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MENDELIAN GENETICS
DR. A. TARAB
DEPT. OF BIOCHEMISTRY
HKMU
Mendel’s Studies of Characters
• Many of your characteristics – or characters –including the colour and shape of your eyes and the texture of your hair resemble those of your parents
• The passing of characters from parents to offspring is called heredity
• From the beginning of recorded history, humans have attempted to alter crop plants and domestic animals to give them traits that are more useful to us
• * trait – a genetically determined
characteristic or condition. Traits may be
physical, such as hair, colour or leaf
shape, or they may be behavioral, such as
nesting in birds and burrowing in rodents
• Before DNA and chromosomes were
discovered, heredity was one of the
greatest mysteries of science
• Humans first applied genetics to the
domestication of plants and animals
between approximately 10,000 and 12,000
years ago
• This domestication led to the development
of agriculture and fixed human settlements
Ancient peoples practiced
genetic techniques in agriculture
• Assyrian bas-relief
sculpture showing
artificial pollination of
date palms at the time
of King
Assurnasirpalli II, who
reigned from 883 –
859 B.C
• Mendelian inheritance (or Mendelian genetics or
Mendelism) is a scientific description of how
hereditary characteristics are passed from
parent organisms to their offspring; it underlines
much of genetics
• This theoretical framework was initially derived
from the work of Gregor Johann Mendel, a 19th
century Austrian priest/monk, published in 1865
and 1866
Gregor Mendel – founder of
modern genetics (1822 – 1884)
• Studied at University of
Vienna from 1851 – 1853
• Conducted breeding
experiments from 1856 –
1863
• Presented his results at
meetings of the Brno
Natural Science Society
in 1865
• Published his results in
1866
• The laws of inheritance were derived by Mendel,
conducting hybridization experiments in garden
peas (Pisum sativum)
• Between 1856 and 1863, he cultivated and
tested some 29,000 pea plants
• From these experiments, he deduced two
generalizations which later became known as
Mendel’s Principles of Heredity or Mendelian
inheritance
Garden peas
Pea plant (Pisum sativum)
Useful Features in Peas
• 1. Several characters of the garden pea exist in two clearly different forms
• For example, the flower colour is either purple or white – there are no intermediate forms
• 2. The male and female reproductive parts are enclosed within the same flower
• You can allow self-fertilization or cross-pollination
• 3. The garden pea is small, grows easily,
matures quickly and produces many
offspring
• Thus, results can be obtained quickly, and
there are plenty of subjects to count
Traits Expressed as Simple
Ratios
• Mendel’s initial experiments were monohybrid
crosses (involves one pair of contrasting traits)
• Mendel carried out the experiments in three
steps:
• Step 1 – allowed each variety of garden pea to
self-pollinate for several generations. This
ensured that each variety was true-breeding for
particular character, i.e, all the offspring would
display only one form of the character
• These true breeding plants serve as the parental generation in Mendel’s experiments
• The parental generation, or P generation, are the first two individuals that are crossed in a breeding experiment
• Step 2 – then cross-pollinated two P generation plants that had contrasting traits, such as purple flowers and white flowers
• Mendel called the offspring the first filial (from
the Latin filialis meaning “of a son or daughter”)
generation or F1 generation
• He then examined each F1 plant and then
recorded the number of F1 plants expressing
each trait
• Step 3 – finally allowed F1 generation to self-
pollinate
• He called the offspring of F1 generation
plants the second filial generation, or F2
generation
• Again, each F2 plant was characterized
and counted
Mendel’s Results
• When Mendel crossed purple flowers with white flowers, all of the offspring in F1 generation had purple flowers
• In F2 generation, 705 plants had purple flowers and 224 plants had white flowers – a ration of 705 to 224, which is then rounded to 3:1
• For each of the seven characters Mendel studied, he found the same 3:1 ratio of plants expressing the contrasting traits in the F2
generation
• Mendel’s conclusions, which were
unappreciated for 45 years, laid the
foundation of our modern understanding of
heredity
• He died at the age of 61 on January 6th,
1884, unrecognized for his contribution to
genetics
• Mendel’s conclusions were largely ignored
• Although they were not completely
unknown to biologists of the time, they
were not seen as generally applicable
• The mechanisms by which characteristics
are transmitted from one generation to the
next remained a mystery until the late
1800s
Early Written Records
• The ancient Greeks gave careful consideration to human reproduction and heredity
• They believed in the concept of pangenesis, which proposed that specific particles, later called gemmules, carry information from the various parts of the body to the reproductive organs, from where they are passed to the embryo at the moment of conception
• Although incorrect, the concept of pangenesis was highly influential and persisted until the late 1800s
The Rise of Modern Genetics
• Dutch spectacles makers began to put together simple microscopes in the late 1500s, enabling Robert Hooke (1653 –1703) to discover cells in 1665
• Excessive enthusiasm for this new world of the very small gave rise to the idea of preformationism
• According to this idea, inside the egg or
sperm existed a tiny miniature adult, a
homunculus, which simply enlarged
during development
• Ovists argued that the homunculus
resided in the egg, whereas spermists
insisted that it was in the sperm
• Another early notion of heredity was blending inheritance, which proposed that the offspring are a blend, or mixture, of parental traits
• This idea suggested that the genetic material itself blends, much as blue and yellow pigments blend to make green paint
• However, we realize today that individual genes do not blend
Preformationism
• The homunculus: A
myth
• Well into the 19th
century, many
prominent
microscopists
believed they saw a
fully formed, miniature
fetus crouched within
the head of a sperm
Preformationism
• Preformationism – a
popular idea of
inheritance in the
seventeenth and
eighteenth centuries
• Homunculus inside a
sperm
• Lamarckism (or Lamarckian
inheritance) is the idea that an organism
can pass on characteristics that it acquired
during its life time to its offspring (also
known as heritability of acquired
characteristics or soft inheritance)
• It is named after the French biologist Jean-
Baptiste Lamarck (1744 – 1829)
• Lamarck did not originate the idea of soft
inheritance, which proposes that individual
efforts during the lifetime of the organisms
were the main mechanism driving species
to adaptation, as they supposedly would
acquire adaptive changes and pass them
on to offspring
• When Charles Darwin published his theory
of evolution by natural selection in “On the
origin of Species”, he continued to give
credence to what he called “use and
disuse inheritance”, but rejected other
aspects of Lamarck’s theories
• Later, Mendelian genetics supplanted the
notion of inheritance of acquired traits,
eventually leading to the development of
the modern evolutionary synthesis, and
the general abandonment of the
Lamarckian theory of evolution in biology
Twentieth Century Genetics
• The year 1900 was a water shed in the history of genetics
• In 1900, Mendel’s work was “re-discovered” by three European scientists
• Its most vigorous promoter was William Bateson who coined the term “genetics” and “allele” to describe many of its tenets
• Walter Sutton proposed in 1902 that genes are located on chromosomes
• Thomas Hunt Morgan and his assistants
later integrated the theoretical model of
Mendel with the chromosome theory of
inheritance, in which the chromosomes of
cells were thought to hold the actual
hereditary material, and create what is
now known as classic genetics, which was
extremely successful and cemented
Mendel’s place in history
• Geneticists began to use bacteria and viruses in the 1940s
• At about this same time, evidence accumulated that DNA was the repository of genetic information
• James Watson and Francis Crick described the three – dimensional structure of DNA in 1953, ushering in the era of molecular genetics
James Watson and Francis
Crick
DNA - Structure
• Mendel suggested that genes come in alternative versions that account for the variations seen in inherited characteristics
• The gene dictating seed colour, for example, exists in two “flavours”; one that directs the production of yellow peas and one that produces green
• Such alternative versions of a gene are today called alleles
• A large contribution to Mendel’s success can be traced to his decision to start his crosses only with plants he demonstrated were true-breeding
• He also only measured absolute (binary) characteristics, such as colour, shape, and position of the offspring, rather than quantitative characteristics
• He expressed his results numerically and subjected them to statistical analysis
• His method of data analysis and his large sample size gave credibility to his data
• He also had the foresight to follow several successive generations (F2,F3) of his pea plants and record their variations
• Finally he performed “test crosses” (back-crossing descendents of the initial hybridization to the initial true breeding lines) to reveal the presence and proportion of recessive characters
• Without his hard work and careful attention to procedure and detail, Mendel’s work could not have had the impact it made on the world of genetics
• Mendel discovered that when crossing white flower and purple flower plants, the result is not a blend
• Rather than being the mix of the two, the offspring was purple flowered
• He then conceived the idea of heredity units, which he called “factors”, one of which is a recessive characteristic and the other dominant
• *dominant – an allele that is expressed in organism’s phenotype; recessive – an allele that is hidden
• Mendel said that factors, later called genes, normally occur in pairs in ordinary body cells, yet segregate during the formation of sex cells
• Each member of the pair becomes part of the separate sex cell
• The dominant gene, such as the purple flower in Mendel’s plants, will hide the recessive gene, the white flower
• After Mendel self-fertilized the F1
generation and obtained the 3:1 ratio, he
correctly theorized that genes can be
paired in three different ways for each trait:
AA, aa and Aa
• The capital “A” represents the dominant
factor and lower case “a” represents the
recessive.
• Mendel stated that each individual has two factors for each trait, one from each parent
• The two factors may or may not contain the same information
• If the two factors are identical, the individual is called homozygous for the trait same allele (BB or bb)
• If the two factors have different information, the individual is called heterozygous different alleles (Bb)
• The alternative forms of a factor are called allele
• The genotype of an individual is made up of the many allele it possesses
• An individual’s physical appearance, or phenotypes, is determined by its alleles as well as by its environment
• An individual possesses two alleles for each trait, one allele is given by the female parent and the other by the male parent
• * genotype – the entire set of genes
• They are passed on when an individual matures
and produces gametes; egg and sperm
• When gametes form, the paired alleles separate
randomly so that each gamete receives a copy
of one of the two alleles
• In heterozygous individuals the only allele that is
expressed is the dominant
• The recessive allele is present but its expression
is hidden
• Mendel summarized his findings into two
laws; the Law of Segregation and the Law
of Independent Assortment
• Law of Segregation (The “first Law”)
• The two alleles for each trait separate
(segregate) during gamete formation, then
unite at random, one from each parent, at
fertilization
• More precisely, the law states that when
any individual produces gametes, the
copies of the gene separate so that each
gamete receives only one copy (allele)
• In meiosis, the paternal and maternal
chromosomes get separated and the
alleles with the trait of the character are
segregated into two different gametes
• Law of Independent Assortment (the
“Second Law”) states that separate genes
for separate traits are passed
independently of one another from parents
to offspring
• More precisely, the law states that alleles
of different genes assort independently of
one another during gamete formation
• That is, if a gene on chromosome 1 has
two alleles, a and b, and a gene on
chromosome 2 has two alleles, c and d,
the combinations a and c, a and d, b and
c, and b and d, are all equally likely
• There is no preference for a to be with
either c or d
• Since chromosome 1 and 2 line up on the metaphase plate independently at the first meiotic division, with equal chance of the maternal or paternal homolog going to one pole for each chromosome, these combinations have an equal chance of occuring
• Thus, alleles of genes that lie on different chromosomes assort independently of one another
• These two laws, the law of segregation and the law of independent assortment, are the basis of Mendelian inheritance
• While Mendel’s experiments with mixing one trait always resulted in a 3:1 ratio (Fig. 1) between dominant and recessive phenotypes, his experiments with mixing two traits (dihybrid cross) showed 9:3:3:1 ratios (Fig. 2)
• But the 9:3:3:1 table shows that each of the two genes are independently inherited of each other, so that there is no relation, between the cats colour and tail length
• * dihybrid cross – when two traits/genes are under consideration
Fig. 1 - Monohybrid Cross
• Dominant and recessive
phenotypes
• (1) Parental generation
• (2) F1 generation
• (3) F2 generation
• Dominant (red) and
recessive (white) look
alike in F1 generation and
show 3:1 ratio in F2
generation
Fig. 2 – Dihybrid Cross
• Two traits/genes are
under consideration
• The phenotype ratio
of the offspring is
9:3:3:1
Fig. 2
• The phenotypes of two independent traits show
a 9:3:3:1 ratio in the F2 generation
• In this example, coat colour is indicated by B
(brown, dominant) or b (white) while tail length is
indicated by S (short, dominant) or s (long)
• When parents are homozygous for each trait
(SSbb and ssBB), their children in F1 generation
are heterozygous at both loci and only show the
dominant phenotypes
Fig. 2
• If the children mate with each other, in the
F2 generation all combination of coat
colour and tail length occur: 9 are
brown/short (purple boxes), 3 are
white/short (pink boxes), 3 are brown/long
(blue boxes) and 1 is white/long (green
box)
• Table showing how
the genes exchange
according to
segregation or
independent
assortment during
meiosis …….
• …….and how this
translates into
Mendel’s laws
• The reasons for these laws is found in the nature
of the cell nucleus. It is made up of several
chromosomes carrying the genetic traits
• In a normal cell, each of these chromosomes
has two parts, the chromatids
• A reproductive cell, which is created in meiosis,
usually contains only one of those chromatids of
each chromosome
• By merging two of these cells (usually one male and one female), the full set is restored and the genes are mixed
• The resulting cell becomes a new embryo
• The fact that this new life has half the genes of each parent (23 from mother, 23 from father for the total of 46 in the case of humans) is one reason for the Mendelian laws
• The second most important reason is the varying
dominance of different genes, causing some
traits to appear unevenly instead of averaging
out (whereby dominant doesn’t mean more likely
to reproduce – recessive genes can become the
most common, too)
• There are several advantages of this method
(sexual reproduction) over reproduction without
genetic exchange
• Instead of nearly identical copies of an organism, a broad range of offspring develops, allowing more different abilities and evolutionary strategies
• Sexual reproduction can help a species survive in an unpredictably variable environment
• If two parents produce many offspring with a wide variety of gene combinations, the chance that at least one of their progeny will have the combination of features necessary for survival is increased
• Sexual reproduction might also speed the
elimination of deleterious genes from a
population: by matting with only the fittest
males, females select for “good” genes
and allow “bad” genes to be lost from the
population more efficiently than they would
otherwise be
Punnett Squares
• A Punnett square is a diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in the cross
• Named for its inventor, Reginald Punnet, the simplest Punnett square consists of four boxes inside a square
• The possible gametes that one parent can produce are written along the top of the square
• The possible gametes that the other parent can produce are written along the left side of the square
• Each box inside the square is filled in with two
letters obtaining by combining the allele along
the top of the box with the allele along the side
of the box. The letters in the boxes indicate the
possible genotypes of the offspring
• Punnett squares can be used to predict the
outcome of a monohybrid cross, where 100% of
the offspring are expected to be heterozygous
Monohybrid Cross -
Heterozygous
• The genotypic ratio of
the offspring is 1:2:1
while the phenotypic
ratio is 3:1
Dihybrid Cross - Heterozygous
• The genotypic ratio of
the offspring between
spherical dented
yellow seeded plants
and spherical dented
green seeded plant is
1:2:1:2:4:2:1:2:1 and
the phenotypic ratio is
9:3:3:1
