SAT II Biology E : Genetics

Study concepts, example questions & explanations for SAT II Biology E

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Example Questions

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Example Question #1 : Genetics

According to the law of independent assortment, what is the possible number of combinations that chromosomes can assort to independently in the gamete?

Possible Answers:

8,388,608

16,777,216

70,368,744,177,664

4,194,304

2,048

Correct answer:

8,388,608

Explanation:

According to the law of independent assortment, there are 2n combinations where chromosomes can assort into different gametes. So where n is the haploid number, you get 223=8,388,608. There are 8,388,608 possible combinations of chromosomes when assorting into gametes.

Example Question #1 : Genetics

A man expresses a certain X-linked recessive genetic disorder. Which of the following must be true?

I. His father passed on the allele for the disorder

II. His mother passed on the allele for the disorder

III. His mother expresses the disorder

Possible Answers:

I, II, and III

II and III only

I only

II only

None of these

Correct answer:

II only

Explanation:

As the disorder is X-linked and the subject is male, he only received an X-chromosome from his mother. Therefore, the allele for the disorder must have been inherited from his mother. However, this does not mean that the mother expressed the disorder herself, as she could have the dominant allele in addition to one recessive allele.

Example Question #1 : Genetics

What contributes to genetic variation during human reproduction?

Possible Answers:

Random fertilization

All of these

Crossing over

Nonrandom mating

Independent assortment

Correct answer:

All of these

Explanation:

All three contribute to giving rise to genetic variation. Independent assortment allows for the chromosomes to assort in millions of random of combinations during fertilization. Crossing over between chromosomes produces recombinant chromosomes, or the combination of chromosomal DNA from two parents into one chromosome. Random fertilization allows aids with variation because it means any sperm can fertilize any egg. It is sometimes easy to overlook, but humans do not mate randomly. Lots of energy is put into choosing an optimal mate with whom to reproduce. 

Example Question #1 : Genetics

For a certain species of flowers, blue petals (P) are dominant to white petals (p) and long stems (Q) are dominant to short stems (q). Flower 1 is the offspring of a purebred long-stemmed, blue flower (PPQQ) and a purebred short-stemmed, white flower (ppqq).

If Flower 1 is crossed with a short-stemmed, white flower, what fraction of its offspring will be white and short-stemmed?

Possible Answers:

Correct answer:

Explanation:

Since the offspring receives one allele from each parent, crossing a purebred dominant organism with a purebred recessive organism (PPQQ x ppqq) will always result in a hybridized offspring (PpQq). Thus, Flower 1's genotype is PpQq. Also remember that a recessive phenotype always indicates double recessive alleles for that trait. And so, crossing Flower 1 with a white, short-stemmed flower will result in the cross PpQq x ppqq. The next step is to draw a 4x4 Punnett square, as seen in the diagram. The desired genotype for this question is ppqq (recessive phenotype), and from the Punnett square you will be able to see that 4/16 of the squares will carry this specific genotype.

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Example Question #1 : Genetics

What is the phenotypic ratio for a cross between a plant with blue flowers BB and a plant with white flowers bb?

Possible Answers:

Half blue, half white

All white

75% blue, 25% white

25% blue, 75% white

All blue

Correct answer:

All blue

Explanation:

The phenotypic ratio is the ratio of one phenotype to another (phenotype is the trait expressed, in this case color, while genotype is the allele combination (BB, bb, Bb, or bB) that produces that phenotype. The capital letters BB signify that the blue allele (B) is dominant to the white allele (b). As such, the only genotype that will produce white plants is bb. All other combinations (BB, Bb, bB) will produce a blue plant. If you cross a homozygous (both dominant or both recessive) dominant plant with a homozygous recessive plant, the dominant allele will be present in all of the offspring, as every possible allele the blue plant could contribute will be dominant to every possible allele the white plant could contribute, making all of the offspring blue. 

Example Question #1 : Genetics

In mammals, which type of phenotypic expression will show recessive traits more frequently in males than females?

Possible Answers:

X-linked

Complete dominance

Co-dominance

Mitochondrial

Incomplete dominance

Correct answer:

X-linked

Explanation:

The correct answer is "X-linked." Mammalian females have two X chromosomes, with recessive alleles often not apparent unless there are two copies. Mammalian males have only one X chromosome, so any recessive alleles on it will be expressed.

Example Question #1 : Inheritance Patterns

Each person can have one of four possible blood types: A, B, AB, or O. Blood type A means that the "A" antigen is present on the red blood cell surface. Blood type B means that the "B" antigen is present on red blood cells' surfaces. Blood type "AB" means that both the "A" antigen and the "B" antigen are present on the red blood cells' surfaces. Blood type "O" means that no antigens are present on the red blood cells' surfaces.

A mother with blood type A and the genotype "A/O" and a father with blood type B and genotype "B/B" have a child with blood type AB. This is an example of what type of inheritance pattern?

Possible Answers:

Incomplete dominance

Autosomal recessive

X-linked autosomal dominance

Complete autosomal dominance

Codominance

Correct answer:

Codominance

Explanation:

The child is blood type AB, meaning that the child has both the "A" antigen and the "B" antigen on his or her red blood cells. The child is able to express the products of both genes simultaneously. The A antigen was inherited from mom, and the B antigen was inherited from dad. The "A" and "B" alleles are codominant because they can both be expressed in the same person at the same time if the person inherits both alleles, as is the case in this example.

Example Question #1 : Genetics

Each person can have one of four possible blood types: A, B, AB, or O. Blood type A means that the "A" antigen is present on the red blood cell surface. Blood type B means that the "B" antigen is present on red blood cells' surfaces. Blood type "AB" means that both the "A" antigen and the "B" antigen are present on the red blood cells' surfaces. Blood type "O" means that no antigens are present on the red blood cells' surfaces.

Someone with genotype "A/A" or "A/O" will have type A blood. Someone with genotype "B/B" or "B/O" will have type B blood. Someone with genotype "A/B" will have AB blood, and someone with genotype "O/O" will have type O blood.

Assuming that blood type is not a sex-linked trait, what is the probability that a mother with genotype "A/O" and a father with genotype "A/B" will have a child with type B blood?

Possible Answers:

Correct answer:

Explanation:

The easiest way to solve this problem is to draw a punnet square. The genotypes of the parents are "AO" and "AB". The potential genotypes of their children are "AA", "AO", "BA", and "BO". Only genotype "BO" will produce type B blood. "BO" is one out of four results of this punnet square, so the probability of this outcome is .

Example Question #101 : Sat Subject Test In Biology

In a certain flower, a blue petal phenotype is dominant to a white petal phenotype. If you cross a heterozygous flower with a homozygous recessive flower, what is the probability of inheritance for the white petal phenotype?

Possible Answers:

Correct answer:

Explanation:

Dominant alleles are referred to with capital letters, so let's call the dominant blue-petal allele B. Recessive alleles are referred to using lower case letters, so we will call the recessive white-petal allele b.

A heterozygous organism has one dominant and one recessive allele, so the heterozygous flower has one B allele and one b allele. Its genotype is Bb. Because B is dominant to b, its phenotype (the trait produced by its genotype) is blue petals.

A homozygous organism has two of the same allele. The homozygous flower will either have two BB alleles or two bb alleles. The question states that the flower with white petals is homozygous recessive, so its genotype is bb and its phenotype is white petals. The only genotype that produces a white phenotype is bb, because you need two recessive alleles in order to express the recessive trait.

When you cross the two flowers, each parent donates one of its two alleles for petal color to the offspring. Accounting for every possible combination of alleles from each parent, there are four possible outcomes from a cross between Bb and bb: Bb, Bb, bb, and bb. (It may also help to draw a punnet square to visualize the four possible combinations). As you can see, these outcomes lead to two possible genotypes: Bb and bb.  The Bb genotype produces flowers with blue petals, and the bb genotype leads to flowers with white petals. Because two of the four possible outcomes are genotype bb, two of the four possible outcomes are for flowers with white petals. Two out of four is equal to , so  is the correct answer.

Example Question #1 : Genetics

Your neighbor has a flower garden in which there are red flowers and white flowers. These flowers are diploid organisms, and flower color is an autosomal trait. The gene for red flowers (R) is dominant, while the gene for white flowers (r) is recessive.

Which of the following is the genotype of a white flower?

Possible Answers:

r

rr

RR

Rr

R

Correct answer:

rr

Explanation:

The correct answer is "rr". In order to express the recessive phenotype (white flowers), the organism must have only the recessive allele. When the question stem says that the organism is "diploid," it means that each flower has two copies of each chromosome. This means that the flower must have two alleles, so there must be two letters, not just one, in the correct answer.

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