High School Biology : Understanding Dominant/Recessive

Study concepts, example questions & explanations for High School Biology

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Example Questions

Example Question #16 : Genetics Principles

Black fur (A) is dominant to brown fur (a) and brown eyes (B) are dominant to blue eyes (b) in mice. Two mice are heterozygous for both traits. If one of the offspring shows a phenotype for green eyes, what is most likely the best explanation for this?

Possible Answers:

Independent assortment must have occurred

The parental genotypes reported initially must have been incorrect

Codominance must have occurred in this particular offspring

A new mutation must have occurred

Correct answer:

A new mutation must have occurred

Explanation:

The best choice is that the individual has a mutation that accounts for his/her green eyes. This mutation would yield a new phenotype that was not a result of direct inheritance from the parent generation.

Codominance cannot only occur in one individual offspring; rather it would occur in some form in all offspring with respect to the given trait. Independent assortment always occurs in cases of typical Mendelian genetics. 

Example Question #11 : Understanding Dominant/Recessive

Red and white are two alleles of a single gene for flower color. A red plant and a white plant are crossed. Some offspring have red flowers and some offspring have white flowers. Which of the following could not be true?

Possible Answers:

The red parent plant is homozygous recessive

The red parent plant is heterozygous

The red parent plant is homozygous dominant

The white parent plant is heterozygous

Correct answer:

The red parent plant is homozygous dominant

Explanation:

If either parent were homozygous dominant, all offspring would exhibit the dominant phenotype and the second phenotype would not be expressed.

Homozygous parent cross: RR x r_

Offspring: all offspring inherit a dominant R allele and will express the dominant phenotype.

If two heterozygotes are crossed, all three genotypes will be expressed, so all possible phenotypes will be expressed.

Both heterozygous: Rr x Rr

Offspring: 1 RR, 2 Rr, 1 rr with both dominant and recessive phenotypes expressed in a 3:1 ratio.

If one heterozygote and one homozygote are crossed the offspring will show only two genotypes, but both possible phenotypes.

One heterozygote: Rr x rr

Offspring: half Rr and half rr, with half showing dominant phenotype and half showing recessive phenotype.

The second and third examples give rise to both colors of offspring, regardless of which allele is dominant. The first example only shows the dominant phenotype, can cannot be possible for this question.

Example Question #18 : Genetics Principles

In a population of fruit flies the allele for large wings is dominant over the allele for small wings. Two heterozygous parents are crossed and produce sixteen offspring. How many of these offspring will have large wings?

Possible Answers:

Twelve

Eight

Sixteen

Four

Correct answer:

Twelve

Explanation:

Biologists use a diagram called a Punnett square to aid them in predicting traits that will be inherited by offspring. In this case both parents are heterozygous, meaning that they both carry one dominant allele (W) and one recessive allele (w). The Punnett square for this cross would look like this:

Punnet_square  

Because the W allele is dominant, squares with a W will produce the large wing trait. Three of the four possible genotypes from this cross carry the dominant allele, meaning that 75% of the offspring will have large wings. If the parents produce sixteen offspring, then twelve of them will show the large wing phenotype.

Example Question #19 : Genetics Principles

A particular flowering plant species exhibits dwarfism upon mutagenesis. Upon further observation, the mutation is exhibited by heterozygotes. The control lacks dwarfism and has a homozygous genotype. What kind of inheritance pattern does this mutation exhibit?

Possible Answers:

Incomplete dominance 

Codominance 

None of the above 

Recessive

Dominant

Correct answer:

Dominant

Explanation:

The trait that is expressed in heterozygotes is the dominant allele. Thus, the normal plant must be homozygous recessive. If dwarfism was a recessive trait, it would not be expressed in the heterozygotes.

Example Question #20 : Genetics Principles

Which of the following describes a phenotype?

Possible Answers:

The atomic composition of a particular molecule

The genetic makeup of an organism

The ratio of adenine, guanine, thymine, and cytosine in a DNA molecule

The composite of an organism’s observable characteristics or traits

Correct answer:

The composite of an organism’s observable characteristics or traits

Explanation:

A genotype is the genetic makeup on an organism, while a phenotype is the composite of an organism’s observable traits. In short, the genotype determines the phenotype. The phenotype can also include any other observable traits, such as morphology, development, biochemical or physiological properties, phenology, behavior, and products of behavior. Environmental factors can also affect the phenotype of an organism (in addition to the inherited genotype).

Example Question #21 : Genetics Principles

In a fictional rabbit there is a gene allele that controls whether its tongue is keratinized—K—or not—k. A rabbit with the genotype of either KK or Kk has a keratinized tongue. A rabbit with the genotype of kk has a non-keratinized tongue. If a heterozygous male and female rabbit mate, then what is the probability that their offspring will be heterozygous for the gene that controls for tongue keratinization?

Possible Answers:

Correct answer:

Explanation:

Both of the parent's genotypes are Kk. This means that the offspring can be either KK, Kk, or kk.

Parental genotype: 

Offspring probability:

Example Question #22 : Genetics Principles

Consider a rare plant that exhibits the phenotype of dark blue leaves (BB) as its dominant trait and and light blue leaves (bb) as its recessive trait. It flowers a bright yellow flower (YY) when dominant, and an orange flower (yy) when recessive. 

When two dihybrid plants of the same species are crossed, what will be the expected phenotypic ratio of offspring that exhibit light blue leaves and yellow flowers? 

Possible Answers:

3:16 

1:4

1:2

1:8

5:16

Correct answer:

3:16 

Explanation:

Dihybridcross

Once you properly set up your punnet square of a dihybrid cross, you should obtain a phenotypic ratio of 9:3:3:1. There will be 9 plants with dark blue leaves/yellow flowers, 3 plants with light blue leaves/yellow flowers, 3 plants with dark blue leaves/orange flowers, and 1 plant with light blue leaves/orange flowers.

Example Question #23 : Genetics Principles

For a monohybrid cross of a gene that exhibits complete dominance, what will be the expected ratios for the phenotype and genotype?

Possible Answers:

Phenotype - 3:1

Genotype - 1:2:1

Phenotype - 3:1

Genotype - 1:1:1:1

Phenotype - 3:1

Genotype - 3:1

There is not enough information given to determine an answer. 

Phenotype - 1

Genotype - 1:1:1:1

Correct answer:

Phenotype - 3:1

Genotype - 1:2:1

Explanation:

For better visualization of this explanation we can use eye color as an example. BB and BB will be brown eyes and bb will be blue eyes. 

In a monohybrid cross you are dealing with only one gene. We can use "Bb" as an example for the gene as it is a hybrid of two alleles. BB & Bb are dominant and bb is recessive. 

Setting up a punnet square of Bb x Bb we will get 4 results: BB, Bb, Bb, and bb

The phenotype is what we see: eye color. Phenotype will be expressed the same for BB and Bb, therefore we have 3 that will express the dominant phenotype and 1 that will express the recessive phenotype; 3:1.

The genotype is the actual gene that creates the phenotype. So for this we have 3 different genes that arise from the monohybrid cross: BB, Bb, and bb. We get 1 homozygous dominant, 2 heterozygous, and 1 homozygous recessive; 1:2:1.

Example Question #61 : Genetics And Evolution

Sharon has blonde hair. Her husband is heterozygous for brown hair, with brown being the dominant autosomal trait. What percent chance will their daughter have blonde hair?

Possible Answers:

50%

25%

0%

75%

100%

Correct answer:

50%

Explanation:

The genotype for Sharon is rr, because blonde is a recessive trait therefore in order to have blonde hair she must be homozygous recessive. Her husband is Rr, because it states that he has brown hair, which is dominant, in addition to being heterozygous. When drawing out a punnet square, you will find the offspring will be Rr, Rr, rr and rr. Therefore, their daughter has 50% chance of having brown hair and 50% chance of having blonde hair.

Example Question #61 : Genetics And Evolution

Sickle cell disease can be terrible and painful; however, the sickle cell trait (heterozygous for the sickle cell gene) is protective against malaria. 

Molly is married to Fred. Molly has the recessive disorder of sickle cell anemia. Fred does not have the disease and is also not a carrier. What is the chance that Molly and Fred's children are all carriers of the disease?

Possible Answers:

75%

100%

0%

50%

25%

Correct answer:

100%

Explanation:

Since Molly is autosomal recessive (ss) and Fred is autosomal dominant (SS), all their children will be heterozygotes, or carriers and they will thus all be protected against malaria.

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