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Summary
Reproduction creates new living things. These new beings are like their parents but also a little different. Even when only one parent is involved, small changes, called variations, happen. When two parents are involved, like in humans, these variations are much more noticeable. This chapter explains how these variations happen and how they are passed down.
When living things reproduce, the new generation gets a basic body plan from its parents. It also gets small changes. This new generation will then reproduce and pass on its own mix of old and new changes. If a single germ, like a bacterium, divides, its children will be very similar. This is because the main way changes happen is through tiny mistakes when their information code, DNA, is copied. But if two parents are involved, much more variety is created. Not all these changes help the new being survive. Some changes might be helpful in certain surroundings, and these help the living thing to live better.
Parents pass down their features, or traits, to their children. A child has basic human features but is not an exact copy of its parents. People show many differences. Both the father and mother give almost equal amounts of their genetic information to the child. This means every trait a child has can be shaped by information from both parents. So, for each trait, a child has two versions of instructions.
A scientist named Mendel studied how traits are passed on using pea plants. He found that if he mixed a tall plant and a short plant, all their children in the first group were tall. There were no medium plants. This meant the “tall” instruction was stronger. When these tall children plants made their own seeds, some of their offspring were short. This showed that the “short” instruction was still there, just hidden in the first group. Mendel suggested that there are two “factors” (now called genes) for each trait. One factor can be stronger, or dominant, and hide the effect of the weaker, or recessive, factor. For a plant to be short, it needed two “short” factors.
Mendel also studied plants with two different traits, like tall plants with round seeds and short plants with wrinkled seeds. He found that these traits were passed down independently. A plant could be tall with wrinkled seeds, or short with round seeds, showing new combinations. This happens because the information for traits is carried on separate units called chromosomes. Each parent gives one chromosome from each pair to its offspring. When the parents’ cells combine, the offspring gets the usual number of chromosomes.
The information for making us who we are is in our DNA. A part of DNA that holds instructions for one protein is a gene. Proteins control our traits. For example, plant height is controlled by hormones, which are made by enzymes. If a gene makes an efficient enzyme, the plant grows tall. If the gene is changed and the enzyme is less efficient, the plant is short.
In humans, sex is decided by genes. Most of our chromosomes come in pairs. We have 22 such pairs. One special pair, the sex chromosomes, decides if we are male or female. Females have two X chromosomes (XX). Males have one X and one Y chromosome (XY). A mother always passes an X chromosome to her child. If the father passes an X chromosome, the child is a girl (XX). If the father passes a Y chromosome, the child is a boy (XY).
Textbook solutions
Intext Questions and Answers I
1. If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?
Answer: Trait B is likely to have arisen earlier. This is because variations are created and inherited, leading to an accumulation of these variations over succeeding generations within a population. A trait that is present in a larger percentage of the population, such as trait B at 60%, suggests it has been inherited and has accumulated through more generations compared to trait A, which exists in only 10% of the population.
2. How does the creation of variations in a species promote survival?
Answer: The creation of variations in a species promotes survival because, depending on the nature of these variations, different individuals would have different kinds of advantages in the environment in which they find themselves. For instance, bacteria that can withstand heat, which is a type of variation, will survive better in a heat wave. The selection of such advantageous variants by environmental factors forms the basis for evolutionary processes.
Intext Questions and Answers II
1. How do Mendel’s experiments show that traits may be dominant or recessive?
Answer: Mendel’s experiments show that traits may be dominant or recessive through observations in pea plants. In the first-generation, or F1 progeny, from a cross of, for example, tall and short plants, there were no ‘medium-height’ plants; all plants were tall. This meant that only one of the parental traits was seen. When these F1 tall plants were allowed to self-pollinate, the second-generation, or F2, progeny were not all tall; instead, one quarter of them were short. This indicates that both the tallness and shortness traits were inherited in the F1 plants, but only the tallness trait was expressed. This led Mendel to propose that two copies of a factor (now called genes) controlling traits are present in a sexually reproducing organism. For instance, both TT and Tt genotypes result in tall plants, while only tt results in a short plant. In other words, a single copy of ‘T’ is enough to make the plant tall, making ‘T’ a dominant trait, while both copies have to be ‘t’ for the plant to be short, making ‘t’ a recessive trait.
2. How do Mendel’s experiments show that traits are inherited independently?
Answer: Mendel’s experiments show that traits are inherited independently by breeding pea plants showing two different characteristics. For example, when a tall plant with round seeds is bred with a short plant with wrinkled-seeds, the F1 progeny are all tall and have round seeds, indicating tallness and round seeds are dominant. When these F1 progeny are used to generate F2 progeny by self-pollination, the experiment reveals that some F2 progeny are tall plants with round seeds, and some are short plants with wrinkled seeds. However, there would also be F2 progeny that showed new combinations: some would be tall but have wrinkled seeds, while others would be short but have round seeds. The formation of these new combinations of traits in F2 offspring, when factors controlling for different traits like seed shape and seed colour recombine to form the zygote, demonstrates that the tall/short trait and the round seed/wrinkled seed trait are independently inherited.
3. 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?
Answer: Yes, this information is enough to tell which trait is dominant. Since the daughter has blood group O, she must have inherited the factor for blood group O from both parents. The mother has blood group O, so her genetic makeup for this trait means she could only pass on the factor for O. Because the daughter has blood group O, she must have also received a factor for O from her father. The father has blood group A, but he passed on a factor for O to his daughter; this means his genetic makeup includes both A and O. Since his blood group is A, the trait A is expressed even when the factor for O is present. Traits that are expressed when only one copy of their gene is present are dominant, while traits that require two copies to be expressed are recessive. Therefore, blood group A is dominant, and blood group O is recessive.
4. How is the sex of the child determined in human beings?
Answer: In human beings, the sex of the individual is largely genetically determined by the genes inherited from the parents. All human chromosomes are not paired perfectly; while most human chromosomes have a maternal and a paternal copy (22 such pairs), one pair, called the sex chromosomes, differs between males and females. Women have a perfect pair of sex chromosomes, both called X (XX). Men have a mismatched pair, with one normal-sized X chromosome and a short one called Y (XY). All children will inherit an X chromosome from their mother. Thus, the sex of the children will be determined by what they inherit from their father. A child who inherits an X chromosome from her father will be a girl (XX), and one who inherits a Y chromosome from him will be a boy (XY).
Exercise Questions and Answers
1. 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
Answer: (c) TtWW
Explanation:
This is because for the progeny to have almost half of them short (genotype tt), when crossed with a short parent (ttww), the tall parent must be heterozygous for height (Tt). For all progeny to bear violet flowers when crossed with a white-flowered parent (ww), the tall parent must be homozygous dominant for violet flower color (WW), ensuring all offspring receive at least one dominant allele for violet color (Ww). Thus, the genetic make-up of the tall parent is TtWW.
2. 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?
Answer: No, on this basis alone, we cannot definitively say whether the light eye colour trait is dominant or recessive. The determination of whether a trait is dominant or recessive, such as in Mendel’s experiments, involves controlled breeding of individuals with contrasting traits and observing the characteristics of subsequent generations, like the F1 and F2 progeny, to see which trait is expressed or reappears. The observation that children with light-coloured eyes are likely to have parents with light-coloured eyes is a correlation from a population study, not the result of such controlled experimental crosses. Without information from specific crosses (for example, the outcome if parents with different eye colours have children, or if two dark-eyed parents have a light-eyed child), a conclusion cannot be reliably made.
3. Outline a project which aims to find the dominant coat colour in dogs.
Answer: A project to find the dominant coat colour in dogs, based on the principles of Mendelian inheritance involve the following steps:
- Select dogs from true-breeding lines with two contrasting, clearly distinguishable coat colours. For example, one line of dogs that consistently produces black coat colour and another line that consistently produces a different distinct coat colour, such as yellow.
- Perform a cross between these two types of dogs: mate a true-breeding black-coated dog with a true-breeding yellow-coated dog. This is the parental (P) generation.
- Observe the coat colour of all the puppies born from this cross (the F1 generation). If all the F1 puppies exhibit only one of the coat colours (e.g., all are black), then that coat colour (black, in this example) would be considered the dominant trait, and the other (yellow) would be considered recessive.
- To further confirm this, interbreed the F1 generation individuals (e.g., mate male and female F1 dogs that all show the dominant coat colour with each other).
- Observe the coat colours of the offspring from this F1 cross (the F2 generation). If the coat colour that was not visible in the F1 generation (e.g., yellow) reappears in the F2 generation, typically in a predictable ratio (such as approximately 3:1, where three-quarters of the F2 dogs show the dominant coat colour and one-quarter show the recessive coat colour), this would confirm the dominance of the first coat colour and the recessiveness of the coat colour that reappeared.
4. How is the equal genetic contribution of male and female parents ensured in the progeny?
Answer: The equal genetic contribution of male and female parents in the progeny is ensured through the mechanism of sexual reproduction involving chromosomes and germ cells. Both the father and the mother contribute practically equal amounts of genetic material to the child. Each body cell in an organism has two copies of each chromosome, one inherited from the male parent and one from the female parent. This means that for each trait, there will be two versions of the gene in the child, one from each parent.
This equal contribution is specifically ensured because:
- Each gene set is present as separate independent pieces called chromosomes.
- During the formation of germ cells (sperm in males and eggs in females), each germ cell receives only one chromosome from each pair. Thus, each germ cell contains only one set of genes.
- When a male germ cell (sperm) and a female germ cell (egg) combine during fertilisation, they restore the normal number of chromosomes in the progeny. The resulting zygote, and subsequently the new individual, will have two copies of each chromosome, one contributed by the male parent (via the sperm) and one by the female parent (via the egg). This ensures that the progeny inherits one complete set of genes from the father and one complete set from the mother, leading to an equal genetic contribution from both parents.
Extras
Additional MCQs (Knowledge Based)
1. In human beings, how many pairs of autosomes (non-sex chromosomes) are typically found?
A. 21
B. 22
C. 23
D. 46
Answer: B. 22
30. The fleshy, lower external part of the human ear, which exhibits variation in its attachment (free or attached) among individuals, is known as the:
A. Helix
B. Tragus
C. Earlobe
D. Auditory canal
Answer: C. Earlobe
Additional MCQs (Competency Based)
1. Assertion (A): In a monohybrid cross between a tall pea plant (TT) and a short pea plant (tt), all F1 progeny are tall.
Reason (R): The allele for tallness (T) is dominant over the allele for shortness (t).
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (a) Both A and R are true and R is the correct explanation of A.
27. A cross between two pea plants produces 120 offspring. 90 of these are tall and 30 are short. What is the most likely genotype of the parent plants, if T (tall) is dominant to t (short)?
(a) TT x tt
(b) Tt x Tt
(c) TT x Tt
(d) Tt x tt
Answer: (b) Tt x Tt
Additional Questions and Answers
1. Which type of reproduction produces very similar offspring with only minor differences?
Answer: Asexual reproduction produces very similar offspring. For instance, if one bacterium divides, and then the resultant two bacteria divide again, the four individual bacteria generated would be very similar. There would be only very minor differences between them, generated due to small inaccuracies in DNA copying.
22. Outline the genetic mechanism of sex determination in human beings, including the roles of X and Y chromosomes.
Answer: In human beings, the sex of the individual is largely genetically determined. Most human chromosomes exist in 22 pairs, with each pair having a maternal and a paternal copy. However, the sex chromosomes are not always a perfect pair. Women have a perfect pair of sex chromosomes, both called X (XX). Men have a mismatched pair with one normal-sized X chromosome and one short Y chromosome (XY). All children inherit an X chromosome from their mother. The sex of the children is determined by what they inherit from their father. A child who inherits an X chromosome from her father will be a girl, and one who inherits a Y chromosome from him will be a boy.
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