Genetics

Gregor Mendel, Austrian ? monk, 1866 uncovered important principles of Inheritance.

Discovered genes and patterns of inheritance
Verified genes come from BOTH parents. (sperm ­ seed)
Discovered that cells must have TWO copies of each gene
Discovered one form of gene (allele) can be dominant to another
Discovered recessive gene expressed only when dominant absent

Work lost in archives for 50 years

Rediscovered in early 1900s

Mendel as "father" of genetics

Recall:

egg (1N) + sperm (1N)	syngamy ----------------->	zygote (2N) --> embryo (2N) --> adult (2N)

Mendel worked with Peas--normally self pollinating...only has to intervene for special crosses

Gene for height in peas...alleles are: D = tall and d = dwarf

Three possible genotypes (= allele combinations)
Two possible phenotypes (= how the plants look)

DD = tall (homozygous dominant)

Dd = tall (heterozygous)
      "when two alleles go walking, the TALL allele does the talking"

dd = dwarf (homozygous recessive)

And he made sure they were homozygous before doing crosses

He was meticulous, kept accurate and numerical records of outcomes

True-breeding tall (DD)	x	True-breeding dwarf (dd)	= Parental Generation
Egg = D	x	Sperm = d
all offspring	Tall (Dd)	= First Filial Generation = F1

Notice that when the two alleles (D&d) go walking, the D allele does the talking ALL ARE TALL

Now Mendel lets the F1s self pollinate as usual:

F1	Tall (Dd)	x	F1	Tall (Dd)

When Mendel published his work he showed how the F2 results came out into special ratios with mathematical formulas. One of the later biologists, named Punnet, helped 20th century biologists understand these formulas with what is now known as a Punnet Square:

	Possible Sperm
Possible Eggs	D	d
D	DD tall	Dd tall
d	Dd tall	dd dwarf

Notice that when both parents are hybrid (Dd) for one gene, this is called a MONOHYBRID cross and it results in an F2 family with a 3:1 ratio of tall to short progeny.

Remember that both DD and Dd genotypes are tall in phenotype.

You cannot "see" genotypes, so all you "see" are 3 "identical" tall plants for every dwarf.

You know the genotype of the dwarfs...right? dd the only possible combination

How can you distinguish which of the tall ones are DD and which are Dd?

Mendel and later workers used what is now known as a TEST CROSS.

TEST CROSS = unknown genotype with dominant phenotype x double recessive

In our case, any unknown tall x dwarf

There could be one of two possible test crosses:

DD x dd	or	Dd x dd

These two have distinct outcomes that tell you what the unknown is:

DD x dd

	Possible Sperm
Possible Eggs	d	d
D	Dd tall	Dd tall
D	Dd tall	Dd tall

notice that the outcome is ALL tall...the unknown has ALL tall alleles = DD!

Dd x dd

	Possible Sperm
Possible Eggs	d	d
D	Dd tall	Dd tall
d	dd dwarf	dd dwarf

notice that the outcome is HALF tall..the unknown has HALF tall alleles = Dd!

Let's try one more example to work through Mendel's ideas:

Mendel crosses truebreeding (homozygous) plants:

He gets plants with all yellow seeds

What does this tell you?

1. Yellow is dominant to green
2. Use G as the symbol
3. F1 are heterozygous

Let's put this down in symbolic logic:

Parental	GG yellow	x	gg green
	F1= all	Gg yellow

Now Mendel self-crosses the F1s to obtain the F2:

F1:	Gg yellow	x	F1:	Gg yellow

	Possible Sperm
Possible Eggs	G	g
G	GG yellow	Gg yellow
g	Gg yellow	gg green

Notice the usual 3:1 ratio of yellow to green expected for a MONOHYBRID cross!

Let's say Mendel has a pod from one of these self crosses, it contains five F2 seeds.
He notices that three of them are green and two are yellow. Is this possible?

Sure SMALL FAMILIES do not fit expected ratios...

1/2 expected female 1/2 expected male in humans
Do you know families of five that are not 2.5 female and 2.5 male?
Any that are 3 female/2 male or 2 male/2 female?

green

green

green

yellow

yellow

For which of these F2s does he know the genotype?..........green = gg

The F2 yellows are unknown.

To determine their genotype, what must Mendel do?

TESTCROSS = cross the unknown yellows with a green.

Let's say Mendel crosses the second yellow F2 with a green and gets a pod with five yellow and one green, what is the yellow parent's most likely genotype?......Gg

What was our expected ratio from this test cross: 1yellow:1green Why didn't Mendel get 3 green and 3 yellow?...........Small family variation!

While many gene situations work out this simply, not all do.

Here is an example from snapdragons:

Petal color alleles: Y = red and y = yellow

YY red	x	yy yellow	Parental
	Yy orange	F1

WOOPS!
When these alleles go walking they both are talking!

Rewrite our ideas!

Petal color alleles: P^R = red and P^Y = yellow

PRPR red	x	PYPY yellow	Parental
	PRPY orange	F1

NOW go on to the F2:

PRPY orange	x	PRPY orange

	Possible Sperm
Possible Eggs	PR	PY
PR	PRPR red	PRPY orange
PY	PRPY orange	PYPY yellow

Mendel's rule of a 3:1 ratio fails here in one sense...
BUT this exception UPHOLDS the rule.

Notice that while the outcome is not 3:1...
The results are completely as predicted through Mendel's logic...1:2:1 ratio!

It gets messier....there are actually three color alleles! The third is p = white

What are all the possible genotypes for snapdragons?

P^RP^R = red
P^Rp = pink
P^YP^Y = yellow
P^Yp = cream
P^RP^Y = orange
pp = white

Human blood types work something like this.
Human hair color works something like this.

Alleles:

H^Bd = blonde
H^Br = brown
h^R = red
h^bl = black

H^Bd H^Bd = blonde
H^Bd H^Br = mousy brown
H^Bd h^R = strawberry blonde
H^Bd h^bl = blonde
H^Br H^Br = brown
H^Br h^R = auburn
H^Br h^bl = brown
h^R h^R = red (orange)
h^R h^bl = red (orange)
h^bl h^bl = black

Important!: Please note double recessive h^blh^bl is MOST common in world!

dominant does not mean common!

recessive may be precisely what we want!

Some characteristics actually operate with MANY genes....

Crop yield is an example:

AABBCCDDEE = highest yielding genotype
aabbccddee = lowest yielding genotype

AaBBccDdEd = moderate yielding genotype

Notice: all kinds of variations between the extremes

Human skin works like this.

Shades of brown from	aabbccddee (very light tan)	to	AABBCCDDEE (dark brown)

Please notice that people are neither white nor black!

MANY characteristics depend upon environment also:

Human Height: Asian population is short but has tall genes...

Ross feels like giant on visit
Asian families who move to US have short parents, tall children
Asia has poor nutritional support...genetically tall people are short
      when they come to good US nutrition, the children grow tall

Of course most obvious for lighter types, but even observable among the darkest types...tan lines on everyone!

Who mates with Whom?

Hybrid Vigor:

wild corn A low yield	x	wild corn B low yield
	hybrid corn high yield

Easily observed within animals: best dog = mongrel = mut       faults of either type are covered by good alleles in other type

Inbreeding Depression:

Laws prevent you from marrying your sister.

European Monarchy was inbreeding: Victoria's mutation for hemophilia

Granddaughters all carriers of hemophilia
Married to Crown Princes of Europe
Male children of Monarchs are hemophiliacs
Monarchy overthrown....good!

Manic Depressive illness among Irish populations

Polydactyly among Hutterites

MESSAGE: OUTCROSS!! Don't marry your own kind (sister/brother)!

Mechanisms assuring outcrossing in plants

Evolution of sex:

Plants male OR female (not both), Dioecious
ex: Holly, Kiwi, need BOTH for fruits.

Timing separation:

Protandrous: Plants male for a few days, when pollen gone, then female
Protogynous: Plants female for awhile, then when "over the hill," produce pollen
Other combinations also: eg: Cucumber, Squash, etc.
Alstroemeria ????

Self-incompatibility:

Own pollen cannot germinate on stigma
Own pollen tube cannot grow through style
Own pollen tube cannot shed sperm
Own sperm cannot unite with egg
eg: Sweet Cherries, Pears, Filberts, Almonds, etc. need two trees!

MECHANISMS ASSURING CORRECT POLLINATION

Do need to be sure the pollen is from another plant of same type!

Use symbolic logic...symbols are whole sets of chromosomes!

Parental	WW Wheat	x	RR Rye
Gametes	W egg	x	R sperm
Offspring		WR sterile hybrid

this hybrid one cannot form seeds = seedless

Scientists want the flavor of wheat with yield of rye so ...
they treat the WR sterile hybrid with...
colchicine
from autumn crocus plants to double chromosomes:

WR sterile hybrid	colchicine ---------------->	WWRR fertile hybrid allopolyploid tetraploid)	= Triticale

Now cross the fertile hybrid with wheat to improve flavor of grain:

WWRR fertile tetraploid	x	WW wheat
WR egg	x	W sperm
	WWR sterile triploid	colchicine ------------>	WWWWRR fertile hybrid hexaploid

Now we have an EXTREMELY VIGOROUS plant with very high yield and good flavor. This is the wheat in most flour today!

This page © Ross E. Koning 1994.

The MLA citation style for this page would be:
Koning, Ross E. "Genetics". Plant Physiology Website. 1994. http://koning.ecsu.ctstateu.edu/plants_human/genetics.html (your visit date).

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