High School Biology : Genetics Principles
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Example Questions
Example Question #1 : Understanding Dominant/Recessive
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 these mice are crossed, what is the phenotypic ratio of the resulting offspring?
When crossing organisms that are heterozygous for two traits, the result is a dihybrid cross.
AaBb x AaBb
Possible offspring: 1 AABB, 1 aaBB, 1 AAbb, 1 aabb, 2 Aabb, 2 aaBb, 2 AaBB, 2 AABb, 4 AaBb
The phenotypic ratio of a heterozygous dihybrid cross for autosomal (none-sex-linked) traits is always 9:3:3:1. Nine offspring will show both dominant traits (AABB, AaBB, AABb, AaBb). Three offspring will show dominance for one trait and recessiveness for the other (Aabb, AAbb) and three offspring will show the reciprocal (aaBb, aaBB). Only one offspring will be recessive for both traits (aabb). For this cross, nine mice will have black fur and brown eyes, three will have black fur and blue eyes, three will have brown fur and brown eyes, and one will have brown fur and blue eyes.
Example Question #1 : Understanding Dominant/Recessive
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 these mice are crossed, what fraction of the offspring will have the genotype Aabb?
When crossing organisms that are heterozygous for two traits, the result is a dihybrid cross.
AaBb x AaBb
Possible offspring: 1 AABB, 1 aaBB, 1 AAbb, 1 aabb, 2 Aabb, 2 aaBb, 2 AaBB, 2 AABb, 4 AaBb
The phenotypic ratio of a heterozygous dihybrid cross for autosomal (none-sex-linked) traits is always 9:3:3:1. The genotype in this question, Aabb, corresponds to black fur and blue eyes. Two mice out of every sixteen will have this genotype.
Example Question #2 : Understanding Dominant/Recessive
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 these mice are crossed, what fraction of offspring will have the genotype AaBb?
When crossing organisms that are heterozygous for two traits, the result is a dihybrid cross.
AaBb x AaBb
Possible offspring: 1 AABB, 1 aaBB, 1 AAbb, 1 aabb, 2 Aabb, 2 aaBb, 2 AaBB, 2 AABb, 4 AaBb
The phenotypic ratio of a heterozygous dihybrid cross for autosomal (none-sex-linked) traits is always 9:3:3:1. The genotype in this question, AaBb, corresponds to black fur and brown eyes. Four mice out of every sixteen will have this genotype.
Example Question #3 : Understanding Dominant/Recessive
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 these mice are crossed, what fraction of offspring can be expected to have blue eyes?
When crossing organisms that are heterozygous for two traits, the result is a dihybrid cross.
AaBb x AaBb
Possible offspring: 1 AABB, 1 aaBB, 1 AAbb, 1 aabb, 2 Aabb, 2 aaBb, 2 AaBB, 2 AABb, 4 AaBb
The phenotypic ratio of a heterozygous dihybrid cross for autosomal (none-sex-linked) traits is always 9:3:3:1. Nine offspring will show both dominant traits (AABB, AaBB, AABb, AaBb). Three offspring will show dominance for one trait and recessiveness for the other (Aabb, AAbb) and three offspring will show the reciprocal (aaBb, aaBB). Only one offspring will be recessive for both traits (aabb). For this cross, nine mice will have black fur and brown eyes, three will have black fur and blue eyes, three will have brown fur and brown eyes, and one will have brown fur and blue eyes. A total of four mice will carry bb and be recessive for eye color.
Example Question #2 : Understanding Dominant/Recessive
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 these mice are crossed, what fraction of offspring can be expected to have black fur?
When crossing organisms that are heterozygous for two traits, the result is a dihybrid cross.
AaBb x AaBb
Possible offspring: 1 AABB, 1 aaBB, 1 AAbb, 1 aabb, 2 Aabb, 2 aaBb, 2 AaBB, 2 AABb, 4 AaBb
The phenotypic ratio of a heterozygous dihybrid cross for autosomal (none-sex-linked) traits is always 9:3:3:1. Nine offspring will show both dominant traits (AABB, AaBB, AABb, AaBb). Three offspring will show dominance for one trait and recessiveness for the other (Aabb, AAbb) and three offspring will show the reciprocal (aaBb, aaBB). Only one offspring will be recessive for both traits (aabb). For this cross, nine mice will have black fur and brown eyes, three will have black fur and blue eyes, three will have brown fur and brown eyes, and one will have brown fur and blue eyes. A total of twelve mice will carry an A allele and be dominant for fur color.
Example Question #11 : 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?
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
A new mutation must have occurred
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 #12 : Inheritance Patterns
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?
The red parent plant is homozygous recessive
The white parent plant is heterozygous
The red parent plant is homozygous dominant
The red parent plant is heterozygous
The red parent plant is homozygous dominant
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 #12 : 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?
Sixteen
Four
Eight
Twelve
Twelve
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:
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 #13 : 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?
Incomplete dominance
Dominant
Recessive
Codominance
None of the above
Dominant
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 #13 : Inheritance Patterns
Which of the following describes a phenotype?
The atomic composition of a particular molecule
The ratio of adenine, guanine, thymine, and cytosine in a DNA molecule
The composite of an organism’s observable characteristics or traits
The genetic makeup of an organism
The composite of an organism’s observable characteristics or traits
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).
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