SAT II Biology E : Inheritance Patterns

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

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Example Questions

Example Question #1 : Inheritance Patterns

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:

None of these

II only

II and III only

I only

I, II, and III

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 : Inheritance Patterns

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.

Screen shot 2016 06 28 at 11.09.49 pm

Example Question #3 : Inheritance Patterns

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

Possible Answers:

75% blue, 25% white

Half blue, half white

All 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 #4 : Inheritance Patterns

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

Possible Answers:

Complete dominance

Mitochondrial

Incomplete dominance

Co-dominance

X-linked

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 #5 : Inheritance Patterns

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:

RR

rr

r

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.

Example Question #6 : Inheritance Patterns

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 could be the genotype of a red flower?

Possible Answers:

Rr or rr only

Rr only

RR, Rr, or rr

RR or Rr only

rr only

Correct answer:

RR or Rr only

Explanation:

A red flower must have the "R" allele, which conveys the red phenotype. Since this red allele is dominant, the flower will be red as long as one copy of this allele is present. Thus, the genotypes "Rr" and "RR" will both produce red flowers. If no red allele is present ("rr"), the flower will be white.

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