Chapter 7: Inheritance

Select the correct answer:

1. Which of the following terms refers to the similarities between parents and their offspring?

A) Variation

B) Evolution

C) Heredity

D) Mutation

Answer: C) Heredity

Explanation: Heredity is the passing of traits from parents to their offspring. It explains why offspring resemble their parents. Variation refers to differences, evolution is change over generations, and mutation is a change in DNA.

2. Which part of the chromosome is responsible for protecting it from degradation?

A) Centromere

B) Telomere

C) Chromatid

D) Histone

Answer: B) Telomere

Explanation: Telomeres are repetitive DNA sequences at the ends of chromosomes that protect the chromosome from degradation, fusion, and damage. They act like the plastic tips on shoelaces.

3. What does each nucleosome consist of?

A) RNA molecules

B) DNA wrapped around eight histone proteins

C) Chromatids and centromeres

D) Telomeres and kinetochores

Answer: B) DNA wrapped around eight histone proteins

Explanation: A nucleosome is the basic unit of DNA packaging. It consists of a segment of DNA wrapped around a core of eight histone proteins (two each of H2A, H2B, H3, and H4).

4. What is the function of chromosomes in protein synthesis?

A) They produce energy for cells

B) They protect the cell from external damage

C) They carry genes that encode proteins

D) They replicate during cell division

Answer: C) They carry genes that encode proteins

Explanation: Chromosomes contain genes, which are segments of DNA that carry the instructions for building proteins. Protein synthesis begins with transcription of genes into mRNA.

5. Which of the following best defines a gene?

A) A segment of RNA that produces proteins

B) A sequence of amino acids

C) A unit of inheritance found in DNA

D) A type of chromosome

Answer: C) A unit of inheritance found in DNA

Explanation: A gene is a specific sequence of DNA that codes for a functional product, typically a protein. It is the basic unit of heredity.

6. Alleles are:

A) Different forms of a gene located at the same locus

B) Different species of organisms

C) Mutated chromosomes

D) Copies of RNA molecules

Answer: A) Different forms of a gene located at the same locus

Explanation: Alleles are alternative versions of a gene that occupy the same position (locus) on homologous chromosomes. They determine variations in a trait.

7. What is a genotype?

A) The visible traits of an organism

B) The genetic makeup of an organism

C) The protein synthesis process in an organism

D) The evolutionary history of a species

Answer: B) The genetic makeup of an organism

Explanation: Genotype refers to the specific combination of alleles an organism possesses for a particular trait. It is the genetic code, while phenotype is the physical expression.

8. In Mendelian genetics, a dominant allele:

A) Is always expressed when present

B) Is never expressed

C) Is only expressed in the presence of a recessive allele

D) Is the same as a recessive allele

Answer: A) Is always expressed when present

Explanation: A dominant allele masks the expression of a recessive allele. It is expressed in the phenotype whether the organism is homozygous dominant or heterozygous.

9. Mendel's conclusion that each parent contributes one factor (allele) for each trait is known as:

A) Law of Mutation

B) Law of Independent Assortment

C) Law of Segregation

D) Law of Evolution

Answer: C) Law of Segregation

Explanation: The Law of Segregation states that the two alleles for a trait separate during gamete formation, so each gamete carries only one allele. Offspring inherit one allele from each parent.

10. In a dihybrid cross, what was the phenotypic ratio Mendel observed in the F2 generation?

A) 3:1

B) 9:3:3:1

C) 1:1

D) 2:1

Answer: B) 9:3:3:1

Explanation: In a dihybrid cross (e.g., RrYy x RrYy), the F2 generation shows a phenotypic ratio of 9:3:3:1, where 9 shows both

dominant traits, 3 show one dominant and one recessive, 3 show the other dominant and recessive, and 1 shows both recessive traits.

11. Mendel's Law of Independent Assortment states that:

A) Alleles of different genes are inherited independently

B) Genes do not change over time

C) Dominant alleles are inherited together

D) All traits are linked to one another

Answer: A) Alleles of different genes are inherited independently

Explanation: The Law of Independent Assortment states that alleles of different genes assort independently during gamete formation. This applies to genes on different chromosomes or far apart on the same chromosome.

12. In a test cross, if all offspring display the dominant trait, the parent with the dominant phenotype is:

A) Heterozygous

B) Homozygous

C) Mutated

D) Recessive

Answer: B) Homozygous

Explanation: If an organism with a dominant phenotype is crossed with a homozygous recessive and all offspring show the dominant trait, the parent must be homozygous dominant (e.g., RR). If any offspring showed the recessive trait, the parent would be heterozygous.

13. What is the purpose of a test cross?

A) To determine the phenotype of an individual

B) To determine the genotype of an individual showing a dominant trait

C) To identify new mutations

D) To observe the effects of environmental factors on inheritance

Answer: B) To determine the genotype of an individual showing a dominant trait

Explanation: A test cross involves crossing an individual with a dominant phenotype (unknown genotype, could be RR or Rr) with a homozygous recessive individual (rr). The phenotypes of the offspring reveal whether the unknown parent was homozygous or heterozygous.

14. Which of the following is true of a homozygous genotype?

A) It contains two different alleles

B) It contains two identical alleles

C) It contains no alleles

D) It contains only one allele

Answer: B) It contains two identical alleles

Explanation: Homozygous means having two identical alleles for a particular gene (e.g., RR or rr). Heterozygous means having two different alleles (e.g., Rr).

15. What ratio did Mendel observe in the F2 generation of a monohybrid cross?

A) 1:1

B) 2:1

C) 3:1

D) 9:3:3:1

Answer: C) 3:1

Explanation: In a monohybrid cross (e.g., Tt x Tt), the F2 generation shows a phenotypic ratio of 3:1, where 3 show the dominant trait and 1 shows the recessive trait.

1. Why do chromosomes need to compact into thick chromatids during cell division?

Answer: Chromosomes need to compact into thick chromatids during cell division to prevent tangling and breakage. The compaction also makes it easier for the spindle fibers to attach to the centromeres and separate the sister chromatids accurately into daughter cells. This ensures the precise distribution of genetic material.

2. How does the structure of telomeres help protect genetic information during cell division?

Answer: Telomeres are repetitive, non-coding DNA sequences at the ends of chromosomes. They act as protective caps, preventing the chromosome ends from being recognized as broken DNA and fused with other chromosomes. They also prevent the loss of essential genes during DNA replication, as the replication machinery cannot copy the very ends of linear chromosomes.

3. How do Mendel's experiments with pea plants support the idea of dominant and recessive traits?

Answer: Mendel's experiments showed that when true-breeding plants with contrasting traits (e.g., tall and short) were crossed, the F1 generation showed only one trait (tall), which he called dominant. The other trait (short) disappeared in F1 but reappeared in the F2 generation in a 3:1 ratio. This supported the idea that traits are controlled by discrete factors (alleles) that can be dominant or recessive.

4. Why did Mendel use true-breeding plants in his experiments?

Answer: Mendel used true-breeding plants to ensure that the traits he was studying were stable and predictable. True-breeding plants are homozygous for a particular trait and produce offspring with the same trait over many generations. This allowed Mendel to control his experiments and clearly observe the patterns of inheritance.

5. How can two parents with free earlobes have a child with attached earlobes?

Answer: Free earlobes are dominant (E) and attached earlobes are recessive (e). If both parents are heterozygous (Ee), they have free earlobes but carry the recessive allele. There is a 25% chance that their child will inherit the recessive allele from both parents and have the genotype ee, resulting in attached earlobes.

6. How does the law of segregation generate genetic diversity in offspring?

Answer: The Law of Segregation states that the two alleles for a gene separate during gamete formation. This means each gamete carries only one allele. When gametes fuse during fertilization, the offspring receives one allele from each parent. The random combination of alleles from different gametes generates genetic diversity.

7. Why did Mendel's dihybrid cross results differ from his monohybrid cross results?

Answer: In a monohybrid cross, Mendel studied one trait and observed a 3:1 phenotypic ratio. In a dihybrid cross, he studied two traits simultaneously and observed a 9:3:3:1 ratio. The difference is because the dihybrid cross involves the independent assortment of two genes, resulting in more possible combinations of traits.

8. In pea plants, the allele for round seeds (R) is dominant over the allele for wrinkled seeds (r). If a heterozygous round-seeded plant (Rr) is crossed with a homozygous wrinkled-seeded plant (rr), what will be the genotypic and phenotypic ratios of their offspring?

Answer:

Cross: Rr x rr Gametes: R, r (from Rr) and r, r (from rr) Offspring Genotypes: 2 Rr : 2 rr (or 1 Rr : 1 rr) Offspring Phenotypes: 2 Round : 2 Wrinkled (or 1 Round : 1 Wrinkled)

9. If a true-breeding tall pea plant (TT) is crossed with a short pea plant (tt), what will be the genotype and phenotype of the F1 generation? What will be the genotypic and phenotypic ratios of the F2 generation if two F1 plants are crossed?

Answer:

F1 Generation: All offspring will be Tt (heterozygous). Phenotype: All tall. F2 Generation (Tt x Tt):

Genotypic Ratio: 1 TT : 2 Tt : 1 tt Phenotypic Ratio: 3 Tall : 1 Short

10. In humans, free earlobes (E) are dominant over attached earlobes (e). A person with free earlobes has a child with attached earlobes. What is the genotype of the parent with free earlobes, and what are the possible genotypes of the offspring?

Answer: For a child to have attached earlobes (ee), they must inherit one 'e' allele from each parent. Since the parent with free earlobes has a child with attached earlobes, that parent must carry the recessive allele. Therefore, the genotype of the parent with free earlobes is Ee (heterozygous). The other parent must be ee. The possible genotypes of the offspring are Ee (free

earlobes) and ee (attached earlobes).

11. Two pea plants, one with yellow round seeds (YYRR) and another with green wrinkled seeds (yyrr), are crossed. What are the phenotypic ratios in the F2 generation if the F1 generation self-fertilizes?

Answer:

F1 Generation: All offspring will be YyRr (yellow round). F2 Generation (YyRr x YyRr): The phenotypic ratio will be 9:3:3:1 (9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled).

12. In a dihybrid cross, two pea plants with the genotype RrYy are crossed. What is the probability (chance) that an offspring will have round, yellow seeds (RRYY) in the F2 generation?

Answer: The probability of getting a gamete with RY from one parent is 14. The probability of getting a gamete with RY from the

other parent is also 14. The probability of an offspring with the genotype RRYY is (14) × (14) = 116.

1. Describe the different parts of a chromosome, including chromatids, centromeres, and telomeres.

Answer: A chromosome is a thread-like structure composed of DNA and proteins. Its main parts are:

1. Chromatids:

Each chromosome consists of two identical sister chromatids. They are joined at the centromere. Each chromatid contains a single DNA molecule.

2. Centromere:

The constricted region where the two chromatids are held together. It is the attachment point for spindle fibers during cell division. The position of the centromere determines the shape of the chromosome (metacentric, submetacentric, acrocentric, telocentric).

3. Telomeres:

The ends of chromosomes. They are repetitive DNA sequences that protect the chromosome from degradation and fusion.

4. Chromatin:

The material of chromosomes, consisting of DNA and proteins (histones).

In summary, the chromosome is a highly organized structure essential for storing and transmitting genetic information.

2. Explain how DNA is organized within the chromosome, including the role of histone proteins and nucleosomes.

Answer: DNA is organized within the chromosome through a hierarchical process of compaction.

1. Nucleosome:

The fundamental unit of DNA packaging. Consists of 147 base pairs of DNA wrapped around an octamer of eight histone proteins (two each of H2A, H2B, H3, H4). H1 binds to the linker DNA between nucleosomes.

2. Chromatin Fiber:

Nucleosomes are coiled into a 30 nm fiber (solenoid).

3. Looped Domains:

The 30 nm fiber is organized into loops attached to a protein scaffold.

4. Condensed Chromosome:

During cell division, the loops are further coiled and condensed to form the visible chromosome.

Role of Histones:

Histones are positively charged proteins that bind to negatively charged DNA. They help package the long DNA molecules into compact structures.

In summary, DNA is organized into nucleosomes, which are further compacted to form chromosomes.

3. Describe Mendel's procedure of crossing a true-breeding round-seeded pea plant with a wrinkled-seeded plant. Also draw the diagram of the cross.

Answer: Procedure:

1. Mendel selected two true-breeding pea plants: one with round seeds (RR) and one with wrinkled seeds (rr).

2. He cross-pollinated the plants by transferring pollen from the round-seeded plant to the stigma of the wrinkled-seeded

plant.

3. He collected the seeds produced (F1 generation) and observed their shape.

4. He then allowed the F1 plants to self-pollinate to produce the F2 generation.

5. He counted the number of round and wrinkled seeds in the F2 generation.

Diagram:

P Generation: RR (Round) x rr (Wrinkled) Gametes: R and r F1 Generation: All Rr (Round) F1 Selfing: Rr x Rr F2 Generation: 1 RR : 2 Rr : 1 rr (Phenotypic ratio: 3 Round : 1 Wrinkled)

4. Explain the results of the F1 and F2 generations in monohybrid cross and describe the conclusions he drew regarding dominant and recessive traits.

Answer: F1 Generation Results:

When Mendel crossed true-breeding tall (TT) and short (tt) plants, all F1 offspring were tall (Tt). The short trait disappeared in the F1 generation.

F2 Generation Results:

When F1 plants were self-pollinated, the F2 generation showed both tall and short plants in a 3:1 ratio. The short trait reappeared.

Conclusions:

1. Dominant and Recessive Traits: Traits are controlled by discrete factors (alleles). The tall allele (T) is dominant, and the

short allele (t) is recessive.

2. Law of Segregation: The two alleles for a trait separate during gamete formation.

3. Law of Dominance: In a heterozygote, the dominant allele is expressed, masking the recessive allele.

In summary, Mendel's monohybrid cross demonstrated the principles of dominance and segregation.

5. Define genotype and phenotype, providing examples to illustrate how different genotypes (homozygous and heterozygous) can lead to different phenotypes.

Answer: Genotype: The genetic makeup of an organism, representing the specific combination of alleles for a trait. Phenotype: The observable physical or biochemical characteristics of an organism, resulting from the interaction of genotype and environment.

Examples:

Homozygous Dominant (TT): Phenotype is tall. Heterozygous (Tt): Phenotype is tall (because T is dominant). Homozygous Recessive (tt): Phenotype is short.

In summary, different genotypes can lead to the same phenotype (TT and Tt both tall), while the recessive phenotype only appears in the homozygous recessive genotype (tt).

6. Explain what a test cross is and how it can be used to determine whether an organism displaying a dominant trait is homozygous or heterozygous.

Answer:

Test Cross: A test cross is a breeding experiment used to determine the genotype of an individual with a dominant phenotype. The individual is crossed with a homozygous recessive individual.

How it Works:

If the individual is homozygous dominant (TT): All offspring will be tall (Tt). If the individual is heterozygous (Tt): 50% of offspring will be tall (Tt) and 50% will be short (tt).

Conclusion: By observing the phenotypes of the offspring, one can determine the genotype of the dominant parent. If any recessive offspring appear, the parent must be heterozygous.

In summary, a test cross is a valuable tool for determining the genotype of an organism with a dominant trait.