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Single Gene Traits Activity
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Monohybrid Genetic Problem Worksheet
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Dihybrid Genetic Problems Worksheet
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Pedigree Chart Worksheet
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Mendel's Monohybrid Cross
adapted from
Kimball's
Biology Pages
Introduction
Gregor Mendel (1822-1884) was an Austrian monk who discovered the basic
rules of inheritance. From 1858 to 1866, he bred garden peas in his
monastery garden and analyzed the offspring of these matings. The garden pea
was good choice of experimental organism because:
- many varieties were available that bred true for clear-cut,
qualitative traits like
- seed texture (round vs wrinkled)
- seed color (green vs yellow)
- flower color (white vs purple)
- tall vs dwarf growth habit
- and three others that also varied in a qualitative - rather than
quantitative - way.
- peas are normally self-pollinated because the stamens and
carpels are enclosed within the petals. By removing the stamens from
unripe flowers, Mendel could brush pollen from another variety on the
carpels when they ripened.
The first cross
Mendel crossed a pure-breeding round-seeded variety with a pure-breeding
wrinkled-seeded one.
Our interpretation
The parents (designated the
P generation) were pure-breeding because
each was homozygous for the alleles at the gene locus (on chromosome 7)
controlling seed texture (
RR for round;
rr for wrinkled).
The results
All the peas produced in the second or
hybrid generation were round.
Our interpretation
All the peas of this
F1 generation have an
Rr genotype.
All the haploid sperm and eggs produced by meiosis received one chromosome
7. All the zygotes received one
R allele (from the round parent) and
one
r allele (from the wrinkled parent). Because the round trait is
dominant, the
phenotype of all the seeds was round.
| |
|
P gametes (round parent) |
| |
|
R |
R |
P gametes
(wrinkled parent) |
r |
Rr |
Rr |
| r |
Rr |
Rr |
The second cross
Mendel then allowed his hybrid peas to self-pollinate.
The results
The wrinkled trait — which had disappeared in his hybrid generation —
reappeared in 25% of the new crop of peas.
Interpretation
Random union of equal numbers of
R and
r gametes produced an F
2
generation with 25%
RR and 50%
Rr - both with the round
phenotype - and 25%
rr with the wrinkled phenotype.
| |
|
F1
gametes |
| |
|
R |
r |
| F1
gametes |
R |
RR |
Rr |
| r |
Rr |
rr |
The third cross
Mendel then allowed some of each phenotype in the F
2 generation
to self-pollinate. His results:
- All the wrinkled seeds in the F2 generation produced only
wrinkled seeds in the F3.
- One-third (193/565) of the round F1 seeds produced only
round seeds in the F3 generation, but
- two-thirds (372/565) of them produced both types of seeds in the F3
and - once again - in a 3:1 ratio.
Interpretation
One-third of the round seeds and all of the wrinkled seeds in the F2
generation were homozygous and produced only seeds of the same phenotype.
But two thirds of the round seeds in the F2 were heterozygous
and their self-pollination produced both phenotypes in the ratio of a
typical F1 cross.
Phenotype ratios are approximate
The union of sperm and eggs is random. So the pod in the color photo
(courtesy of Cathie Martin from
Cell 12 January 1990) — with its 9
smooth seeds and 3 wrinkled seeds! — represents something of a statistical
fluke. As the size of the sample gets larger, however, chance deviations
become minimized and the ratios approach the theoretical predictions more
closely. The table shows the actual seed production by ten of Mendel's F
1
plants. While his individual plants deviated widely from the expected 3:1
ratio, the group as a whole approached it quite closely.
 |
| |
Round |
Wrinkled |
| 1. |
45 |
12 |
| 2. |
27 |
8 |
| 3. |
24 |
7 |
| 4. |
19 |
16 |
| 5. |
32 |
11 |
| 6. |
26 |
6 |
| 7. |
88 |
24 |
| 8. |
22 |
10 |
| 9. |
28 |
6 |
| 10. |
25 |
7 |
| Total |
336 |
107 |
|
To explain his results, Mendel formulated a hypothesis that included the
following:
- In the organism there is a pair of factors that controls the
appearance of a given characteristic. (We call them genes.)
- The organism inherits these factors from its parents, one from each.
- Each is transmitted from generation to generation as a discrete,
unchanging unit. (The wrinkled seeds in the F2 generation
were no less wrinkled than those in the P generation although they had
passed through the round-seeded F1 generation.)
- When the gametes are formed, the factors separate and are
distributed as units to each gamete. This statement is often called
Mendel's rule of segregation.
- If an organism has two unlike factors (we call them
alleles) for a characteristic, one may be expressed to the total
exclusion of the other (dominant vs recessive).
The Testcross: A Test of Mendel's Hypothesis
A good hypothesis meets several standards.
- It should provide an adequate explanation of the observed facts.
- If two or more hypotheses meet this standard, the simpler one is
preferred.
- It should be able to predict new facts.
So if a generalization is valid, then certain specific consequences can
be deduced from it.
In order to test his hypothesis, Mendel predicted the outcome of a
breeding experiment that he had not yet carried out. He crossed heterozygous
round peas (Rr) with wrinkled (homozygous, rr) ones. He
predicted that in this case one-half of the seeds produced would be round (Rr)
and one-half wrinkled (rr)
| |
|
F1
gametes |
| |
|
R |
r |
| P gametes |
r |
Rr |
rr |
| r |
Rr |
rr |
To a casual observer in the monastery garden, the cross appeared no
different from the P cross described above: round-seeded peas being
crossed with wrinkled-seeded ones. But Mendel predicted that this time he
would produce both round and wrinkled seeds and in a 50:50 ratio. He
performed the cross and harvested 106 round peas and 101 wrinkled peas.
This kind of mating is called a testcross. It "tests" the genotype
in those cases where two different genotypes (like RR and Rr)
produce the same phenotype.
Mendel did not stop here.
- He went on to cross pea varieties that differed in six other
qualitative traits. In every case, the results supported his hypothesis.
- He crossed peas that differed in two traits. He found that
the inheritance of one trait was independent of that of the other and so
framed his second rule: the rule of independent assortment.
Mendel's rules today
Little attention was paid when Mendel published his findings in 1866. Not
until 1900, 34 years later and 16 years after his death, was his work
brought to light. By then, three men — working independently — discovered
the same principles. So the present remarkable development of genetics dates
from only the start of the 20th century.
The discovery of chromosomes — and their behavior during meiosis (2n
-> n) and fertilization (n + n -> 2n) —
established the structural basis for Mendel's rules.
What is the status today of Mendel's rules? Although many important
exceptions to them have been discovered — two examples:
- both members of many allelic pairs affect the phenotype; that is,
neither is fully dominant;
- many gene loci are not inherited independently but show linkage
(because they are relatively close together on the same chromosome),
his rules still form the foundation upon which the science of genetics
rests.
.