Gregor Mendel
1860’s
Garden Pea
Flower produces both gametes.
stamens: male
carpel: female
Experimental Design
Experimental Design
1. Ensure true-breeding varieties.
these are the parental (P) generation
2. Cross two varieties.
produces the F1 generation (hybrids)
Experimental Design
3. Allow F1 plants to self-fertilize
a. produces the F2 generation
b. Mendel counted F2
Results
1. F1 all looked the same
a. purple flowers (no white)
b. green pods (no yellow)
c. yellow seeds (no green)
d. only one of each trait appeared
Results
2. This identified the dominant traits.
3. The recessive traits did not appear in
F1.
Results
4. F2 generation (self-fertilization)
a. the recessive traits reappeared
b. Mendel counted the results: dominant
vs. recessive
c. 3:1 ratio
Results
F3 generation
a. all recessives were true-breeders
b. only 1/3 dominant F2 plants were true-breeders
c. other 2/3 of dominants produced 3:1 ratio
Results
5. The F2 genotype was really 1:2:1
1 true-breeding dominant
2 not true-breeding dominant
1 true-breeding recessive
Mendel’s Model
A. Four hypotheses
1. There are alternative forms of heritable factors (genes) that
account for variations in inherited characteristics
alleles: alternative forms of a gene (for a trait)
2. For each characteristic, an organism inherits two alleles, one
from each parent. These alleles may be the same.
a. homozygous: organism has two identical alleles
b. heterozygous: organism has two different alleles
Mendel’s Model
3. If the alleles of an inherited pair differ, then one determines
the organism’s appearance and is called the dominant allele, the
other has no noticeable effect on the organism’s appearance and
is called the recessive allele.
Having a recessive allele does not necessarily mean it will be
expressed.
Mendel’s Model
4. A sperm or egg carries only one allele for each inherited trait
because allele pairs separate (segregate) from each other during
Meiosis.
This is known as Mendel’s Law of Segregation.
Test Cross
Mendel’s Law of Independent Assortment
Is illustrated by dihybrid crosses:
Inheritance of one trait does not influence the inheritance of any other.
Not valid for genes on the same chromosome.
Genes located on different chromosomes are inherited independently.
Multiple Alleles (9.12)
Diploid individuals can possess only two alleles at one time
(one on each member of a homologous pair).
There are almost always more than two alleles for any given
trait in the population.
More than one can be dominant (codominance)
Sex Linkage (9.21)
Some genes are carried on sex chromosomes
in humans, mostly on X