Inheritance — Short Questions

For a plant with genotype Rr YY, the possible gamete combinations are RY and rY, because the plant is heterozygous for the R gene (Producing R or r) but homozygous for the Y gene (only producing Y), resulting in two distinct types of gametes.

Mendel explained that each parent has two alleles of a gene. But a parent can only pass one allele to the offspring. During gamete formation, the alleles separate and there is only one allele in each gamete. When fertilization occurs, the offspring has the two alleles again. It is called the law of segregation. It states that "the alleles are separated during gamete formation and each gamete receives one or the other allele, but not both"

The F1 plants (RrYy), produced four types of gametes i.e., RY, Ry, rY, and ry. When these plants were allowed to self-pollinate, there were 16 combinations of alleles in F2 generation. It means all alleles R and r segregated independently of the alleles Y and y. Mendel's discovery is referred to as the law of independent assortment. It states that "alleles separate independently of one another during the formation of gametes"

a. Monohybrid Cross
• Definition: A cross between organisms different in only one heritable character or trait.
• Example: Crossing a pure tall pea plant (TT) with a pure dwarf pea plant (tt) to study stem length.
• F2 Phenotypic Ratio: 3:1

b. Dihybrid cross
Definition: A cross between organisms differing in two heritable characters or traits.

To draw the cross between round green (RRyy) pea and a wrinkled yellow (rr YY) pea (a dihybrid cross).
Parental (P) gametes (Ry and rY), resulting in all round yellow (Rr Yy) F1 offspring which then self-pollinate to produce the classic 9 round yellow: 3 round green: 3 wrinkled yellow: 1 wrinkled green (9:3:3:1) phenotypic ratio in the F2 generation, using a punnet square for the F2.

Genes and Alleles Genes: (i) Genes are segments of DNA. (ii) Each specific gene contains instructions for the synthesis of specific protein.
Alleles: (i) The alternative forms of a gene are called alleles. (ii) Gene occur in pairs on homologous chromosomes. Each member of a gene pair is called as allele.

Dominant and recessive Dominant Allele: In the heterozygous condition, one allele masks or prevents the expression of the other, such allele is called as dominant allele. Dominant alleles are represented by capital letters e.g. A and B
Recessive Allele: The allele which is not expressed is called as recessive allele. Recessive alleles are represented by lower case letters e.g. a and b

Genotype and phenotype Genotype: (i) The combination of the alleles is called genotype. (ii) It is internal, not visible. (iii) It passed directly from parents. (iv) Alleles (e.g., AA, Aa, aa)
Phenotype: (i) The observable outcome of genotype in the form of characteristics is called phenotype. (ii) It is external or measurable. (iii) It is inferred by both genes and environment. (iv) Traits (e.g: blue eyes, tall height, blood type).

F1 and F2 generations F1: (i) The first generation of offspring resulting from a cross between two distinct parent (P) plants, often with contrasting traits (e.g. homozygous dominant crossed with homozygous recessive) (ii) Example (pea plants) when two F1 tall (Tt) plants self-pollinate, the F2 generation results in approximately 3 tall plants for every 1 dwarf plant (TT, Tt, Tt, tt) bring back the trait.
F2: (i) The generation produced by crossing two F1 individuals (e.g. self-pollinating F1 plants or crossing F1 siblings) (ii) Example (pea plants) when two F1 tall (Tt) plants self-pollinate, the F2 generation results in approximately 3 tall plants for every 1 dwarf plant (TT, Tt, Tt, tt) bring back the trait.

Homozygous and heterozygous Homozygous: (i) A genetic makeup that consists of two identical alleles of a gene for a trait in a diploid organism. (ii) Produce single type of gametes. (iii) Two phenotypes are expressed. (iv) Self-breeding produces the same trait over the generations. (v) Present in true breeds. (vi) Homozygous dominant and homozygous recessive are the two types.
Heterozygous: (i) A genetic makeup that consists of two different alleles of a gene for a trait in a diploid organism. (ii) Produces two types of gametes. (iii) One phenotype is expressed. (iv) Self-breeding produces a combination of different traits. (v) Present in cross breeds. (vi) Complete dominance, incomplete dominance and codominance are the types.

Nucleus, Chromosome, Ribosome, Transcription, Translation, Protein

Chromatid, Centromere, Chromosome, Protein Histones, DNA

(i) Chromosomes need to compact into thick chromatids during cell division to ensure accurate segregation of genetic material to daughter cells. (ii) This compaction helps to prevent tangling and breakage of the long DNA strands, allowing them to be moved along the mitotic spindle without damage. Essentially, it makes the chromosomes easier to handle and distribute during the cell division process.

X chromosome is bigger than Y chromosome. Y chromosomes are about 1/3rd the size of X chromosome. The X chromosomes represent about the 5% of blood DNA in cells while Y chromosome represent about 2%.

DNA (Deoxyribonucleic Acid) i. DNA is a double-stranded, helical molecule
ii. It is made of deoxyribonucleotides.
iii. Each nucleotide of DNA is made of a deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, or guanine).
iv. The two strands are held together by base pairing (A with T, and C with G).

RNA (Ribonucleic Acid) i. RNA is single stranded chain of nucleotides.
ii. It is made of ribonucleotides.
iii. Its nucleotides have a sugar called ribose, a phosphate group, and nitrogenous bases (adenine, uracil, cytosine, and guanine).
iv. The two strands are held together by base pairing (A with U, and G with C).

(i) When he crossed two true breeding parent plants with contrasting traits, one trait consistently appeared in the first generation (F1) of offspring while the other trait is masked. (ii) Importantly, the masked trait (recessive) reappears in the second generation (F2) offspring in a predictable ratio, showing that traits are not blended but remain separate.

He used true-breeding plants (i) To ensure predictable and consistent inheritance patterns, allowing him to isolate and study the principles of inheritance.
(ii) By using plants where each generation consistently exhibited the same trait, Mendel could observe how traits were passed down from parent to offspring, leading to his discoveries about dominant and recessive traits.

The law of segregation ensures genetic diversity in offspring by ensuring that each gamete (sperm or egg cell) receives only one allele from each pair of alleles that an individual carries. This process, occurring during meiosis, leads to a variety of allele combinations in the offspring, contributing to the unique genetic makeup of each individual.

Dihybrid cross results differed from monohybrid cross results because dihybrid crosses consider two traits at the same time, while monohybrid crosses only consider one trait. This difference in the number of traits studied leads to distinct phenotypic ratios in the F2 generation. Monohybrid crosses typically produce a 3:1 phenotypic ratio, whereas dihybrid crosses typically produce a 9:3:3:1 phenotypic ratio.

If a heterozygous round-seeded plant (Rr) is crossed with homozygous wrinkled seeded plant (rr), then half of their offspring will be heterozygous round (Rr) and half of their offsprings will be homozygous wrinkled (rr).

If a tree breeding tall pea plant (TT) is crossed with a short pea plant (tt), we will have following results;

P1 Generation
Homozygous Tall (TT) × Homozygous short (tt)

Gametes T and t

F1 Generation
All plants with Round Yellow seeds

P2 Generation
F1 × F1, Tt × Tt

Gametes T t, T t

F1 Generation
Phenotypic ratio = All Tall (04 · 00)
Genotypic ratio = All heterozygous (04 · 00)

F2 Generation
Phenotypic ratio = 3:1
Genotypic ratio = 1:2:1

If a person with free earlobes has a child with attached earlobes, the parent with free earlobes must have the genotype Ee (heterozygous). The possible genotypes of the offspring are Ee (free earlobes) and ee (attached earlobes).

If F1 generation self-fertilizes to produce the F2 generation, four combinations will be formed:

• Round yellow
• Round green
• Wrinkled yellow
• Wrinkled green The genotypic ration will be 9:3:3:1.

In a dihybrid cross, the probability of discussion an offspring having round yellow seeds (RRYY) in the F2 generations is 1/16 or 6.25%. This is determined by the phenotypic ration of 9:3:3:1 observed in dihybrid crosses.

Self-fertilization is a type of reproduction where an organism's own sperm fertilizes its own eggs. This process commonly occurs in plants and some hermaphroditic animals. In self-fertilization, the offspring usually inherit genes from a single parent, leading to less genetic variation compared to cross-fertilization.

(i) AaBB
This genotype produces two types of gametes i.e. AB and aB.
(ii) aaBB
This genotype produces only one type of gamete i.e; aB.
(iii) AAbb
This genotype produces only one type of gamete i.e., Ab.

Genes control traits by providing DNA instructions to build proteins, which perform most cell functions, determining physical (eye colour, height) and bio chemical (blood type) characteristics (traits), there instructions are copied into RNA, then translated into amino acid chains that fold into specific proteins.

Different alleles of the same gene lead to different physical traits because they can result in variations in the structure or function of the protein that the gene codes for.