Heredity and Variation

159 questions

All types159 Multiple choice85 True / False26 Numeric7 Matching41

An organism's phenotype consists solely of its outwardly visible structural features and is entirely independent of environmental influences during development.

True

False

Show answer & explanation

Answer: False

Answer

The statement is False. Phenotype encompasses all observable traits (morphological, physiological, and biochemical) and is determined by the interaction between an organism's genotype and its environment.

The correct evaluation is False. Phenotype refers to all observable features of an organism, including internal physiological and biochemical properties, and results from the interaction of the genotype with environmental conditions.

Step-by-Step Solution

Define phenotype within basic genetics terminology.

Phenotype refers to the total observable operational, structural, biochemical, and physiological characteristics of an organism.

A complete definition clarifies that phenotype is not restricted strictly to external physical appearance.

Examine the role of environmental factors in phenotypic expression.

Environmental factors (such as nutrient availability, temperature, and exposure to light) interact with the genotype to influence how genes are expressed.

Phenotypic expression is a product of both genetic makeup and environmental influences.

Evaluate the statement's validity.

Because phenotype includes non-visible internal traits and depends on environmental interaction, the statement is false.

The statement incorrectly narrows the scope of phenotype and denies environmental impact.

Key Concept

Phenotype Definition and Environmental Interaction

Estimated Time:1m 0s

Question 102Question →

In clinical medicine, understanding ABO blood group inheritance is essential for safe blood transfusions. A patient with blood group O requires a blood transfusion. Which of the following genotypes must a donor possess to ensure that their red blood cells express neither A nor B surface antigens?

ii

I^A I^B

I^A i

I^B i

Show answer & explanation

Answer: $ii$

Answer

The donor must possess the homozygous recessive genotype

ii

The allele

i

is recessive to both codominant alleles

I^A

and

I^B

. Therefore, an individual must be homozygous recessive with genotype

ii

to have blood group O, which lacks both A and B surface antigens on red blood cells.

Step-by-Step Solution

Identify the genetic basis of blood group O

Blood group O red blood cells lack both A and B agglutinogens (antigens).

The production of surface antigens is controlled by the

I^A

I^B

, and

i

alleles.

Determine the allele dominance relationship

Alleles

I^A

and

I^B

are codominant with respect to each other, and both are completely dominant over allele

i

Allele

i

is a recessive allele that does not code for any functional surface antigen enzyme.

Select the genotype producing no antigens

Only individuals who are homozygous recessive (

ii

) express neither antigen.

Individuals with genotype

ii

belong to blood group O, making their un-agglutinated cells safe for recipients requiring antigen-free donor blood.

Key Concept

ABO blood group genetics and medical application in transfusion safety

Question 103Question →

Match each basic genetics term on the left with its corresponding definition on the right.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Gene locus matches the specific physical position of a gene on a chromosome; Allele matches alternative molecular form of a gene located at a specific position on homologous chromosomes; Phenotype matches the observable physical and physiological expression of an organism's genetic makeup; Homozygous matches the condition of possessing identical alleles for a given gene on homologous chromosomes.

Each genetic term correctly pairs with its fundamental biological definition: Gene locus is the physical chromosome position; Allele is an alternative form of a gene; Phenotype represents observable expressed characteristics; Homozygous describes having identical alleles for a specific gene.

Step-by-Step Solution

Identify the definition of gene locus.

Gene locus corresponds to the specific physical position of a gene on a chromosome.

The term 'locus' originates from the Latin word for place, denoting the specific site of a gene on a chromosome.

Identify the definition of an allele.

Allele corresponds to an alternative molecular form of a gene located at a specific position on homologous chromosomes.

Genes often exist in multiple variant forms called alleles that dictate alternative versions of a trait.

Identify the definition of phenotype.

Phenotype corresponds to the observable physical and physiological expression of an organism's genetic makeup.

Phenotype is the external manifestation of genetic instructions combined with environmental influence.

Identify the definition of homozygous.

Homozygous corresponds to the condition of possessing identical alleles for a given gene on homologous chromosomes.

The prefix 'homo-' means same, indicating identical genetic alleles present at a particular locus.

Key Concept

Basic Genetics Terminology and Concepts

Question 104Question →

Match each biological trait or population distribution profile on the left with its correct underlying genetic mechanism or characteristic variation pattern on the right.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Human ABO blood group system matches Monogenic inheritance resulting in clear-cut qualitative phenotypic categories uninfluenced by environmental factors; Human adult height range matches Polygenic additive inheritance producing a quantitative continuum of phenotypes significantly modified by nutrition and environment; Bell-shaped normal distribution curve matches Graphical representation of continuous phenotypic variation exhibiting complete graduation between extreme values; Discrete bar graph with non-overlapping columns matches Graphical representation of discontinuous phenotypic variation showing distinct, isolated phenotypic classes.

Matching each item based on underlying genetic architecture demonstrates that continuous variation is polygenic, environmentally influenced, and bell-curve distributed, whereas discontinuous variation is monogenic, environmentally stable, and represented by discrete categorical bars.

Step-by-Step Solution

Analyze the genetic basis of qualitative versus quantitative biological traits.

Discontinuous traits like ABO blood groups show distinct phenotypic classes (monogenic), whereas continuous traits like height span a spectrum of intermediate phenotypes (polygenic).

Single-gene inheritance produces discrete phenotypic groups, while multi-gene additive inheritance produces continuous phenotypic ranges.

Evaluate environmental influence on phenotypic variance for each trait type.

ABO blood groups remain constant regardless of environment, whereas adult height is heavily influenced by environmental factors like nutrition during growth.

Continuous traits interact with environmental variables, altering phenotypic outcome, while discontinuous traits are genetically fixed.

Associate each variation category with its characteristic graphical representation.

Continuous variation forms a smooth, bell-shaped normal distribution curve; discontinuous variation forms distinct, isolated bars on a histogram.

Smooth transition between phenotypes generates a curve, while clear phenotypic gaps yield separated bars.

Key Concept

Continuous and Discontinuous Variation Mechanisms and Graphical Distributions

Question 105Question →

In diploid eukaryotic organisms, an individual that is heterozygous for a specific gene carries two distinct alleles located at corresponding loci on non-homologous chromosomes.

True

False

Show answer & explanation

Answer: False

Answer

The statement is false because distinct alleles of a single gene reside at corresponding loci on homologous chromosomes, not non-homologous chromosomes.

The statement is false because alleles are alternative forms of the same gene that occupy corresponding positions (loci) on homologous chromosomes. Non-homologous chromosomes belong to separate chromosome pairs and harbor completely different sets of genes.

Step-by-Step Solution

Define homologous chromosomes in diploid organisms

Homologous chromosomes are matching pairs of chromosomes (one maternal, one paternal) that possess identical structural features and gene loci in the same linear order.

Genetics terminology establishes that gene pairs and their alternative forms (alleles) reside on homologous pairs.

Examine the organization of alleles in a heterozygous genotype

A heterozygous organism has two different alleles of a given gene positioned at the same locus on homologous chromosomes.

Non-homologous chromosomes represent completely different chromosome pairs carrying distinct, unrelated genes.

Evaluate the validity of the statement

The statement incorrectly places alleles of the same gene on non-homologous chromosomes.

Because alleles of a single gene exist only on homologous chromosome pairs, the statement is false.

Key Concept

Homologous Chromosomes and Allelic Loci

Estimated Time:1m 0s

Question 106Question →

A paracentric inversion is a structural chromosomal aberration involving two double-strand breaks on a chromosome followed by a 180-degree rotation of the detached segment including the centromere, thereby altering the chromosome's relative arm length ratio.

True

False

Show answer & explanation

Answer: False

Answer

The statement is False. A paracentric inversion occurs within a single arm of a chromosome and does not involve the centromere. A pericentric inversion is the type of inversion that includes the centromere and can change the arm length ratio.

The statement is false because a paracentric inversion is restricted to a single chromosome arm and does not incorporate the centromere. Therefore, it cannot alter the centromere's position or the arm ratio of the chromosome.

Step-by-Step Solution

Define structural chromosomal inversions

Inversions occur when a segment of a chromosome breaks at two points, rotates 180 degrees, and reinserts into the chromosome.

Establishing the general mechanism of inversion aberrations.

Distinguish between paracentric and pericentric inversions based on centromere involvement

Para- means 'next to' or 'beside' (confined to one arm, excluding the centromere). Peri- means 'around' (spans across the centromere).

Identifying which structural inversion type contains the centromere.

Evaluate the morphological impact on chromosome arm ratios

Because paracentric inversions take place entirely within one arm, the centromere remains intact in its original location, leaving the relative lengths of the short (

p

) and long (

q

) arms unchanged. Pericentric inversions can change the centromere position relative to the ends, changing arm ratios.

Determining the truth value of the stem's claim.

Key Concept

Paracentric versus Pericentric Chromosomal Inversions

Estimated Time:1m 30s

Question 107Question →

Match each example of human physiological or morphological variation on the left with its corresponding underlying characteristic or pattern of inheritance on the right.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Sickle cell hemoglobin trait matches with Physiological variation maintained in populations via heterozygote advantage against malaria; Fingerprint ridge pattern matches with Morphological discontinuous variation determined fully before birth and permanent throughout life; Adult body height distribution matches with Morphological continuous variation governed by polygenic inheritance and environmental factors; ABO blood group classification matches with Physiological discontinuous variation characterized by discrete biochemical phenotypes governed by multiple alleles.

Each matching pair accurately connects the specific human variation type to its physiological or morphological classification, genetic basis, and environmental sensitivity.

Step-by-Step Solution

Distinguish between morphological (structural/external physical form) and physiological (functional/biochemical process) variations.

Fingerprint patterns and height are identified as morphological variations, while sickle cell trait and ABO blood group are identified as physiological variations.

Classification relies on whether the variation is visible externally (structural) or operates internally at the cellular/biochemical level (functional).

Classify each trait by distribution pattern (continuous vs discontinuous).

Height displays continuous variation across a spectrum; blood groups, fingerprints, and hemoglobin traits show clear-cut discontinuous categories.

Continuous traits show a range of intermediate phenotypes, whereas discontinuous traits fall into distinct, non-overlapping phenotypic classes.

Correlate specific biological mechanisms and environmental interactions to each matched pair.

Heterozygote advantage corresponds to sickle cell carrier status; polygenic inheritance and nutrition correspond to height; complete genetic determination before birth corresponds to fingerprint patterns; and multiple alleles at a single locus correspond to ABO blood groups.

Matching requires pairing the precise physiological/morphological trait to its specific genetic and evolutionary behavior.

Key Concept

Human Morphological and Physiological Variations

Question 108Question →

During DNA replication, exposure to an alkylating agent causes the insertion of a single extra nucleotide base into the coding region of a functional gene, while an error during spindle fiber assembly in meiosis causes two homologous chromosomes to fail to separate. Which of the following statements correctly distinguishes the molecular nature and scope of these two genetic events?

The nucleotide insertion causes a frameshift gene mutation that alters the reading frame of a single protein, whereas non-disjunction causes a numerical chromosomal aberration altering total chromosome count.

The nucleotide insertion results in a chromosomal structural aberration, whereas non-disjunction leads exclusively to a point mutation within a single locus.

Both events alter the karyotype of the organism without affecting the amino acid sequence of synthesized polypeptides.

The insertion leads to polyploidy across all autosomes, whereas non-disjunction alters only the nucleotide sequence of the nitrogenous bases.

Show answer & explanation

Answer: The nucleotide insertion causes a frameshift gene mutation that alters the reading frame of a single protein, whereas non-disjunction causes a numerical chromosomal aberration altering total chromosome count.

Answer

A single nucleotide addition within a gene alters the codon triplet reading frame (frameshift mutation), affecting only that specific gene product. In contrast, non-disjunction involves the failure of chromosome separation, leading to aneuploidy, which is a numerical chromosomal aberration.

Step-by-Step Solution

Classify the single nucleotide insertion event.

Insertion of a single base into a gene's coding sequence alters the triplet codon reading frame during translation (frameshift gene mutation).

Gene mutations involve chemical or sequence changes within localized nucleotides of a single gene.

Classify the homologous chromosome non-separation event during meiosis.

Failure of homologous chromosomes to separate is termed non-disjunction, resulting in gametes with extra or missing whole chromosomes (aneuploidy).

Non-disjunction affects macro-structures (whole chromosomes), making it a numerical chromosomal aberration.

Compare the scope and classification of both events.

The insertion is a gene-level mutation affecting one polypeptide, while non-disjunction is a chromosomal aberration affecting total chromosome number.

Gene mutations affect nucleotide sequences, whereas chromosomal aberrations affect gross chromosome structure or number.

Key Concept

Distinction between Gene Mutations and Chromosomal Aberrations

Estimated Time:1m 30s

Question 109Question →

In snapdragon plants (*Antirrhinum majus*), flower color inheritance exhibits incomplete dominance between the red allele ( $C^R$ ) and white allele ( $C^W$ ). A plant breeder crosses a pink-flowered snapdragon ( $C^R C^W$ ) with a red-flowered snapdragon ( $C^R C^R$ ). What percentage of the resulting offspring is predicted to have pink flowers?

25%

50%

75%

100%

Show answer & explanation

Answer: 50%

Answer

50% of the offspring are predicted to have pink flowers.

In incomplete dominance, heterozygous individuals (

C^R C^W

) express an intermediate pink phenotype. A cross between a pink plant (

C^R C^W

) and a red plant (

C^R C^R

) produces equal numbers of

C^R C^R

(red) and

C^R C^W

(pink) progeny, resulting in a 50% probability for pink flowers.

Step-by-Step Solution

Determine the parental genotypes

Pink parent genotype is

C^R C^W

; Red parent genotype is

C^R C^R

In incomplete dominance, the intermediate phenotype (pink) is heterozygous, while the red phenotype is homozygous dominant.

Formulate gametes for each parent

Pink parent produces 50%

C^R

and 50%

C^W

gametes. Red parent produces 100%

C^R

gametes.

Mendel's Law of Segregation dictates that allele pairs separate during gamete formation.

Determine offspring genotypes and phenotypes

Offspring genotypes are 50%

C^R C^R

(Red) and 50%

C^R C^W

(Pink).

Combining

C^R

from the red parent with

C^R

C^W

from the pink parent gives a 1:1 ratio of red to pink flowers.

Key Concept

Incomplete Dominance Phenotypic Ratios

Estimated Time:1m 0s

Question 110Question →

In medical genetics and blood transfusion compatibility, an individual with blood group AB expresses both A and B antigens on their red blood cells. Which of the following best explains the genetic relationship between the $I^A$ and $I^B$ alleles?

The

I^A

and

I^B

alleles are codominant, allowing both antigens to be fully expressed.

The

I^A

allele is completely dominant over the recessive

I^B

allele.

The

I^A

and

I^B

alleles undergo incomplete dominance to produce a blended trait.

The trait is sex-linked and expressed only on the X chromosome.

Show answer & explanation

Answer: The $I^A$ and $I^B$ alleles are codominant, allowing both antigens to be fully expressed.

Answer

The

I^A

and

I^B

alleles display codominance, meaning both alleles are fully and independently expressed in individuals with blood group AB.

The correct option identifies codominance as the inheritance pattern where both

I^A

and

I^B

alleles are expressed equally and simultaneously in blood group AB individuals.

Step-by-Step Solution

Identify the genotype associated with blood group AB.

An individual with blood group AB possesses the heterozygous genotype

I^A I^B

Each parent contributes one allele (

I^A

I^B

) to the offspring.

Analyze how the alleles manifest in the phenotype.

Both antigen A and antigen B are present on the membrane of red blood cells.

When two different alleles are both expressed in a heterozygous organism, the genetic phenomenon is called codominance.

Key Concept

Codominance in ABO Blood Groups

Estimated Time:45s

Question 111Question →

In human population genetics, traits that display continuous variation—such as adult height and skin color—are characterized by distinct, non-overlapping phenotypic classes because they are governed by polygenic inheritance.

True

False

Show answer & explanation

Answer: False

Answer

The statement is false. Traits showing continuous variation exhibit a continuous spectrum of phenotypic values without distinct categories.

The statement incorrectly combines the genetic cause of continuous variation (polygenic inheritance) with the phenotypic pattern of discontinuous variation (distinct, non-overlapping classes). Traits exhibiting continuous variation present a continuous range of phenotypes without distinct gaps.

Step-by-Step Solution

Analyze the trait characteristics described in the statement.

The statement mentions continuous variation traits (adult height and skin color) and claims they produce 'distinct, non-overlapping phenotypic classes'.

Identifying the phenotypic pattern is necessary to evaluate whether it matches continuous or discontinuous variation.

Differentiate between continuous and discontinuous phenotypic patterns.

Continuous variation presents an unbroken gradient of intermediate forms with no sharp boundaries. Discontinuous variation presents discrete, clear-cut categories without intermediate phenotypes.

This establishes the correct biological rule regarding phenotypic distribution.

Evaluate the underlying genetic mechanisms.

Polygenic inheritance (multiple genes with additive effects combined with environmental factors) generates continuous variation, not discrete classes.

Matching the phenotypic distribution to the underlying mode of inheritance reveals that claiming continuous traits form non-overlapping classes is factually incorrect.

Key Concept

Continuous vs Discontinuous Variation

Question 112Question →

In tomato plants (*Solanum lycopersicum*), red fruit color ( $R$ ) is dominant over yellow fruit color ( $r$ ), and tall stem height ( $T$ ) is dominant over dwarf stem height ( $t$ ). A geneticist crosses two heterozygous tall, red-fruited tomato plants ( $RrTt \times RrTt$ ). If this dihybrid cross yields a total of $1,600$ offspring in the $F_2$ generation, how many plants are expected to exhibit both yellow fruit and dwarf stems?

Show answer & explanation

Answer: 100

Answer

The expected number of offspring with yellow fruit and dwarf stems is 100 plants.

In a dihybrid cross of two heterozygous individuals (

RrTt \times RrTt

), allele pairs segregate independently. The probability of obtaining recessive yellow fruit (

rr

) is

\frac{1}{4}

, and the probability of obtaining recessive dwarf stem (

tt

) is

\frac{1}{4}

. By the product rule of probability, the combined probability of both recessive traits (

rrtt

) occurring simultaneously is

\frac{1}{4} \times \frac{1}{4} = \frac{1}{16}

. Multiplying this fraction by the total offspring count (

1,600

) gives

100

plants.

Step-by-Step Solution

Determine the genotype of the specified phenotype

Yellow fruit and dwarf stem phenotype corresponds to the double recessive genotype

rrtt

Yellow (

r

) and dwarf (

t

) are both recessive alleles, requiring homozygous recessive conditions at both loci.

Determine the phenotypic ratio for a dihybrid cross of two heterozygotes (

RrTt \times RrTt

)

The expected

F_2

phenotypic ratio according to Mendel's Law of Independent Assortment is

9:3:3:1

The double recessive phenotype (

rrtt

) makes up

\frac{1}{16}

of the total offspring.

Calculate the expected count in a population of 1,600 offspring

1,600 \times \frac{1}{16} = 100

plants.

Multiplying the total offspring count by the probability of the double recessive phenotype yields the expected number of individuals.

Key Concept

Mendel's Law of Independent Assortment and F2 Dihybrid Phenotypic Ratios

Question 113Question →

Match each type of genetic alteration listed on the left with its precise molecular or cytogenetic mechanism on the right.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Transition mutation matches replacement of a purine by another purine (or pyrimidine by pyrimidine). Transversion mutation matches substitution of a purine with a pyrimidine (or vice versa). Pericentric inversion matches chromosomal breaks flanking the centromere with

180^\circ

inversion containing the centromere. Robertsonian translocation matches centromeric fusion of two acrocentric long arms resulting in loss of short arms and reduced chromosome number.

Transition mutation corresponds to swapping purine-for-purine (

A \leftrightarrow G

) or pyrimidine-for-pyrimidine (

C \leftrightarrow T

). Transversion mutation corresponds to swapping purines for pyrimidines (

A/G \leftrightarrow C/T

). Pericentric inversion includes the centromere between two break points prior to rotation. Robertsonian translocation specifically joins the

q

arms of acrocentric chromosomes near the centromere, shedding the non-essential heterochromatic

p

arms.

Step-by-Step Solution

Classify point mutations by chemical base structure alteration

Transition mutations exchange like-for-like ring structures (purine to purine or pyrimidine to pyrimidine), whereas transversion mutations swap single-ring pyrimidines with double-ring purines or vice versa.

This establishes the precise molecular distinction between point substitution categories.

Differentiate structural chromosomal inversions

Pericentric inversions involve breaks on both sides of the centromere (including it in the inverted segment), unlike paracentric inversions which occur entirely within one chromosome arm (excluding the centromere).

Including the centromere can alter arm ratios and morphological appearance of the chromosome.

Identify special translocation mechanisms involving acrocentric chromosomes

Robertsonian translocation specifically involves breakage near centromeres of acrocentric chromosomes, causing long arms to fuse into a single metacentric or submetacentric chromosome.

This reduces the overall functional chromosome count (

2n = 45

in balanced carriers).

Key Concept

Distinction between point gene mutation mechanisms (transitions vs transversions) and structural/numerical chromosomal aberrations (pericentric inversions vs Robertsonian translocations).

Estimated Time:2m 0s

Question 114Question →

In a molecular genetics analysis of a patient, a single nucleotide substitution is detected where adenine is replaced by thymine in the sixth codon of the $\beta$ -globin gene, causing glutamic acid to be replaced by valine. Which of the following correctly classifies this genetic change and the pattern of variation its resulting phenotype displays in human populations?

A gene mutation resulting in discontinuous variation

A gene mutation resulting in continuous variation

A sex-linked chromosomal aberration resulting in discontinuous variation

A somatic chromosomal aberration inherited via environmental adaptation

Show answer & explanation

Answer: A gene mutation resulting in discontinuous variation

Answer

The genetic change is classified as a gene mutation and the resulting phenotype displays discontinuous variation.

The substitution of a single nitrogenous base in a codon alters the amino acid sequence of a specific polypeptide without altering the macroscopic structure or number of chromosomes, defining it as a gene (point) mutation. Because the resulting sickle-cell condition produces distinct, clear-cut phenotypic classes (normal, carrier, affected) without intermediate continuum states, it represents discontinuous variation.

Step-by-Step Solution

Classify the type of genetic modification

Replacing a single nucleotide base (adenine with thymine) within the coding sequence of the

\beta

-globin gene constitutes a point mutation (gene mutation), as it affects only the nucleotide sequence of one gene without altering chromosome structure or number.

Gene mutations involve localized alterations in the DNA base sequence of a single gene locus.

Determine the resulting pattern of genetic variation

Sickle-cell trait/anemia displays distinct, non-overlapping phenotypic categories (unaffected, sickle-cell trait carrier, or sickle-cell anemia).

Traits governed by single gene loci with clear discrete phenotypic classes exemplify discontinuous variation.

Key Concept

Gene mutation vs chromosomal aberration and its phenotypic expression as discontinuous variation

Estimated Time:2m 0s

Question 115Question →

Match each application of genetics in medicine or agriculture on the left with its corresponding biological mechanism or practical objective on the right.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Genetic counseling matches with analyzing parental genotypes to evaluate inheritance risks of blood disorders; Induction of polyploidy matches with using colchicine to multiply chromosome sets for larger or seedless crops; Rhesus factor compatibility screening matches with preventing maternal antibody sensitization against fetal red blood cells; Hybrid vigor matches with crossing distinct inbred lines for enhanced progeny yield and vigor.

Each application accurately matches its defined genetic procedure or outcome. In medicine, genetic counseling determines inheritance probability while Rhesus screening prevents hemolytic disease of the newborn. In agriculture, polyploidy modification enhances organ size or seedlessness, and heterosis produces high-performing hybrid crops.

Step-by-Step Solution

Differentiate medical genetic applications from agricultural genetic applications.

Genetic counseling and Rhesus factor compatibility are medical applications, whereas polyploidy induction and hybrid vigor are agricultural applications.

Categorizing items by domain simplifies finding their underlying genetic mechanisms.

Align medical concepts with their clinical targets.

Genetic counseling assesses carrier probability (e.g., sickle-cell trait). Rhesus compatibility screening avoids immune rejection of fetal erythrocytes (

Rh^+

) by sensitized maternal (

Rh^-

) antibodies.

Both procedures prevent or mitigate hereditary and developmental blood disorders.

Align agricultural techniques with their biotechnological methods.

Polyploidy uses mitotic inhibitors like colchicine to induce chromosome doubling for crop improvement. Hybrid vigor exploits heterosis from crossing inbred lines.

These techniques increase crop biomass, fruit quality, and resistance to environmental stress.

Key Concept

Applications of Genetics in Medicine and Agriculture

Estimated Time:1m 30s

Question 116Question →

In cattle (*Bos taurus*), the polled (hornless) condition ( $P$ ) is dominant to the horned condition ( $p$ ), and black coat color ( $B$ ) is dominant to red coat color ( $b$ ). If a heterozygous polled, heterozygous black bull ( $PpBb$ ) is crossed with a horned, heterozygous black cow ( $ppBb$ ), what proportion of the offspring is expected to display the horned, black coat phenotype?

\frac{3}{8}

\frac{9}{16}

\frac{1}{4}

\frac{1}{8}

Show answer & explanation

Answer: $\frac{3}{8}$

Answer

The expected proportion of offspring displaying the horned, black coat phenotype is

\frac{3}{8}

According to Mendel's Law of Independent Assortment, the inheritance of horn condition and coat color are independent events. The cross between

Pp

and

pp

yields a

\frac{1}{2}

probability of horned offspring (

pp

). The cross between

Bb

and

Bb

yields a

\frac{3}{4}

probability of black-coated offspring (

BB

Bb

). Multiplying these independent probabilities (

\frac{1}{2} \times \frac{3}{4}

) gives

\frac{3}{8}

as the expected fraction of horned, black-coated offspring.

Step-by-Step Solution

Analyze the cross for the horn condition trait independently.

The cross

Pp \times pp

produces genotypes

Pp

(polled) and

pp

(horned) in a

1:1

ratio, giving a probability of

P(pp) = \frac{1}{2}

Mendel's Law of Segregation dictates that alleles segregate independently into gametes.

Analyze the cross for the coat color trait independently.

The cross

Bb \times Bb

produces phenotypes in a

3:1

ratio, giving a probability of

P(\text{black coat}, B\_) = \frac{3}{4}

Crossing two heterozygotes results in

\frac{1}{4} BB

\frac{1}{2} Bb

, and

\frac{1}{4} bb

genotypes.

Combine the independent probabilities using Mendel's Law of Independent Assortment.

P(\text{horned and black coat}) = P(pp) \times P(B\_) = \frac{1}{2} \times \frac{3}{4} = \frac{3}{8}

Because the two gene pairs assort independently, the combined probability is the product of their separate probabilities.

Key Concept

Mendel's Law of Independent Assortment and Probability Calculations in Dihybrid Crosses

Estimated Time:1m 30s

Question 117Question →

Sickle cell anaemia is a well-known inherited disease caused by a point mutation in the gene encoding the $\beta$ -globin chain of human haemoglobin. Which of the following alterations at the primary protein structure level directly causes the formation of abnormal haemoglobin S (HbS)?

Replacement of glutamic acid with valine at the sixth position of the

\beta

-globin polypeptide chain

Deletion of a nucleotide triplet resulting in the total absence of the

\beta

-globin chain

Nondisjunction resulting in an extra copy of the chromosome bearing the globin gene family

Inversion of a chromosome segment containing the genes responsible for ABO blood grouping

Show answer & explanation

Answer: Replacement of glutamic acid with valine at the sixth position of the $\beta$ -globin polypeptide chain

Answer

The formation of abnormal haemoglobin S (HbS) in sickle cell anaemia is caused by the replacement of glutamic acid with valine at the sixth position of the

\beta

-globin polypeptide chain.

Sickle cell anaemia is caused by a single nucleotide substitution in the

\beta

-globin gene located on chromosome 11, where adenine is substituted by thymine. This alters the mRNA codon from GAG to GUG, causing valine to replace glutamic acid at the 6th position of the

\beta

-chain, leading to polymerisation of haemoglobin molecules under low oxygen tension.

Step-by-Step Solution

Identify the genetic nature of sickle cell anaemia

It is a gene (point) mutation caused by a single base pair substitution in the DNA sequence of the

\beta

-globin gene.

Understanding whether a trait is caused by a point mutation or structural/numerical chromosomal aberration narrows down the biological mechanisms.

Determine the molecular consequence at the protein level

The codon GAG (coding for glutamic acid) is mutated to GUG (coding for valine) at codon position 6 of the

\beta

-globin polypeptide.

Replacing a hydrophilic amino acid (glutamic acid) with a hydrophobic amino acid (valine) alters the solubility and structural properties of haemoglobin under low oxygen conditions.

Key Concept

Gene Point Mutation and Molecular Consequences in Sickle Cell Anaemia

Estimated Time:1m 0s

Question 118Question →

Match each human variation trait on the left with its correct genetic, morphological, or physiological classification on the right.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Sickle-cell status matches discontinuous physiological variation with codominance and malaria advantage; PTC tasting sensitivity matches discontinuous physiological variation determined by monogenic taste perception; Total fingerprint dermal ridge count matches continuous morphological variation under polygenic control unaffected by post-natal environment; Skin melanin concentration matches continuous morphological variation under polygenic control with environmental modification.

The correct pairing matches each human variation according to whether it affects physical anatomy (morphological) or internal biological function (physiological), whether it shows continuous quantitative distribution or distinct categorical groups (discontinuous), and its specific mode of genetic control and environmental interaction.

Step-by-Step Solution

Distinguish between morphological and physiological variations

Sickle-cell hemoglobin status and PTC taste perception involve internal cellular biochemistry and chemoreception (physiological traits). Fingerprint ridge count and skin color involve outward structural and physical features (morphological traits).

Morphological traits describe anatomical form, whereas physiological traits describe internal function and biochemical processes.

Differentiate between continuous and discontinuous variation distributions

Sickle-cell status and PTC tasting split individuals into distinct, non-overlapping phenotypic groups (discontinuous variation). Fingerprint ridge counts and skin melanin levels form a smooth gradient of continuous numerical values across a population (continuous variation).

Discontinuous traits are controlled by one or few genes with major effects, while continuous traits are quantitative and governed by polygenes.

Determine specific genetic inheritance and environmental influences

Sickle-cell status involves codominance (

Hb^A

and

Hb^S

) providing heterozygote protection against malaria. PTC tasting follows monogenic Mendelian inheritance. Fingerprint ridge counts are polygenic yet unaffected by post-natal factors, while skin color is polygenic and modified by environmental UV radiation.

Each variation combines distinct modes of gene expression (monogenic vs polygenic, codominance) and environmental susceptibility.

Key Concept

Classification and underlying mechanisms of human morphological and physiological variations

Question 119Question →

In pea plants (*Pisum sativum*), seed shape (round $R$ dominant to wrinkled $r$ ) and seed color (yellow $Y$ dominant to green $y$ ) inherit independently according to Mendel's Second Law. Match each parental genetic cross on the left with its corresponding phenotypic ratio of offspring on the right.

Click a left item, then click its matching right item

Show answer & explanation

Answer

Cross between two heterozygous dihybrids (

RrYy \times RrYy

) matches

9 : 3 : 3 : 1

; Dihybrid test cross (

RrYy \times rryy

) matches

1 : 1 : 1 : 1

; Cross between

RrYy

and

Rryy

matches

3 : 3 : 1 : 1

; Cross between

RrYY

and

RrYy

matches

3 : 1

(Round Yellow : Wrinkled Yellow).

Each cross produces a specific phenotypic distribution based on independent assortment during meiosis.

RrYy \times RrYy

gives the classical

9:3:3:1

dihybrid F2 ratio.

RrYy \times rryy

gives the equal

1:1:1:1

test cross ratio.

RrYy \times Rryy

produces

3:3:1:1

across all four phenotypes.

RrYY \times RrYy

produces only yellow seeds in a

3:1

ratio of round to wrinkled.

Step-by-Step Solution

Determine gamete combinations for each parent in the cross.

Identify the types and frequency of gametes produced via independent assortment.

Mendel's Law of Independent Assortment states that alleles for different traits segregate independently during gamete formation.

Construct Punnett squares or calculate product rule probabilities for each parental cross pair.

Obtain the genotypic frequencies and translate them into phenotypic ratios.

Crosses involving different combinations of homozygous and heterozygous loci produce characteristic phenotypic frequency distributions.

Match each cross with its calculated phenotypic ratio.

RrYy \times RrYy \rightarrow 9:3:3:1

RrYy \times rryy \rightarrow 1:1:1:1

RrYy \times Rryy \rightarrow 3:3:1:1

, and

RrYY \times RrYy \rightarrow 3:1

Comparing predicted proportions to the listed choices establishes the accurate pairings.

Key Concept

Mendel's Law of Independent Assortment and Dihybrid Phenotypic Ratios

Question 120Question →

During a plant breeding experiment, a researcher treats dividing meristematic cells of a diploid crop species ( $2n = 14$ ) with the chemical mutagen colchicine. Colchicine inhibits microtubule polymerization and prevents the assembly of the mitotic spindle apparatus during cell division, preventing chromatid separation into daughter nuclei. Which of the following correctly describes the chromosome count and the class of mutation present in the resulting cells?

28 chromosomes, resulting from polyploidy which is a chromosomal aberration

15 chromosomes, resulting from aneuploidy which is a chromosomal aberration

28 chromosomes, resulting from a frameshift gene mutation

14 chromosomes, resulting from a point substitution gene mutation

Show answer & explanation

Answer: 28 chromosomes, resulting from polyploidy which is a chromosomal aberration

Answer

28 chromosomes, resulting from polyploidy which is a chromosomal aberration

Colchicine interferes with microtubule assembly, disabling spindle fibers during mitosis. As a result, duplicated sister chromatids are retained within a single cell nucleus instead of being pulled to opposite poles. For an initial diploid cell (

2n = 14

), this failure of segregation results in tetraploidy (

4n = 28

). Because this change involves entire sets of chromosomes rather than alterations within single nucleotide chains, it is classified as a numerical chromosomal aberration (polyploidy).

Step-by-Step Solution

Analyze the action of colchicine on dividing cells

Colchicine prevents spindle fiber formation, blocking anaphase separation of sister chromatids.

Microtubules cannot assemble, so duplicated chromosomes remain together in a single nucleus without cytokinesis.

Calculate the resulting chromosome number

The diploid number

2n = 14

is doubled to

4n = 28

Since DNA replication occurred before mitosis and sister chromatids failed to segregate into separate daughter cells, the chromosome set doubles.

Classify the type of genetic mutation

Numerical doubling of whole chromosome sets is classified as polyploidy, a major chromosomal aberration.

Gene mutations affect nucleotide sequences within individual genes, whereas numerical alterations of whole chromosome sets are chromosomal aberrations.

Key Concept

Polyploidy and Numerical Chromosomal Aberrations

Estimated Time:1m 30s

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