1
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M ENDEL' S HYPOTHESES TO EXPLAIN INHERITANCE.
1. There are alternative forms for genes, the units that determine heritable
characteristics.
The gene for pod color, for example exists in 2 alternative forms, one for green pods and the
other for yellow pods. Such alternative forms of genes are now called alleles.
2. For each inherited characteristic, an organism has two alleles, one
inherited from each parent.
In Mendel=s experiments, one parental variety had a pair of alleles for green pod color,
while the other had a pair of alleles for yellow pod color. Mendel=s F1 hybrids had inherited
from the parental plants one allele for green pod color and one allele for yellow pod color.
3. A sperm or egg carries one allele for each inherited characteristic,
because allele pairs separate (segregate) from each other during the
production of gametes.
When a sperm and egg unite during fertilization, both contribute their alleles, thus restoring
the gene to the paired condition. In Mendel=s experiments, each gamete of a parental plant
carried one allele for that plants= pod color, specifying either green or yellow. Cross-pollination
resulted in the Amixed@ gene combination of the F1 hybrids.
4. When the two alleles of a pair are different, one is fully expressed and the
other completely masked. These are called the dominant allele and
the recessive allele, respectively.
According to this idea, the F1 hybrids had green pods because the allele for the trait is
dominant over the allele for yellow pods, which is recessive.
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MENDEL=S LAW OF SEGREGATION:
Allele pairs segregate during gamete formation, and the paired condition is restored by the
random fusion of gametes at fertilization.
Expressed traits are called PHENOTYPE
Genetic makeup called GENOTYPE
2
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3:1 HYPOTHESIS
F1 generation predicts that two classes of gametes produced. When genes separate 2 of the
gametes receive a green-pod allele (G) the other 2 gets a yellow-pod allele (g). During selfpollination these two classes of gametes unite randomly. This is true both for sperm carrying a
green-pod gamete as well as a sperm carrying a yellow-pod gamete. Since this is also true for
the egg gamete there is a total of four likely combination that arise between sperm and egg.
Since green-pod gametes are dominant over yellow-pod gametes all pea pods will appear green
in the first generation. We can diagram these combinations using a PUNNET SQUARE.
Parent alleles
Green pod (GG)
Yellow pod (gg)
F 1 generation
G
g Gg
G
Gg
g Gg
Gg
What will the appearance of the F2 generation be??
F1 backcross
Gg x Gg
G
g
G GG Gg
g Gg
gg
Clearly, 2 of the alleles from the F1 generation are green-pod alleles while the 2 are for
yellow-pod alleles. Separating the alleles and using the Punnet Square we see 3 of the progeny
ped pods will have inherited two green alleles, 2 of the pea pods will be a mix of green and
yellow alleles and 3 of the remaining pea pods will inherit alleles yellow-pods only thus
expressing a 3:1 ratio (phenotype) of green- to yellow-pod.
One parent appears to have disappeared (not expressed) in the F1 generation only to
reappear (expressed) in 3 the F2 generation.
3
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THE TEST CROSS
What is the point to the test cross??
Can we tell whether a green pea pods are homozygous or heterozygous for green//yellow
pods?? In another words, are they true breeding for green pea pods?
Yellow-pods (gg) are recessive a pea plants expressing yellow pods has to be homozygous
for yellow alleles. It then follows that the progeny of GG x gg will express as phenotypically
green-pods (Gg). However, if green pea pods are heterozygous (Gg) then test crossing Gg x gg
will produce a phenotypic expression of Gg and gg or 1:1 ratio.
Gg x gg------->
G g
g Gg gg
g Gg gg
M ENDEL LAW OF INDEPENDENT ASSORTMENT
Monohybrid crosses Pea shape such as round (RR) which is dominant over wrinkled (rr)
seeds or yellow (YY) seed collor which is dominant over green (yy) seeds.
What happens if we examine two traits (Dihybrid cross) being transmitted from parents to
offspring?
RRYY x rryy = F1 progeny will be RrYy
Question: do the F1 progeny transmit their genes in the same combinations to their offspring?
Four classes of gametes are formed from RrYy ----> RY, Ry, rY, ry
4
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If the four classes of sperm are mixed with the four classes of egg gametes there will be
a 16 (4 x 4) equal probable ways that genes can be combined in the F2 generation.
Experimental results of dihybrid crosses give an observable phenotypic ratio 9:3:3:1
supporting Mendel=s law of independent assortment of genes.