RBSE Solutions Class 12 Biology Chapter 36 Man-Chromosomal Aberrations

RBSE Class 12 Biology Chapter 36 Multiple Choice Questions

 

Question 1. Number of autosomes found in man are:
(a) 42
(b) 44
(c) 46
(d) 48
Answer: (b) 44
In simple words: Humans have 44 autosomes, which are non-sex chromosomes. These chromosomes carry genes for all traits except those that determine sex.

🎯 Exam Tip: Remember that humans typically have 46 chromosomes in total: 44 autosomes and 2 sex chromosomes (XX for females, XY for males).

 

Question 3. Progeny of normal female and colourblind father will be:
(a) Normal vision in all children and no carrier
(b) Colour blind boys and normal girls
(c) Girls are a carrier but boys normal
(d) All children colourblind
Answer: (c) Girls are a carrier but boys normal
In simple words: A colourblind father passes his X chromosome to his daughters. Since the mother is normal, daughters will be carriers. Sons get the Y chromosome from the father and a normal X from the mother, so they will not be colourblind.

🎯 Exam Tip: For sex-linked traits like colourblindness, always trace the X chromosome inheritance carefully. Fathers pass their X to all daughters and their Y to all sons.

 

Question 4. A number of chromosomes in Turner's syndrome:
(a) 44
(b) 45
(c) 46
(d) 47
Answer: (b) 45
In simple words: Turner's syndrome is a genetic condition where a female is missing all or part of one X chromosome, resulting in a total of 45 chromosomes (44 autosomes + X0). This is a numerical abnormality.

🎯 Exam Tip: Key genetic syndromes are often associated with specific chromosome numbers or deletions; memorize these common examples.

 

Question 5. Which of the following disease is sex-linked?
(a) Diarrhoea
(b) Edward syndrome
(c) Mongolism
(d) Haemophilia
Answer: (d) Haemophilia
In simple words: Haemophilia is a genetic disorder where blood does not clot properly. It is primarily inherited through the X chromosome, meaning it's a sex-linked disease.

🎯 Exam Tip: Sex-linked diseases like haemophilia and colourblindness are usually more common in males because they only have one X chromosome.

 

Question 7. When linkage is found in a group of genes than they:
(a) Cannot show independent assortment
(b) Stimulate cell division
(c) Did not show chromosomal mapping
(d) Show recombination during meiosis
Answer: (a) Cannot show independent assortment
In simple words: Genes that are linked are located close together on the same chromosome. Because they are so close, they tend to be inherited together and do not sort independently during meiosis like genes on different chromosomes.

🎯 Exam Tip: Linkage is an exception to Mendel's law of independent assortment, as closely located genes are inherited as a unit.

 

Question 8. Hereditary recombination is found in higher animals as the result of crossing over, In which of the following it occurs:
(a) Between sister chromatids of any bivalent
(b) Between non-sister chromatids of any bivalent
(c) Between two daughter nuclei
(d) Between two different bivalents
Answer: (b) Between non-sister chromatids of any bivalent
In simple words: Crossing over is when homologous chromosomes exchange genetic material. This exchange happens specifically between non-sister chromatids of a bivalent (a pair of homologous chromosomes).

🎯 Exam Tip: Crossing over is a crucial process that increases genetic variation in sexually reproducing organisms by creating new combinations of alleles on chromosomes.

 

Question 9. Which of the following enzyme cannot be synthesized in phenylketonuria disease?
(a) Phenylalanine hydroxylase
(b) Phenylalanine dehydrogenase
(c) Phenyl oxygenase
(d) Phenyl hydroxylases
Answer: (a) Phenylalanine hydroxylase
In simple words: Phenylketonuria (PKU) is a genetic disorder where the body cannot produce the enzyme phenylalanine hydroxylase. This enzyme is needed to break down the amino acid phenylalanine.

🎯 Exam Tip: Genetic disorders often involve a deficiency or absence of a specific enzyme, leading to a build-up of certain substances in the body.

RBSE Class 12 Biology Chapter 36 Very Short Answer Questions

 

Question 1. What is Down syndrome?
Answer: Down syndrome is a genetic condition caused by an extra 21st chromosome, leading to a total of 47 chromosomes instead of the usual 46. This condition is also known as Trisomy 21 or mongolism, and it results in various physical and intellectual developmental delays. It is one of the most common chromosomal abnormalities.
In simple words: Down syndrome happens when a person has an extra copy of chromosome 21, which causes developmental differences.

🎯 Exam Tip: When describing genetic syndromes, always mention the specific chromosomal abnormality (e.g., which chromosome is extra or missing) and the total chromosome count.

 

Question 2. What is the function of Phenylalanine hydroxylase?
Answer: The enzyme phenylalanine hydroxylase is responsible for breaking down (catabolizing) the amino acid phenylalanine into another amino acid called tyrosine. This process is essential for proper metabolism in the body. If this enzyme is not working correctly, phenylalanine can build up.
In simple words: Phenylalanine hydroxylase helps change phenylalanine, an amino acid, into tyrosine.

🎯 Exam Tip: Enzymes are proteins that act as catalysts for specific biochemical reactions; understanding their function is key to explaining metabolic disorders.

 

Question 3. Which type of mutation occurs in the haemoglobin of sickle cell anaemia?
Answer: Sickle cell anaemia is caused by a point mutation in the gene for haemoglobin. This specific mutation leads to the replacement of glutamic acid by valine amino acid at the 6th position of the beta (B) chain of the haemoglobin molecule. This single change affects the shape of red blood cells.
In simple words: In sickle cell anaemia, a tiny change in one gene causes glutamic acid in haemoglobin to become valine.

🎯 Exam Tip: Point mutations are changes to a single nucleotide in DNA and can have significant effects, as seen in sickle cell anaemia.

 

Question 4. How many chromosomes may be present in Klinefelter's syndrome?
Answer: In Klinefelter's syndrome, individuals typically have an extra X chromosome. Therefore, a person with Klinefelter's syndrome may have 47, 48, or even 49 chromosomes. The most common form is 47, XXY, but rarer forms with more X or Y chromosomes also exist.
In simple words: People with Klinefelter's syndrome usually have extra X chromosomes, so they can have 47, 48, or 49 chromosomes in total.

🎯 Exam Tip: Remember the normal chromosome count (46) and how conditions like Klinefelter's or Turner's syndrome involve deviations from this number.

 

Question 5. When and in which type of cell division crossing over occurs?
Answer: Crossing over occurs during meiosis, specifically in the Pachytene phase of Meiosis-I. This is the stage where homologous chromosomes pair up and exchange genetic material, leading to genetic recombination. This process is crucial for genetic diversity.
In simple words: Crossing over happens during meiosis, in the pachytene stage of Meiosis-I, when parts of chromosomes are swapped.

🎯 Exam Tip: Accurately identifying the specific stage of cell division where key events like crossing over take place is important for full marks.

 

Question 6. What is a linkage group?
Answer: A linkage group refers to all the genes located on the same chromosome that tend to be inherited together. Each chromosome represents a linkage group, meaning that genes on the same chromosome are physically connected and often passed down as a single unit during inheritance.
In simple words: A linkage group is all the genes found on one chromosome that are usually inherited together.

🎯 Exam Tip: The number of linkage groups in an organism is typically equal to its haploid number of chromosomes.

 

Question 8. Write a number of chromosomes found in a man and woman.
Answer: A man typically has 44 autosomes plus XY sex chromosomes, making a total of 46 chromosomes (44 + XY). A woman typically has 44 autosomes plus XX sex chromosomes, also making a total of 46 chromosomes (44 + XX). The sex chromosomes determine the individual's biological sex.
In simple words: A man has 44 regular chromosomes and XY sex chromosomes, while a woman has 44 regular chromosomes and XX sex chromosomes, both totaling 46.

🎯 Exam Tip: Clearly distinguish between autosomes (non-sex chromosomes) and sex chromosomes when stating total chromosome numbers.

RBSE Class 12 Biology Chapter 36 Short Answer Questions

 

Question 1. What is colourblindness?
Answer: Colour blindness is an inherited condition where a person cannot tell the difference between certain colours. This happens because some cells in the eye that detect colour do not work correctly. There are two main types:

• Red-Green Colourblindness: Individuals with this type struggle to distinguish between shades of red and green. It is often caused by the absence of red or green cones in the eye.
• Blue Colourblindness: People with this type find it hard to see the difference between violet, indigo, blue, and green, often seeing them all as green. It is due to a lack of blue cones.

This condition is more common in men because the genes responsible for colour vision are located on the X chromosome. Since males have only one X chromosome, a single recessive allele on it will cause colour blindness. Females have two X chromosomes, so they need recessive alleles on both to be colourblind, or they can be carriers if only one X chromosome carries the allele.
In simple words: Colour blindness is an inherited problem where a person cannot see some colours apart, mostly red and green. It affects more men because it is linked to the X chromosome.

🎯 Exam Tip: When explaining genetic disorders, always mention the type of inheritance (e.g., hereditary, sex-linked) and its phenotypic expression (how it affects the individual).

 

Question 2. Express the inheritance of disease in the progeny of carrier of colourblind mother and a normal father.
Answer: When a carrier colourblind mother (who has one normal X and one X with the colourblindness gene) has children with a normal father (who has a normal X and a Y chromosome), the likely outcomes for their children are:

• 50% of daughters will be carriers (carrying the gene but not colourblind).
• 50% of daughters will be normal (not carrying the gene).
• 50% of sons will be colourblind (inheriting the affected X from the mother).
• 50% of sons will be normal (inheriting the normal X from the mother).

This pattern shows how sex-linked traits are passed on, where sons are more likely to express the condition.
In simple words: From a mother who carries the colourblind gene and a normal father, half of the daughters will be carriers, half will be normal, half of the sons will be colourblind, and half will be normal.

🎯 Exam Tip: Practice drawing Punnett squares for sex-linked inheritance to visually confirm these ratios and understand the gene transmission patterns.

 

Question 4. Explain the experiment of Bateson and Punnet.
Answer: In 1906, Bateson and Punnet performed a dihybrid cross using sweet pea plants to study the inheritance of two traits: flower colour (Blue = B, Red = b) and pollen shape (Long = L, Round = l). They crossed plants with blue flowers and long pollens (BBLL) with plants having red flowers and round pollens (bbll). In the first generation (F₁), all offspring showed blue flowers and long pollens (BbLl), as expected for dominant traits.

Next, they conducted a test cross between the heterozygous F₁ offspring (BbLl) and a homozygous recessive parent (bbll). According to Mendel's law of independent assortment, they expected four types of gametes and offspring phenotypes in equal proportions (1:1:1:1 ratio). However, Bateson and Punnet observed different results, suggesting that the genes for flower colour and pollen shape did not assort independently. This led them to propose the concept of gene linkage. Their experiment showed that genes located on the same chromosome tend to be inherited together, which explained the deviation from the expected Mendelian ratios. This work was foundational in understanding how genes are physically connected on chromosomes.

Their specific results for F₁ generation were:
(Test cross of F₁ (BbLl) x recessive parent (bbll)):

In total, 12.6% recombinants (Blue round and Red long) were observed.
In simple words: Bateson and Punnet crossed sweet peas and found that some traits, like flower color and pollen shape, were often inherited together. This showed that genes on the same chromosome don't always separate independently, a concept called linkage.

🎯 Exam Tip: When explaining experiments, clearly state the organisms used, the traits studied, the crosses performed, the expected results (Mendelian), and the actual observed results that led to new conclusions (like linkage).

 

Question 5. Explain various types of linkage.
Answer: Linkage describes the tendency of genes located on the same chromosome to be inherited together during cell division. There are mainly two types of linkage:

• 1. Complete Linkage: This occurs when genes are located very close to each other on the same chromosome and are always inherited together without any new combinations forming in successive generations. They never separate. This is rare in nature, as some level of crossing over usually occurs.
• 2. Incomplete Linkage: This is more common and happens when genes are on the same chromosome but are far enough apart that crossing over can occur between them. While they tend to be inherited together, there is a chance of separation due to genetic exchange during meiosis, leading to some new combinations. The farther apart the genes are, the higher the probability of crossing over and therefore the higher the chances of being isolated.

These two types of linkage help explain the variations in inheritance patterns observed in different organisms.
In simple words: Linkage is when genes on the same chromosome are inherited together. Complete linkage means they never separate, while incomplete linkage means they can sometimes separate through crossing over.

🎯 Exam Tip: Differentiating between complete and incomplete linkage often depends on the frequency of recombination observed; complete linkage shows no recombination.

RBSE Class 12 Biology Chapter 36 Essay Type Questions

 

Question 1. What do you mean by sex-linked inheritance? Discuss it with reference to colourblindness and haemophilia diseases.
Answer: Sex-linked inheritance refers to the pattern of genetic trait transmission where the genes responsible for the trait are located on the sex chromosomes, typically the X chromosome. Because males have one X and one Y chromosome (XY) while females have two X chromosomes (XX), sex-linked traits often show different inheritance patterns between genders.

Key aspects of Sex-linked Inheritance:

• Humans have 44 autosomes and 2 sex chromosomes. Females are XX and males are XY.
• Genes on the X chromosome can affect both males and females, showing what is called crisscross heredity, where a father passes an X-linked trait to his daughters, who then can pass it to their sons.
• Genes on the Y chromosome only affect males, as only males possess a Y chromosome.
• About 20 different traits are associated with genes on the human X chromosome. Fathers cannot pass X-linked traits directly to their sons, only to their daughters.

Sex-linked hereditary diseases include:
1. Colourblindness:

• This is a hereditary condition where a person cannot distinguish between specific colours.
• The two main types are Red-Green colourblindness (inability to differentiate red and green due to missing cones) and Blue colourblindness (difficulty seeing violet, indigo, blue, and green, often appearing green).
• Males are more prone to colourblindness because the responsible gene is on the X chromosome. If a male inherits an X chromosome with the recessive allele for colour vision, he will be colourblind.
• Females have two X chromosomes. If one X has the recessive allele, they are usually carriers and do not show symptoms because the other normal X chromosome compensates. They only become colourblind if both X chromosomes carry the recessive allele.

Inheritance of colourblindness:

• Example 1: Cross between a normal woman and a colourblind man.
  If a colourblind male mates with a normal female, all their children (sons and daughters) will appear normal. However, all daughters will be carriers of the colourblindness gene because they receive the affected X chromosome from their father and a normal X from their mother.
• Example 2: Cross between a carrier woman and a colourblind man.
  In this scenario, there is a 50% chance for daughters to be colourblind and a 50% chance to be carriers. For sons, there's a 50% chance of being normal and a 50% chance of being colourblind.

Conclusions for colourblindness heredity:

• Colour identification is an X-linked trait, with its gene on the X chromosome, and no corresponding allele on the Y chromosome in males.
• Males are more affected because a single recessive gene on their X chromosome leads to the condition.
• Women have two X chromosomes, and a recessive gene on both is needed for them to be colourblind.
• If a woman has only one X chromosome with the recessive gene, she becomes a carrier, usually with normal vision.
• The gene for colour identification is dominant, so if a woman has both X chromosomes carrying the recessive gene, she will be colourblind.

2. Haemophilia:

• Haemophilia is a sex-linked hereditary disorder where blood clotting is delayed or absent. It was first described by John Otto and later studied by Haldane in Queen Victoria's family.
• This disease is also known as "Royal disease" or "bleeder's disease" and primarily affects males. Females usually act as carriers. Recessive genes on both X chromosomes would cause severe haemophilia in females, which is often fatal, meaning such females typically do not survive.
• A father with haemophilia will always pass the affected X chromosome to his daughters.
• There are two main types:
  — Haemophilia-B (Christmas disease): Caused by a deficiency of clotting factor-IX, a milder form.
  — Haemophilia-A: Caused by a deficiency of clotting factor-VIII (AHG), which is more common and severe.
• If a haemophilic man mates with a normal woman, 50% of their daughters will be carriers, and 50% of their sons will be normal.
• If a carrier woman mates with a normal man, 25% of their daughters will be carriers, 25% will be normal daughters, 25% of their sons will be haemophilic, and 25% will be normal sons.
• Haemophilia is predominantly observed in males because they possess only one X chromosome. Females with both X chromosomes carrying the recessive gene for severe haemophilia usually cannot survive, especially during menstruation due to unstoppable blood flow.
• Females who carry only one recessive gene for haemophilia on one X chromosome, with the other X being normal, are carriers of the disease.

In simple words: Sex-linked inheritance means traits are passed through sex chromosomes, mainly the X chromosome. Diseases like colourblindness and haemophilia are common examples, affecting males more often because they have only one X chromosome. Females can be carriers or affected if both X chromosomes have the faulty gene.

🎯 Exam Tip: For essay questions involving diseases, provide a clear definition, discuss inheritance patterns, and explain how the disease manifests differently in males and females due to chromosomal differences.

 

Question 2. Differentiate between linkage and crossing over.
Answer: Linkage and crossing over are two fundamental concepts in genetics that describe how genes are transmitted from parents to offspring, but they represent opposite tendencies.

1. Linkage:

• Linkage is the phenomenon where genes located on the same chromosome tend to be inherited together during meiosis and are passed on to the next generation as a unit.
• Mendel's principle of independent assortment applies to genes on different chromosomes or genes very far apart on the same chromosome. Linkage is an exception where genes do not assort independently.
• The concept of linkage was first observed by Sutton and later explained by Bateson and Punnet in their sweet pea experiments.
• Each chromosome can be considered a linkage group, containing many genes that are physically linked.
• Linked genes have a tendency to remain together during inheritance, reducing the chances of new combinations.

2. Crossing Over:

• Crossing over is the process of exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase-I of meiosis.
• This process leads to recombination, creating new combinations of alleles on the chromosomes that differ from those found in the parents.
• Bateson and Punnet observed recombinant offspring in their experiments, which could only be explained by a process like crossing over.
• During crossing over, homologous chromosomes align and form chiasmata, which are points of physical connection where chromatids break and rejoin.
• The occurrence of crossing over separates linked genes, leading to the formation of recombinant chromatids and increasing genetic diversity.

Significance of Crossing Over:

• Crossing over takes place during the pachytene stage of prophase-I of meiosis.
• It involves the exchange of segments between non-sister chromatids of homologous chromosomes.
• The amount of crossing over depends on the length of the chromosome and the distance between genes; longer chromosomes and greater gene distances lead to more frequent crossing over.
• Crossing over is crucial for increasing genetic variation within a species, providing raw material for evolution.

In simple words: Linkage means genes on the same chromosome stick together during inheritance. Crossing over is the opposite: it's when parts of homologous chromosomes swap places, mixing up genes and creating new combinations.

🎯 Exam Tip: Clearly define both terms and highlight their contrasting effects on genetic variation: linkage reduces variation, while crossing over increases it by forming new allele combinations.

 

Question 3. Describe various chromosomal aberrations in man.
Answer: Chromosomal aberrations, also known as chromosomal disorders or abnormalities, are changes in the number or structure of chromosomes in a cell. These changes can lead to various genetic conditions in humans. They are broadly categorized into two types:

1. Autosomal Abnormalities: These involve changes in the number or structure of autosomes (non-sex chromosomes).
— 1.1. Mongolism or Down's Syndrome:

• First described by Langdon Down in 1866.
• Individuals with Down syndrome typically have 47 chromosomes because of an extra 21st chromosome, a condition known as Trisomy 21.
• It occurs in about 1 out of 600 births.
• This defect is more common in children born to women over 40 years old, often due to an error during meiotic division of the ovum.

In simple words: Chromosomal aberrations are changes in our chromosomes, either in their number or structure. Down syndrome is an example, caused by an extra copy of chromosome 21, resulting in 47 chromosomes instead of 46.

🎯 Exam Tip: When describing chromosomal aberrations, always specify whether the abnormality is numerical or structural, and which specific chromosome (or set of chromosomes) is affected.

 

Question 1. Describe Edward's Syndrome, its cause, and main characteristics.
Answer: Edward's Syndrome is a genetic condition caused by having an extra 18th chromosome. This means individuals with the syndrome have 47 chromosomes (45 + 2 = 47) instead of the usual 46. It is also known as 18th Trisomy. This syndrome occurs in about 1 out of every 6000 births. Babies with Edward's Syndrome often have serious health issues like congenital heart disease and extra fingers or toes (polydactyly). Sadly, their life expectancy is very short, typically a maximum of 6 months. This extra chromosome interferes with normal development, leading to severe physical and intellectual challenges.
In simple words: Edward's Syndrome happens when a baby has an extra chromosome 18, leading to severe health problems like heart defects and extra digits. Babies with this condition usually live for only a few months.

🎯 Exam Tip: Remember that trisomy 18 refers to an extra chromosome 18, which causes severe developmental problems and a very short lifespan.

 

2. Allosomal Abnormalities

 

Question 2. Describe Turner's Syndrome, its cause, and symptoms.
Answer: Turner's Syndrome is a chromosomal disorder that affects only females. It is caused when a female has only one X chromosome instead of the usual two. So, a person with Turner's Syndrome has a total of 45 chromosomes (44 autosomes + X0). This condition is seen in approximately 1 out of every 3000 girl births. Women with Turner's Syndrome often have a short stature, a broad chest, a short neck, and underdeveloped mammary glands. They also typically have undeveloped sexual characteristics and are sterile (unable to have children). In some cases, mental development can also be slightly slowed. The absence of a second X chromosome significantly impacts various developmental processes.
In simple words: Turner's Syndrome is a condition in females where they have only one X chromosome. This causes short height, underdeveloped sexual traits, and often means they cannot have children.

🎯 Exam Tip: Key for Turner's Syndrome is understanding it affects females and is characterized by the presence of only a single X chromosome (X0).

 

Question 3. Explain Klinefelter's Syndrome, its chromosomal makeup, and characteristics.
Answer: Klinefelter's Syndrome is a genetic disorder that affects males. Instead of the typical 46 chromosomes, men with this syndrome have an extra X or sometimes an extra Y chromosome, resulting in a total of 47, 48, or even 49 chromosomes. Common chromosomal patterns include \( 44 + XXY = 47 \) (one extra X chromosome), \( 44 + XXXY = 48 \) (two extra X chromosomes), \( 44 + XXXXY = 49 \) (three extra X chromosomes), \( 44 + XXYY = 48 \) (one extra X and one extra Y chromosome), or \( 44 + XXXYY = 49 \) (two extra X and one extra Y chromosome). Males with Klinefelter's Syndrome often develop feminine characteristics, such as enlarged breast tissue (a condition called Gynaecomastia). They tend to have longer arms and legs compared to their body size. The presence of extra genetic material disrupts normal male development. While often associated with physical traits, mental development can also be affected.
In simple words: Klinefelter's Syndrome is a male condition where there are extra X or Y chromosomes, leading to traits like enlarged breasts and longer limbs.

🎯 Exam Tip: For Klinefelter's Syndrome, remember it affects males and is usually caused by an extra X chromosome (XXY), leading to a mix of male and some female physical traits.

 

Question 4. What is Super Female Syndrome (XXX syndrome) and what are its features?
Answer: Super Female Syndrome, also known as Triple X syndrome, occurs in women who have extra X chromosomes. This leads to a total chromosome count of 47 to 49 (e.g., \( 44 + XXX = 47 \), \( 44 + XXXX = 48 \), or \( 44 + XXXXX = 49 \)). Women with XXX syndrome typically have normal external reproductive organs. However, they may experience reduced fertility and can sometimes have slightly delayed or slowed mental development. The effects can become more noticeable with a higher number of extra X chromosomes. This condition arises from an error during egg formation or early embryo development. The presence of additional X chromosomes can subtly influence overall development.
In simple words: Super Female Syndrome happens when a woman has more than two X chromosomes. She usually looks typical but might have some fertility issues or mild learning difficulties.

🎯 Exam Tip: When discussing Super Female (XXX) syndrome, emphasize that it involves extra X chromosomes in females and that physical impacts are often milder than other chromosomal disorders.

 

Question 5. Describe XYY Syndrome (Supermale) and its typical symptoms.
Answer: XYY Syndrome, sometimes referred to as 'Supermale' syndrome, is a genetic condition found in males. It is caused by the presence of an extra Y chromosome, giving these individuals a karyotype of \( 44 + XYY = 47 \) chromosomes. Men with XYY syndrome are often characterized by being exceptionally tall and having a generally robust or stout body build. They may also have minor abnormalities in their sex organs. Historically, this syndrome was controversially associated with lower mental abilities and an increased tendency towards criminal behavior, though modern research indicates a wide range of outcomes and these links are largely debated. The extra Y chromosome can influence growth and development in subtle yet distinct ways.
In simple words: XYY Syndrome occurs when a male has an extra Y chromosome. They are usually very tall and sturdy, and past studies, though debated, linked it to behavioral traits.

🎯 Exam Tip: For XYY syndrome, focus on the extra Y chromosome in males and its association with increased height, while noting that links to behavioral issues are complex and often debated.