1. Assume you are performing a cross between a plant that is heterozygous for the dominant traits red flowers and wrinkled seeds, with a plant that is homozygous for the recessive traits white flowers and smooth seeds. Assume the alleles are R for red flowers, r for white flowers, W for wrinkled seeds, and w for smooth seeds. The offspring of this cross show the following phenotypes:
43 red flowers and wrinkled seeds
7 red flowers and smooth seeds
9 white flowers and wrinkled seeds
41 white flowers and smooth seeds
What are the traits of the recombinant offspring in terms of flower color and seed shape? Explain how you know that these offspring are recombinant. What alleles would you expect to see in the individual chromosomes of each parent before and after crossing-over takes place? What is the recombination frequency? What is the distance between the genes in map units?
To determine the traits of the recombinant offspring and understand why they are considered recombinant, we need to examine the phenotypes and genotypes of the parent plants and their offspring.
First, let's assign the alleles to the parent plants:
- Plant 1 (heterozygous for red flowers and wrinkled seeds): RrWw
- Plant 2 (homozygous for white flowers and smooth seeds): rrww
The offspring phenotypes are as follows:
- 43 offspring have red flowers and wrinkled seeds (R-_W-_): These individuals have inherited the red flower allele (R) from plant 1 and the wrinkled seed allele (W) from plant 1.
- 7 offspring have red flowers and smooth seeds (R-_ww): These individuals have inherited the red flower allele (R) from plant 1 but have the smooth seed allele (w) from plant 2. This combination is the result of a recombination event during meiosis.
- 9 offspring have white flowers and wrinkled seeds (rrW-_): These individuals have inherited the white flower allele (r) from plant 2 but have the wrinkled seed allele (W) from plant 1. This combination is the result of a recombination event during meiosis.
- 41 offspring have white flowers and smooth seeds (rrww): These individuals have inherited the white flower allele (r) and the smooth seed allele (w) from plant 2.
To determine if the offspring are recombinant, we compare their phenotypes to the parental phenotypes. The parental combination would be red flowers and wrinkled seeds (R-W-) from plant 1, and white flowers and smooth seeds (rww) from plant 2. Since the recombinant offspring show a mix of these parental traits, they are considered recombinant.
Before crossing-over takes place, each parent would have two sets of chromosomes, with alleles arranged as follows:
- Plant 1: RW and rw
- Plant 2: rw and rw
During meiosis, crossing-over can occur between homologous chromosomes, resulting in the exchange of genetic material, including alleles. The occurrence of crossing-over produces recombinant chromosomes with new combinations of alleles.
To determine the recombination frequency, we need to calculate the proportion of recombinant offspring among the total number of offspring. In this case, there are 7 offspring with the recombinant phenotype (red flowers and smooth seeds), out of a total of 100 offspring. The recombination frequency is therefore 7/100, which is 0.07 or 7%.
The distance between genes in map units can be determined from the recombination frequency. One map unit (centimorgan) represents a recombination frequency of 1%. In this case, the recombination frequency is 7%, so the distance between the genes is 7 map units.
In summary:
- The traits of the recombinant offspring are red flowers and smooth seeds (R-_ww), white flowers and wrinkled seeds (rrW-_), respectively.
- These offspring are considered recombinant because they possess a combination of traits that differ from the parental traits.
- Before crossing-over, the parental chromosomes contain the original combinations of alleles (RW and rw for plant 1, and rw and rw for plant 2).
- The recombination frequency is calculated by dividing the number of recombinant offspring by the total number of offspring (7/100 or 0.07).
- The distance between the genes in map units is the same as the recombination frequency (7 map units).