Heredity and Evolution — Important Questions

Answers

• F1 Generation: All plants were tall, with the genotype Tt. This shows that the allele for tallness (T) is dominant over the allele for shortness (t).
• F2 Generation: When F1 plants (Tt) were self-pollinated, the F2 generation showed both tall and short plants. The genotypes were TT, Tt, and tt in the ratio 1:2:1. The phenotypes were tall (TT and Tt) and short (tt) in the ratio 3:1. This demonstrates the principle of segregation.
  2. The probability of having a boy or a girl is approximately 50% each, irrespective of the sex of previous children. This is because the sex of a child is determined by the type of sex chromosome (X or Y) inherited from the father. The mother always contributes an X chromosome. If the father contributes an X chromosome, the child will be a girl (XX). If the father contributes a Y chromosome, the child will be a boy (XY). Therefore, the sex of each child is an independent event.
  3. (a) Genotype of F1 generation: Tt. Phenotype of F1 generation: Tall.
  (b) In the F2 generation, the genotypic ratio is TT:Tt:tt = 1:2:1. The phenotypic ratio of tall plants to short plants is 3:1.
  (c) This illustrates Mendel's Law of Segregation and the concept of dominance and recessiveness.
  4. Variations arise during reproduction due to:
• Asexual Reproduction: Small inaccuracies during DNA copying.
• Sexual Reproduction: Combination of genetic material from two parents and crossing over during gamete formation.
  Importance for survival: Variations provide different traits that might be advantageous in a changing environment. For example, if the environment becomes hotter, individuals with variations that allow them to tolerate heat will survive better, ensuring the species' continuity.
  5. Mendel's systematic approach involved:
• Controlled Crosses: He carefully selected pea plants with distinct contrasting traits (e.g., tall/short, round/wrinkled seeds).
• Quantitative Analysis: He meticulously counted the number of plants exhibiting each trait in successive generations (F1, F2, etc.).
• Mathematical Ratios: He analyzed these counts to derive consistent mathematical ratios (e.g., 3:1 in monohybrid crosses, 9:3:3:1 in dihybrid crosses).
  This quantitative and statistical approach allowed him to identify patterns of inheritance and formulate his laws, which were a significant departure from earlier qualitative observations.
  6. In human beings, sex determination is based on sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
• The mother always contributes an X chromosome to her offspring.
• The father contributes either an X chromosome (resulting in a female child, XX) or a Y chromosome (resulting in a male child, XY).
• Diagram:
  Gametes from Mother: X, X
  Gametes from Father: X, Y
  Possible Zygotes:
  Mother's X + Father's X = XX (Girl)
  Mother's X + Father's Y = XY (Boy)
  Therefore, the sex of the child is determined by the chromosome inherited from the father.
  7. (a) Genotype of F1 generation: TtRr
  (b) Phenotype of F1 generation: Tall with round seeds
  (c) In the F2 generation, the genotypes will be TTRR, TTRr, TtRR, TtRr, TTrr, Ttrr, ttRR, ttRr, ttrr. The phenotypic ratio of tall round : tall wrinkled : short round : short wrinkled is 9:3:3:1.
  8. Dominant Trait: A trait that is expressed even when only one copy of the corresponding allele is present. Example: In pea plants, the allele for tallness (T) is dominant over the allele for shortness (t). So, plants with genotypes TT and Tt are tall.
  Recessive Trait: A trait that is expressed only when both copies of the corresponding allele are present. Example: In pea plants, the allele for shortness (t) is recessive. So, only plants with genotype tt are short.
  9. Temperature-dependent Sex Determination: In some reptiles (like turtles and crocodiles), the incubation temperature of the eggs determines the sex of the developing embryo. Different temperature ranges trigger the development of either male or female reproductive organs. This shows that sex is not always genetically determined.
  Contrast with Human Sex Determination: In human beings, sex is determined by sex chromosomes (X and Y) inherited from parents, not by environmental factors like temperature.
  10. Mendel's experiments on the inheritance of two pairs of traits (dihybrid cross) involved crossing pea plants differing in two characteristics, e.g., seed shape (round/wrinkled) and seed colour (yellow/green).
• He crossed plants with pure round yellow seeds (RRYY) with plants with pure wrinkled green seeds (rryy).
• The F1 generation showed all round yellow seeds (RrYy).
• When F1 plants were self-pollinated, the F2 generation exhibited four types of phenotypes: round yellow, round green, wrinkled yellow, and wrinkled green.
• The ratio of these phenotypes was 9:3:3:1.
• This led to the conclusion that the factors (genes) for seed shape and seed colour assort independently of each other during gamete formation, meaning the inheritance of one trait does not influence the inheritance of the other. This is Mendel's Law of Independent Assortment.
  11. Haemophilia is a sex-linked recessive trait, meaning the gene is located on the X chromosome. Let $X^H$ be the allele for normal blood clotting and $X^h$ be the allele for haemophilia. The Y chromosome does not carry the gene for haemophilia.
• Father's genotype: $X^{H}Y$ (normal male)
• Mother's genotype: $X^H{X}^h$ (carrier female)
  Possible offspring genotypes:
• $X^H{X}^H$ (Normal daughter)
• $X^H{X}^h$ (Carrier daughter)
• $X^{H}Y$ (Normal son)
• $X^{h}Y$ (Affected son)
  The probability of their son being affected by haemophilia ($X^{h}Y$) is 1 out of 2 (50%).
  The probability of their daughter being a carrier ($X^H{X}^h$) is 1 out of 2 (50%).
  12. Genes are segments of DNA that provide the information for making specific proteins. Proteins, in turn, perform various functions in the cell, thereby controlling characteristics.
  For example, in plant height:
• A gene controls the production of a specific plant hormone responsible for growth.
• If the gene codes for an enzyme that efficiently produces this hormone, the plant will be tall.
• If an alteration in the gene leads to a less efficient enzyme, less hormone is produced, resulting in a short plant.
  Thus, the gene influences the characteristic of plant height by controlling protein (enzyme) synthesis.
  13. Let 'F' represent the allele for free earlobes (dominant) and 'f' represent the allele for attached earlobes (recessive).
• Man with attached earlobes has genotype: ff
• Woman with free earlobes: Since her children can have attached earlobes, she must be a carrier of the recessive allele. Her genotype is Ff.
• Cross: ff x Ff
• Possible offspring genotypes: Ff (free earlobes) and ff (attached earlobes).
• If 50% of their children have free earlobes and 50% have attached earlobes, this confirms the genotypes of the parents.
• Genotype of father: ff
• Genotype of mother: Ff
• Possible genotypes of children: Ff, ff.
  14. Assertion (A): Asexual reproduction produces more variation than sexual reproduction. False. Reason (R): DNA copying is highly accurate in asexual reproduction. True.
  Explanation: Sexual reproduction generally leads to greater variation due to the mixing of genetic material from two parents and crossing over, while asexual reproduction relies on mutations and DNA copying errors, which typically produce fewer variations.
  15. Germ cells (sperm and egg) are specialized reproductive cells that carry only half the set of chromosomes (one from each pair) from the parent organism. During fertilization, when a male germ cell fuses with a female germ cell, they combine to form a zygote with the complete diploid number of chromosomes. This ensures that:
• The characteristic number of chromosomes for the species is maintained across generations.
• Each new individual receives a unique combination of genes from both parents, contributing to genetic diversity.
  This mechanism prevents the doubling of chromosomes in each generation and maintains genetic stability.
  16. (a) The trait of red flowers is dominant, and the trait of white flowers is recessive.
  (b) Let 'R' be the allele for red flowers (dominant) and 'r' be the allele for white flowers (recessive).
• Purebred red plant genotype: RR
• Purebred white plant genotype: rr
• F1 generation genotype: Rr (all red flowers)
• If F1 plants (Rr) are self-pollinated, the F2 generation will have genotypes RR, Rr, and rr in the ratio 1:2:1.
• The phenotypes will be red flowers (from RR and Rr) and white flowers (from rr). The ratio of red-flowered plants to white-flowered plants in the F2 generation will be 3:1.
  17. Genes: Genes are specific segments of DNA that code for a particular trait or characteristic. They are the functional units of heredity.
  Chromosomes: Chromosomes are thread-like structures found inside the nucleus of a cell, composed of DNA tightly coiled around proteins. They carry genes. Humans have 23 pairs of chromosomes.
  Role in Heredity: Genes located on chromosomes are passed from parents to offspring. Each individual inherits one set of chromosomes (and thus one set of genes) from each parent. These inherited genes determine the traits and characteristics of the offspring. Chromosomes act as carriers of genetic information in the form of genes.
  18. Mode of Inheritance: Autosomal Dominant.
  Justification:
• The trait appears in every generation (from generation I to II and II to III). This is characteristic of dominant inheritance.
• Both males and females are affected, and it is passed from affected parents to offspring, suggesting it is not sex-linked.
• In generation I, an affected father and unaffected mother have affected children, which is consistent with a dominant trait.
• In generation II, an affected mother and unaffected father have unaffected children. This is possible if the affected mother is heterozygous (carries one dominant allele and one recessive allele) and passes the recessive allele to her unaffected children.
  19. Genetic variation is crucial for the long-term survival of a species because it increases the probability that some individuals within a population will possess traits that are advantageous in a changing environment. If the environment changes (e.g., due to climate change, new diseases, or predators), individuals with favourable variations are more likely to survive and reproduce, passing on these advantageous traits to their offspring. This allows the population to adapt and persist over time, preventing extinction.
  20. One gene can control more than one trait through a phenomenon called pleiotropy. This occurs when a single gene affects multiple phenotypic characteristics. For example, the gene responsible for phenylketonuria (PKU) in humans, when mutated, not only leads to impaired cognitive development but also affects skin and hair pigmentation. Another example is the gene controlling pigment production in mice, which influences coat colour, eye colour, and susceptibility to certain diseases.