Class 12 Biology Chapter 4 Principle of Inheritance and Variation

Key Concept: Symptom Analysis

b) Down’s Syndrome

[Solution Description] The symptoms described match those commonly associated with Down’s Syndrome, which includes characteristics like short stature, specific facial features, and cognitive impairment. Hence, option b) is correct.

Key Concept: Mechanism of Mutation

b) UV radiation
[Solution Description] Mutations can be induced by physical and chemical factors known as mutagens. Ultraviolet (UV) radiation is a well-known physical mutagen that can cause changes in the DNA sequence by forming pyrimidine dimers, leading to errors during DNA replication.

Key Concept: Basic Definition

c) Traits controlled by three or more genes
[Solution Description] Polygenic inheritance refers to traits controlled by multiple genes rather than a single gene. These traits show a range of phenotypes due to the additive effect of alleles.

Key Concept: Punnett Square Application

b) 3:1
[Solution Description] By constructing a Punnett square for Aa x Aa, the phenotypes can be predicted: – AA: Homozygous dominant
– Aa: Heterozygous
– aa: Homozygous recessive

Phenotypically, both AA and Aa will show the dominant trait, while aa shows the recessive trait. The phenotypic ratio thus becomes 3:1 for dominant to recessive.

Key Concept: Historical Figure

c) Langdon Down
[Solution Description] Down Syndrome was first described by Langdon Down in 1866. He identified the distinct set of symptoms and characteristics associated with the disorder, leading to its initial recognition and study.

Key Concept: Phenotypic Ratio

c) 0% red, 0% white, 100% pink
[Solution Description] In cases of incomplete dominance, the phenotype of the F1 generation is not identical to either parent; instead, it exhibits an intermediate trait. Here, crossing RR (red) with rr (white) will result in all heterozygous Rr offspring, showing the intermediate pink phenotype. Therefore, the phenotypic ratio of red to white to pink in F1 is 0:0:100% pink.

Key Concept: Real-world Application

c) A volcanic island is colonized by a single bird species.
[Solution Description] Adaptive radiation often occurs when a single species colonizes a new area with different ecological opportunities, such as a volcanic island. The bird species can diversify into multiple forms to exploit the varied habitats and food sources on the island, unlike the stable forest or desert scenarios where there are limited opportunities for diversification.

Key Concept: Genetic Probability

c) 25%
[Solution Description] Cystic fibrosis being an autosomal recessive disorder means both parents must be carriers of the recessive allele. The possible genotypes for their children are:
– 25% chance of having two recessive alleles (affected).
– 50% chance of being a carrier (one normal and one recessive allele).
– 25% chance of having two normal alleles.
Thus, there is a 25% probability the next child will have cystic fibrosis.

Key Concept: Basic Concept

b) Ultraviolet radiation
[Solution Description] Ultraviolet (UV) radiation is a physical mutagen that can cause mutations by inducing changes in the DNA structure, such as the formation of thymine dimers.

Key Concept: Mendel’s Experiment

b) Pea
[Solution Description] Gregor Mendel conducted his hybridization experiments using the garden pea plant Pisum sativum. He chose this plant due to its distinct characteristics and ease of controlling pollination.

Key Concept: Pedigree Analysis

d) X-linked recessive inheritance
[Solution Description] A trait that appears mainly in males and can skip generations is indicative of an X-linked recessive pattern. Males are affected because they have only one X chromosome; hence any mutation on the X chromosome will manifest as the disorder.

Key Concept: Mutation Impact

d) Sickle-cell anaemia

[Solution Description]

Sickle-cell anaemia is caused by a point mutation at the sixth position of the beta globin chain where Glutamic acid (GAG) is replaced by Valine (GTG). This substitution results in the production of abnormal hemoglobin known as Hemoglobin S (HbS), causing red blood cells to sickle.

Key Concept: Inheritance Pattern

c) 1:2:1
[Solution Description] According to Mendel’s Law of Segregation, each parent can pass one of their two alleles for a given trait to their offspring. When crossing two heterozygous individuals (e.g., Aa x Aa), we use a Punnett square to determine the potential genetic combinations: AA, Aa, Aa, and aa. This results in a genotypic ratio of 1:2:1.

Key Concept: Independent Assortment

b) Genetic variation
[Solution Description] Independent assortment refers to how different genes independently separate from one another when reproductive cells develop. During meiosis, this leads to genetic variation because each gamete contains a different set of DNA. This occurs because homologous chromosomes line up randomly at the cell’s equator in metaphase I and then are separated into different gametes. Therefore, the correct answer is genetic variation.

Key Concept: F1 and F2 Generations

d) The segregation of alleles allows recessive traits to appear.
[Solution Description] In the F2 generation, alleles segregate independently and recombine during fertilization, leading to a 3:1 ratio where three-quarters display the dominant trait and one-quarter display the recessive trait.

Key Concept: Pedigree Analysis

b) Autosomal dominant

[Solution Description] An autosomal dominant trait typically appears in every generation because a single copy of the mutant allele is sufficient for expression of the phenotype. Autosomal recessive traits tend to skip generations as they require two copies of the mutant allele for phenotypic expression. X-linked traits may or may not skip generations depending on whether or not there are enough males or females carrying the trait.

Key Concept: Genetic Mapping

b) It estimates the distance between genes on a chromosome
[Solution Description] Recombination frequency is used in genetic mapping to estimate the distance between genes on a chromosome. A higher recombination frequency indicates greater distance, while a lower frequency suggests closer proximity.

Key Concept: Dominance Concept

a) Tall
[Solution Description]
In Mendel’s pea plant experiment, the trait for tall plants (represented by T) is dominant over the trait for dwarf plants (represented by t). This means that if a plant has at least one allele for tallness, it will exhibit the tall phenotype.

Key Concept: Phenotypic Ratio

b) 3:1

[Solution Description] According to the Law of Dominance, in Mendel’s monohybrid cross, the F2 generation shows a phenotypic ratio of 3:1, with 3 tall plants for every dwarf plant.

Key Concept: Homozygous vs Heterozygous

c) Tt
[Solution Description] A heterozygous genotype refers to having two different alleles for a specific gene, such as one dominant and one recessive allele (e.g., Tt).

Key Concept: Dominance Concept

a) Tall
[Solution Description] In a cross between Tt x Tt, the resulting phenotypes are influenced by dominance. Tall (T) being dominant, the majority of the offspring will express the tall trait. The genotypic breakdown would give TT, Tt, and tt, but since T is dominant, both TT and Tt show the tall phenotype.

Key Concept: Complex Genotype Analysis, Environmental Impact Analysis

d) High UV exposure

[Solution Description]

To determine how environment affects the expression of the polygenic trait related to skin color, we recognize that each dominant allele contributes to a darker phenotype. The given genotype AaBbCcDd includes several dominant alleles: A, B, C, and D. Environmental factors like UV exposure can increase melanin production, leading to a darker skin tone. Therefore, high UV exposure enhances the effect of these dominant alleles on skin pigmentation.

Key Concept: Comparison with Other Inheritance

a) Pleiotropy involves one gene affecting multiple traits, while polygenic inheritance involves multiple genes affecting a single trait.
[Solution Description]
Pleiotropy differs from polygenic inheritance in the way genes influence traits. In pleiotropy, a single gene affects multiple traits. Conversely, polygenic inheritance involves multiple genes collectively influencing a single trait, such as height or skin color.

Understanding these differences helps in distinguishing between complex genetic patterns observed in various conditions.

Key Concept: Complex Pedigree Analysis, Environmental Influence

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description] In this case, we are dealing with a recessive sex-linked disorder which is typically found on the X chromosome. Let’s consider a man with genotype $$X^dY$$ where $d$ denotes the allele for the disorder, and a woman who is a carrier with genotype $X^DX^d$. For their sons, they receive one X chromosome from the mother and a Y chromosome from the father. So, the possible offspring genotypes for sons can be either $X^DY$ or $X^dY$. – If a son receives the $X^d$ from the mother and the Y from the father (i.e., X^dY), he will express the disorder since there is no dominant allele to mask the recessive trait. Thus, Assertion is true because there is a possibility for having affected sons. However, the Reason suggests that environmental factors influence the disorder’s expression. While environment might affect phenotypic severity in some cases, it does not determine inheritance patterns for recessive sex-linked disorders. Therefore, the assertion is independent of the reason provided. Hence, both statements are true, but Reason is NOT the correct explanation of Assertion.

Key Concept: Phenotype Identification

a) Tall
[Solution Description] Genotype TT is homozygous dominant, resulting in the expression of the dominant trait, which is tall in pea plants.

Key Concept: Law Application

c) All plants are yellow

[Solution Description] According to the Law of Dominance, when two different alleles are present in an organism, one allele will mask the expression of the other. Here, the yellow color (Y) is dominant over green (y). Thus, crossing YY with yy produces heterozygous Yy offspring. All F1 plants will express the dominant yellow phenotype.

Key Concept: Gamete Types

b) 50%
[Solution Description] During spermatogenesis in males, two types of sperm are formed. According to the syllabus, 50% of the sperm carry the X chromosome while 50% carry the Y chromosome.

Key Concept: Cross Analysis

b) Linkage
[Solution Description]
When a genetic cross results in offspring that exhibit predominantly parental phenotypes, it suggests that the genes controlling these traits are likely linked. Linkage refers to genes being located close together on the same chromosome, which reduces the chance of recombination occurring between them during meiosis. Hence, most offspring will inherit the parental combination of alleles rather than recombinant combinations, indicating strong linkage.

Key Concept: Mutation Types

b) Frame-shift mutation
[Solution Description] A frame-shift mutation occurs when nucleotide insertions or deletions are not in multiples of three, causing a shift in the reading frame of codons. This can lead to significant changes in the protein product due to altered amino acid sequences.

Key Concept: Linkage Strength

a) White and miniature wing
[Solution Description] In Morgan’s experiments, it was observed that some gene pairs had lower recombination frequencies than others, suggesting stronger linkage. For example, the white eye and miniature wing genes showed a stronger linkage compared to other pairs like yellow body and white eyes.

Key Concept: Complex Cross Analysis, Linkage and Independent Assortment

a) 80% parental types, 20% recombinant types

[Solution Description] The recombination frequency represents the percentage of recombinant offspring produced from a parental cross. In genetic terms, it can be calculated by the formula:

$$Recombination \; Frequency = \frac{\text{Number of Recombinant Offspring}}{\text{Total Number of Offspring}} \times 100.$$

Given a recombination frequency of 20%, this implies that out of 100 total offspring, 20 will have non-parental allele combinations due to recombination. Therefore, 80 offspring will retain the parental gene combinations.

Thus, the outcome can be described as:- 80% will show parental (non-recombinant) combinations- 20% will show recombinant combinationsThe correct option is a), which accurately describes the expected distribution according to linkage and recombination principles.

Key Concept: Punnett Square Application

c) 1 pink: 1 white
[Solution Description]
To solve this, we use a Punnett square. The gametes from the pink-flowered plant (Rr) are R and r, while those from the white-flowered plant (rr) are both r.

$$
\begin{array}{c|c c}
& r & r \\
\hline
R & Rr & Rr \\
r & rr & rr \\
\end{array}
$$

From the Punnett square, the offspring genotypes are:
– 2 Rr (pink)
– 2 rr (white)

Therefore, the expected phenotypic ratio is 1 pink : 1 white.

Key Concept: Linkage and Genetic Variation, Complex Genetic Mapping

c) Accumulate progeny genotype data over time
[Solution Description] For precise genetic mapping based on recombination frequencies, it is essential to consistently observe progeny over multiple generations, collecting data on offspring genotypes and calculating recombinant versus parental classes.

Key Focus:
1. Use selective breeding to maintain control groups allowing comparison.
2. Collect extensive data across several generations to mitigate statistical error and account for environmental factors.
3. Calculate recombination frequencies regularly to update gene maps accurately.

Given the different recombination percentages, focusing on progeny analysis will best reveal the layout of these genes on the chromosome.

Key Concept: Point Mutation Details

c) Replacement of glutamic acid with valine.

[Solution Description] Sickle cell anemia is caused by a point mutation where the amino acid glutamic acid is replaced with valine at position 6 of the beta-globin chain of hemoglobin. This substitution leads to abnormal sickle-shaped red blood cells.

Key Concept: Critical Evaluation, Advanced Genetic Concepts

c) They introduce diversity, allowing adaptation to new conditions.
[Solution Description] Genetic mutations introduce new genetic variations into a population. In rapidly changing environments, some of these mutations may provide beneficial adaptations that enhance an organism’s ability to survive and reproduce under new conditions. Through natural selection, these advantageous mutations become more common in the population over successive generations, contributing to the species’ long-term survival and adaptability.

Key Concept: Genotype-Phenotype Link

a) Tall

[Solution Description] In Mendel’s experiments, the allele for tall plants (T) is dominant over the allele for dwarf plants (t). Therefore, in a heterozygous condition (Tt), the dominant trait will be expressed. The phenotype of the plant will be tall.

Key Concept: Cross Outcome Prediction

d) All round and yellow

[Solution Description] For this cross, the genotypes involved are RRYy and rryy. The possible gametes are: – From RRYy: RY and Ry

– From rryy: ry only

Crossing these gives the following possible genotypes in the F1 generation:

– RY $\times$ ry = RrYy

– Ry $\times$ ry = Rryy

Both outcomes show that all potential offspring will have at least one dominant allele for each trait, which results in a phenotype that is round (due to R) and yellow (due to Y).

Key Concept: Basic Definition

a) Skin color in humans
[Solution Description] A polygenic trait is controlled by three or more genes, each contributing to the phenotype in an additive manner. An example of a polygenic trait is human skin color, which is influenced by multiple genes.

Key Concept: Detailed Example Analysis

b) Mental retardation and reduced pigmentation
[Solution Description] In Phenylketonuria, a single gene mutation affects multiple traits: mental retardation and reduced pigmentation are major consequences due to the accumulation of phenylalanine affecting brain development and melanin production.

Key Concept: Male Heterogamety

d) Males produce two types of gametes
[Solution Description]
Male heterogamety refers to the fact that males produce two different types of gametes: one carrying an X chromosome and the other carrying a Y chromosome. This results in variability in determining the sex of the offspring.

Key Concept: Simple Disorders

d) Sickle-cell anemia
[Solution Description] Sickle-cell anemia is a classic example of an autosomal recessive disorder where two copies of the mutated gene are required for the disease to manifest.

Key Concept: Chromosomal Inheritance

c) Males inherit chromosomes only from their mothers.
[Solution Description] Male bees in honeybee colonies are haploid, meaning they possess only one set of chromosomes, totaling 16. They develop from unfertilized eggs through a process called parthenogenesis, and therefore, inherit all their chromosomes solely from their mothers.

Key Concept: Genetic Relationships

c) Males do not have fathers but can have grandsons.

[Solution Description] Male honey bees, also known as drones, are produced from unfertilized eggs and hence do not have fathers. They develop through parthenogenesis, where no fertilization occurs. Therefore, they can have grandfathers but cannot have sons since they themselves produce sperm by mitosis and do not contribute genetically to offspring.

Key Concept: Chromosomal Count

c) 45, X0
[Solution Description]
In Turner Syndrome, there is one less X chromosome, leading to a chromosomal count of 45, X0. The normal female chromosomal composition is 46, XX, but due to the monosomy, it results in 45 chromosomes.

Key Concept: Sex Determination in Insects

c) Honey bee

[Solution Description] The haplodiploid sex determination system is primarily observed in honey bees. In this system, males develop from unfertilized haploid eggs and are therefore haploid, while females develop from fertilized diploid eggs and are diploid.

Key Concept: Gamete Formation

c) Four
[Solution Description] An organism with the genotype RrYy can produce four different types of gametes due to independent assortment. The alleles for each gene separate independently, resulting in gametes that contain one allele from each gene. For the genotype RrYy, these gametes are RY, Ry, rY, and ry.

Key Concept: Definition Recall

c) To determine the genotype of an organism showing a dominant phenotype
[Solution Description] A test cross is used to determine the genotype of an organism that exhibits a dominant phenotype by crossing it with a homozygous recessive organism.

Key Concept: Dominance Concept

a) T
[Solution Description] In the context of pea plant height, the dominant allele is represented by “T” for tallness. This dominant allele masks the expression of the recessive allele “t”, which stands for dwarfness.

Key Concept: Symptom Analysis

d) Gynaecomastia
[Solution Description] Individuals with Klinefelter Syndrome typically exhibit overall masculine development but also show feminine traits such as gynaecomastia. Gynaecomastia is the enlargement of breast tissue in males, which is a distinguishing feature for this syndrome.

Key Concept: Phenotype Prediction

d) Round and yellow
[Solution Description] The genotype RrYy indicates that the plant possesses one allele each for the dominant (R, Y) and recessive (r, y) traits. As per Mendelian dominance, the phenotype will express the dominant traits. Hence, the phenotype will be round seed shape and yellow seed color.

Key Concept: Frame-Shift Mutation

b) The addition of one amino acid without a frame shift.
[Solution Description] Frame-shift mutations typically occur when insertions or deletions are not in multiples of three, causing a shift in the reading frame. However, inserting three nucleotides adds one whole amino acid without altering the reading frame, thus maintaining the sequence integrity beyond that point.

Key Concept: Pedigree Analysis

c) Autosomal dominant
[Solution Description]
The trait being present in all generations suggests it could be a dominant trait. Since it affects both males and females equally, it indicates it is autosomal rather than sex-linked. Therefore, the most likely pattern of inheritance is autosomal dominant.

Key Concept: Mutation Definition

b) A change in the genetic material
[Solution Description] A mutation is a change in the genetic material of an organism, which can occur in various forms like point mutations or frame-shift mutations.

Key Concept: Gamete Formation

b) 25%

[Solution Description] For a dihybrid organism with genotype RrYy, four types of gametes are produced: RY, Ry, rY, ry. Each type has an equal probability of occurring, hence there is a 25% chance for each gamete type.

Key Concept: Advanced Linkage, Complex Crosses

c) Assertion is true, but Reason is false.

[Solution Description] In this question, we’re examining the impact of gene linkage on Mendelian ratios and understanding how genetic distance affects recombination rates.

The assertion states that in a dihybrid cross with linked genes, the presence of linkage alters the expected phenotypic ratio of 9:3:3:1 observed in independently assorting genes. This is true because when genes are located close together on the same chromosome, they tend not to separate during meiosis, thus altering the expected independent assortment pattern described by Mendel.

The reason given suggests that the proximity of genes on a chromosome increases recombination frequency. However, this statement is false; the closer two genes are, the lower the probability of crossover events occurring between them, resulting in decreased recombination frequency.

Therefore, while the assertion is correct (linked genes do alter expected Mendelian ratios), the reason provided is incorrect as it misstates the relationship between genetic distance and recombination frequency.

Key Concept: Pedigree Complexity

c) 50%
[Solution Description] Haemophilia is an X-linked recessive disorder. An affected male has one X chromosome carrying the haemophilia allele (X^h Y). The daughter must receive the haemophilia allele on one of her X chromosomes from her father, so her genotype is X^h X if she is unaffected. When she has a child, she passes on one of her X chromosomes. For a son, he receives his father’s Y chromosome and one of his mother’s X chromosomes. Therefore, there is a 50% chance her son will inherit the X^h chromosome and thus have haemophilia.

Key Concept: Punnett Square Analysis

c) 3:1 (Tall:Dwarf)
[Solution Description] The mathematical expression given is used to determine the phenotype ratio.

Expanding the expression:
$$\left( \frac{1}{2}T + \frac{1}{2}t \right)^2 = \left( \frac{1}{2}T + \frac{1}{2}t \right) \times \left( \frac{1}{2}T + \frac{1}{2}t \right)$$.

Applying the distributive property:
\begin{align*}
& = \frac{1}{4}TT + \frac{1}{4}Tt + \frac{1}{4}tT + \frac{1}{4}tt \\
& = \frac{1}{4}TT + \frac{1}{2}Tt + \frac{1}{4}tt
\end{align*}

The phenotypic ratio considering dominance (where $T$ is dominant over $t$):
– Tall (both $TT$ and Tt): $\frac{3}{4}$
– Dwarf (tt): $\frac{1}{4}$

Thus, the phenotypic ratio in the F2 generation is 3:1 (Tall:Dwarf).

Key Concept: Codominance: Genotypic and Phenotypic Ratios

c) 3:1 (Tall:Dwarf)
[Solution Description]
The phenotypic ratio refers to the ratio of different phenotypes observed from a genetic cross. In this case, we have a monohybrid cross between two heterozygous tall plants (Tt). Using the information provided in the syllabus, the phenotypic ratio for Tall to Dwarf in such a cross is 3:1.

Key Concept: Phenotypic Expression

b) Multiple unrelated phenotypic expressions
[Solution Description] When a mutation in a gene with pleiotropic effects occurs, it can lead to multiple unrelated phenotypic expressions. This is due to the gene’s involvement in different biological functions or pathways, influencing various physical and physiological traits.

Key Concept: Application of Laws

c) 3:1
[Solution Description] According to Mendel’s Law of Segregation, each parent contributes one allele for each trait at random during reproduction. When crossing heterozygous parents (Aa x Aa), the Punnett Square shows possible combinations: AA, Aa, Aa, and aa. The phenotypic ratio, where ‘A’ is dominant and ‘a’ is recessive, is 3:1, representing three dominant phenotype expressions to one recessive.

Key Concept: Genetic Mutation Analysis, Genetic Linkage

a) 25%
[Solution Description] Sickle-cell anemia is an autosomal recessive disorder. For a child to inherit sickle-cell anemia, they must receive two recessive alleles (one from each parent). If both parents are carriers (heterozygous), the genetic cross can be represented as follows: Let’s denote $A$ as the normal allele and $a$ as the allele causing sickle-cell anemia. The genotype possibilities for the offspring are:
– AA: Normal (no symptoms)
– Aa: Carrier but not affected
– aa: Affected by sickle-cell anemia

Using a Punnett square, we calculate the probabilities:
$$
\begin{array}{c|cc}
& A & a \\
\hline
A & AA & Aa \\
a & Aa & aa \\
\end{array}
$$

From this grid, there is 1 chance in 4 $(\frac{1}{4})$ that the child will inherit the recessive alleles from both parents (aa), thus having sickle-cell anemia.

Key Concept: Independent Assortment

b) It allows for random distribution of maternal and paternal chromosomes into gametes.
[Solution Description] During meiosis, homologous chromosomes are distributed randomly into separate gametes. This means each pair of homologous chromosomes is sorted independently of other pairs, leading to a variety of combinations of maternal and paternal chromosomes. So, if an organism has two pairs of chromosomes (e.g., AaBb), they could produce gametes with AB, Ab, aB, or ab combinations due to the random arrangement, thus increasing genetic diversity.

Key Concept: Chromosome Behavior

a) Meiosis
[Solution Description] Sutton and Boveri noted that chromosomes behave in ways that parallel Mendel’s laws during meiosis, where homologous chromosomes segregate and assort independently.

Key Concept: Chromosomal Mutation Effects

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description]

To determine the correct response, we need to consider both statements separately and then analyze their connection.

Assertion (A) states that chromosomal mutations can affect phenotype by altering gene expression. This is true because changes in chromosome structure or number can influence the transcription and translation of genes, which ultimately affects the observable traits, or phenotype, of an organism.

Reason (R) states that mutations in regulatory regions of DNA can change how genes are expressed. This statement is also true because regulatory regions control the initiation and rate of transcription, and mutations in these areas can lead to increased, decreased, or altered expression patterns of genes.

Now, let’s evaluate if Reason (R) correctly explains Assertion (A). The reason provides a specific mechanism through which chromosomal mutations could alter gene expression: mutations in regulatory regions. Thus, Reason (R) does provide a valid explanation for why chromosomal mutations can alter gene expression and consequently phenotype.

Therefore, both Assertion and Reason are true, and Reason is indeed the correct explanation of Assertion.

Key Concept: Allele Expression

c) Assertion is true, but Reason is false.

[Solution Description] The assertion states that in individuals with the genotype $I^A I^B$, both A and B antigens are expressed. This is true because $I^A$ and $I^B$ are co-dominant alleles, meaning they both express their traits when present together, resulting in the AB blood group. The reason claims that the allele $i$ is co-dominant to $I^A$ and $I^B$. This statement is false as the allele $i$ is recessive, not co-dominant. It does not express any antigen if either $I^A$ or $I^B$ is present. Thus, the reason provided is incorrect. Therefore, while the assertion is true, the reason is false.

Key Concept: Simple Probability

c) 50%
[Solution Description] A carrier mother has one affected X chromosome and one normal X chromosome (X^cX). The father is normal with a XY genotype. The son’s probability of being affected depends solely on the mother’s X chromosome he inherits, because he will inherit a Y chromosome from his father. There’s a 50% chance he will inherit the affected X chromosome from his mother, which would make him color blind.

Key Concept: Disease Mechanism

c) Mutation in phenylalanine hydroxylase preventing metabolism of phenylalanine

[Solution Description] Phenylketonuria is caused by a mutation in the gene responsible for coding the enzyme phenylalanine hydroxylase. This enzyme deficiency results in the accumulation of phenylalanine, leading to various symptoms.

Key Concept: Phenotypic Ratios

a) 9:3:3:1
[Solution Description] For a dihybrid cross $TtRr \times TtRr$, we consider each trait separately and then combine them using the product rule. Height trait:

$$
Tt \times Tt \implies TT, Tt, tt \quad (\text{Phenotypic ratio for height: } 3:1)
$$

Seed shape trait:

$$
Rr \times Rr \implies RR, Rr, rr \quad (\text{Phenotypic ratio for seed shape: } 3:1)
$$

Using the product rule for independent assortment, multiply the phenotypic ratios:

$$\text{Tall Round : Tall Wrinkled : Short Round : Short Wrinkled = }$$

$$(3 \times 3):(3 \times 1):(1 \times 3):(1 \times 1) = 9:3:3:1$$

Hence, the phenotypic ratio is $9:3:3:1$.

Key Concept: Comparison with Humans

a) Females are heterogametic in birds, while males are heterogametic in humans.
[Solution Description] In the ZW-ZZ system found in birds, females are heterogametic with ZW chromosomes, whereas in the XY system in humans, males are heterogametic with XY chromosomes. This fundamental difference highlights the opposite roles of gender in heterogamety between these two systems.

Key Concept: Gamete Types

b) 4
[Solution Description] An organism with genotype RrYy can produce four types of gametes due to the independent assortment of alleles. The possible combinations are RY, Ry, rY, and ry, each occurring with equal frequency $\frac{1}{4}$.

Key Concept: Basic Definitions

c) ZW
[Solution Description] Birds exhibit female heterogamety, where females have one Z and one W chromosome, while males have two Z chromosomes. Therefore, in birds, females have ZW chromosomes.

Key Concept: Linkage Concept

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

[Solution Description] Both linkage and independent assortment concepts are fundamental in genetics. Genes that are physically close on the same chromosome tend to be inherited together, which is what linkage describes. This proximity reduces the chances that they will be recombined independently during meiosis, leading to traits being passed down together more frequently than if they were on separate chromosomes. Thus, both statements are true, and the reason correctly explains why linked genes do not assort independently.

Key Concept: Basic Concept

c) The equal expression of both alleles in the phenotype
[Solution Description] Codominance occurs when both alleles in a heterozygous organism contribute to the phenotype, resulting in the expression of both traits simultaneously. In the ABO blood group system, this is seen when both $I^A$ and $I^B$ alleles are present, leading to the AB blood type, which reflects characteristics of both A and B antigens.

Key Concept: Mechanisms

c) It allows advantageous traits to become more common over generations
[Solution Description] Natural selection acts on genetic variations within a population, allowing individuals with traits better suited for their environment to survive and reproduce more successfully. Over time, these advantageous traits become more common in the population, leading to adaptation.