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Summary Fundamentals of Genetics
- Instelling
- Wageningen University (WUR)
The document is a summary of the mentioned book chapters, integrated with the information given during the lectures.
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FUNDAMENTALS OF GENETICS AND MOLECULAR BIOLOGYCHAPTER 1 – THE GENETICS REVOLUTION
THE BIRTH OF GENETICS
Genetics → science that tries to understand the rules that govern the transmission of genetic information
at three levels:
From parent to offspring within families (transmission genetics);
From DNA to gene action at cellular level (molecular developmental genetics);
Over generations within populations (population-evolutionary genetics).
Before Mendel → blending theory of inheritance: belief that inheritance works like mixing fluids (e.g.
mixing red flowers with white flowers gives pink, or tall parent + short parent = middle height child). Later
on, it was clear that this is NOT how nature works:
There were exceptions
If it really was like this, there would have been loss of variations (e.g. all flowers would be pink at a
certain point).
Mendel → theory of inheritance, published in 1866 but unappreciated until 1900. From 1856 he crossed-
pollinated different varieties of pea plants. He crossed a variety with purple flower to one with white
flowers:
First generation: all plants with purple flowers;
Second generation (from pollination of first generation): 3 purple flower plants for every 1 white
flower plant (3:1).
From this, Mendel discovered that:
1. The factors that control traits act like particles and not fluids (like in the blending theory), where
the particles are genes. Each pea plant has two copies of the gene controlling colour in each
somatic cell, but there is only one copy in gametes.
2. The gene for colour has two variants or alleles, of which the purple one is dominant and the white
one is recessive → a plant with one purple allele and one white allele will have purple flowers,
while only plants with two white alleles will have white flowers.
Mendel rediscovered → in 1900 Mendel’s laws of inheritance were rediscovered and a new era begun,
when geneticists resolved many fundamental questions:
Where in the cell are genes? In 1901, Morgan discovered that genes are located in chromosomes
using the fruit fly Drosophila → chromosome theory of inheritance.
Can Mendelian genes explain the inheritance of continuously variable traits like human height? In
1918, Fisher discovered that these traits are controlled by multiple genes → multifactorial
hypothesis.
How do genes function in cells in a way that enables them to control different states for a trait like
flower colour? In 1941, Tatum and Beadle proposed that genes encode enzymes that have
metabolic functions → one gene – one enzyme hypothesis.
, What is the physical nature of a gene? In 1944, Avery et al. found an evidence that genes are made
of DNA.
How can DNA store information? In 1950s, Watson and Crick determined that DNA is a double
helix, where each strand is made of sugar, phosphate groups and bases A, T, G and C.
How are genes regulated? In 1961, Jacob and Monod discovered that genes have regulatory
elements that regulate gene expression, acting as activators or repressors (e.g. lac operon).
How is the information stored in DNA decoded to make proteins? In 1960s, it was deduced that a
string of DNA encodes a set of 20 amino acids, and that RNA carries information for protein
synthesis.
Central dogma → introduced by Crick in 1958. DNA transcribed → (m)RNA translated → proteins.
AFTER CRACKING THE CODE
At the end of 1960s, a new era of genetics begun, where model organisms have been studied and now tools
for analysing DNA have been developed.
Model organisms → species used in experimental biology with the belief that what is learned from this
species could be valid for other species. Examples are the fruit fly Drosophila, E. coli and mice.
Characteristics that a model organism should have:
Small, easy and cheap to maintain;
Reproduce fast.
With small genome, so looking for genes is easier.
Easy to cross and mate, and that produce large quantities of offspring.
Tools for genetic analysis → to characterize and manipulate DNA, RNA and proteins.
Isolation of cellular enzymes (DNA polymerases, nucleases – cut or degrade DNA molecules, ligases
– join two DNA molecules together end-to-end), to copy, paste, cut and transcribe DNA, and also to
label/tag it with fluorescent dyes.
Methods to clone DNA and genes. DNA is inserted into a host organism where it is replicated.
Methods to insert foreign DNA molecules into the genomes of other species → transformation,
genetically modified organisms (GMO).
Methods to hybridise DNA molecules to one another or with RNA.
Methods to determine the sequence of bases of an organism → DNA sequencing.
Tools to analyse the entire genome → genomics, field that studies the structure and function of
entire genomes.
GENETICS TODAY
From classical genetics to medical genomics → the patient Benge has an illness that caused her arteries to
be calcified, and this gives pain in her legs. It was discovered that the disease, called ACDC, was caused by
the lack of the enzyme CD73. This enzyme sends signals to other cells to block calcification. It was
discovered that Benge’s parents are third cousins: when parents are close relatives there is a high chance
that they both have inherited a defective gene from their common ancestor, and they will pass it on to
,their children. In fact, Benge and her siblings all have this disease because they all have two defective
copies (inherited from their parents) of the CD73 gene. (if they had only a defective copy, they wouldn’t
have the disease). This means that Benge has inherited a copy of the gene from her mother which is
identical to that inherited from her father. These identical regions are very rare, as generally a segment of
DNA has several differences in the base sequence between the copy inherited from the mother and from
the father, called single nucleotide polymorphism (SNP). The diagnosis of this disease was possible thanks
to the integration of classical transmission genetics and genomics.
Investigating mutation and disease risk → long ago, a German physician reported that there seems to be a
higher incidence of short-limbed dwarfism in children born last than in those born first. A British geneticist
suggested that mutations causing haemophilia were more common in men than in women. These
observations suggested that the risk of inheriting disorders is greater when parents are older and that
fathers contribute to it more.
In 2012 a team of geneticists proved these theories, by studying 78 “trios” in Iceland and determining the
genome sequence of each individual:
They focussed on point mutations – changes of one base in the DNA code (e.g. A replaced with G).
point mutations are so called “new mutations” or “de novo mutations” – DNA mutations that exist
in a child but not in his parents. These mutations occur in the germline of one of the parents and
are transmitted to the child.
They knew the age of parents at the time of conception of the child – older mothers did not pass on
more new point mutations to their child than younger ones, but older fathers passed on more new
point mutations than younger ones. For each year of increase in his age, a father passes on two
additional mutations to his child (e.g. a 20yo father passes about 25 mutations, whereas a 40yo
father passes on about 65 mutations). It happens because sperm cells are produced continuously
during a man’s life and there are more cell divisions with more possibilities of mutations, whereas
egg cells are formed before a woman is born.
Flood-intolerant and flood-tolerant rice → rice is commonly grown in flooded fields called paddies.
However, when rain is too heavy there is the risk that plants are submerged and die. There is a variety of
rice called FR13A which is tolerant to submergence thanks to a gene called SUB1. This variety has a low
yield, so geneticists worked to transfer the flood tolerance gene to superior varieties.
Recent evolution in humans → genetics seeks to understand how genes change over generations within a
population. Genetic changes enabled populations to adapt to different conditions all over the world. Three
factors have been powerful in shaping the gene variants:
Pathogens such as malaria or smallpox;
Local climatic conditions such as solar radiation, temperature, altitude;
Diet.
Examples:
Adaptation to high altitude: the gene called EPAS1 regulates the number of red blood cells
according to the level of oxygen in body tissues. In oxygen levels are low, EPAS1 signals to produce
more red blood cells. Tibetans and Andeans have a variant of the EPAS1 gene that helps them
adapt to life at high elevations. If a person adapted to low elevation moves to high elevations, their
, tissues would get less oxygen. EPAS1 would cause the production of more red blood cells, but since
that person already has enough of them, their blood would become overloaded. This can cause
pulmonary hypertension and the formation of blood clots.
Lactose tolerance: the enzyme lactase enables to digest lactose. In ancient times, milk was only
drunk by children during childhood and was then switched off. When dairy products started to be
produced and milk consumed more regularly, adults of that generation could not digest lactose
because they did not produce lactase anymore. Nowadays, some people have a variant of the
lactase gene that has a T instead of a C (in Europe). These people can produce lactase during
adulthood, because the variant enables a regulatory protein OCT1 to bind near the lactase gene,
and cause its expression in adults. Today, the T variant has a frequency of 90% in northern Europe
where cattle farming was common.
Key concepts
Mendel demonstrated that genes behave like particles and not fluids.
The rediscovery of Mendel’s laws launched a new era in which geneticists resolved many fundamental
questions about genes and genetic information. They learned that genes are located on chromosomes and
are made of DNA, and that genes encode proteins.
Most genetic studies are conducted on a limited number of model organisms, which have features that make
them suitable for genetic analysis.
Progress in genetics has produced and has been caused by the development of molecular and mathematical
tools for the analysis of single genes and whole genomes.
Classical transmission genetics provides the foundation for modern medical genetics. The integration of
classical genetics and genomic technologies allows to identify the causes of inherited diseases.
Genome sequences of parents and their children clarify the factors that contribute to new point mutations.
Fathers contribute four times as many new mutations to their offspring as do mothers. The number of new
mutations passed on from a father rises with the age of the father.
Genetics and genomics play an important role in improving crop plants and domestic animals.
Evolutionary genetics provides the tools to document how gene variants that provide a beneficial effect can
rise in frequency in a population and make individuals better adapted to the environment in which they live.
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