HeredityClass 10 Biology NCERT Solutions
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Q1E X E R C I S E S
A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as
(a)
TTWW
(b)
TTww
(c)
TtWW
(d)
TtWw
Solution
The correct option is (c) TtWW.
Explanation:
Let's denote the alleles as:
T = Tall, t = short
W = Violet, w = white
The short pea plant with white flowers has a genotype of ttww since both traits are recessive.
Analyzing the progeny:
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Flower Colour: All progeny have violet flowers. This means that the violet trait (W) is dominant over white (w). Since the short parent is 'ww', all progeny must inherit a 'w' allele from it. For all progeny to have violet flowers, they must all inherit a 'W' allele from the tall, violet-flowered parent. This is only possible if the tall parent is homozygous dominant for this trait, i.e., its genotype for flower colour is WW.
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Plant Height: Almost half of the progeny were short (tt). This 1:1 ratio of tall to short offspring occurs when one parent is heterozygous (Tt) and the other is homozygous recessive (tt). Since we know the short parent is 'tt', the tall parent must be heterozygous for height, i.e., its genotype for height is Tt.
Combining the genotypes for both traits, the genetic make-up of the tall parent is TtWW.
Q2E X E R C I S E S
A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?
Solution
No, on the basis of this information alone, we cannot definitively determine whether the light eye colour trait is dominant or recessive.
Reasoning:
- Case 1: If light eye colour is recessive. Parents with light-coloured eyes would both have a homozygous recessive genotype (e.g., ee). They can only pass on the recessive allele to their children, so all their children must have light-coloured eyes. This scenario is consistent with the finding.
- Case 2: If light eye colour is dominant. Parents with light-coloured eyes could be either homozygous dominant (EE) or heterozygous (Ee). If both parents are heterozygous (Ee), they can have children with both light eyes (EE or Ee) and dark eyes (ee), but it is statistically likely that most of their children will have light eyes (a 3 in 4 chance). This scenario is also consistent with the finding.
To determine dominance, we would need more information, such as observing the eye colour of children from parents where one has light eyes and the other has dark eyes, or knowing if two dark-eyed parents can have a light-eyed child (which would prove that light eye colour is recessive).
Q3E X E R C I S E S
Outline a project which aims to find the dominant coat colour in dogs.
Solution
A project to determine the dominant coat colour in dogs can be designed based on Mendelian principles.
Objective: To determine the dominant coat colour between two contrasting colours (e.g., black and brown) in a specific dog breed.
Procedure:
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Selection of Parents (P generation): Select a pure-breeding male dog with a black coat and a pure-breeding female dog with a brown coat. 'Pure-breeding' means that they come from a long line of dogs with the same coat colour and have consistently produced offspring of only that colour when bred with their own kind.
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First Cross (P generation F1 generation): Breed the selected black male and brown female. Observe the coat colour of all the puppies in the resulting litter (the F1 generation).
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Analysis of F1 Generation: According to Mendel's Law of Dominance, the trait that appears in the F1 generation is dominant. For example, if all the puppies are black, then black is the dominant coat colour and brown is recessive.
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Confirmation Cross (F1 generation F2 generation): To confirm the result, cross two individuals from the F1 generation (e.g., a brother and sister from the litter, both of whom will be heterozygous for coat colour).
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Analysis of F2 Generation: Observe the coat colours of the puppies in this second litter (the F2 generation). If the trait follows Mendelian inheritance, the dominant and recessive phenotypes should reappear in a ratio of approximately 3:1 (e.g., 3 black puppies for every 1 brown puppy). This result would confirm that black is the dominant coat colour.
Q4E X E R C I S E S
How is the equal genetic contribution of male and female parents ensured in the progeny?
Solution
The equal genetic contribution of male and female parents is ensured in the progeny through the process of sexual reproduction, which involves the formation of gametes and fertilisation.
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Chromosome Number: The body cells of both male and female parents are diploid, meaning they contain two complete sets of chromosomes. One set is inherited from their mother and one from their father.
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Gamete Formation (Meiosis): During the formation of reproductive cells (gametes – sperm in males and eggs in females), a special type of cell division called meiosis takes place. Meiosis halves the number of chromosomes, so each gamete becomes haploid, containing only one complete set of chromosomes.
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Fertilisation: When a male gamete (sperm) and a female gamete (egg) fuse during fertilisation, they form a zygote. This zygote receives one set of chromosomes from the father (via the sperm) and one set of chromosomes from the mother (via the egg).
This fusion restores the diploid number of chromosomes in the zygote, which then develops into the progeny. As a result, the offspring inherits an equal amount of genetic material from both the male and female parent, ensuring an equal genetic contribution.
Q1In-text Questions 1
If a trait A exists in of a population of an asexually reproducing species and a trait B exists in of the same population, which trait is likely to have arisen earlier?
Solution
In asexually reproducing species, variations arise due to inaccuracies in DNA copying and are passed on to subsequent generations. A new trait arises and then spreads through the population over time.
A trait that exists in a larger percentage of the population has had more time to accumulate and spread across more individuals through successive generations. Therefore, trait B, which exists in of the population, is likely to have arisen earlier than trait A, which is present in only of the population.
Q2In-text Questions 1
How does the creation of variations in a species promote survival?
Solution
The creation of variations in a species is crucial for its survival. Environments are subject to change, and these changes can include variations in climate, food availability, or the presence of new predators or diseases.
When variations exist within a species, there is a higher probability that some individuals will possess traits that make them better suited to survive and reproduce in the new environmental conditions. For example, if a heat wave occurs, bacteria with a variation that allows them to withstand heat will survive while others might die. These surviving individuals will reproduce and pass on the advantageous trait to their offspring. This process, known as natural selection, allows the species as a whole to adapt and avoid extinction, thus promoting its long-term survival.
Q1In-text Questions 2
How do Mendel's experiments show that traits may be dominant or recessive?
Solution
Mendel's experiments demonstrate the concept of dominant and recessive traits through his monohybrid crosses with pea plants.
- Parental Cross: Mendel crossed pure-breeding tall pea plants (TT) with pure-breeding short pea plants (tt).
- F1 Generation: He observed that all the plants in the first generation (F1) were tall. The trait for shortness seemed to have disappeared. This suggested that one trait (tallness) was masking the effect of the other (shortness).
- F2 Generation: When Mendel allowed the F1 generation plants (Tt) to self-pollinate, the resulting second generation (F2) had both tall and short plants in a ratio of approximately 3:1.
This reappearance of the short trait in the F2 generation proved that the trait was not lost but was merely unexpressed in the F1 generation. Mendel concluded that the trait expressed in the F1 generation is the dominant trait (tallness, 'T'), while the trait that is suppressed or hidden is the recessive trait (shortness, 't'). A recessive trait is only expressed when both copies of the gene are recessive (tt).
Q2In-text Questions 2
How do Mendel's experiments show that traits are inherited independently?
Solution
Mendel's dihybrid cross experiments show that traits are inherited independently.
- Parental Cross: Mendel crossed pea plants having two different contrasting traits. For example, he crossed plants with round, green seeds (RRyy) with plants having wrinkled, yellow seeds (rrYY).
- F1 Generation: All the plants in the F1 generation had round, yellow seeds (RrYy). This showed that round shape and yellow colour were dominant traits.
- F2 Generation: When he self-pollinated the F1 plants, the F2 generation produced plants with four different phenotypes: round yellow, round green, wrinkled yellow, and wrinkled green, in a ratio of approximately 9:3:3:1.
The appearance of new combinations of traits—round green and wrinkled yellow—which were not present in the parental generation, demonstrated that the inheritance of seed shape (round/wrinkled) was not linked to the inheritance of seed colour (yellow/green). Each trait was inherited independently of the other. This is known as Mendel's Law of Independent Assortment.
Q3In-text Questions 2
A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits - blood group A or O - is dominant? Why or why not?
Solution
Yes, this information is enough to determine which trait is dominant.
Reasoning:
- An individual inherits one allele (gene version) for a trait from each parent.
- The woman has blood group O. The only genotype for blood group O is homozygous recessive, which can be represented as 'ii'. Therefore, she can only pass on an 'i' allele to her child.
- The daughter also has blood group O, so her genotype must also be 'ii'.
- To have the 'ii' genotype, the daughter must have inherited one 'i' allele from her mother and one 'i' allele from her father.
- The father has blood group A, but he must carry the recessive 'i' allele to pass it to his daughter. Therefore, his genotype must be heterozygous, represented as ''.
Conclusion:
Since the father has both the allele for blood group A () and the allele for blood group O (i) but expresses blood group A, the allele for blood group A must be dominant over the allele for blood group O. Thus, blood group A is the dominant trait and blood group O is the recessive trait.
Q4In-text Questions 2
How is the sex of the child determined in human beings?
Solution
In human beings, the sex of a child is genetically determined by the sex chromosomes inherited from the parents.
- Sex Chromosomes: Humans have 23 pairs of chromosomes. One pair, the sex chromosomes, determines the sex. Females have a pair of identical sex chromosomes, XX. Males have a mismatched pair, XY.
- Gamete Formation: During gamete formation (meiosis), females produce eggs that all contain a single X chromosome. Males produce two types of sperm in equal proportions: half contain an X chromosome, and the other half contain a Y chromosome.
- Fertilisation: The sex of the child depends on which type of sperm fertilises the egg.
- If a sperm carrying an X chromosome fertilises the egg (X), the resulting zygote will have the genotype XX and will develop into a female.
- If a sperm carrying a Y chromosome fertilises the egg (X), the resulting zygote will have the genotype XY and will develop into a male.
Therefore, it is the genetic contribution from the father (whether an X or a Y chromosome is passed on) that determines the sex of the child.