Question 1

Mention the advantages of selecting pea plant for experiment by Mendel.

Accepted Answer

Mendel selected pea plants for his experiments because they have several advantages:
- Pea plants have easily distinguishable traits (such as flower color, seed shape).
- They have a short life cycle, allowing multiple generations in a short time.
- Pea plants can self-pollinate as well as cross-pollinate, enabling controlled breeding.
- They produce a large number of offspring, providing sufficient data for statistical analysis.
- The traits studied are inherited in a clear dominant-recessive pattern, simplifying analysis. — These advantages allowed Mendel to observe clear patterns of inheritance and formulate his laws of genetics.

Question 2

Differentiate between the following -
(a) Dominance and Recessive
(b) Homozygous and Heterozygous
(c) Monohybrid and Dihybrid.

Accepted Answer

(a) Dominance and Recessive:
- Dominance: The allele that expresses its trait even when present in a single copy (heterozygous condition).
- Recessive: The allele whose trait is masked in the presence of a dominant allele and only expressed when homozygous.

(b) Homozygous and Heterozygous:
- Homozygous: An organism having two identical alleles for a particular gene (e.g., AA or aa).
- Heterozygous: An organism having two different alleles for a particular gene (e.g., Aa).

(c) Monohybrid and Dihybrid:
- Monohybrid: A cross involving a single pair of contrasting traits (one gene locus).
- Dihybrid: A cross involving two pairs of contrasting traits (two gene loci). — These distinctions are fundamental to understanding inheritance patterns and genetic crosses.

Question 3

A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?

Accepted Answer

If an organism is heterozygous for 4 loci, each locus has two different alleles.
The number of types of gametes produced = 2^n, where n = number of heterozygous loci.
Here, n = 4.
Therefore, number of gametes = 2^4 = 16.

Hence, 16 types of gametes can be produced. — Each heterozygous locus can produce two types of alleles, and the gametes receive one allele from each locus independently, so total combinations are 2^4 = 16.

Question 4

Explain the Law of Dominance using a monohybrid cross.

Accepted Answer

Law of Dominance states that when two contrasting traits are crossed, only one trait (dominant) is expressed in the F1 generation, while the other (recessive) remains hidden.

Using a monohybrid cross of pea plants:
- Cross pure tall (TT) with pure dwarf (tt).
- F1 generation: All plants are tall (Tt) because tall (T) is dominant.
- F2 generation (Tt x Tt): Phenotypic ratio is 3 tall : 1 dwarf.

This shows that the dominant trait masks the recessive trait in heterozygous condition. — The monohybrid cross demonstrates that dominant alleles express their trait over recessive alleles in heterozygotes, confirming the Law of Dominance.

Question 5

Define and design a test-cross.

Accepted Answer

Test-cross is a cross between an individual showing the dominant phenotype (but unknown genotype) and a homozygous recessive individual.

Purpose: To determine the genotype of the dominant phenotype individual.

Design:
- Cross the individual with dominant phenotype (genotype could be homozygous dominant or heterozygous) with a homozygous recessive individual.
- Analyze the offspring:
  - If any offspring show recessive phenotype, the tested individual is heterozygous.
  - If all offspring show dominant phenotype, the tested individual is homozygous dominant. — Test-cross helps in identifying the genotype of an organism showing dominant phenotype by crossing it with a homozygous recessive individual and observing the phenotypes of progeny.

Question 6

Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.

Accepted Answer

Let the gene locus have alleles A (dominant) and a (recessive).
Female genotype: AA (homozygous dominant)
Male genotype: Aa (heterozygous)

Punnett Square:
          A       a
    A |  AA     Aa
    A |  AA     Aa

Genotypes of offspring:
- 50% AA
- 50% Aa

Phenotypes:
- Both AA and Aa show dominant phenotype.
- Therefore, 100% offspring show dominant phenotype. — Crossing homozygous dominant female with heterozygous male results in all offspring showing dominant phenotype, with genotypes split equally between homozygous dominant and heterozygous.

Question 7

When a cross in made between tall plant with yellow seeds (TtYy) and tall plant with green seed (Ttyy), what proportions of phenotype in the offspring could be expected to be
(a) tall and green.
(b) dwarf and green.

Accepted Answer

Given:
Parent 1 genotype: TtYy (tall, yellow seeds)
Parent 2 genotype: Ttyy (tall, green seeds)

Genes:
T = tall (dominant), t = dwarf (recessive)
Y = yellow seeds (dominant), y = green seeds (recessive)

Step 1: Determine gametes
Parent 1 (TtYy) gametes: TY, Ty, tY, ty
Parent 2 (Ttyy) gametes: Ty, ty

Step 2: Make Punnett square with 4 x 2 = 8 combinations

Offspring genotypes and phenotypes:
1) TY x Ty = TTYY (tall, yellow)
2) TY x ty = TTYy (tall, yellow)
3) Ty x Ty = TTYy (tall, yellow)
4) Ty x ty = TTyy (tall, green)
5) tY x Ty = TtYY (tall, yellow)
6) tY x ty = TtYy (tall, yellow)
7) ty x Ty = TtYy (tall, yellow)
8) ty x ty = Ttyy (tall, green)

Note: All offspring have at least one T allele, so all are tall.

But question asks for dwarf and green (dwarf = tt), so no dwarf offspring expected.

Phenotype proportions:
(a) Tall and green: TTyy and Ttyy genotypes
From above, TTyy (1/8), Ttyy (1/8) = 2/8 = 1/4
(b) Dwarf and green: tt yy genotype
No tt genotype in offspring, so 0.

Answer:
(a) Tall and green = 1/4
(b) Dwarf and green = 0 — By determining gametes and crossing them, we find all offspring have at least one dominant T allele (tall), so no dwarf offspring. Green seed phenotype requires yy genotype, which occurs in 1/4 of offspring.

Question 8

Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in  $[1mF_1[0m$  generation for a dihybrid cross?

Accepted Answer

When two loci are linked, the genes are located close together on the same chromosome and tend to be inherited together.

Given: Both parents heterozygous for two loci (e.g., AaBb x AaBb), but loci are linked.

In linked genes, the expected phenotypic ratio deviates from the classic 9:3:3:1 ratio of independent assortment.

The phenotypic distribution depends on the recombination frequency:
- If no crossing over, only parental combinations appear.
- If crossing over occurs, recombinant phenotypes appear but less frequent.

Therefore, the F1 generation will have more parental type phenotypes and fewer recombinant phenotypes.

Exact distribution depends on recombination frequency, but the key point is linked genes do not assort independently, altering phenotypic ratios. — Linked genes violate Mendel's law of independent assortment, leading to phenotypic ratios skewed towards parental types in F1 generation.

Question 9

Briefly mention the contribution of T.H. Morgan in genetics.

Accepted Answer

T.H. Morgan's contributions:
- Discovered sex-linked inheritance through experiments with fruit flies (Drosophila melanogaster).
- Provided evidence that genes are located on chromosomes.
- Demonstrated the concept of gene linkage and recombination.
- Developed the chromosomal theory of inheritance.
- His work laid the foundation for modern genetics. — Morgan's experiments established the chromosomal basis of inheritance and explained how genes are transmitted through chromosomes.

Question 10

What is pedigree analysis? Suggest how such an analysis, can be useful.

Accepted Answer

Pedigree analysis is the study of inheritance patterns of traits through several generations of a family using a pedigree chart.

It is useful to:
- Trace the inheritance of genetic disorders.
- Determine whether a trait is dominant, recessive, autosomal, or sex-linked.
- Predict the probability of offspring inheriting a trait.
- Assist in genetic counseling. — Pedigree charts help visualize family relationships and trait inheritance, aiding in understanding and managing genetic conditions.

Question 11

How is sex determined in human beings?

Accepted Answer

Sex determination in humans is based on the presence of sex chromosomes:
- Females have two X chromosomes (XX).
- Males have one X and one Y chromosome (XY).

During reproduction, the female produces eggs with an X chromosome.
The male produces sperm carrying either an X or a Y chromosome.
If the sperm carrying X fertilizes the egg, the offspring is female (XX).
If the sperm carrying Y fertilizes the egg, the offspring is male (XY). — The presence or absence of the Y chromosome determines the sex of the offspring in humans.

Question 12

A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.

Accepted Answer

Blood group alleles: IA, IB (codominant), and i (recessive).

Child's blood group: O (ii genotype).

Father's blood group: A (could be IAIA or IAi).
Mother's blood group: B (could be IBIB or IBi).

Since child is ii, both parents must carry i allele.
Therefore, father genotype: IAi
Mother genotype: IBi

Possible offspring genotypes from IAi x IBi cross:
- IAIB (blood group AB)
- IAi (blood group A)
- IBi (blood group B)
- ii (blood group O)

Phenotypic ratio:
- 1 AB : 1 A : 1 B : 1 O — Parents must be heterozygous to produce a child with blood group O. The cross shows all four blood groups are possible in offspring.

Question 13

Explain the following terms with example
(a) Co-dominance
(b) Incomplete dominance

Accepted Answer

(a) Co-dominance:
Both alleles in a heterozygote are fully expressed, resulting in a phenotype that shows both traits simultaneously.
Example: Human blood group AB, where both A and B alleles are expressed.

(b) Incomplete dominance:
The heterozygote shows an intermediate phenotype between the two homozygotes.
Example: In Mirabilis jalapa (four o'clock plant), crossing red flower (RR) with white flower (rr) produces pink flowers (Rr). — Co-dominance shows both traits together, while incomplete dominance shows blending of traits.

Question 14

What is point mutation? Give one example.

Accepted Answer

Point mutation is a change in a single nucleotide base in the DNA sequence.

Example: Sickle cell anemia is caused by a point mutation in the beta-globin gene where adenine is replaced by thymine, resulting in substitution of valine for glutamic acid. — Point mutations can alter protein structure and function, leading to genetic disorders.

Question 15

Who had proposed the chromosomal theory of the inheritance?

Accepted Answer

The chromosomal theory of inheritance was proposed by Walter Sutton and Theodor Boveri independently in the early 20th century. — They proposed that genes are located on chromosomes, which segregate during meiosis, explaining Mendel's laws.

attention in biology for the next century. The entire body of

attention in biology for the next century. The entire body of — Study Notes

4.1 Mendel's Laws of Inheritance

4.2 Inheritance of One Gene

4.2.2.1 Incomplete Dominance

Practice Questions — attention in biology for the next century. The entire body of

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