Biology Non-mendelian Genetics Practice Problems

Question

1. Complete, Mendelian Genetics- In roses, Red (RR) and white flowers (rr) were crossbred. What phenotype would be seen in what % of the offspring?

2. Incomplete dominance- In pansies, Red (RR) and white flowers (rr) were crossbred.

3. Codominance- In rhododendron, a Red (CRCR) flower and a white flower (CWCW) were crossbred. What phenotype would be seen in what % of the offspring?

Answer 1

1. Complete, Mendelian Genetics- In roses, Red (RR) and white flowers (rr) were crossbred. In this case, the offspring would be expected to show a 50%/50% split between red and white flowers. This is because the red flower (RR) is dominant over the white flower (rr), so the offspring will all have the genotype Rr (one copy of the red allele and one copy of the white allele). Since the red allele is dominant, the flowers will appear red, but they will carry the white allele as well. When these flowers are crossed with each other, half of the offspring will receive the dominant red allele and half will receive the recessive white allele, resulting in a 50%/50% split between red and white flowers.
2. Incomplete dominance- In pansies, Red (RR) and white flowers (rr) were crossbred. In this case, the offspring would be expected to show a mix of red, white, and pink flowers. This is because the red and white alleles are both expressed in the offspring, but they are not fully dominant over each other. Instead, the offspring will show a blend of the two colors, resulting in pink flowers.
3. Codominance- In rhododendron, a Red (CRCR) flower and a white flower (CWCW) were crossbred. In this case, the offspring would be expected to show a 100% split between red and white flowers. This is because the red and white alleles are both fully expressed in the offspring and are not dominant or recessive to each other. As a result, all of the offspring will have both the red and white alleles and will show both colors in their flowers.

Answer 2

Hello, Asma Y:

When we say "complete dominance," we mean that having the homozygous recessive genotype (rr) must display the recessive phenotype (white flower) but having either the homozygous dominant genotype (RR) or the heterozygous genotype (Rr) must display the dominant phenotype (red flower). So, in the case of complete dominance, if you see a white flower, you know that individual has the rr (recessive) genotype. But if you see a red flower, you don't automatically know the genotype--that individual could be RR or Rr. The only way to tell is to do a test cross with a recessive individual. So, as the question asks, if you crossbreed an rr individual (white flowers) with an individual known to be RR, all the offspring will be Rr (heterozygous genotype). Therefore ALL will show the dominant phenotype, red flowers, and 0% will have white flowers. In other words, since the R allele has complete dominance over the r allele, the white flower color is never expressed in the presence of R, and so, you will never see a white flower among the offspring of rr x RR, only red flowers. However, say that you crossbred that same rr flower with a red flower with unknown genotype, and you saw that 50% of the offspring are white and 50% are red. Then, you would know that the red parent flower was Rr and not RR, because half of the offspring would have to be rr (white) and half would have to be Rr (red).

In the case of incomplete dominance, you will see an intermediate phenotype in individuals with the heterozygous genotype. As in the case of complete dominance, the RR genotype pansy (homozygous genotype and phenotype) will always have red flowers and the rr genotype (homozygous genotype and phenotype) pansy will always have white flowers. But, the difference is, in the case of incomplete dominance, the Rr (heterozygous genotype) pansy will always have pink flowers, which is the color in between white and red and therefore an intermediate phenotype. So, the heterozygous genotype Rr is pink because phenotype is influenced by the presence of both alleles (forms, R and R) of the flower color gene. So, as the question asks, in the case of incomplete dominance, 100% of the offspring of a red (RR) pansy and a white (rr) pansy will be pink Rr.

In the case of codominance, there is no recessive allele. This is indicated in the question by the fact that the alleles are listed in different capital letters (W for white and R for red) instead of R and r. Since there is no recessive allele, one allele does not mask the other allele in the expressed phenotype (flower color) of the offspring. In this case, crossing one homozygous individual (red, RR) with another homozygous individual (white, WW) will produce 100% of the offspring that are the heterozygous genotype (RW) and the intermediate phenotype unlike either of the parent plants (spotted flowers instead of white or red flowers). An example of codominance is human blood types in which the A allele and the B allele are codominant, so that people who inherit one A allele and one B allele have the AB blood type, which is unlike either the A or the B blood types.

I hope this helps--Jill