1. ACKNOWLEDGEMENT
I would like to express my special thanks of gratitude to my teacher
Mr GR Arya as well as our principal Lt Col Shyam Krishna TP who
gave me the golden opportunity to do this wonderful project on the
topic STUDY OF MENDELIAN TRAITS, which also helped me in
doing a lot of Research and i came to know about so many new
things I am really thankful to them.
Secondly I would also like to thank my parents and friends who
helped me a lot in finalizing this project within the limited time
frame.
2. GREGOR MENDEL
GregorJohann Mendel ( 20 July 1822 – 6 January 1884)was a
meteorologist, mathematician, biologist, Augustinian friar and abbot of St.
Thomas' Abbeyin Brno, Margraviate of Moravia. Mendel was born in a German-
speaking family in the Silesian part of the Austrian Empire (today's Czech
Republic)and gained posthumous recognitionas the founder of the modern
science of genetics.Though farmers had known for millennia that crossbreeding
of animals and plants could favour certain desirable traits, Mendel's pea plant
experiments conducted between1856 and 1863 established many of the rules
of heredity, now referred to as the laws of Mendelian inheritance.
Mendel worked with seven characteristics of pea plants: plant height, pod shape
and colour, seed shape and colour, and flower position and colour. Taking seed
colour as an example, Mendel showed that when a true-breeding yellow pea and
a true-breeding green pea were cross-bredtheir offspring always produced
yellow seeds.However, in the next generation, the greenpeas reappeared at a
ratio of 1 green to 3 yellow. To explain this phenomenon,Mendel coined the
terms "recessive" and "dominant" in reference to certain traits. In the preceding
example, the green trait, which seems to have vanished in the first filial
generation, is recessive and the yellow is dominant. He published his work in
1866,demonstrating the actions of invisible "factors"—nowcalled genes—in
predictably determining the traits of an organism.
The profound significance of Mendel's work was not recognized until the turn of
the 20th century (more than three decades later) with the rediscoveryof his
laws. Erich von Tschermak, Hugo de Vries and Carl Correns independently
verified several of Mendel's experimental findings in 1900,ushering in the
modern age of genetics.
3. Mendelian trait
A Mendelian trait is one that is controlled by a single locus in an inheritance
pattern. In such cases, a mutation in a single gene can cause a disease that
is inherited according to Mendel's principles.
Dominant diseases manifest in heterozygous individuals. Recessive ones are
sometimes inherited unnoticeably by genetic carriers.
Examples include sickle-cell anemia, Tay–Sachs disease, cystic
fibrosis and xeroderma pigmentosa.
A disease controlled by a single gene contrasts with a multi-factorial
disease, like heart disease, which is affected by several loci (and the
environment) as well as those diseases inherited in a non-
Mendelian fashion.
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. If the two factors have different information, the individual is called
heterozygous. The alternative forms of a factor are called alleles
Non Mendelian trait
Non-Mendelian inheritance is any pattern of inheritance in which traits do not
segregate in accordance with Mendel's laws. These laws describe the
inheritance of traits linked to single genes on chromosomes in the nucleus.
In Mendelian inheritance, each parent contributes one of two possible alleles
for a trait.
They include inheritance of multiple allele traits, traits with codominance or
incomplete dominance, and polygenic traits.
4. Experiment with garden pea plants (Pisumsativum)as
did Austrian monk Gregor Mendel (1822-1884).
Mendel choseto experiment with peas becausethey possessed four important
qualities:
i. Peas had been shown to be true-breeding (all offspring will have the
same characteristic generation after generation).
ii. Peas exhibit a variety of contrasting traits (purple vs. white flowers;
round vs. wrinkled seeds).
iii. The shape of the pea flower protected it from foreign pollen. Peas
usually reproduce by self-pollination, in which pollen produced by a
flower fertilizes eggs in the same flower.
iv. Pea plants grow quickly and do not require much space.
The traits that Mendel studied are listed below:
Form of ripe seed (R) – smooth or wrinkled
Color of seed albumen (Y) – yellow or green
Color of flower (P) – purple or white
Form of ripe pods (I) – inflated or constricted
Color of unripe pods (G) – green or yellow
Position of flowers (A) – axial or terminal
Length of stem (T) – tall or dwarf
These traits are all pictured in the plants below:
5. The following table shows each of the traits and which
traits are dominant and which recessive.
Trait Dominant Expression Recessive Expression
Form of ripe seed (R) Smooth Wrinkled
Colour of seed albumen (Y) Yellow Green
Colour of flower (P) Purple White
Form of ripe pods (I) Inflated Constricted
Colour of unripe pods (G) Green Yellow
Position of flowers (A) Axial Terminal
Length of stem (T) Tall Dwarf
Some Abbreviations
Axial: Flowers located near the middle of the plant.
Dominant: Traits that appear to mask (or hide) other traits.
Pedigree: A diagram of a family history used for tracing a trait
through several generations.
Recessive: Traits that can be hidden in one generation and then
appear in the next.
Terminal: Flowers located at the ends of the stems.
Trait: A distinguishing characteristic.
6. Mendel's genetic discoveries
Five parts of Mendel's discoveries were an important divergence from
the common theories at the time and were the prerequisite for the
establishment of his rules.
1. Characters are unitary, that is, they are discrete e.g: purple vs. white, tall vs. dwarf. There is no
medium sized plant or light purple flower.
2. Genetic characteristics have alternate forms, each inherited from one of two parents. Today, we call
these alleles.
3. One allele is dominant over the other. The phenotype reflects the dominant allele.
4. Gametes are created by random segregation. Heterozygotic individuals produce gametes with an
equal frequency of the two alleles.
5. Different traits have independent assortment. In modern terms, genes are unlinked.
According to customary terminology we refer here to the principles of
inheritance discovered by Gregor Mendel as Mendelian laws, although
today's geneticists also speak of Mendelian rules or Mendelian
principles, as there are many exceptions summarized under the collective
term Non-Mendelian inheritance.
Mendel selected for the experiment the following characters of pea plants:
Form of the ripe seeds (round or roundish, surface shallow or wrinkled)
Colour of the seed–coat (white, grey, or brown, with or without violet spotting)
Colour of the seeds and cotyledons (yellow or green)
Flower colour (white or yellow)
Form of the ripe pods (simply inflated, not contracted, or constricted between the seeds and wrinkled)
Colour of the unripe pods (yellow or green)
Position of the flowers (axial or terminal)
Length of the stem
When he crossed purebred white flower and purple flower pea plants (the parental
or P generation) by artificial pollination, the resulting flower colour was not a
blend. Rather than being a mix of the two, the offspring in the first generation (F1-
generation) were all purple-flowered. Therefore, he called this biological
trait dominant. When he allowed self-fertilization in the uniform looking F1-
generation, he obtained both colours in the F2 generation with a purple flower to
white flower ratio of 3 : 1. In some of the other characters also one of the traits
was dominant.
He then conceived the idea of heredity units, which he called hereditary "factors".
Mendel found that there are alternative forms of factors—now called genes—that
account for variations in inherited characteristics. For example, the gene for
flower colour in pea plants exists in two forms, one for purple and the other for
white. The alternative "forms" are now called alleles. For each trait, an organism
inherits two alleles, one from each parent. These alleles may be the same or
different. An organism that has two identical alleles for a gene is said to
be homozygous for that gene (and is called a homozygote). An organism that has
two different alleles for a gene is said be heterozygous for that gene (and is called
a heterozygote).
7. Mendel hypothesized that allele pairs separate randomly, or segregate,
from each other during the production of the gametes in the seed plant (egg
cell) and the pollen plant (sperm). Because allele pairs separate during
gamete production, a sperm or egg carries only one allele for each
inherited trait. When sperm and egg unite at fertilization, each contributes
its allele, restoring the paired condition in the offspring. Mendel also found
that each pair of alleles segregates independently of the other pairs of
alleles during gamete formation.
The genotype of an individual is made up of the many alleles it possesses.
The phenotype is the result of the expression of all characteristics that are
genetically determined by its alleles as well as by its environment. The
presence of an allele does not mean that the trait will be expressed in the
individual that possesses it. If the two alleles of an inherited pair differ (the
heterozygous condition), then one determines the organism's appearance
and is called the dominant allele; the other has no noticeable effect on the
organism's appearance and is called the recessive allele.
Mendel's laws of inheritance
LAW DEFINITION
a. Lawof
dominance
and
uniformity
Some alleles are dominant while others are recessive; an organism with at least
one dominant allele will display the effect of the dominant allele.
b. Lawof
segregation
During gamete formation, the alleles for each gene segregate from each other
so that each gamete carries only one allele for each gene.
c. Lawof
independent
assortment
Genes of different traits can segregate independently during the formation of
gametes.
8. Mendelian traits in humans
Mendelian traitsin humans concernshow, in Mendelian inheritance, a child
receivinga dominant allele from either parentwill have the dominant form of
the phenotypic trait or characteristic.Only those that received the recessive
allele from both parents, known as zygosity, will have the
recessivephenotype. Those that receive a dominant allele from one parent
and a recessive allele from the other parent will have the dominant form of
the trait. Purely Mendelian traits are a tiny minority of all traits, since most
phenotypic traits exhibit incomplete dominance, codominance,
and contributions from many genes.
The recessive phenotypemay theoreticallyskip any number of generations,
lying dormantin heterozygous "carrier" individuals until they have children
with someone who also has the recessive allele and both pass it on to their
child.
These traits include:
i. Albinism
ii. Brachydactyly
iii. Colourblindness
iv. Haemophilia
v. Sickle-cell diseaseetc