Classical assumption 1 about heredity
- Heredity occurs within species. [people realized that heredity occurs within species only] you cannot create bizarre creatures by cross breeding. Species were maintained without significant change from the time of their creation.
Classical assumption 2 about heredity
- Direct transmission of traits.
-They assumed that – Information on each body part was transmitted separately to
the child and child formed once all information from each
body part was gathered.
– Assumed that male and female contribution resulted in
blend of offspring eg. Tall father and short mother resulted in
medium height children
What was the paradox that the assumptions led to?
The question was: If there is no genetic variation entering a species from outside and if the variation in each species blends in each generation, then shouldn’t all members of the species develop the same characteristics? e.g. humans all looking the same after many generations. [paradox]
Who was Koelreuter?
A German in 1760 who carried out hybridisation experiments with tobacco plants. The offspring appeared different from either parent. Crosses of the offspring [hybrids] resulted in further variation.
- The parent traits were NOT blended. The traits either resembled parents or grand parents. Sometimes the traits would be masked for a generation , and reappear in the next. # Alternative forms segregating among offspring
Classical theories [assumptions 2] fail. How?
- If it was as the second assumption stated, the direct transmission, How did Koelreuter get disappearance of trait in 1 gen. and reappearance in the next?
-There was no blending of traits occurring. - Alternative forms were ‘segregating’ amongst offspring. i.e. one offspring was exhibiting hairy
leaves, for example while another smooth leaves.
Classical theories [assumption 1] fail. How?
[Assumption 1 = they said that you could not cross breed and get bizarre creatures]
During the T.A experiments, they crossed true-breeding peas, purple and white flowers. results were:
- All offspring of first cross had purple flowers
– Offspring of next cross had both color flowers
– Purple flowers predominated over white flowers[more purple]
Early, pre-Mendel Genetic concepts.
– Some forms of inherited traits masked in one generation
– Forms of a trait segregate among offspring
– Some forms represented more frequently than others.
Gregor Mendel
- Father of modern genetics
- Carried out first Quantitative Studies.
- Used garden pea
- Expected segregation among offspring
- There are many true-breeding traits, he studied only 7.
- He used small plants, which are easy to grow and have short generation time.
How was Mendel’s Experimental Design?
- Allowed Several Generations of Self-Fertilization
• Progeny produced only a single form of a trait
• Assured that forms of traits were transmitted regularly
• Conducted Crosses Between Alternate Forms of a Trait
• Removed male parts from a flower with white flowers
• Fertilized with pollen from plant with purple flowers
• Performed reciprocal crosses white flower pollen on
purple flower plant
• Allowed Self-Fertilization of Hybrids
• Allowed segregation of alternate forms of traits
• Counted number of offspring of each type per
generation
• Quantification of results most important to studies
Mendel’s Findings
First Filial (F1) Progeny Resembled One of the Parents
• Trait expressed in F1 called dominant
• Trait masked in F1 called recessive
• All seven traits had dominant and recessive forms
• Planted F1 Seeds To Produce F2 (Second Filial)
Generation
• Determined proportion of dominant to recessive
• Three-fourths of plants exhibited dominant form
• One-fourth of plants exhibited masked, recessive form
• Dominant: recessive ratio was close to 3:1 for all seven
traits• Subsequent Generations
• Recessive individuals bred true
• One-third of dominant individuals bred true
• Two-thirds of dominant individuals produced 3:1
progeny
• 3:1 ratio really 1:2:1 ratio, separating dominant
genotypes
How did Mendel interpret his results?
Understood Four Things About Nature of Heredity:
- Alternatives of traits are inherited intact
- One form did not appear in F1 but reappeared in F2
- Pairs of alternative forms segregated among progeny
- Characteristic Mendelian Ratio of segregation is 3:1
Mendel’s Model [summary]
Parents transmit factors that provide information
about traits
• Each individual contains two factors for each trait
• May code for same form or alternative forms
Alternate forms of factor called alleles
• Individual is homozygous when both alleles are the
same
• Individual is heterozygous when alleles are different
• Position of gene on DNA is called its locus
- Alleles from each parent do not influence one another
- Genotype is the totality of the genes (blueprint)
• Phenotype is the expression of the genes (outcome)
-
The F1 Generation
Mendel's first cross = pp x PP • Each parent can produce gametes of only its kind • Purple gametes contain only P allele • White gametes contain only p allele • Resulting progeny all Pp, all purple
The F2 Generation
All are heterozygous, purple, cross = Pp x Pp
• Alleles segregate randomly in gametes, either P or p
The Further Generations
Three kinds of F2 individuals • Pure-breeding white flowers (pp) • Heterozygous purple flowers (Pp) • Pure-breeding purple flowers (PP) • Closer examination of 3:1 ratio indicates 1:2:1 genotypic ratio
Mendels first law of heredity: the law of Segregation
- Explained segregation without cellular knowledge
• Behavior of alternative alleles
• Alternative forms encoded by discrete alleles
• Alternative alleles separate in gametes formation
• Each gamete has equal possibility to get either
allele
Mendel’s Second Law of heredity: The Law of Independent Assortment
Genes located on different chromosomes assort
independent of one another
• Mendel picked traits on different chromosomes
From Genotype –> Phenotype: How genes interact.
Complex genetic patterns: Difficult to determine phenotypic classes
Multiple alleles
There’s also gene interaction: where genes act sequentially or jointly.
Modified Mendelian ratios
Epistasis: One gene modifies expression of the other gene.
Emerson 9:7 ratio
Continuous variation
multiple genes act jointly – varying
heights of people
Pleiotropy
Gene has more than one effect on phenotype.
Incomplete dominance
Alternative alleles not dominant or recessive.
Environmental effects
Modify gene products
Chromosomes: sex linkage
Proof of Chromosomal Theory of Inheritance - Morgan 1910
• Discovery of mutant, white-eyed male fruit fly
• Crossed with wild type red-eyed female
• All progeny had red eyes, concluded red eye colour dominant
• Cross of F1 generation
• 3:1 ratio red to white eyes
• All recessive white eye flies were male
• Testcross F1 to white-eyed male
• 1:1:1:1 ratio
• Eye colour and sex equally represented
• Explanation: eye colour gene related to sex chromosome
• Eye Colour Gene Located on Sex Chromosome in Fruit Flies
• Two kinds of sex chromosomes, X and Y
• XX = female, XY = male
• Eye colour gene located on the X chromosome
• Sex linked trait
