Linked Genes: Genes on the Same Chromosome and Crossing Over 

The genetic problems you have been involved with up to this point are concerned with genes on separate chromosomes. Therefore these genes assort  independently during meiosis forming the various genotypes of gametes. Examine the previous problem you encountered in the dihybrid web page shown  below. 

Previous example: 

cross between the parents to the right          RrLl           x         RRLl 
 
 
The first step is to set up the Punnett square with the correct  haploid genotypes for sperm and egg gametes in cells along the top and down the left side. 
 

/////////   RL   Rl   rL   rl
  RL  RRLL RRLl RrLL RrLl
  Rl  RRLl RRll  RrLl Rrll
 
Note that the "R's" and  "L's" segregate independently of one another resulting in  the formation of six different genotypes. 

What would happen if the "R" allele was on the same chromosome as the "l" allele? 

This is the concept which is being investigated on this web page, linkage between two or more genes. 

Read the pages indicated by D-10 and study behavioral objectives 23 and 24 concerning linked genes. 
 

1. What would happen if two or more genes are located on the same chromosome (linked genes)? 

Click here to check answer.   click 
 
 
 
 

2. Are linked genes very common when dealing with living systems? 

Click here to check answer.   click 
 
 
 
 
 
 
 
 
 
 
 

Basically, linked gene problems may be solved by with the Punnett square using the same number of cells when solving monohybrid problems. Remember linked genes will always involve two or more sets of genes (dihybrid or greater).  Study the example below: 

The following cross involves two sets of genes located on the same chromosome. The gene 'AA or Aa' produces red hair color and 'aa' produces brown hair color. The gene 'BB or Bb' produces facial freckles and 'bb' produces no facial freckles.  The genotype of the two parents are: 

                                             AaBb             x          AABb 

3. What would be the phenotypes of the parents? 

Click here to check answer.   click 
 
 
 


                         AaBb             x          AABb 

In parent 1 allele "A is linked to B"  and allele "a is linked to b". 
In parent 2 allele "A is linked to A" and allele  "B is linked to b" 

These are redrawn as follows: 
 
             A  |    | a                                            A  |   |A 
                  |    |                     x                            |   | 
             B  |    | b                                             b  |   | B 

              Parent 1                                           Parent 2 

The three vertical lines represent a single chromosome containing two alleles. The first chromosome contains the alleles "A and B" , the second chromosome contains the alleles "a and b", the third chromosome contains the alleles "A and b" and the fourth chromosome contains the alleles "A and B". 

In parent 1, where allele "A" goes allele "B" will go and where allele "a" goes "b" will go. 

4. Explain what happens to parent 2 has far as the movement of these alleles. 
 
Click here to check answer.   click 
 
 
 
 
 


Using the stick figures above, set these alleles up as they would appear on each  chromosomes in a Punnett square top row or column as previously described.  See below:   Draw Punnett square in notes. 
 
 

                                                                Parent 1: Genotype of  haploid gametes 
                                                                              (top  row) 
  
 

/////////////////////////////////////////////         A | 
            | 
        B  |		              a | 
                | 
             b |
               A | 
                   | 
                b |
               A | 
                   | 
                B |
Parent 2: Genotypes of haploid gametes 
              (first column) 
 

5. Fill in the cells of the Punnett square. Each cell contains one chromosome from each parent. 
 

Click here to check answer.   click 
 
 
 
 


 

        A | 
            | 
        B  |		              a | 
                | 
             b |
               A | 
                   | 
                b |		         A || A 
            || 
         b || B                    1		             A || a 
                || 
             b || b               3
               A | 
                   | 
                B |		         A || A 
            || 
        B || B                     2		             A || a 
                || 
            B || b               4
 

6. What are the genotypes and phenotypes of cells 1, 2, 3, and 4?  Write in your notes. 
 
Click here to check answer.   click 
 
 
 
 
 

 

Have you mastered working  problems involving linked genes? 

7. What would be the phenotype ratio of the offspring for the following cross concerning linked genes? 
    CC and Cc would produce a widow's peak hairline cc would produce a straight hairline, DD and  Dd  would form middigital hair and dd would not produce any middigital hair.   Show work in notes. 

Parent 1 having a genotype of  CcDd with the C allele linked to the D allele. 
Parent 2 having a genotype  of ccdd with the c allele linked to d allele. 
 

Click here to check answer.   click 

 
 
 
 
 
 
 
 
 
 
 
 

 

8. What would be the phenotype ratio of the offspring for the same cross above if the alleles were not linked?   Show work in notes. 

 Parent 1 having a genotype of CcDd. 
 Parent 2 having a genotype of ccdd. 
 
 
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9. Compare the ratios for questions 7 and 8. 
 
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Does this mean that in the example above, parent 1 can only produce  'AB' alleles linked in one chromosome and 'ab' alleles linked in a second chromosome? 

No!  During the process of crossing over which was discussed in miniunit Gamma linked genes may be segregate into different chromosomes. You may want to go back and review crossing over in miniunit gamma. Click here. 

Pieces of homologous chromosomes can cross over between two genes producing a new genetic combination. Remember that crossing over occurs during meiosis at the prophase I stage. 

Example: 

             A  |    | a 
                  |    | 
             B  |    | b 

 Original homologous pair of chromosomes with two linked genes, 'A linked to B' on one chromosome and 'a linked to b' on its homologue. See below. 
 
 

             A  |                                                                   | a 
                  |                                                                   | 
             B  |                                                                   | b 

       Gamete 1                                                             Gamete 2 

 Two types of gametes (haploid) may be formed if no crossing over occurred. 
 

 If crossing over occurred than the following types of gametes may be produced. 
 

             A  |                                                                   | a 
                  |                                                                   | 
             b   |                                                                   | B 

       Gamete 3                                                             Gamete 4 
 

Therefore, four types of gametes may occur. Two types  from the non crossovers and two types as a result of crossing over. 

Would these be in a 1:1:1:1 ratio?    No, their ratio would depend on how often crossing over occurs. If crossing over occurs   during 20 percent of the meiotic divisions, than the ratio would be 80 non crossovers to 20 crossovers. See below. 

 
             A  |                       | a                        A |                            a | 
                  |                       |                              |                               | 
             B  |                       | b                         b |                            B | 

   40 = Gamete 1    40 = Gamete 2              10 = Gamete 3         10 = Gamete 4 
   result of non crossovers                           result of crossing over 
 
Draw these in you notes and study!!!  
 

Have you mastered working  problems involving linked genes with crossing over? 

Work the following problem: 

10. 'C and d' are linked on one chromosome and  'c and D' are linked on its homologue. Assume that crossing over occurs 8 percent of the time between these two alleles.  List the genotype for this individual,  the genotypes of the gametes for the non crossovers produced by this individual, the genotypes of the gametes for the crossovers produced by this individual. Also give the ratio you would expect of the gametes produced . 
 

 Click here to check answer.   click 
 

  

This is the end of lesson six. In the next lesson you will learn how genes work at a molecular level. This process will involve DNA being translated into mRNA which in turn will direct the synthesis of proteins. In this lesson you will learn the molecular structure of DNA and how it replicates. If you would like to work additional genetic problems continue below on this page. The answers are provided below each problem set.  If you do want to work additional problems click here to go back to the home page and lesson seven, Biochemistry of Genes.   click 

  Additional Problems:  
1.   Red (R) and white (r) are incompletely dominant, and hybrid (Rr) cattle have 
      an intermediate roan color: 
                                       Rr X Rr 

  Answers:       1 red to 2 roan to 1 white 
 

2.  Roman nose (R) dominant over straight nose (r): 
                                       Rr X rr 

  Answers:      1 roman nose to 1 straight nose 
 

3.  Brown eyes is dominant to blue eyes and dark hair is dominant to blond hair. A man and woman marry who are both  heterozygous for brown eyes (Bb) and dark  hair (Dd).  BbDd X BbDd   What are  the chances that a given  child will be: 

              a) brown-eyed and dark-haired 
              b) brown-eyed and blond 
              c) blue-eyed and dark-haired 
              d) blue-eyed and blond 

  Answers:
     a) 9/16  brown-eyed  and dark-haired 
     b) 3/16  brown- eyed   and blond- haired 
     c) 3/16  blue-eyed  and dark-haired 
     d) 1/16  blue-eyed  and blond-haired 
 
 4.  If there were incomplete dominance between genes G and g for pea coloring. 
      for the cross below what fraction of the offspring would likely be : 
a) dark green 
b) light green 
c) yellow 
                                            Gg X Gg     
Answers:

a) 25% is dark green 
b) 50% is light green 
c) 25% is yellow 

5.  For the next couple of frames, determine from the Punnett square the probability that a child will be hemophilic male or hemophilic female. 
A)  Normal man  x  "carrier" woman                   XHY    x    XHXh 
 
 
\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ XH Xh
XH XHXH XHXh
Y XHY XhY

Answers:
50% of males are hemophilic. 
0% of females are hemophilic. 

B)   Hemophilic man X normal woman with hemophilic father 
                               XhY    x     XHXh 
 
 
\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ Xh Y
XH XHXh XHY
Xh XhXh XhY

answers:
50% of females are hemophilic 
50% of males are hemophilic 
 
 

6.  In guinea pigs, rough coat (R) is dominant to smooth (r) and short hair (S) is dominant to long (s).   What were the genotypes of the quinea pigs which produce the offspring described below?   Using one of the following answers: 

     a) RrSs X RrSs 
     b) RrSs X rrss 
     c) not "a or b" but has some other genotype (RRSS, RrSS or rrSS) 

  1) 9/16 were rough and short haired         answer: a 
  2) 1/4 were smooth and long haired          answer: b 
  3) 3/16  were rough and long haired         answer: a 
  4) 1/4 were rough and short haired           answer: b 
  5) 1/2 were smooth and long haired         answer: c 
 
7.  A type A woman and her husband with type B blood give birth to children one with type B and another with type O blood.  Given this information and the blood factor alleles of IA (type A), IB (type B) and ii (type O), show the genotypes of the parents and children. 

                                                    IAi  X  IBi 
 
///////////////////////////// IB i
IA IAIB IAi
i IBi ii

8.  In sweet peas, purple flower color (P) is dominant to white (p), but the purple color cannot develop in the absence of a dominant gene C.  That is, the production of purple flower color is at least a two step process, involving both genes P and C.

Indicate whether the genotypes below are associated with purple or with white flowers. 

a) CCPP              answer:  purple 
b) CCpp              answer:  white 
c) CcPp               answer:  purple 
d) ccPP               answer:  white 
e) ccPp                answer:  white 
f) C_P_               answer: purple 

9.  In a cross between two CcPp plants, what fraction of the offspring will be purple? and what fraction will be white? 
                                        CcPp X CcPp 
 
CP Cp cP cp
CP CCPP CCPp CcPP CcPp
Cp CCPp CCpp CcPp Ccpp
cP CcPP CcPp ccPP ccPp
cp CcPp Ccpp ccPp ccpp

Answers:
Purple is 9/16 
White is 7/16 

Click here to go back to the home page and lesson seven, Biochemistry of Genes.   click 

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Created by the Center for Learning Technologies, Academic Technology Services.  
Last modified October 22, 1997.