I. Introduction
II. Gregor Mendel -- See also this
III. Terms to know
IV. Monohybrid cross
A. TT X tt T=Tall, t=dwarf
results of cross - true breeding TT X tt -> Tt tall plants
tall dwarf
1. results of cross - Punnett square
TT tt
_______________
| | |
TT | TT | Tt |
| | |
----------------
| | |
tt | tT | tt |
|______|_______|
Monohybrid cross with a human trait
The rules of inheritance discovered by Mendel in Pea are universally applicable to all organisms, including humans. Consider the inherited disease of humans, Cystic Fibrosis. This is the most common lethal genetic disease afflicting Caucasians. It is caused by a mutant recessive gene carried by one in 20 people of European descent (over 12 million people in the United States alone). This means that one in 400 Caucasian couples will both be carriers, and as can be expected by the monohybrid inheritance pattern, one in four of their children will have the disease.
The disease affects tissues called secretory epithelia which are responsible for transporting water, and salt at the interface between the bloodstream and the external environment (lungs, intestine, and sweat glands). The abnormal secretory cells in cystic fibrosis patients fail to carry out this transport properly, which causes buildup of thick mucus in the affected organs. This is particularly significant in the lungs where bacteria find a fertile environment for growth. Lung infections, are very serious and the primary reason cystic fibrosis patients do not survive past their twenties.
Inheritance Pattern
If two parents who are carriers of the recessive gene Cfcf, (heterozygous) produce offspring, as can be seen by the Punnett square below, it is expected that one in four of their children will be homozygous and have cystic fibrosis.
| Cf | cf | |
| Cf | Cf Cf normal | Cf cf symptom less carrier |
| cf | Cf cf symptom less carrier | cf cf cystic fibrosis |
Of course, it should be noted, that this one in four probability is just that, an expectation, and that in fact two carriers could produce any number of perfectly normal children. However, the greatest probability is for one in four children to be affected. This factor is very important when prospective parents are concerned about the chance of having affected children. Presently one out of every 29 Americans is a symptom less carrier of the gene.
IV. Dihybrid cross - involves two pairs of genes
RRYY x rryy
gametes RY x ry
F1 RrYy
then RrYy x RrYy
RY rY Ry ry
_______________________________
| | | | |
RY | RRYY | RrYY | RRYy | RrYy |
-------------------------------
rY | RrYY | rrYY | RrYy | RrYy |
|______|_______|_______|_______|
Ry | RRYy | RrYy | RRyy | Rryy |
|______|_______|_______|_______|
ry | RrYy | rrYy | Rryy | rryy |
|______|_______|_______|_______|
2. Results of cross (F2)
Genotype Phenotype
ratio ratio
1 RRYY
2 RRYy 9 R*Y* - round yellow
1 RRyy
2 RrYY
4 RrYy 3 R*yy - round green
2 Rryy
1 rrYY 3 rrY* - wrinkled yellow
2 rrYy
1 rryy 1 rryy - wrinkled green
V. Test Cross
VI. Incomplete dominance - neither gene is completely dominant over the other one
VI. Summary
Other sites of interest:
An introduction
to Mendelian genetics
An
historical overview of Mendel's work
"What
did Gregor Mendel think he discovered?"
"The
nine lives of Gregor Mendel"
A thorough review
of Mendelian genetics: Rather advanced
I. MONOHYBRID CROSSES and INCOMPLETE DOMINANCE
1. Most individuals of a certain wild flower have white petals, but a
few are blue. Crosses have shown that the blue color is due to a recessive
allele. Use "W" for white and "w" for blue.
2. Four o'clocks exhibit incomplete dominance. Thus, if a plant with red flowers is crossed with a plant with white flowers, the offspring will all produce pink flowers. What offspring would you expect if you crossed a red-flowered plant with a pink-flowered plant?
II. DIHYBRID CROSSES
3. How many different kinds of gametes can an organism of genotype RrSS
produce? List them.
4. In Jimsonweed, purple flower color (P) is dominant over white (p) and
spiny pods (S) over smooth pods (s). These pairs of alleles are not linked.
A Jimsonweed of genotype PPss is crossed with one of ppss. What is the genotype
of the F1 generation? The phenotype?
5. In peas, the gene for tall plants (T) is dominant over the gene for dwarf
plants (t) and round seed (R) is dominant over wrinkled seed (r). If one
crosses a homozygous tall, wrinkled-seeded plant with a homozygous dwarf,
round-seeded plant, what is the phenotype of the F1? The genotype of the
F1? The phenotypic ratio of the F2?
6. If the cross BbDD X bbDd is made, which of the following would not be
represented in the offspring?
a. BbDD; b. BbDd; c. bbDD; d. bbDd; e. BBDd
III. WORKING BACKWARDS
7. In corn, one gene controls the height of the plant. Normal height
(N) is dominant over dwarf (n). A normal corn plant is crossed with a dwarf
plant. Of the progeny, 247 are normal and 256 are dwarf. What are the genotypes
of the parent plants?
8. Brown eyes "B" are dominant over blue eyes "b". A brown-eyed man whose
mother was blue-eyed marries a brown-eyed woman whose father was blue-eyed
and they produce three blue-eyed children. What are the genotypes of the
brown-eyed man and woman? Can they have a brown-eyed child?
9. In peas, the gene for red flowers (R) is dominant over the gene for white
flowers (r). A pea plant with white flowers is crossed with one that has
red flowers. Of the offspring, 147 have white flowers and 161 have red flowers.
What are the genotypes of each parent?
10. In sesame, the one-pod condition "P" is dominant over the three-pod condition
"p", and normal leaf "L" is dominant over wrinkled leaf "l". The two characters
are inherited independently. Determine the genotypes and phenotypes of the
two parents that produce the following progeny: 318 one-pod normal; 98 one-pod
wrinkled; 323 three-pod normal; and 104 three-pod wrinkled.
1.
2. * red, * pink
3. Two- RS and rS
4. Ppss
5. all tall, round seeds; TtRr; 9:3:3:1
6. e (BBDd)
7. Nn X nn
8. Bb X Bb; yes
9. rr X Rr
10. PpLl X ppLl
