question archive In rabbits, chocolate-colored fur (w+) is dominant to white fur (w), straight fur (c+) is dominant to curly fur (c), and long ear (s+) is dominant to short ear (s)
In rabbits, chocolate-colored fur (w+) is dominant to white fur (w), straight fur (c+) is dominant to curly fur (c), and long ear (s+) is dominant to short ear (s)
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In rabbits, chocolate-colored fur (w+) is dominant to white fur (w), straight fur (c+) is dominant to curly fur (c), and long ear (s+) is dominant to short ear (s). The cross of a trihybrid rabbit with straight, chocolate-colored fur and long ears to a rabbit that is white, curly fur and short ears produces the following results:
| Phenotype | Number |
| White, short, straight | 13 |
| Chocolate, long, straight | 165 |
| Chocolate, long, curly | 13 |
| White, long, straight | 82 |
| Chocolate, short, straight | 436 |
| Chocolate, short, curly | 79 |
| White, short, curly | 162 |
| White, long, curly | 450 |
- Determine the order of the genes on the chromosome and identify the alleles that are present on each of the homologous chromosomes in the trihybrid rabbits.
- Calculate the recombination frequencies between each of the adjacent pairs of genes.
- Determine the interference value for this cross.
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1. The gene order can be c+ w+ s+ or s+ w+ c+
| Phenotype | Alleles |
| White, short, straight | c+ w s |
| Chocolate, long, straight | c+ w+ s+ |
| Chocolate, long, curly | c w+ s+ |
| White, long, straight | c+ w s+ |
| Chocolate, short, straight | c+ w+ s |
| Chocolate, short, curly | c w+ s |
| White, short, curly | c w s |
| White, long, curly | c w s+ |
2. Recombination frequency between gene c and w = 25.21 %
Recombination frequency between gene s and w = 13.36 %
3. Interference = 0.449 or 0.45
Step-by-step explanation
1)
First lets find the parental genotypes, double crossover genotypes, single crossover genotypes.
The phenotype expressed by majority of the progeny form the parental phenotypes. Eg:
| Chocolate, short, straight | 436 |
| White, long, curly | 450 |
The phenotype expressed by least number of offspring form the DCO phenotypes. Eg:
| White, short, straight | 13 |
| Chocolate, long, curly | 13 |
By comparing the parental and DCO phenotypes we can find the middle gene. During DCO events, the middle gene is usually exchanged. Here, the chocolate (w+) and white gene (w) is exchange. Hence the middle gene is fur color gene w+.
The gene order can be c+ w+ s+ or s+ w+ c+
The parental genotypes are Chocolate, short, straight and White, long, curly.
Parent 1: Chocolate, short, straight = c+ w+ s
Parent 2: White, long, curly = c w s+
| Phenotype | Alleles |
| White, short, straight | c+ w s |
| Chocolate, long, straight | c+ w+ s+ |
| Chocolate, long, curly | c w+ s+ |
| White, long, straight | c+ w s+ |
| Chocolate, short, straight | c+ w+ s |
| Chocolate, short, curly | c w+ s |
| White, short, curly | c w s |
| White, long, curly | c w s+ |
| Phenotype | Number | Type |
| White, short, straight | 13 | Double crossover (DCO) |
| Chocolate, long, straight | 165 | Single crossover (SCO) between s (ear length) and w (fur color) gene |
| Chocolate, long, curly | 13 | Double crossover (DCO) |
| White, long, straight | 82 | Single crossover (SCO) between c (fur texture) and w (fur color) gene |
| Chocolate, short, straight | 436 | Parental type |
| Chocolate, short, curly | 79 | Single crossover (SCO) between c (fur texture) and w (fur color) gene |
| White, short, curly | 162 | Single crossover (SCO) between s (ear length) and w (fur color) gene |
| White, long, curly | 450 | Parental type |
2)
Recombination frequency:
Recombination frequency between gene c and w = (number of SCO between c and w + DCO / total progeny) x 100
= (165+162+13+13 /1400) x 100
=(353/1400) x 100
= 25.21 %
Recombination frequency between gene s and w = (number of SCO between s and w + DCO / total progeny) x 100
= (82+79+13+13 /1400) x 100
=(187/1400) x 100
= 13.36 %
3)
Interference:
Step 1: Find the expected frequency of double crossover:
Expected frequency of double crossover = SCO frequency between genes c and w x SCO frequency between genes s and w
SCO frequency between genes c and w = Recombination frequency between gene c and w / 100 = 25.21/100 = 0.2521
SCO frequency between genes s and w = Recombination frequency between gene s and w /100 = 13.36/100 = 0.1336
Expected frequency of double crossover = 0.2521 x 0.1336 = 0.0337
Step 2: Find the expected number of double crossover
Expected number of double crossover = Expected frequency of double crossover x total progeny
= 0.0337 x 1400
= 47.18
Step 3: Find the actual double crossover
Actual number of double crossover = total DCO = 13+13 = 26
Step 4: Coefficient of coincidence
Coefficient of coincidence = observed DCO/ expected DCO = 26/ 47.18 = 0.551
Step 5: Find interference
Interference = 1 - coefficient of coincidence = 1- 0.551 = 0.449
