VARIATION & GENETICS
FILL IN THE BLANKS
Q.01: Fill in the blanks:
(i) ______ is the basic unit of biological information. (gene)
(ii) A sudden change in the structure of a gene is called ______. (mutation)
(iii) _______ is the chance of an event to occur. (probability)
(iv) A cross among monohybrids is a ______ cross. (monohybrid)
(v) An individual with a homozygous genotype is called ______. (homozygote)
(vi) Different alleles of a gene that are both expressed in a heterozygote are called ______. (codominant)
(vii) When a heterozygote exceeds the phenotypic expression of both the homozygotes the phenomenon is called ______. (over dominance)
(viii) When a single gene affects two or more traits, the phenomenon is called ______. (pleiotropy)
(ix) A gene with multiple phenotypic effect is called _______. (pleiotropic)
(x) The phenomenon of staying together of all the genes of a chromosome is called ______. (linkage)
(xi) _______ minimizes the chances of genetic recombination. (gene linkage)
(xii) _______ is an exchange of segments between non-sister chromatids of homologous chromosomes during meiosis. (crossing over)
(xiii) All chromosomes other than sex chromosomes are called _______. (autosomes)
(xiv) _______ is the maleness determining gene in man. (SRY gene)
(xv) Type _______ of diabetes mellitus is non-insulin dependent. (II)
(xvi) Polygenic inheritance with environmental influence is called _______ inheritance. (multifactorial)
TRUE / FALSE
Q.02: Indicate True or False,
(i) In grasshopper, the male has XY and the female has XX types of sex chromosomes. (TRUE)
(ii) Pea is normally a self-fertilizing plant. (TRUE)
(iii) Dihybrids are offspring of the parents who differ in one contrasting pair of trait. (FALSE)
CORRECT: Dihybrids are offspring of the parents who differ in two contrasting pair of traits.
(iv) X-linked traits pass direct from father to son. (FALSE)
CORRECT: Y-linked traits pass direct from father to son.
(v) A person suffering from blue cone monochromacy cannot see blue colour. (FALSE)
CORRECT: A person suffering from blue cone monochromacy can see only blue colour.
(vi) In birds and moths, eggs determine sex. (TRUE)
(vii) A homozygote forms all gametes of the same type. (TRUE)
(viii) The allele for a sex-limited trait is dominant in one sex but recessive in the other. (FALSE)
CORRECT: The allele for a sex-influenced trait is dominant in one sex but recessive in the other.
(ix) Pattern baldness is a sex influenced trait. (TRUE)
(x) Carriers of haemophilia show no symptoms of the disease. (TRUE)
MCQs
Q.03: Encircle the correct option from multiple choices.
(i) When a single gene has multiple genotypic effect, the phenomenon is called:
(a) Codominance
(b) Epistasis
(c) Pleiotropy
(d) Sex – linkage
ANSWER: (c) Pleiotropy
EXPLANATION: Pleiotropy simply means one gene affecting multiple traits. For example, a single gene might control both eye color and hair color in an organism. This means that changes (mutations) in that one gene could not only affect eye color, but also hair color.
(ii) What happens when both alleles of a gene pair independently express in a heterozygote?
(a) Dominance
(b) Incomplete dominance
(c) Over dominance
(d) Codominance
ANSWER: (d) Codominance
EXPLANATION: Codominance is a genetic phenomenon where both alleles of a gene are expressed in the phenotype, resulting in a visible and simultaneous display of the traits associated with each allele. An example of codominance is the human blood type system, where the alleles for blood type A and B are codominant.
(iii) A heterozygote offspring quantitatively exceeds the phenotypic expression of both the homozygote parents due to:
(a) Dominance
(b) Incomplete dominance
(c) Over dominance
(d) Codominance
ANSWER: (c) Over dominance
EXPLANATION: In over dominance, heterozygote exceeds in quantity the phenotypic expression of both the homozygotes. For example, in fruit fly Drosophila, the heterozygote (w+/w) has more quantity of fluorescent pigments in eyes than wild (w+/w+) or white eye (w/w) homozygotes.
(iv) How many gene pairs contribute to the wheat grain colour?
(a) One
(b) Two
(c) Three
(d) Four
ANSWER: (c) Three
EXPLANATION: The genetics of wheat grain colour is a polygenic trait which is influenced by three gene pairs, Aa, Bb, Cc.
(v) Who for the first time found white eye mutant in Drosophila?
(a) Morgan
(b) Bridges
(c) Correns
(d) de Varies
ANSWER: (b) Bridges
EXPLANATION: Morgan raised cultures of Drosophila flies to study different traits, such as colour of the eye. Normal fruit flies, the wild type, have bright red eyes. One of his coworkers Calvin Bridges, observed an unusual white eye mutant male fly.
(vi) Which of the following traits is transmitted directly from an affected father to only his sons?
(a) Autosomal
(b) X–linked
(c) Y–linked
(d) X and Y linked
ANSWER: (c) Y–linked
EXPLANATION: Y-linked traits are genetic characteristics that are determined by genes located on the Y chromosome. Since, the Y chromosome is passed exclusively from father to son, Y-linked traits are inherited in a strictly paternal lineage, i.e., father to son.
(vii) Which phenomenon reduces the chance of genetic recombination and variations among offspring?
(a) Linkage
(b) Crossing over
(c) Independent assortment
(d) Dominance
ANSWER: (a) Linkage
EXPLANATION: If genes are tightly linked on the same chromosome, they are more likely to be inherited as a unit, limiting the potential for new combinations of alleles. This decreases the genetic diversity among offspring, as the genes tend to be inherited together as a linked group.
(viii) Which of the following traits is not sex-linked recessive?
(a) Haemophilia
(b) Colour blindness
(c) Hypophosphatemic ricket
(d) tfm syndrome
ANSWER: (c) Hypophosphatemic ricket
EXPLANATION: Hypophosphatemic rickets is not a sex-linked recessive trait. Rather it is sex linked dominant trait.
(ix) Which of these traits zigzags from maternal grand father through a carrier daughter to a grandson?
(a) Autosomal
(b) X–linked
(c) Y–linked
(d) X and Y linked
ANSWER: (b) X–linked
EXPLANATION: Men only have one X chromosome and they always pass it on to their daughters. The daughter receives the affected X chromosome from her father but doesn’t express the trait herself because she likely has a normal X chromosome from her mother. However, she is a carrier. The grandson has a 50% chance of inheriting his mother’s affected X chromosome. If he does, he will express the trait, since males only have one X chromosome.
(x) When a haemphilic carrier woman marries a normal man, who among her offspring may be affected.
(a) All her children
(b) All her daughters
(c) Half of her daughters
(d) Half of her sons
ANSWER: (d) Half of her sons
EXPLANATION: When a hemophilic carrier woman (heterozygous for the hemophilia gene) marries a normal man (who has a normal X chromosome), there are two possible outcomes for their offspring: (1) Sons: Each son has a 50% chance of inheriting the X chromosome carrying the hemophilia gene from his carrier mother and a 50% chance of inheriting the Y chromosome from his father. Therefore, half of the sons may be affected. (2) Daughters: Each daughter has a 50% chance of inheriting the X chromosome carrying the hemophilia gene from her carrier mother and a 50% chance of inheriting the X chromosome from her father. Therefore, half of the daughters may be carriers of the hemophilia gene, but they are not affected as they have a normal X chromosome from their father.
(xi) What is risk of a colour-blind child in a family when mother is colour- blind but father is normal?
(a) 100%
(b) 75%
(c) 50%
(d) 25%
ANSWER: (c) 50%
EXPLANATION: When a color-blind mother (XcXc) and a normal father (XY) have children, all sons will be color-blind (100% risk), while all daughters will be carriers of the color blindness trait but not affected. Therefore, the overall risk of having a color-blind child in the family is 50%.
(xii) What is the risk of a colour-blind child in a family when father is colour-blind but mother is normal?
(a) Zero%
(b) 25 %
(c) 50%
(d) 100%
ANSWER: (a) Zero%
EXPLANATION: When the father is color-blind (XY) and the mother is normal (XX), the risk of having a color-blind child is zero. Sons inherit the Y chromosome from the father, and daughters inherit one X chromosome from each parent, none of which carries the color blindness trait. The correct option is (a) Zero%.
Q.04: SHORT QUESTIONS
(i) Differentiate between:
- Phenotype and genotype
- Homozygous and heterozygous
- Autosome and sex chromosome
- Allele and multiple allele
- Incomplete dominance and codominance Continuous and discontinuous variations
- Gene and allele
- Monohybrid and dihybrid
- Dominance and epistasis
- X-linked trait and Y-linked trait
- Sex limited and sex influenced trait
- Dominant trait and recessive trait
- Wild type and mutant
ANSWER:
Phenotype and Genotype:
Phenotype is the form of appearance of a trait.
Example: A flower may be red or white in colour. Redness or whiteness is its phenotype.
Genotype is the genetic complement i.e., the genes in an individual for a particular trait.
Example: A flower may be red or white in colour. Flower colour is a trait, determined by a different allele of the colour gene.
ANSWER:
Homozygous and Heterozygous:
Homozygous: When both the alleles of a gene pair in an organism are same, the organism is homozygous for that gene pair. An individual with a homozygous genotype is a homozygote.
Heterozygous: When both the alleles of a gene pair in an organism are different, the organism is homozygous for that gene pair. An individual with a homozygous genotype is a heterozygote.
ANSWER:
Allele and Multiple Allele
Allele: Partners of a gene pair are called alleles.
Multiple Alleles: All the altered alternative forms of a gene, whose number is more than two, are called multiple alleles.
Incomplete dominance and codominance
ANSWER:
Incomplete dominance:
When the phenotype of the heterozygote is intermediate between phenotypes of the two homozygotes, it is called incomplete or partial dominance.
Codominance:
Codominance occurs when both the alleles express independently in heterozygote (A1A2) and form their respective products X and Y. The codominant heterozygote would have both substances at the same time.
Continouos Variations and Discontinuous Variations:
ANSWER:
Continouos Variations:
Nature: Continuous quantitative variations involve traits that show a smooth and gradual range of phenotypic differences across a population.
Examples: Height, weight, blood pressure, and other measurable traits.
Genetic Basis: These traits are often polygenic, influenced by multiple genes.
Discontinuous Variations:
Nature: Discontinuous qualitative variations involve traits that exhibit distinct, separate phenotypes with little to no intermediates.
Examples: Blood types (A, B, AB, O), eye color (brown, blue, green), and certain genetic disorders.
Genetic Basis: Discontinuous traits are often controlled by one or a few major genes.
Gene and Allele:
ANSWER:
GENE:
A gene is a specific sequence of DNA that encodes information for the synthesis of a protein or RNA molecule. Genes are the basic units of biological information and are located on chromosomes.
ALLELE:
Genes form pairs on pairs of homologous chromosomes. “Partners of a gene pair are called alleles.” Each allele of a gene pair occupies the same gene locus on its respective homologue. Both alleles on one locus may be identical, or different from each other.
Monohybrid and Dihybrid:
ANSWER:
MONOHYBRID:
“Monohybrid refers to the study or cross involving a single trait.”
Example: In Mendel’s pea plant experiments, a monohybrid cross between purple and white flower plants.
DIHYBRID:
“Dihybrid refers to the study or cross involving two different traits.”
Example: In Mendel’s pea plant experiments, a dihybrid cross between two plants having different flower color and seed shape.
Dominance and Epistasis:
ANSWER:
DOMINANCE:
“Dominance is a physiological effect of an allele over its partner allele on the same gene locus.”
Dominance is the relationship between alleles of the same gene occupying the same locus.
Examples: Complete dominance, incomplete dominance, co-dominance, over dominance.
EPISTASIS:
“When an effect caused by a gene or gene pair at one locus interferes with or hides the effect caused by another gene or gene pair at another locus, such a phenomenon of gene interaction is called epistasis.”
Epistasis is the interaction between different genes occupying different loci.
Examples: Bombay phenotype.
X-linked Trait and Y-linked Trait:
ANSWER:
X-LINKED TRAIT:
“X-linked traits are traits that are determined by genes located on the X chromosome.”
Males inherit X-linked traits from their mothers because they receive their X chromosome from their mother.
Examples: Color blindness and hemophilia are examples of X-linked traits.
Y-LINKED TRAIT:
“Y-linked traits are traits that are determined by genes located on the Y chromosome.”
These traits are passed exclusively from fathers to their sons because only males carry the Y chromosome.
Examples: Male-pattern baldness is an example of a Y-linked trait.
Sex Limited Trait and Sex Influenced Trait:
ANSWER:
SEX LIMITED TRAIT:
A sex-limited trait is limited to only one sex due to anatomical differences. Such trait affects a structure or function of the body present in only males or only females. These trails may be controlled by sex-linked or autosomal genes.
Examples:
Genes for milk yield in dairy cattle affect only cows.
Similarly beard growth in humans is limited to men. A woman does not grow a beard herself but she can pass the genes specifying heavy beard growth to her sons.
SEX INFLUENCED TRAIT:
“Sex influenced trait occurs in both males and females but it is more common in one sex.” It is controlled by an allele that is expressed as dominant in one sex but recessive in the other.
This difference in expression is due to hormonal difference between the sexes.
Examples:
Pattern baldness is a sex influenced trait. Many more men than women are bald. It is inherited as an autosomal dominant trait in males but as an autosomal recessive trait in females. A heterozygous male is bald but a heterozygous female is not. A woman can be bald only when she is homozygous recessive
Dominant Trait and Recessive Trait:
ANSWER:
DOMINANT TRAIT:
“Dominant traits are the traits that are expressed when only one copy of the dominant allele is present.”
In a heterozygote, dominant trait masks the recessive trait.
Example: Brown eye color in humans (dominant) can mask the expression of blue eye color (recessive).
RECESSIVE TRAIT:
“Recessive traits are the traits that are expressed only when two copies of the recessive allele are present.”
In a heterozygote, recessive trait is masked by the dominant trait.
Example: Blue eye color in humans (recessive) is only expressed when an individual inherits two recessive alleles for eye color.
Wild Type and Mutant Type:
ANSWER:
WILD TYPE:
“Wild type refers to the standard or naturally occurring form of a gene, trait, or organism in a population.” It represents the most common phenotype observed in nature.
Example: In fruit flies, the red eye color is considered the wild type.
MUTANT TYPE:
“Mutant type refers to a variant or altered form of a gene, trait, or organism resulting from a mutation.” Mutations can lead to changes in phenotype compared to the wild type.
Example: In the same population of fruit flies, the appearance of a white eyed male is a mutant type fly.
(ii) What is a gene pool?
ANSWER:
GENE POOL:
“All the genes/alleles found in a breeding population at a given time are collectively called the gene pool.”
It is the total genetic information encoded in the total genes in a breeding population existing at a given time.
(iii) Was pea a lucky choice for Mendel? What would have happened if he had studied an eighth character?
ANSWER:
Yes, pea was a lucky choice for Mendel to conduct experiments. This is because Pea has seven homologous pairs of chromosomes. The allelic pairs for each of the seven characters studied by Mendel were luckily on different homologous pairs of chromosomes. If he had studied an eighth character, it alleles would have been linked to alleles of another trait on the same homologous pair, and could have never assorted independently.
(iv) What is a test cross? Why did Mendel devise this cross?
ANSWER:
TEST CROSS:
“Test cross is a cross or mating in which an individual showing a dominant phenotype is crossed with an individual showing its recessive phenotype.”
Mendel devised this cross to test the genotype of an individual showing a dominant phenotype. This cross finds out the homozygous or heterozygous nature of the genotype.
(v) What would happen if alleles of a pair do not segregate at meiosis? How would it affect the purity of gamete?
ANSWER:
If alleles don’t segregate during meiosis, gametes become impure and contain an abnormal number of chromosomes. This reduces genetic diversity and increases the risk of developmental problems in offspring, like miscarriages and genetic disorders.
(vi) If the alleles do not assort independently, which type of combination is missing in the progeny.
ANSWER:
Independent assortment means genes mix freely, creating new trait combinations in offspring. If genes are linked (located close on a chromosome), they are usually inherited together. This limits the creation of new combinations (recombinant genotypes). Therefore, a lack of independent assortment means fewer recombinant genotypes in the offspring.
(vii) Why has each gamete equal chance of getting one or the other allele of a pair?
ANSWER:
Each gamete has an equal chance of getting one or the other allele during meiosis due to random segregation. During this process, the paired homologous chromosomes line up randomly and separate. Since each gamete receives one chromosome from each pair, the chance of receiving either allele from the pair becomes 50/50. This process, called Mendel’s Law of Segregation, ensures that each gamete carries a unique combination of alleles.
(viii) Does the dominant allele modify the determinative nature of its recessive partner? What sort of relationship do they have?
ANSWER:
A dominant allele doesn’t modify the recessive one, rather masks its expression. They have a hierarchical relationship, where the dominant allele dictates the observable trait while the recessive remains hidden unless present in a homozygous state.
(ix) Which type of traits can assort independently?
ANSWER:
Independent assortment primarily occurs for traits located on different chromosomes, as chromosomes assort independently during meiosis. Traits on the same chromosome may exhibit linkage and tend to be inherited together.
(x) Why does the blood group phenotype of a person remain constant throughout life?
ANSWER:
The blood group phenotype remains constant because the genes determining blood type are inherited from parents and do not change over an individual’s lifetime. These genes dictate the presence of specific antigens on red blood cells, which remain consistent once determined during development.
(xi) What is a universal blood donor?
ANSWER:
An individual with blood type O negative is a universal blood donor. They can donate red blood cells to recipients of any blood type because their blood lacks A, B, and Rh antigens, minimizing the risk of immune reactions.
(xii) How can ABO – incompatibility protect the baby against Rh – incompatibility?
ANSWER:
Sometimes a mild ABO incompatibility protects the baby against a more severe Rh incompatibility. If O– mother conceives A+ or B+ baby, any foetal A or B type RBCs entering the mother’s blood are quickly destroyed by her anti – A or anti – B antibodies, before she can form anti – Rh antibodies.
(xiii) Which type of genes do not obey law of independent assortment?
ANSWER:
Genes located on the same chromosome and in close physical proximity do not obey the law of independent assortment; instead, they demonstrate genetic linkage.
(xiv) How can linked genes be separated from each other?
ANSWER:
Linked genes can be separated through the process of recombination or crossing over during meiosis, where homologous chromosomes exchange genetic material.
(xv) What is multifactorial inheritance?
ANSWER:
Multifactorial Inheritance:
Multifactorial inheritance involves the combined influence of genetic and environmental factors on the expression of traits or the occurrence of diseases.
Example: Diabetes mellitus.
(xvi) What is MODY?
ANSWER:
MODY:
“About 2% – 5% of type II diabetics get the disease early in life, before 25 years of age. It is called Maturity Onset Diabetes of the Young or MODY.”
MODY can be inherited as an autosomal dominant trait.
About 50% of cases of MODY are caused by mutations in glucokinase gene.
MODY can also be caused by mutations in any of the four other genes which encode transcription factors involved in pancreatic development and insulin regulation
(xvii) Can a child have more intelligence (IQ score) than his parents?
ANSWER:
Yes, a child can have a higher intelligence quotient (IQ) than their parents. This is because a child’s IQ is influenced by a combination of genetic factors inherited from both parents and environmental influences, allowing for the potential of surpassing the IQ scores of their parents.
Q.05: EXTENSIVE QUESTIONS
(i) What is incomplete dominance? Explain it with an example.
ANSWER:
Consult textbook at page 171 — 172.
(ii) Define Mendel’s law of segregation. Explain it with an example.
ANSWER:
Consult textbook at page 164 — 167.
(iii) Define Mendel’s law of independent assortment. Explain it with an example.
ANSWER:
Consult textbook at page 168 — 170.
(iv) Define probability. Derive 9:3:3:1 F2 ratio of independent assortment through product rule.
ANSWER:
Consult textbook at page 169 — 170.
(v) What is codominance? Explain the phenomenon of codominance with an example.
ANSWER:
Consult textbook at page 172.
(vi) Define multiple alleles. Describe multiple allelic blood group system of man.
ANSWER:
Consult textbook at page 173 — 175.
(vii) What is Rh factor? Describe the genetic basis of Rh – blood group system of man.
ANSWER:
Consult textbook at page 175 — 176.
(viii) What is erythroblastosis foetalis? Discuss this adverse effect of Rh incompatibility? Also suggest a therapy to avoid Rh sensitization of an Rh” mother married to an Rh+ man.
ANSWER:
Consult textbook at page 176.
(ix) Define epistasis. Explain epistatic gene interaction with an example.
ANSWER:
Consult textbook at page 176 — 177.
(x) What is a pleiotropic gene? Discuss pleiotropy with examples.
ANSWER:
Consult textbook at page 177.
(xi) What are polygenes? Explain polygenic inheritance.
ANSWER:
Consult textbook at page 178 — 181.
(xii) What is crossing over? Define recombination frequency and explain its significance.
ANSWER:
Consult textbook at page 182 — 184.
(xiii) What are sex-chromosomes? Discuss the chromosomal patterns of sex determination in organisms.
ANSWER:
Consult textbook at page 184 — 188.
(xiv) Compare chromosomal determination of sex between Drosophila and humans.
ANSWER:
Consult textbook at page 186.
(xv) Define gene pool. Explain the concept of gene pool in a sample population.
ANSWER:
Consult textbook at page 164.
(xvi) What is sex linkage? Explain T. H. Morgan’s study of sex – linkage in Drosophila
ANSWER:
Consult textbook at page 188 — 192.
(xvii) Compare the pattern of inheritance of an X – linked dominant trait with an X – linked recessive trait in humans.
ANSWER:
Consult textbook at page 192 — 196.
(xviii) Explain diabetes mellitus and its genetic basis.
ANSWER:
Consult textbook at page 197.
(xix) Discuss the genetics of colour-blindness or haemophilia.
ANSWER:
Consult textbook at page 194 — 195.
NOTE: Please go through these solutions and give your opinion about the quality of the work. You may also give suggestions to improve the contents. Thanks!