1
A village in a remote hilly region primarily relied on traditional farming practices. Over generations, they noticed that a specific group of villagers consistently had darker skin tones compared to the rest. This darker skin tone was associated with better tolerance to the harsh sunlight during long hours of field work. However, a few younger individuals who had migrated to the city and adopted indoor lifestyles showed a slightly lighter skin tone.(a) Based on the information provided and your understanding of heredity, explain how the darker skin tone might have become prevalent in this specific group of villagers over generations. [2 marks](b) The scenario suggests that variations can impact survival. Explain how variations in skin tone could have provided an advantage to the villagers working in the fields. [2 marks](c) The younger individuals who moved to the city showed a slightly lighter skin tone. Does this suggest that skin tone is a trait that can be influenced by environmental factors, even if it has a genetic basis? Justify your answer. 1mark
2
In pea plants, the gene for round seeds (R) is dominant over the gene for wrinkled seeds (r), and the gene for yellow seeds (Y) is dominant over the gene for green seeds (y). A plant with genotype RrYy is self-pollinated.(a) If you were to perform a dihybrid cross experiment with this plant, what would be the expected phenotypic ratio of the offspring in the F2 generation? [1 mark](b) Explain the principle of inheritance that this ratio demonstrates. [2 marks](c) If 1600 offspring were produced in the F2 generation, approximately how many would you expect to have wrinkled seeds and green seeds? 1mark
3
Assertion (A): In sexually reproducing organisms, traits like height in plants are influenced by only one parent's genetic material.Reason (R): Each trait is controlled by factors (genes) that are inherited from both parents, and these factors may have different versions. 1mark
4
Consider a family where the father has attached earlobes and the mother has free earlobes. They have children, and it is observed that 50% of the children have attached earlobes and 50% have free earlobes.(a) If the gene for free earlobes is dominant (F) and the gene for attached earlobes is recessive (f), determine the genotype of the father and the mother. [2 marks](b) Explain why the children have a mix of attached and free earlobes, and what are the possible genotypes of the children. 2marks
5
A group of students observed that in their class, some students had attached earlobes while others had free earlobes. They decided to conduct a small survey among their families.(a) Outline a simple project the students could undertake to investigate the inheritance pattern of earlobe type within their families. What data would they need to collect? [3 marks](b) Based on their observations, can they definitively conclude whether free earlobes are dominant or recessive without further genetic information or crosses? Explain. 2marks
6
In many species of reptiles, the temperature at which the eggs are incubated determines the sex of the developing offspring. For example, in certain turtles, higher temperatures produce females, while lower temperatures produce males.(a) Explain why this phenomenon indicates that sex determination is not always genetically determined. [2 marks](b) Compare this method of sex determination with that in human beings, highlighting the primary difference. 2marks
7
Gregor Mendel, through his experiments on pea plants, laid the foundation for our understanding of heredity. He observed that certain traits were expressed in the first generation (F1), while others seemed to disappear only to reappear in the second generation (F2).(a) Explain Mendel's observation that in a cross between a tall pea plant and a short pea plant, all F1 progeny were tall. What does this observation suggest about the relationship between the genes for tallness and shortness? [2 marks](b) How did Mendel's experiment using the F1 progeny to produce F2 generation reveal the reappearance of the short trait? What ratio did he observe in the F2 generation for this trait? 2marks
8
Assertion (A): Asexual reproduction leads to greater diversity within a species compared to sexual reproduction.Reason (R): Asexual reproduction involves the fusion of gametes from two parents, introducing new combinations of genes. 1mark
9
A scientist is studying the inheritance of fur color in a newly discovered species of rodent. She crosses a true-breeding brown-furred rodent with a true-breeding white-furred rodent. All the offspring in the first generation (F1) are brown. When these F1 individuals are interbred, the second generation (F2) shows a ratio of 3 brown-furred rodents to 1 white-furred rodent.(a) Based on this experiment, which fur color is dominant and which is recessive? Justify your answer. [2 marks](b) What are the genotypes of the true-breeding brown-furred parent and the F1 offspring? 1mark
10
In humans, the sex of a newborn baby is determined by the chromosomes inherited from the parents.(a) Explain the role of the father's sex chromosomes in determining the sex of the child. [2 marks](b) Why is the pair of sex chromosomes in human males described as a "mismatched pair"? 1mark
11
A farmer is concerned about the potential health issues in a breed of chickens that has been interbred for many generations, resulting in very little genetic variation.(a) Explain how a lack of variation in a species can make it more vulnerable to environmental changes or diseases. [2 marks](b) How does sexual reproduction, as opposed to asexual reproduction, contribute to the generation of variations within a species? 2marks
12
Mendel conducted experiments where he crossed pea plants that differed in two traits simultaneously, such as seed shape and seed color.(a) Describe the outcome of Mendel's dihybrid cross in the F1 generation when crossing plants with round, yellow seeds and wrinkled, green seeds. [1 mark](b) What new combinations of traits were observed in the F2 generation, and what principle of inheritance do they demonstrate? 2marks
13
Assertion (A): The mechanism of heredity ensures that germ cells contain only one set of genes.Reason (R): This ensures the restoration of the normal number of chromosomes in the progeny during sexual reproduction, maintaining species stability. 1mark
14
A geneticist is studying a trait in a population that appears in approximately 25% of the individuals. They suspect it is a recessive trait.(a) If it is a recessive trait, what would be the genotypes of the individuals who exhibit the trait? [1 mark](b) If the frequency of the dominant allele is 0.5, and assuming simple Mendelian inheritance, what would be the expected frequency of individuals showing the dominant phenotype? 2marks
15
A couple is planning for a family. The husband has blood group B and the wife has blood group A. They are concerned about the possible blood groups of their children.(a) If both husband and wife are homozygous for their respective blood groups, what will be the blood group of all their children? [1 mark](b) If the wife is heterozygous for blood group A (genotype ) and the husband is heterozygous for blood group B (genotype ), what are the possible blood groups and their respective genotypes of their children? 2marks
16
In the context of genetics, the term "gene" and "allele" are often used.(a) Define the term 'gene' in your own words. [1 mark](b) Explain the difference between a dominant allele and a recessive allele using an example from the chapter. 2marks
17
A species of bird exhibits a distinct difference in beak shape between males and females, with males having larger, more robust beaks used for display and competition.(a) Explain how this sexual dimorphism in beak shape might have evolved over time through natural selection, considering reproductive success. [3 marks](b) If beak size in this bird species were influenced by genetics, what would happen if a male with a large beak and a female with a small beak produced offspring? 1mark
18
Assertion (A): The basic body design of an organism is inherited from the previous generation.Reason (R): Reproductive processes ensure that subtle variations are also passed on, leading to diversity among individuals. 1mark
19
A student is trying to understand how traits are passed down. They observe that a trait present in their grandmother is also seen in their father and themselves.(a) Using a relevant example from the chapter (like earlobes or pea plant traits), explain how this trait could have been inherited across three generations. [3 marks](b) What role does DNA play in this inheritance process? 1mark
20
Consider the statement: "All human chromosomes are not paired perfectly."(a) Explain the significance of the 'sex chromosomes' in this context, specifically referring to males and females. [2 marks](b) How does the inheritance of these sex chromosomes from parents determine the genetic sex of a child? 1mark
Answers
1
(a) The darker skin tone became prevalent due to natural selection. Individuals with darker skin had a survival advantage in the harsh sunlight, allowing them to work longer and more effectively, thus producing more offspring who inherited this trait. Over generations, this advantageous trait became more common in the group.(b) Variations in skin tone that provided better tolerance to sunlight would have allowed these villagers to survive and reproduce more successfully in their environment compared to those with less tolerant skin tones. This is an example of adaptation.(c) Yes, this suggests that environmental factors can influence the expression of a trait. While the genetic predisposition for skin tone is inherited, factors like prolonged exposure to sunlight can affect its appearance or degree of expression (e.g., tanning). The lighter skin tone in those who adopted indoor lifestyles indicates an environmental influence.
2
(a) The expected phenotypic ratio in the F2 generation of a dihybrid cross is 9:3:3:1 (9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green). 1mark(b) This ratio demonstrates Mendel's Law of Independent Assortment, which states that the alleles of two different genes separate independently of each other during gamete formation.(c) Total offspring = 1600. The proportion of wrinkled green seeds is 1/16. So, the number of wrinkled green seeded offspring would be .
3
Assertion (A) is false. Reason (R) is true. Proteins control traits, and genes are inherited from both parents. The trait of height, for instance, is influenced by multiple genes and parental contributions. 1mark
4
(a) The father has attached earlobes, which is a recessive trait. Therefore, his genotype must be 'ff'. The mother has free earlobes (dominant trait) and has 50% children with attached earlobes (ff). For her to have 'f' alleles to pass on, she must also carry the recessive allele. Thus, her genotype must be 'Ff'. 2marks(b) The father's genotype is ff and the mother's genotype is Ff. Their possible gametes are f (from father) and F or f (from mother). When these combine, the possible genotypes of the children are:f + F Ff (free earlobes)f + f ff (attached earlobes)This explains why 50% of the children have free earlobes (Ff) and 50% have attached earlobes (ff). [2 marks]
5
(a) The students could create a family tree for each student, mapping the earlobe type of parents, grandparents (if possible), and siblings. They would collect data on:
- Earlobe type of each student (free or attached).
- Earlobe type of their parents.
- Earlobe type of their siblings.
They would then try to identify patterns, such as whether attached earlobes appear only when both parents have them, or if they can appear when one parent has free earlobes. [3 marks]
(b) No, they cannot definitively conclude dominance or recessiveness solely from observation without performing controlled crosses or analyzing a large enough sample size to establish clear ratios. For example, if free earlobes were recessive, and attached earlobes were dominant, observing free-earlobes in parents and attached in children would indicate this. However, without controlled experiments, it's hard to be certain, especially if the trait is not strictly Mendelian or if there are incomplete penetrance issues. [2 marks]
6
(a) This phenomenon shows that the sex of the offspring is determined by an external environmental factor (temperature) rather than by specific genetic chromosomes inherited from parents that dictate sex. This contrasts with genetic determination where chromosomes carry the sex-determining information. 2marks(b) In human beings, sex is genetically determined by the sex chromosomes (X and Y). Females have XX chromosomes, and males have XY chromosomes. The father contributes either an X or a Y chromosome, thus determining the sex of the child. In reptiles, temperature dictates sex, not specific chromosomal combinations. [2 marks]
7
(a) Mendel observed that in the F1 generation, only one parental trait (tallness) was expressed, while the other (shortness) seemed to be masked. This suggests that the allele for tallness is dominant over the allele for shortness. The genetic makeup of the F1 plants contained both the 'tall' and 'short' factors, but only the dominant trait was visible. 2marks(b) When the F1 tall plants were self-pollinated, they produced F2 progeny where the short trait reappeared. Mendel observed that approximately one-quarter (1/4) of the F2 plants were short, while three-quarters (3/4) were tall. This indicated that the recessive trait (shortness) was present but unexpressed in the F1 generation and got expressed when the F1 plants reproduced. The ratio of tall to short plants in F2 was approximately 3:1. [2 marks]
8
Assertion (A) is false. Reason (R) is false. Sexual reproduction leads to greater diversity due to recombination and mixing of genes from two parents, while asexual reproduction produces offspring genetically identical to the parent (barring mutations). 1mark
9
(a) Brown fur color is dominant, and white fur color is recessive. This is because in the F1 generation, all offspring displayed the dominant trait (brown), indicating that the allele for brown fur masked the allele for white fur. The reappearance of the white fur trait in the F2 generation in a 3:1 ratio (dominant phenotype: recessive phenotype) confirms that the white fur allele was present but not expressed in F1. 2marks(b) The genotype of the true-breeding brown-furred parent would be BB (assuming B for brown is dominant). The genotype of the F1 offspring would be Bb, as they inherited one 'B' allele from the brown parent and one 'b' allele from the white parent. [1 mark]
10
(a) In humans, males have XY sex chromosomes, and females have XX sex chromosomes. The father contributes either an X chromosome or a Y chromosome to his child. If the child inherits an X chromosome from the father, it will be a girl (XX). If the child inherits a Y chromosome from the father, it will be a boy (XY). Thus, the father's sex chromosomes determine the sex of the child. 2marks(b) The pair of sex chromosomes in human males is described as a "mismatched pair" because it consists of one X chromosome and one Y chromosome (XY). These chromosomes differ in size and the genes they carry. In contrast, females have a "perfect pair" of two X chromosomes (XX). [1 mark]
11
(a) A lack of variation means that most individuals in the species have very similar genetic makeup. If a new disease emerges or the environment changes drastically, there may be no individuals with genetic traits that allow them to survive or resist the new conditions. This can lead to a high mortality rate and potentially threaten the survival of the entire species. 2marks(b) Sexual reproduction involves the fusion of gametes (sperm and egg) from two different parents. During meiosis, genetic recombination occurs, and during fertilization, genetic material from both parents is combined. This mixing and reassortment of genes lead to new combinations of traits in the offspring, thus generating variations within the species. [2 marks]
12
(a) In the F1 generation, when crossing plants with round, yellow seeds (e.g., RRYY) and wrinkled, green seeds (rryy), all the offspring had round and yellow seeds. This indicated that the alleles for round shape (R) and yellow color (Y) were dominant over the alleles for wrinkled shape (r) and green color (y). 1mark(b) In the F2 generation, new combinations of traits were observed: round seeds with green color, and wrinkled seeds with yellow color, in addition to the parental combinations of round yellow and wrinkled green. This demonstrates Mendel's Law of Independent Assortment, which states that factors (genes) for different traits segregate independently of each other during gamete formation. [2 marks]
13
Assertion (A) is true. Reason (R) is true. The reason correctly explains the assertion. 1mark
14
(a) If it is a recessive trait, individuals who exhibit the trait must have two copies of the recessive allele. Therefore, their genotype would be 'rr' (assuming 'r' is the recessive allele). 1mark(b) If the frequency of the dominant allele (let's say 'R') is 0.5, then the frequency of the recessive allele ('r') must also be 0.5 (assuming only two alleles for the trait in the population, and their frequencies add up to 1: p + q = 1). According to the Hardy-Weinberg principle, the frequency of the homozygous recessive genotype (rr) would be . The frequency of the heterozygous genotype (Rr) would be . The frequency of the homozygous dominant genotype (RR) would be .Individuals showing the dominant phenotype are those with genotypes RR and Rr. Their combined frequency is . So, approximately 75% of individuals would show the dominant phenotype. [2 marks]
15
(a) If the husband has blood group B (genotype BB) and the wife has blood group A (genotype AA), then all their children will inherit one 'B' allele from the father and one 'A' allele from the mother. Therefore, all their children will have the genotype AB and thus blood group AB. 1mark(b) Husband's genotype: . Wife's genotype: .Possible gametes from husband: .Possible gametes from wife: .Possible offspring genotypes and blood groups:
- (Blood group AB)
- (Blood group A)
- (Blood group B)
- (Blood group O)
So, the possible blood groups are AB, A, B, and O. [2 marks]
16
(a) A gene is a specific segment of DNA that carries the instructions for building a particular protein or performing a specific function. It represents a unit of heredity. 1mark(b) A dominant allele is an allele that expresses its trait even when only one copy is present in the genotype. For example, in pea plants, the allele for tallness (T) is dominant over the allele for shortness (t). A plant with genotype TT or Tt will be tall. A recessive allele, on the other hand, only expresses its trait when two copies of the allele are present in the genotype (i.e., in the homozygous state). For instance, a pea plant with the genotype tt will be short because the allele for shortness (t) is recessive. [2 marks]
17
(a) Sexual selection can drive the evolution of sexual dimorphism. In this case, males with larger, more robust beaks might be more successful at attracting mates or defending territories, leading to greater reproductive success. If this beak size is heritable, the genes for larger beaks in males will be passed on to more offspring, leading to an increase in the average beak size in males over generations, while females may retain smaller beaks if they do not confer a similar advantage or if there is a trade-off. 3marks(b) If beak size is genetically determined, the offspring would likely inherit a combination of genes from both parents. Depending on the inheritance pattern (e.g., dominance, codominance, or polygenic inheritance), the offspring might have intermediate beak sizes, or express traits from one parent more strongly than the other. For instance, if larger beak size is dominant, the offspring might have larger beaks. [1 mark]
18
Assertion (A) is true. Reason (R) is true. The reason explains how variations, alongside the basic body design, are passed on through reproduction. 1mark
19
(a) Let's consider the example of earlobe type. Suppose free earlobes (F) are dominant and attached earlobes (f) are recessive.
- Grandmother: Could have genotype Ff (free earlobes, passes 'f' to father) or ff (attached earlobes, passes 'f' to father).
- Father: If he inherited 'f' from his mother and 'F' from his other parent (or his mother if she was Ff), he could have genotype Ff (free earlobes). If he inherited 'f' from both parents (less likely given the grandmother scenario), he could be ff. Assuming he has free earlobes (Ff) and passes 'f' to his child.
- Student: Inherits 'f' from the father and could inherit 'F' or 'f' from the mother. If the mother also contributes 'F', the student would be Ff (free earlobes). If the student inherited 'f' from both parents, they would have attached earlobes (ff).
This illustrates how a trait can be passed down across generations, with dominant traits potentially masking recessive ones until they reappear when an individual inherits two copies of the recessive allele. [3 marks]
(b) DNA contains the genetic information (genes) that codes for proteins. These proteins are responsible for various characteristics and traits. Inheritance occurs because DNA is replicated and passed from parents to offspring through gametes, carrying the blueprints for these traits. [1 mark]
20
(a) In humans, females have a pair of identical sex chromosomes, XX. Males have a pair of non-identical sex chromosomes, XY. The Y chromosome carries genes that initiate the development of male characteristics. The presence of a Y chromosome from the father determines the genetic sex as male. 2marks(b) A child inherits one sex chromosome from their mother (always an X chromosome) and one sex chromosome from their father. If the father passes on an X chromosome, the child's genotype will be XX, making it genetically female. If the father passes on a Y chromosome, the child's genotype will be XY, making it genetically male. [1 mark]
