Mendelian Genetics

Mendelian Genetics

• The branch of biology that deals with the rules that govern pattern of inheritance of heredity over the generations.
• Bateson coined the term genetics but Mendel is known as Father of genetics who performed hybridization experiments on pea plant.
• Mendel was born in Austria on 22nd July 1822 and was graduated in 1843 then he joined monastry & taught science and maths. In 1851 he joined university of Vienna where he was taught mathematical models.
• After returning from the university he join the monastry and performed hybridization experiments on pea plant from 1856 to 1863. From the results he proposed the rules for inheritance in 1865. His laws were overlooked due to the discussion on Origin of Species proposed by Charles Darwin.
• In 1884 he was died due to kidney failure. Later In 1900 De Vries, Correns and Tschermak rediscovered Mendels laws and called them Mendelism.
• Based on above observations Sutton and Boveri proposed chromosomal theory of Inheritance.
• Bateson and Punnett Performed an experiment in Sweet Pea which has purple flower and long anther and white flower and short anther.

Genetics:

• Study of Genes and their inheritance pattern
• Pattern of inheritance of Genes = Gregor Mendel
• 1822 born and 1843 graduated
• Monk and taught science and math.
• 1851-1853 university of Vienna.
• 1856-1863 experiments on peas
• Result chronological data.
• Studied the data.
• Pattern
• Laws of inheritance
• 1865 ⇒ Proceeding ⇒ Present
• 1856 ⇒ Origin of Species.

1907
De Vries
Correns
Tschermak

Phenotype = appear.

Genetics is study of genes and their inheritance pattern. The term genetics coined by Bateson but Gregor Mendel is know as Genetics. Later many scientist have performed experiments on genetics but studies of T.H Morgan have Significance impact therefore Morgan is known as father of experimental genetics.

Biography of Mendels:

Mendel was born in Austria in 1822 and was graduated. He then join monestery and became monk where he taught sciences + math. He went to University of Vienna for higher studies and stayed there for two years from 1851 to 1853. After returning from the university he joint the monestry where he Performed hybridization experiments on pea plants from 1856-1863.

From the experiments he collected the data and analysed them and obtained a specific pattern this pattern was called as law of inheritance which were presented in a preceeding in 1865.

The laws of inheritance proposed by mendel were overlooked due to the discussion on Origin of Species proposed by Charles Darwin.

De Vries, Correns and Tschermak independently worked and found the result similar to mendel after re-discovery of mendelism new branch of biology called genetics flourishes.

Genetic terminology

1. Gene:
   A segment of DNA that determine Specific character like height, colour, nail size etc.
2. Character:
   A feature which can be observed or measured a character is always found in Population and has alternative forms.
3. Traits:
   The alternative form of a character that an individual posseses is called trait. A character height has two traits tall and dwarf (short).
4. Alleles:
   The alternative form of a gene is called allele. A gene has always two alleles one from father and another from mother.
5. Dominant allele:
   The Allele which can synthesis protein is called dominant allele. It is always expressed and is denoted by Capital letter.
6. Recessive allele:
   The allele which cannot synthesis protein is called recessive allel it is not expressed and is represented by small form of letter that represent dominant allele.
7. Homozygous:
   When both alleles of a gene are found at same state It is called homozygous. It always forms same kind of gamets. (TT) and (tt) are homozygous.
8. Heterozygous (Hybrid):
   When alleles of a gene are different kind it is called Heterozygous (TT) or (Tt) for height is heterozygous which always produces two kind of gamet. One kind of gamete contains dominant allele and another kind of gamete Recessive allele.
9. Locus (Cross):
   The matting betn two homozygous parents with constrasting trait is called Cross. When tall Pea and dwarf pea bread. It is called Cross.
10. Filial generation:
    The generation obtain after mating a cross betn homozygous parent with contrasting trait is called filial generation. It might be f1, f2, f3 etc.
11. Locus:
    The position of a gene in a chromosome is called locus. It is always fixed for a gene.
12. Back Cross:
    A cross breed betn f1 generation and any one of the parents is called Back Cross. It is performed to provide preserve the desire traits.
13. Test Cross:
    A cross mate betn f1 and recessive parent is called test Cross. It is performed in order to find wheather dominant trait is available at homozygous and heterozygous.
    When we have been given tall plant and are requested to find wheather tall is homozygous (TT) or heterozygous (Tt), we make a Cross betn the tall and dwarf when offspring are tall then the tall is homozygous when both tall and dwarf are available the tall is heterozygous.
14. Phenotype:
    The appearance of a trait in an individual is called phenotype. Tall and Dwarf are phenotype for height.
15. Genotype:
    The allele combination of a gene for a trait is called Genotype. The trait tall height is due to (TT) & (Tt).
16. Selfing:
    The cross which takes place betn members of same generation is called selfing.
17. Punnette square:
    The square board on to which result of crossing are presented is called punnette square. It was introduced by R.C Punnette.

Reasons for Selecting Pea's as Experimental unit

Mendel was Successful to formulate laws of inheritance due to the selection of Pea plant (Pisum Sativum) as experimental unit it has following features.

1. It can be easily cultivated both naturally and artificially.
2. It has short life cycle.
3. It has several variety like white flower or Purple flower, Yellow seeds or green seeds, round seed or winkled seed, tall height or dwarf height etc.
4. The flower is bisexual and allows both self and cross pollination.
5. Cross pollination is easily performed.
6. He allow f1 for selfing to obtain the second generation. These seeds were grown into the pea plants which were both tall & dwarf in the ratio of 3:1. It shows that the allele which was Subdued at f1 generation was seperated during gamete formation and is expressed at f2 generation.
7. The hybrid (Heterozygous) are fertiles.

If is called segregation or law of Purity of Gamete.

Monohybrid Cross

When a cross is made betn two homozygous parents Considering a part of contrasting traits of a character.

Mendels cross tall height (TT) with dwarf height pea plant (tt) and obtained the seeds which were grown into tall pea plant. This generation of pea plant is called f1 generation which always contains heterozygous. It shows that tall height is dominant over dwarf height in Pea Plant.

Phenotype: Tall:Dwarf = 3:1
Genotype: TT:Tt:tt = 1:2:1

Law of independent assortment

From the result of dibybrid crosses mendel proposed a law called law of independent assortment. It states that Inheritance of a gene is independent to inheritance of another gene. They form both parental and non-parental traits. In pea plant mendel observe the ratio of parental and non-parental traits to be 10:6.

• Mendel crossed pea having yellow Coloured and round shaped seeds with the pea having green coloured and wrinkled shaped seeds. He obtain all f1 pea as yellow coloured and round shaped seeds. It shows that yellow coloured and round shaped are dominant over green colour and wrinkled shaped in Pea plant.
• He allowed f1 for selfing and obtain f2 as yellow colour round shaped, yellow colour wrinkled shaped, green colour round shaped and green colour wrinkled shaped in the ratio of 9:3:3:1 out of 4 yellow Coloured round shaped and green coloured wrinkled shaped are parental traits and others are non-parental traits.

Non-parentals traits are resulted only when inheritance of one gene is independent of inheritance of another gene.
Character = gene = two allele

- Epistasis
- Pleiotrophy
- Polygenic

Genetic Interaction

Mendel believes that one gene one character and inheritance of one gene is independent to inheritance of another gene. Recent experiment have proved that this law is not always true.
Sometime allele of genes interact with each other and produce new traits which cannot be explain by mendelism. In other cases allele of one gene interacts with allele of another gene to determine a traits. This phenomenon of allelic interaction is called gene interaction.

Gene interaction is of two types:
When interaction occurs betn allele of same gene It is called intra-allelic genetic interaction. Incomplete dominance, co-dominance and multiple allelism are example.
When allele of one gene interacts with allele of another gene it is called intergenic interaction. Epistatic, pleitropy and polygenic inheritance are example.

(1) Intra-allelic interaction

1) Incomplete Dominance:

It is intragenic interaction and it also called partial dominance or blending inheritance. It doesnot follow mendels law of dominance.
In heterozygous both allele of a gene partially synthesized protein and the protein becomes intermediate This protein determines a traits which is intermediated of both parents. Therefore f1 appears intermediate of both parents.

• When red flower 4 o'clock plant (Mirabilis Jalapa) is crossed with white flower 4 o'clock plant all f1 is found to be pink which is intermediate of red and white in this case f1 doesnot resemble to any of parent but is intermediate of both parents when f1 is allow for selfing f2 appears red pink and white in the ratio of 1:2:1 in this case genotype and phenotype are same.
• When purple flower egg plant (brinjal) is crossed with white flower egg plant all f1 is found to be violet flowers which is intermediate of purple and white when f1 is allowed for selfing f2 appears purple, violet and white in the ratio of 1:2:1.

4 o'clock plant with red Flower (RR) x 4 o'clock plant with white flower (rr)

So In which genotype and phenotype is same = Incomplete.
Genotype = 1:2:1
Phenotype = 1:2:1

Co-Dominance

• It is an example of intragenic Interaction and also doesn't obey mendels law of dominance. it states that both allele of a gene in Heterozygous are equally expressed and resemble traits of both parents. Therefore f1 generation appears as both parents.
• In human blood group is determine by a gene called isochaemoggolutinogen gene (I) which has 3 alleles I^A, I^B & I^O but an individuals has only 2 alleles one from father and another from mother.
• Out of three alleles I^A and I^B are dominant over I^O because they synthesize protein I^A synthesizes protein named antigen (A) I^B synthesizes a protein named antigen (B) and I^O doesnot Synthesizes protein.
• When I^A and I^B come along with I^O blood group (A) and (B) are determined blood group (O).
  • I^A I^A = A (homozygous)
  • I^A I^O = A (heterozygous)
  • I^O I^O = O (homozygous only)
• Blood group O is formed only when both allele are I^O.
• When allele I^A and I^B come together then both allele synthesize protein and blood group becomes AB. it shows co-dominance.

Multiple Allelism

• Mendel believes that one gene has two alleles but experiment have proved that one gene may have three or more alleles This condition is called multiple allelism.
• Multiple allelism is shown by population of a species but an individual always has two alleles for a gene, one allele from father & other allele from mother.
• Multiple allelism is shown by the isohaemoglutinogen gene (I) which has three allele I^A, I^B, and I^O. This gene determine the following are possible genotype for blood group.

Chromosomal Theory of Inheritance

• Mendel belives that genes are inheritance and dependent of each other due to this both parental and non-parental traits are appear in f2 generation in the ration of 10:6.
• Recent experiment have proved than gene's are linearly arranged in chromosome and chromosome is inherited, not the genes it was explained by Bateson and Punnett. They crossed sweet pea having purple Flower & long anther with the Sweet Pea having white Flower + short round anther, they obtained all f1 as purple flower and long anther He allowed f1 for selfing and obtained f2 as purple flower long anther, purple Flower round anther, white flower long anther and white flower round anther in the ration of 7:1:1:7 in stead of 9:3:3:1.
• The ratio of parental and non-parental traits is 14:2.
• Sutton & Boveri name above pheromenon as chromosomal theory of inheritance. It tells that gene of a chromosome always tend to stay together therefore Frequency of parental traits is always higher.