Inheritance

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GRE Subject Test: Biology › Inheritance

Questions 1 - 10
1

Hemophilia A is a blood clotting disorder that is usually inherited as an X-linked recessive trait. If a woman who does not have hemophilia A has a daughter with a man who does not have hemophilia A, what is the probability that the daughter will have hemophilia A if the maternal grandfather did have hemophilia A?

Explanation

X-linked recessive inheritance dictates that expression of themutant phenotype will only occur if the individual is homozygous for the mutation on the X-chromosomes. Therefore, a female must have inherited two mutant X-chromosomes to have hemophilia A, while a male only requires one mutant X-chromosome to have the disorder. By virtue of the father not having hemophilia A, we know the daughter is inheriting at least one wild-type X-chromosome, and therefore there is zero chance she will be homozygous and have hemophilia A.

2

Hemophilia A is a blood clotting disorder that is usually inherited as an X-linked recessive trait. If a woman who does not have hemophilia A has a daughter with a man who does not have hemophilia A, what is the probability that the daughter will have hemophilia A if the maternal grandfather did have hemophilia A?

Explanation

X-linked recessive inheritance dictates that expression of themutant phenotype will only occur if the individual is homozygous for the mutation on the X-chromosomes. Therefore, a female must have inherited two mutant X-chromosomes to have hemophilia A, while a male only requires one mutant X-chromosome to have the disorder. By virtue of the father not having hemophilia A, we know the daughter is inheriting at least one wild-type X-chromosome, and therefore there is zero chance she will be homozygous and have hemophilia A.

3

Two parents are heterozygous for an allele that determines hair color for this species. These parents have offspring with the following genotypic ratios:

50% Bb, 25% BB, 25% bb

The B allele produces black hair, and the b allele produces white hair. However, the phenotypic expression of this gene's traits do not follow Mendelian patterns. If allele B exhibits incomplete dominance, which of the following is true for the offspring?

All three genotypes (BB, bb, Bb) produce a different hair color phenotype

All three genotypes produce black hair

All three genotypes produce an intermediate phenotype

The homozygotes will have black hair, and the heterozygotes will have gray hair

There is not enough information provided to determine the offspring phenotypes

Explanation

Incomplete dominance indicates that there is no dominant allele. In these cases, the phenotype associated with inheriting one copy of each allele (the heterozygotes, Bb) is often a blending of the phenotypes associated with homozygosity of each allele. As such, a genotype of BB will result in black hair, bb will produce white hair, and Bb will result in grey hair.

The incorrect answers are too limited in scope to be cases of incomplete dominance. The correct answer identifies that there will be three unique phenotypes.

4

Two parents are heterozygous for an allele that determines hair color for this species. These parents have offspring with the following genotypic ratios:

50% Bb, 25% BB, 25% bb

The B allele produces black hair, and the b allele produces white hair. However, the phenotypic expression of this gene's traits do not follow Mendelian patterns. If allele B exhibits incomplete dominance, which of the following is true for the offspring?

All three genotypes (BB, bb, Bb) produce a different hair color phenotype

All three genotypes produce black hair

All three genotypes produce an intermediate phenotype

The homozygotes will have black hair, and the heterozygotes will have gray hair

There is not enough information provided to determine the offspring phenotypes

Explanation

Incomplete dominance indicates that there is no dominant allele. In these cases, the phenotype associated with inheriting one copy of each allele (the heterozygotes, Bb) is often a blending of the phenotypes associated with homozygosity of each allele. As such, a genotype of BB will result in black hair, bb will produce white hair, and Bb will result in grey hair.

The incorrect answers are too limited in scope to be cases of incomplete dominance. The correct answer identifies that there will be three unique phenotypes.

5

Peas in pea plants can be either yellow or green, with yellow being the dominant color. The peas can also be smooth or wrinkled, with smooth being the dominant shape. Suppose that a pea plant that is heterozygous for both traits is self crossed.

What proportion of the next generation will have smooth, green peas?

Explanation

The shortcut for this problem involves the standard phenotypic ratios for a dihybrid cross. Nine offspring will show both dominant traits. Three will show one dominant trait and the other recessive trait. Three will show the inverse phenotypes, with the opposite dominant trait and recessive trait combination. One offspring will show both recessive traits. Based on these ratios, we can see that three of the sixteen offspring will show the dominant smooth trait and the recessive green phenotype.

We can also solve by using a dihybrid punnett square. The cross described will be AaBb x AaBb, in which the A alleles signify color and the B alleles signify shape.

Consider the color of the peas. In order to have green peas, two recessive alleles must combine in the next generation. According to a punnett square where both sides are heterozygous for the trait, there is only a one in four chance of this taking place. Since smooth is the dominant shape for the peas, a punnett square where each side is heterozygous shows a three in four chance that pea plants will have this shape. By multiplying these two probabilities, we determine that three out of sixteen pea plants will have smooth, green peas.

6

Peas in pea plants can be either yellow or green, with yellow being the dominant color. The peas can also be smooth or wrinkled, with smooth being the dominant shape. Suppose that a pea plant that is heterozygous for both traits is self crossed.

What proportion of the next generation will have smooth, green peas?

Explanation

The shortcut for this problem involves the standard phenotypic ratios for a dihybrid cross. Nine offspring will show both dominant traits. Three will show one dominant trait and the other recessive trait. Three will show the inverse phenotypes, with the opposite dominant trait and recessive trait combination. One offspring will show both recessive traits. Based on these ratios, we can see that three of the sixteen offspring will show the dominant smooth trait and the recessive green phenotype.

We can also solve by using a dihybrid punnett square. The cross described will be AaBb x AaBb, in which the A alleles signify color and the B alleles signify shape.

Consider the color of the peas. In order to have green peas, two recessive alleles must combine in the next generation. According to a punnett square where both sides are heterozygous for the trait, there is only a one in four chance of this taking place. Since smooth is the dominant shape for the peas, a punnett square where each side is heterozygous shows a three in four chance that pea plants will have this shape. By multiplying these two probabilities, we determine that three out of sixteen pea plants will have smooth, green peas.

7

What is the structure in prokaryotic cells that often holds antibiotic resistance genes?

Plasmid

Nucleus

Prophage

Nucleoid

Nuclear area

Explanation

Antibiotic resistance genes are often found on plasmids, which are small DNA molecules which are easily transfered to other prokaryotic cells.

8

What is the structure in prokaryotic cells that often holds antibiotic resistance genes?

Plasmid

Nucleus

Prophage

Nucleoid

Nuclear area

Explanation

Antibiotic resistance genes are often found on plasmids, which are small DNA molecules which are easily transfered to other prokaryotic cells.

9

Frederick Griffith's experiments showed that formerly harmless IIR bacteria had become deadly in mice. This change took place when the harmless IIR bacteria were exposed to the remains of heat killed IIIS bacteria. Based on this experiment, what was the process that converted the harmless bacteria to a deadly strain?

Transformation

Conjugation

Transduction

Binary fission

Mutation

Explanation

The IIR bacteria became deadly when exposed to the remains of the IIIS bacteria. This means that the IIR bacteria managed to receive genetic material from the environment and incorporate it into their genome. This is an example of transformation, a process that results in genetic recombination. In this case, the recombination made the formerly harmless bacteria deadly in mice.

Transduction is the process by which new genetic information is introduced to a bacterium via a vector, such as a bacteriophage. Conjugation is the transfer of genetic material between bacteria via a sex pilus. Binary fission is not a means of recombination; rather, the parent cell divides to produce two identical copies of itself.

10

What type of prokaryotic genetic recombination requires the presence of plasmids?

Conjugation

Transformation

Transduction

Binary fission

Explanation

Conjugation is a type of genetic recombination that requires one bacterium to have the F-plasmid in order to create a sex pilus. This sex pilus will connect with another bacterium and allow DNA to pass between the bacteria.

Transduction is the transfer of genetic information to a bacterium via a vector, such as a bacteriophage. Transformation occurs when a bacterial cell receives genetic material from its surrounding environment. Binary fission does not involve recombination, and is the term for bacterial cell division that results in two identical offspring from a single parental cell.

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