DNA structure and function

• Mendel's work, and others, led to insights on inheritance patterns.
• These factors represent the genetic basis of inheritance.
• Focus shifted to understanding the genetic material's structure and how genotype relates to phenotype.
• Molecular biology became a major field, thanks to contributions from Watson, Crick, Nirenberg, Khorana, and Kornbergs.
• The mechanisms of evolution were also explored, molecular genetics, structural biology, and bioinformatics advanced the understanding of evolution's molecular basis.
• This unit examines DNA structure, function, and the story/theory of evolution.

James Watson

• Born in Chicago on April 6, 1928
• Received a B.Sc. in Zoology in 1947.
• A desire to learn genetics started with bird-watching
• Received a Fellowship for graduate study in Zoology at Indiana University, Bloomington.
• Earned a Ph.D. in 1950, studying the effect of hard X-rays on bacteriophage multiplication

Francis Crick

• Born on June 8, 1916, in Northampton, England.
• Studied physics at University College, London and obtained a B.Sc. in 1937.
• Completed a Ph.D. in 1954 with a thesis on “X-ray Diffraction: Polypeptides and Proteins.”
• Friendship with James Watson influenced Crick's career
• They proposed the double-helical structure for DNA and the replication scheme in 1953.
• Crick became an F.R.S. in 1959.

Watson and Crick Honors

• John Collins Warren Prize of the Massachusetts General Hospital in 1959
• Lasker Award in 1960
• Research Corporation Prize in 1962
• Nobel Prize in 1962.

Genetics

• Genetics deals with inheritance and variation of characters from parents to offspring.
• Inheritance is the process of characters being passed from parent to progeny
• It forms the basis of heredity.
• Variation is the degree to which progeny differ from their parents.
• Humans understood that variation stems from sexual reproduction as early as 8000-1000 B.C.
• Variations in wild plant and animal populations were exploited through selective breeding for desirable traits.

Mendel's Laws of Inheritance

• Gregor Mendel conducted pea plant hybridization experiments over seven years (1856-1863).
• Mendel proposed the laws of inheritance in living organisms.
• Mendel's investigations applied statistical analysis and mathematical logic to biology for the first time.
• Large sampling sizes in Mendel's experiments added credibility to his data.
• Confirmation of inferences from successive generations proved his results indicated general inheritance rules.
• Mendel studied characters in the garden pea plant that manifested as two opposing traits.
• This created a basic framework of inheritance rules.
• True-breeding pea lines, which show stable trait inheritance and expression after continuous self-pollination, were used in artificial pollination/cross-pollination experiments
• Mendel selected 14 true-breeding pea plant varieties.
• Varieties were similar except for one character with contrasting traits.
• Contrasting traits included smooth/wrinkled seeds, yellow/green seeds, and inflated/constricted green/yellow pods.

Contrasting Traits in Pea Plants

• Stem height contrasting traits are tall and dwarf
• Flower color contrasting traits are violet and white
• Flower position contrasting traits are axial and terminal
• Pod shape contrasting traits are inflated and constricted
• Pod color contrasting traits are green and yellow
• Seed shape contrasting traits are round and wrinkled
• Seed color contrasting traits are yellow and green

Inheritance of One Gene

• Mendel crossed tall and dwarf pea plants to study the inheritance of one gene.
• Seeds from this cross were collected and grown to produce the first hybrid generation, known as Filial1 progeny or F1
• All F1 progeny plants were tall, like one of the parents, so having none that were dwarf.
• Similar observations for other pairs of traits found that the F1 always resembled one parent.
• The other parent's trait was not seen.
• A cross was made in pea plants by removing the anthers to avoid unwanted pollination- Emasculation
• Tall F1 plants were self-pollinated and in the Filial, generation offspring were 'dwarf'
• The character that was not seen in the F1 generation was now expressed.
• Dwarf plants were 1/4th of the F2 plants, while 3/4th of the F2 plants were tall.
• Tall and dwarf traits were identical to their parental type and did not show any blending
• All offspring were either tall or dwarf, none were of in-between height.
• Similar results from other traits showed that only one of the parental traits was expressed in the F1 generation.
• That both traits were expressed in the proportion 3:1 at the F2 stage.
• Contrasting traits did not show any blending at either F1 or F2 stage.

Genes

• Mendel proposed that something was being stably passed down from parent to offspring through gametes and called these things ‘factors'.
• 'Factors’ are now called genes.
• Genes are the units of inheritance because they contain the information for characteristics
• Genes coding for contrasting traits are alleles, are slightly different forms of the same gene.
• Capital letters represent traits which are expressed at the F1 stage while small letters represent that of other traits.
• T is used for the Tall trait and the ‘dwarf is represented as t
• T and t are alleles of each other.
• Plants have a pair of alleles for height.
• The height would be TT, Tt, or tt.
• Allelic pair of genes for height are identical or homozygous in a true breeding, tall or dwarf pea variety meaning that they are TT and tt.
• TT and tt signify the genotype of plant, while descriptive terms represent the phenotype.
• Phenotype is the visible character like Tall and Dwarf
• Tt is heterozygote.
• Tt was exactly like the TT parent and proposed that in a pair of dissimilar factors, one dominates the other and in this case T (for tallness) is dominant over t (for dwarfness),
• T for tall and d for dwarf should not be used which will make it hard to differentiate if T and d are allels of the same gene.
• Alleles can be similar is homozygotes or dissimilar as in heterozygote

Genetics

• A Tt plant is heterozygous, and since it controls one character (height), it is a monohybrid
• A cross between TT and tt is a monohybrid cross.
• Recessive parental traits can be expressed with no blending in the F2 generation
• Tall and dwarf plants produce gametes through meiosis
• Alleles of the parental pair separate or segregate from each other
• Only one allele is transmitted to a gamete, allele segregation is a random process, with a 50% chance of a gamete containing either allele.
• Tall TT plants have gametes with the allele T and dwarf tt plants have gametes with the allele t.
• During fertilization, the two alleles, T and t, unite to produce zygotes that have one T allele and one t allele and hybrids become Tt
• Hybrids are heterozygous plants
• Punnett Square helps to describe the parents from parents through to the formation of the gametes, formation of the zygotes, the F1 and F2
• Developed by British geneticist, Reginald C. Punnett to calculate the probability of all possible genotypes of offspring in a genetic cross.

Phenotypes on a Punnett Square

• Possible gametes are written on two sides, which is usually the top row and left columns.
• Combinations are represented in each box in the squares to form a square output form.
• Parental tall is shown as TT and dwarf is tt
• The gametes produced by them, and F1 progeny are represented as Tt
• F1 plants that are Tt are self pollinated, & and %, are used to show that female represents an(eggs) and pollen represents male of F1 generation
• Plant of genotype Tt, when self-pollinated produces gametes of the phenotype T and t equally.
• When fertilisation occurs, the pollen of the phenotype T has 50% chance to pollinate the eggs(T) or phenotype t.
• Random fertilisation produces genotypes TT, Tt, or tt.

Outcomes from using the Punnett square

• Through the Punnet square is easily seen that ¼ random fertilisations will lead to TT or lead to Tt/tt.
• Although the phenotype of Tt, is Tall the F2 will have Tall with TT and Tt.
• To specify within Tt, T Tall character is dominant in the allele t or Dwarf character, Tall F1 is shown dominance of 1 allele in the phenotype.
• Plants in the F2, will have to phenotypes that have 1/4TT and Phenotype of genotype Tt is ¾ from ¼ (TT + Tt) , Dwarf which is ¼ (tt).
• Genotypic ratio is 1:2:1 and phenotypic ratio is 3:4
• 1/4:1/2 and ¼ of TT / Tt and tt has two forms, (ax+by)2 contains phenotype that contains Genes T and t which also determines the expresseion
• T is denoted with (1/2T +1/2t)2 = (1/2T + ½ T x (+ ½ T = ¼ TT + ½ Tt + ¼ tt
• Genotype = tall

Mendel's conclusion

• Mendel self-pollinated the F2 plants and found that dwarf F2 can continued generating dwarf plants like the F3 and 4 as well which concludes that dwarf phenotype will generate homogenous (tt).
• The F2 of T plant that self pollilnates, there is still no specific information wether is is genetically homogenous.
• To show that phenotype probability is more of a math concept than by actually showing them through dominant phenotype that the genotype and therefore can be calculated even with one trait.
• A test cross determines is genotype of a tall pant of F2
• Self crossing is a result of testing to see if an organism cross test ( pea plant) can show a type of dominant phenotype and its genotype is known to be crossed with parents recesseivness.

Law of Dominance, 4.2.1

• Factors control character and that those with pairs are in dominance to others.
• It goes on to prove how important one parent character is by F1 while showing both parents being seen in F2.

Law of Segregation, 4.2.2

• The allels aren't mixed and thus both with traits are again in effect with the F2 generation though trait though is not seen at the F1.
• A homosygous will provide similiar gamete to that to a heterosygous production from 2 parents