a Barr body.
o At an early stage of embryonic development, one X chromosome in each cell will deactivate.
What is Our Genome?
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The Human Genome Project has found the following:
o Only about 2% of the nucleotides in the human genome make up our genes and code for all the
proteins in the body.
o The estimated 25,000 total number of genes is much less than scientists predicted. They had
originally thought we had between 80,000 and 140,000 genes.
o Over 50% of our DNA consists of stretches of repeated sequences.
o There is very little genetic variation within our species. About 99.9% of the DNA sequence is almost
exactly the same in all people.
GENOMICS is the study of genomes and how genes work together to control phenotype.
The Human Genome Project allowed scientists the ability to consider multiple genes and the genome as a
whole. It allows scientists to study the interactions among many genes and how they all contribute to a
phenotype.
Even though there is little variation in the sequence of the human genome, that 0.1% difference represents
potential for variation in about 3 million nucleotides!
o Some of this variation is associated with diseases.
o The most common type of variation between people is differences in individual nucleotides.
o SNP (single nucleotide polymorphism) can act as a marker for a gene or be associated with a gene.
There are almost 10 million different SNPs that commonly occur in the human genome.
Genetic Information: Public Benefits and Concerns
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Some of the most important benefits of genetic research are in the area of human medicine.
Studying the human genome as a whole may make it possible to develop drugs that are tailored to the
expression of the genes associated with particular disorder.
If the cost of DNA sequencing continues to decrease, people may have access to their GENETIC PROFILE
– their complete genotype, including all of the various mutations linked to disease.
With a genetic profile, doctors will be able to provide more specific risk assessments, design individualized
prevention plans, and design genetically precise treatment programs.
Some possible downfalls of genetic profiling could be: insurance companies or employers refusing to deal
with certain individuals who are predisposed to particular diseases.
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