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Genetics summary chapter 4 VU Amsterdam
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- Vrije Universiteit Amsterdam (VU)
This summary covers complete chaper 4 of genetics.
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Chapter 4 geneticsMany eukaryotic species follow a Mendelian inheritance pattern. Such traits obey two laws:
1. The law of segregation
2. Law of independent assortment.
Furthermore, the genes that influence such traits are not altered (except by rare mutations) as they are passed from parent
to offspring. The traits displayed by the offspring depend on the alleles they inherit and also on environmental factors.
- Simple Mendelian inheritance: one gene with two different alleles. One allele is dominant over the other allele
(recessive).
Geneticists want to understand Mendelian inheritance for two reasons:
1. One goal is to predict the outcome of the crosses. Many of the inheritance patterns do not produce a 3:1
phenotypic ratio when two heterozygotes produce offspring.
2. To understand how the molecular expression of genes can account for an individual’s phenotype. What is the
underlying relationship between molecular genetics – the expression of genes to produce functional proteins –
and the traits of individuals that inherit the genes.
Recessive mutant alleles often cause a reduction in the amount or function of the encoded protein. Geneticists refer to
prevalent alleles in natural population as wild-type alleles. In large populations, more than one wild-type allele may occur –
a phenomenon known as genetic polymorphism. At the molecular level, a wild-type allele typically encodes a protein that is
made in the proper amount and functions normally. Wild-type alleles tend to promote the reproductive success of
organisms in their native environments. In addition, random mutations occur in populations and alter pre-existing alleles –
these are referred to as mutant alleles to distinguish them from the wild-type alleles. Because random mutations are more
likely to disrupt gene function, mutant alleles are often defective in their ability to express a functional protein. Such
mutant alleles tend to be rare in natural populations. They are typically, but not always, inherited in a recessive manner.
The idea that recessive mutant alleles usually cause a decrease in the expression of a functional protein is supported by the
analysis of many human genetic diseases. A genetic disease is typically caused by a mutant allele. Molecular techniques
have enables researchers to clone these genes and determine the differences between wild-type and mutant alleles. They
have found that recessive alleles usually contain a mutation that causes a defect in the synthesis of a fully functional
protein. Diploid individuals have two copies of every gene. In a simple dominant/recessive relationship, the recessive allele
does not affect the phenotype of a heterozygote. A simple copy of a dominant allele is sufficient to mask the effects of the
recessive allele. If the recessive allele cannot produce a functioning protein, how do we explain the wild-type phenotype of
the heterozygote? A common explanation is that 50% of the functional protein is adequate to provide a wild-type
phenotype. PP homozygote and Pp heterozygote each make sufficient amounts of the functional protein to yield purple
flowers. A second possible explanation for other dominant alleles is that the heterozygote actually produces 50% of the
functional protein. Due to gene regulation, the expression of the normal gene may be increased, or up-regulated in the
heterozygote to compensate for the lack of function of the defective allele.
Though dominant mutant alleles are much less common than recessive mutant alleles, they do occur in natural populations.
How can a mutant allele be dominant over a wild-type allele? One of the three mechanisms accounts for most dominant
mutant alleles:
1. Gain-of-function mutation: this type of mutation changes the gene or the protein encoded by a gene so that it
gains a new or abnormal function. For example, a mutant gene may be overexpressed or may be expressed in the
wrong cell type.
2. Dominant-negative mutation: this type of mutation changes the protein such that the mutant protein acts
antagonistically to the normal protein. In a heterozygote, the mutant protein counteracts the effects of the
normal protein, thereby altering the phenotype.
3. Haploinsufficiency: the dominant mutant allele is a loss-of-function allele. Haploinsufficiency is used to describe
patterns of inheritance in which a heterozygote (with one functional allele and one inactive allele) exhibits an
abnormal or disease phenotype. An example is polydactyly.
Dominant alleles are expected to influence the outcome of a trait when they are present in heterozygotes, but this does not
always happen. This is called incomplete penetrance – a situation in which an allele that is expected to cause a particular
phenotype does not.
Polydactyly
Polydactyly causes the individual to have extra fingers or toes and it is due to an autosomal dominant allele – the allele is
found in the gene located on an autosome (not a sex chromosome) and a single copy of this allele is sufficient to cause this
disease. In the case of polydactyly, the dominant allele does not always ‘penetrate’ into the phenotype of the individual and
therefore shows incomplete penetrance.
, Chapter 4 genetics
The measure of penetrance is described at the population level. If 60% of the heterozygotes carrying a dominant allele
exhibit the trait, we say that this trait is 60% penetrant. At the individual level, the trait is either present or not. Another
term used to describe the outcome of traits is the degree in which the trait is expressed, or its expressivity. In the case of
polydactyly, the number of extra digits can vary. For example, one individual may have an extra toe whereas a second
individual may have extra digits on the hand. A person with several extra digits would have a high expressivity and a person
with only one extra digit, in this case the one extra toe, would have a low expressivity of this trait. The range of phenotypes
is often due to environmental influences and/or modifier genes (genes that alter the phenotypic effects of other genes).
- Temperature-sensitive allele: allele where the phenotypic effects are dependent on the temperature. In the
winter, the fox is white and in the summer, the fox is brown.
The relationship between environmental and phenotype can be seen in the human genetic disease known as PKU. This
autosomal recessive disease is caused by a defect in the gene that encodes the enzyme phenylalanine hydroxylase.
Homozygous individuals with this defective allele are unable to metabolize the amino-acid phenylalanine, which is found in
most protein rich foods. Since the 1960s, testing methods have been developed that can determine if an individual is
lacking the phenylalanine hydroxylase enzyme. The diets of infants with PKU can be modified before the harmful effects of
phenylalanine ingestion have occurred. The fox and the PKU individuals provide examples of the effects of different
environmental conditions.
- Norm of reaction: refers to the effects of environmental variation in phenotype. Specifically, it is the phenotypic
range seen in individuals with a particular genotype. To evaluate the norm of reaction, researchers begin with
true-breeding strains that have the same genotypes and subject them to different environmental conditions.
Although many alleles display a simple dominance/recessive relationship, geneticists have also found cases in which a
heterozygote exhibits incomplete dominance – a condition in which the phenotype is intermediate between those of the
corresponding homozygous individuals. An example of this is when homozygous white flowers are crossed with
homozygous red flowers and produce pink offspring. When these F1 offspring where allowed to self-fertilize, the F2
generation consisted of ¼ red flowers, ½ pink flowers and ¼ white flowers. The pink flowers were heterozygous with an
intermediate phenotype. The F2 generation displayed a 1:2:1 ratio which is different from the 3:1 ratio observed by simple
Mendelian inheritance. At the molecular level, the allele that causes the white phenotype is expected to result in a lack of a
functional protein required for pigmentation. Depending on the gene regulation, the heterozygotes may produce 50% of
the normal protein, but this amount is insufficient to produce the same phenotype as the C RCR homozygote, which may
make twice as much of this protein.
Overdominance
As we have seen, environment plays a key role in the outcome of traits. For some genes, heterozygotes may display
characteristics that are more beneficial for their survival in a particular environment. They might be more likely to survive
and reproduce, or are better able to withstand harsh environmental conditions. The phenomenon in which a heterozygote
has greater reproductive success compared with either of the corresponding homozygotes is called overdominance, or
heterozygote advantage. An example is the human allele that causes sickle cell disease in homozygous individuals. This
autosomal recessive disease produces an altered form of the protein hemoglobin. Most individuals make Hb A, whereas
people with this disease are homozygous for Hb S and only make hemoglobin S. This causes that the red blood cells to
deform into a sickle shape under low oxygen conditions. The life span of these red blood cells becomes shortened due to
this disease and therefore anemia results. In addition, abnormal sickled cells can become clogged in the capillaries
throughout the body, leading to localized areas of oxygen depletion. Sickle cell is found in a high frequency among the
malaria people. The protest genus that causes malaria, Plasmodium, spends part of his life cycle within the mosquito and
another part in the red blood cells of humans who have been bitten by this mosquito. However, red blood cells of
heterozygotes, HAHs are likely to rupture when infected with this parasite, thereby preventing this parasite from
propagating. People who are heterozygous have better resistance to malaria than Hb AHbA homozygotes and they do not
suffer from the ill effects of this disease. Even though the homozygotes Hb SHbS condition is detrimental, the greater survival
of the heterozygotes has selected that the presence of Hb S allele within populations where malaria is prevalent.
Overdominance is usually due to two alleles that produce proteins with slightly different amino acid sequences. How can
we explain the observation that two proteins variants in the Hb AHbS heterozygote produce a more favourable phenotype?
1. Disease resistance: in the case of the sickle cell disease, the phenotype is related to the infectivity of the
Plasmodium. In the heterozygote, the infection agent is less likely to propagate within red blood cells. Other
alleles in the body may confer disease resistance in the heterozygote condition, but are detrimental in the
homozygote state. These include PKU, in which the heterozygote fetus may be resistant to miscarriage caused by
a fungal toxin, and Tay-Sachs disease, in which the heterozygote may be resistant to tuberculosis.
2. Subunit composition of Proteins: A second way to explain overdominance is related to the subunit composition
of proteins. In some cases, a protein functions as a complex of multiple subunits: each subunit is composed of
only polypeptide. a protein composed of two subunits is called a dimer. When both subunits are encoded by the
same gene, the protein is a homodimer. The prefix homo comes from the fact that both subunits come from the
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