Short & Long Question Answers with Solution: Principles of Inheritance & Variation | Biology for ACT PDF Download

Short Answer Type Questions

Q1: Explain Down’s syndrome.
Ans: Down's syndrome is a genetic disorder resulting from an extra copy of chromosome 21, known as trisomy 21. This condition has physical and intellectual impacts. Individuals with Down's syndrome typically exhibit flattened nasal features, small ears, and experience cognitive challenges, including difficulties in thinking, understanding, and reasoning that persist throughout their lives. Additionally, they may face issues with hearing and vision and often have a shorter stature.

Q2: Differentiate between dominance, co-dominance and incomplete dominance.
Ans: 

• Dominance refers to a genetic phenomenon where one form of a gene hides the impact of another form of the same gene.
• Co-dominance describes a genetic interaction between two alleles of a gene where neither allele is recessive, and the traits associated with both alleles are visibly expressed in the phenotype.
• Incomplete dominance represents a type of intermediate inheritance where one allele for a particular trait doesn't entirely overshadow the expression of its paired allele.

Q3: Why is it that women exceeding 40 years of age have more chances of having a child with Down’s syndrome?
Ans: Women over the age of 40 are at a higher risk of giving birth to a child with Down's syndrome due to the adverse effects of increased age on chromosome meiosis. With age, meiosis tends to remain incomplete and can stall at prophase-I, resulting in unpaired chromosomes. If fertilization takes place after an extended period, the chromosomes have to remain unpaired for an extended duration. The longer this unpairing persists, the higher the likelihood of non-disjunction occurring, leading to conditions like trisomy.

Q4: What are chromosomes and who discovered chromosomes?
Ans: Chromosomes are thread-like structures located in the cell nucleus, and the number of chromosomes varies among different species. Humans, for instance, possess 23 pairs of chromosomes.
The discovery of chromosomes in plant cells dates back to 1842 when Swiss botanist Carl Wilhelm von Nageli made this observation.

Q5: The human male never passes on the gene for haemophilia to his son. Why is it so?
Ans: Due to the location of the haemophilia gene on the X chromosome, males do not pass it on to their sons. This is because males inherit one X chromosome from their mother and one Y chromosome from their father. As a result, they pass on the X chromosome to their daughters (female progeny) but not to their sons (male progeny).

Q6: What are Sex chromosomes?
Ans: Sex chromosomes are a pair of chromosomes responsible for determining an individual's gender as either male or female. In mammals, including humans, there are two types of sex chromosomes: X and Y. Females typically have two X chromosomes (XX), while males possess one X and one Y chromosome (XY). These sex chromosomes play a crucial role in determining an individual's biological sex and the development of secondary sexual characteristics.

Q7: What is a test cross?
Ans: A test cross is a breeding experiment in which an individual with a dominant phenotype but an unknown genotype is crossed with an individual that is homozygous recessive for a specific trait. This type of cross is conducted to determine whether the dominant phenotype individual is homozygous dominant or heterozygous for that particular trait. Analyzing the phenotypic ratios of the offspring from the test cross can help deduce the genotype of the dominant phenotype individual.

Q8: Define artificial selection. Has it affected the process of natural selection?
Ans: Artificial selection is a purposeful breeding method where humans choose specific traits in plants and animals to produce offspring with desired characteristics. It is an ancient form of genetic engineering and influences the natural selection process by prioritizing traits based on human preferences rather than natural fitness in the environment. This practice can impact genetic diversity and biodiversity.

Q9: Why does sickle-cell anaemia persist in the human population when it is believed that the harmful alleles get eliminated from the population after a certain time?
Ans: Sickle cell anemia is an autosomal recessive disorder where red blood cells take on a sickle shape, reducing their ability to carry oxygen. Interestingly, it provides some protection against malaria. People with heterozygous HbAS (one normal and one sickle cell gene) have a higher survival rate compared to homozygous HbSS (two sickle cell genes) individuals due to reduced malaria risk.

Q10: How was it known that the genes are located on chromosomes?
Ans: The chromosomal theory of inheritance, first put forth by Bovine and Sutton, suggests that genes reside at specific positions on chromosomes. Thomas Morgan later conducted experiments with fruit flies, noting mutations in eye color, and deduced from the inheritance patterns that the gene controlling eye color is situated on the X-chromosome.

Long Answer Type Questions

Q1: Why did scientists select fruit flies for his genetics experiments?
Ans: 

• Drosophila melanogaster, a small and commonly found fly species in the Drosophilidae family, is often referred to as the vinegar fly or fruit fly.
• In 1830, Drosophila melanogaster became a significant model organism for biomedical research due to its strong biological resemblance to mammals and the availability of numerous genetic tools.
• Similar to humans, these fruit fly species also possess a comparable chromosomal distribution. A female fruit fly has a pair of X chromosomes, while a male has one X and one Y chromosome.

Q2: How is sex determined in humans?
Ans: Females possess a pair of X-chromosomes, while males have one X-chromosome and a smaller Y-chromosome. Consequently, in a cross between a male and a female, there is an equal probability of producing male and female offspring. The presence of a Y-chromosome in the offspring determines their sex; the presence of a Y-chromosome results in a male child, while its absence results in a female child.

Q3: Why is colour blindness more prominent in males than females?
Ans: Color blindness is a sex-linked disorder, and the genes responsible for it are located on the X-chromosome. In order for a female to exhibit the disease, she must have the alleles for color blindness on both of her X-chromosomes. If the allele is present on just one chromosome, the female becomes a carrier of the disease. However, males, who possess only one X-chromosome, will be affected if it carries the allele. This is why males are more susceptible to color blindness.

Q4: What are Sex-linkage?
Ans: Sex linkage is the phenomenon where the expression of an allele depends on an individual's gender. It is associated with the patterns of inheritance linked to the sex chromosomes. Sex linkage is closely connected to the sex chromosomes, specifically the homogametic (XX) and heterogametic (XY) sexes in mammals, with females being homogametic and males being heterogametic. Consequently, sex-linked genes are located on the X chromosome.

Q5: Define autosome, hemizygous, homozygous, and heterozygous?
Ans: 

• Autosomes are the chromosomes excluding the sex chromosomes, and the quantity of autosomes can vary between different organisms. In humans, there are 44 autosomes, which form 22 pairs.
• Hemizygous refers to a state in which an organism possesses only one instance of a gene or DNA sequence in diploid cells.
• Homozygous is a state where an organism has two identical alleles for a particular gene (XX).
• Heterozygous is a state where an organism possesses two distinct alleles for a specific gene (XY).

Q6: List out the characteristics of the chromosome theory of Inheritance.
Ans: The key features of the chromosome theory of inheritance include:

• Fertilization returns the organism to a diploid state.
• Chromosomes separate and assort independently during inheritance.
• During meiosis, homologous chromosomes segregate from each other.
• Within diploid cells, both chromosomes and genes are found in pairs.
• A gamete carries only one type of chromosome and just one of the two alleles for a given trait.

Q7: Why is Drosophila used extensively for genetic studies?
Ans: Drosophila is commonly used in genetic research due to the following traits:

• They have a short lifespan of two weeks.
• They can be easily cultured in the lab using a basic synthetic diet.
• A single mating can result in the production of numerous offspring.
• Distinguishing between male and female Drosophila is straightforward.
• Many observable variations are present in Drosophila and can be seen with a simple microscope.

Q8: What is aneuploidy? Differentiate between aneuploidy and polyploidy.
Ans: Aneuploidy is a chromosomal anomaly that results from the gain or loss of one or more chromosomes during meiosis due to non-disjunction.
Contrast between aneuploidy and polyploidy:
Polyploidy is a chromosomal irregularity characterized by the presence of an entire additional set of chromosomes, which can be either triploid or tetraploid. This occurrence is frequently observed in plants but tends to be lethal in animals.

Q9: Describe the individuals with the following chromosomal abnormalities:

• Trisomy at chromosome 21
• XXY
• XO

Ans: 

• Trisomy: Trisomy leads to an autosomal genetic disorder called Down's syndrome, which is characterized by the following traits:
  • Protruding tongue
  • Round head
  • Slanting eyes
  • Short stature
  • Open mouth
  • Short neck
  • Intellectual disability
  • Underdeveloped genitals and gonads
• XXY: Having an extra X-chromosome results in Kleinfelter's syndrome, which manifests in the following ways:
  • Male individuals display feminine characteristics.
  • Development of breast tissue in males
  • Sterility in males
  • Poor beard growth
  • Feminine-sounding voice
• XO: Losing one X-chromosome leads to Turner's syndrome, which is marked by the following features:
  • Sterility in females
  • Immature ovaries
  • Webbed neck
  • Shield-shaped chest
  • Underdeveloped breasts
  • Swollen fingers
  • Short stature
  • Small uterus

Q10: A colour-blind father has a daughter with normal vision. The daughter marries a man with a normal vision. What is the probability of her children to be colour blind? Explain with the help of a pedigree chart.
Ans:
50% of daughters are carrier while 50% have a normal vision. 50% of sons are diseased while the other 50% will have normal vision.