Dawning of Genetics

For pdf files of all the early papers in genetics

http://www.esp.org/foundations/genetics/classical/browse/title-lst.html

How is phenotype transmitted from parent to offspring?

“Blending inheritance”

Variation declines with each generation

Francis Galton

Biographical links

http://www.mugu.com/galton/

http://www.maps.jcu.edu.au/hist/stats/galton/index.htm

First worked with sweet peas

Sent each of 10 friends a set of bags (labelled K through Q) and a set of instructions

Summary of parental seed information

Seed class	Weight of one seed (grains)	Length of row of 100 seeds (inches)	Diam. of one seed (1/100s of an inch)
K	1.750	21.0	21
L	1.578	20.2	20
M	1.406	19.2	19
N	1.234	17.9	18
O	1.062	17.0	17
P	0.890	16.1	16
Q	0.718	15.2	15

Results

Within each group of parental seeds, variation conformed to “the law of error”

Like that of balls falling in a quincux

See a quincux in action:

http://www.ms.uky.edu/~mai/java/stat/GaltonMachine.html

Relationship of progeny to parents

Suggested inheritance, but subject to influence of many minute causes that produced “regression to the mean”

Diameter of Parent Peas	Mean Diameter of Offspring Peas
21	17.26
20	17.07
19	16.37
18	16.40
17	16.13
16	16.17
15	15.98

… for each increase of one unit on the part of the parent seed,

… there is a mean increase of only one-third part of a unit in the filial seed

… the mean filial seed resembles the parental when the latter is about 15.5 hundredths of an inch in diameter.

Regression towards mediocrity

“It appeared from these experiments that the offspring did not tend to resemble their parent seeds in size, but always to be more mediocre than they

“To be smaller than the parents, if the parents were large

“To be larger than the parents, if the parents were very small

Galton later focused on human data

The right-hand plot is his attempt to summarize the data and fit a line.

He multiplied the womens' heights by 1.08 to make them comparable to mens' heights.

Defined the parent's height as the average of the two parents (midparent value)

Similar result: “regression towards mediocrity”

Slope less than one

“Regression” was disturbing to Galton, because the parentals were always more deviant that the offspring

He thought that it suggested a form of “blending inheritance” that caused variation to be lost

Gregor Mendel

Links

http://www.stg.brown.edu/webs/MendelWeb/MWref.html

http://www.mendel-museum.org/eng/1online/room4.htm

Review of the crosses (basic Mendelian genetics review)

http://ntri.tamuk.edu/homepage-ntri/lectures/biology/lecture12.html

Worked with peas

Why?

Pea flower

Figure from: http://ntri.tamuk.edu/homepage-ntri/lectures/biology/lecture12.html

Female structure: stigma arising from the ovary, which contains the ovules

Male structures: filaments holding the anthers, which contain the pollen

First Cross

Two unlike plants (e.g. yellow seeded by green seeded)

Yielded all yellow seeds in first filial generation (F₁)

Second cross

Cross two seeds from the first filial generation (F₁)

Monohybrid cross

This type of cross is now called a monohybrid cross

Some problem sets with monohybrid crosses

http://www.biology.arizona.edu/mendelian_genetics/problem_sets/monohybrid_cross/monohybrid_cross.html

Mendel’s results from his monohybrid crosses

Mendel’s first three Postulates

Mendel’s First Postulate

Particulate inheritance

Mendel’s Second Postulate

Each individual inherits two heritable units, one from each parent

Mendel’s Third Postulate

When an individual has an unmatched pair, one unit is dominant to the recessive unit

Mendel’s work not recognized by contemporaries

Contemplate interesting counterfactual: What if Darwin had read and understood Mendel’s paper?

Rediscovery

In 1900, papers were published by three different botanists who had each independently “rediscovered” Mendel’s postulates

All three papers were translated into English in 1950 and republished in a supplement to the journal Genetics

The Birth of Genetics: Mendel, de Vries, Correns and Tschermak in English Translation. Supplement to Genetics 35, September 1950, No. 5 pt. 2

Extensions

Incomplete dominance

Correns first described incomplete dominance from crosses in 4-o’clock (Mirabilis jalapa)

Incomplete dominance

Punnett Square and probability theory

e.g. Product Law

Chance of multiple independent events occuring together is equal to products of probabilities of each independent event occuring individually

E.g. flip a coin

Chance of flipping head = P(head) = 1/2

Chance of flipping two heads = P(head, head) = 1/2 x 1/2 = ¼

Sex linkage tidbit

Punnett was a poultry geneticist

Produced first autosexing poultry breed in 1929 (Gold Campine X Barred Rock).