BSCI 124 Lecture Notes

Undergraduate Program in Plant Biology, University of Maryland


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


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?

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

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.



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

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Last revised: 7 Feb 1997 - Bottino