Mendel and the Gene Idea. Inheritance. The passing of traits from parents to offspring. Humans have known about inheritance for thousands of years. Genetics. The scientific study of the inheritance. Genetics is a relatively “new” science (about 150 years). Genetic Theories.
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1. Blending Theory -
traits were like paints and mixed evenly from both parents.
2. Incubation Theory -
only one parent controlled the traits of the children.
Ex: Spermists and Ovists
parents pass on traits as discrete units that retain their identities in the offspring.
Mendel was a pea picker. by Science until the early 1900’s.
He used peas as his study organism.
Example - Tall X short
Example - Tall X short
P1 Tall X short (TT x tt)
F1 all Tall (Tt)
F2 3 tall to 1 short
(1 TT: 2 Tt: 1 tt)
1. Genes can have alternate versions called alleles.
2. Each offspring inherits two alleles, one from each parent.
3. If the two alleles differ, the dominant allele is expressed. The recessive allele remains hidden unless the dominant allele is absent.
Comment - do not use the terms “strongest” to describe the dominant allele.
4. The two alleles for each trait separate during gamete formation. This now called: Mendel's Law of Segregation
TT X tt all Tt all Dom
Tt X Tt 1TT:2Tt:1tt 3 Dom: 1 Res
TT X TT all TT all Dom
tt X tt all tt all Res
TT X Tt 1TT:1Tt all Dom
Tt X tt 1Tt:1tt 1 Dom: 1 Res
If TT - all dominant
If Tt - 1 Dominant: 1 Recessive
The formula 2n can be used, where “n” = the number of heterozygous traits.
Ex: TtRr, n=2
22 or 4 different kinds of gametes are possible.
TR, tR, Tr, tr
TtRr X TtRr
Each parent can produce 4 types of gametes.
TR, Tr, tR, tr
Cross is a 4 X 4 with 16 possible offspring.
75% chance of the dominant
25% chance of the recessive
“T’s” = Tt X Tt = 3:1
“R’s” = rr X Rr = 1:1
“G’s” = GG x gg = 1:0
(3:1) X (1:1) X (1:0 ) = 3:3:1:1
1. Incomplete Dominance
3. Multiple Alleles
5. Polygenic Inheritance
Ex. Red X White snapdragons
F1 = all pink
F2 = 1 red: 2 pink: 1 white
A IA IA or IAi
B IB IB or IBi
CcBb X CcBb
Brown X Brown
F1 = 9 brown (C_B_)
3 black (C_bb)
4 albino (cc__)
Question - Is this possible?
Comment - Wife’s boss is type O
Therefore, the child is the offspring of the wife and her husband (and not the boss).
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Person with trait
Recessive Trait by Science until the early 1900’s.
R = F X M X D
R = risk
F = probability that the female carries the gene.
M = probability that the male carries the gene.
D = Disease risk under best conditions.
R = 1 X 1 X .25
R = .25
There is a .25 chance that every child will be albino.