Mendelism Questions in English

(N/A) test cross is a genetic cross between an individual with an unknown genotype (showing a dominant phenotype) and a homozygous recessive parent. It is used to determine whether the individual is homozygous or heterozygous for a specific trait.
If the progeny produced by a test cross shows $50 \%$ dominant trait and $50 \%$ recessive trait (a $1:1$ ratio),then the unknown individual is heterozygous $(Rr)$.
On the other hand,if all the progeny produced show the dominant trait,then the unknown individual is homozygous dominant $(RR)$.

(N/A) We study Genetics to understand fundamental biological questions such as:
- Why does an elephant always give birth to a baby elephant and not another animal?
- Why does a mango seed produce only a mango plant?
- Are offspring identical to their parents,or do they show variations?
- Why do siblings sometimes look similar and sometimes very different?
Genetics is the branch of biology that deals with the inheritance and variation of characters from parents to offspring.
Inheritance is the process by which characters are passed from parent to progeny; it is the basis of heredity.
Variation is the degree by which progeny differ from their parents or among members of the same species.
Historical Background:
Humans have known since $8000-1000$ $B.C.$ that sexual reproduction is a cause of variation.
They exploited these natural variations in wild populations to selectively breed organisms with desirable traits.
For example,through artificial selection and domestication from ancestral wild cows,we have developed breeds like the $Sahiwal$ cows in $Punjab$.
Although our ancestors understood the existence of inheritance and variation,they lacked knowledge of the scientific basis behind these phenomena.

(N/A) Gregor Mendel conducted hybridisation experiments on garden peas for seven years ($1856$-$1863$) and proposed the laws of inheritance in living organisms.
During Mendel's investigations into inheritance patterns,it was for the first time that statistical analysis and mathematical logic were applied to problems in biology.
His experiments had a large sampling size,which gave greater credibility to the data that he collected.
Also,the confirmation of his inferences from experiments on successive generations of his test plants proved that his results pointed to general rules of inheritance rather than being unsubstantiated ideas.
Mendel investigated characters in the garden pea plant that were manifested as two opposing traits,e.g.,tall or dwarf plants,yellow or green seeds.
This allowed him to set up a basic framework of rules governing inheritance,which was expanded on by later scientists to account for all the diverse natural observations and the complexity inherent in them.
Mendel conducted such artificial pollination/cross-pollination experiments using several true-breeding pea lines.
$A$ true-breeding line is one that,having undergone continuous self-pollination,shows stable trait inheritance and expression for several generations.
Mendel selected $14$ true-breeding pea plant varieties as pairs which were similar except for one character with contrasting traits.

(A) Mendel conducted experiments on pea plants using true-breeding lines.
True-breeding lines are those that,having undergone continuous self-pollination,show the stable trait inheritance and expression for several generations.
Mendel selected $14$ true-breeding pea plant varieties,as pairs which were similar except for one character with contrasting traits.
The seven pairs of contrasting traits studied by Mendel are:
$1$. Stem height: Tall $(T)$ / Dwarf $(t)$
$2$. Flower colour: Violet $(V)$ / White $(v)$
$3$. Flower position: Axial $(A)$ / Terminal $(a)$
$4$. Pod shape: Inflated $(I)$ / Constricted $(i)$
$5$. Pod colour: Green $(G)$ / Yellow $(g)$
$6$. Seed shape: Round $(R)$ / Wrinkled $(r)$
$7$. Seed colour: Yellow $(Y)$ / Green $(y)$

(N/A) Mendel selected two pea plants,one with a tall stem and the other with a dwarf stem. These plants were considered parent plants $(P)$,which were true-breeding (maintaining purity of traits over several generations).
In the female parent plant,the stamens were removed from the immature flower. This flower was covered with a small paper bag until it matured. Then,pollen grains collected from the male dwarf plant were dusted onto its stigma.
Pea flowers are bisexual. In this method,one plant is treated as male and the other as female.
The stamens of the plant designated as female are removed at the juvenile (immature) stage. This process is called emasculation,which is done to prevent self-pollination.
The emasculated flower is covered with a bag to prevent unwanted cross-pollination. This is called bagging.
Mature pollen grains from the male plant are collected and dusted onto the stigma of the emasculated flower.
The seeds from this plant are collected,sown,and grown into a new generation of plants.
This generation is called the first filial generation ($F_1$ generation).

(N/A) Mendel proposed that something was being passed down unchanged from parents to offspring through the gametes over successive generations. He called these things 'factors'. Today,we call them 'genes'. Therefore,genes are the units of inheritance.
Genes that code for a pair of contrasting traits are known as alleles.
In alphabetical symbols,capital letters are used for the trait expressed in the $F_1$ generation,and small letters are used for the other trait. For example,$T$ is used for tallness and $t$ for dwarfness. Now,$T$ and $t$ are alleles of each other,which can be expressed as $TT, Tt,$ or $tt$.
If both alleles are identical,the organism is homozygous ($TT$ or $tt$).
$TT$ and $tt$ represent the genotype of the plants,while the terms 'tall' and 'dwarf' represent the phenotype.
In a pair of dissimilar alleles,one dominates the other and is expressed in the $F_1$ generation. This is called the dominant factor,and the one that is not expressed is called the recessive factor.
In a homozygous condition,the alleles are identical ($TT$ or $tt$),but in a heterozygous condition,they are dissimilar,such as $Tt$. Crosses between $Tt$ and $tt$ are known as monohybrid experiments.

(A) The results obtained in a Punnett square,such as the $TT : Tt : tt$ ratio of $1 : 2 : 1$,can be expressed mathematically by the binomial expansion $(ax + by)^2$.
In this case,the gametes $T$ and $t$ are present in equal frequencies of $1/2$ each.
Therefore,the expression is $(1/2T + 1/2t)^2 = (1/2T + 1/2t) \times (1/2T + 1/2t) = 1/4TT + 1/2Tt + 1/4tt$.

(N/A) test cross is a genetic cross between an individual exhibiting a dominant phenotype (but with an unknown genotype) and an individual that is homozygous recessive for the trait.
It is necessary because,while genotypic ratios can be calculated using mathematical probability,it is not possible to determine the exact genotypic composition (homozygous dominant or heterozygous) simply by observing the phenotype of a dominant trait.
For example,it is impossible to distinguish between a tall plant with a $TT$ genotype and one with a $Tt$ genotype just by looking at it.
To solve this,Mendel devised the test cross.
$A$ back cross is defined as a cross of $F_{1}$ progeny back to one of their parents.
$A$ special type of back cross,where the $F_{1}$ progeny is crossed with the recessive parent,is known as a test cross.
This method allows us to determine if the dominant phenotype is homozygous or heterozygous.
For instance,in a monohybrid cross between a violet-flowered plant $(W)$ and a white-flowered plant $(w)$,the $F_{1}$ hybrid is a violet-flowered plant.
If all $F_{1}$ progeny are violet,the dominant parent is homozygous $(WW)$. If the progeny appear in a $1:1$ ratio of violet to white,the dominant parent is heterozygous $(Ww)$.
In the case of a dihybrid cross,such as a heterozygous violet and axial flower $(WwAa)$ crossed with a double recessive white and terminal flower $(wwaa)$,the resulting ratio will be $1:1:1:1$.

(A) To understand dominance,we must consider how genes function. $A$ gene contains the information required to express a particular trait.
In diploid organisms,each gene exists as a pair of alleles. These alleles may not always be identical; they can be heterozygous. $A$ variation in one allele often arises from a mutation that modifies the information it carries.
Consider a gene that codes for an enzyme. The two alleles of this gene are its two forms. The normal allele produces the enzyme required for the transformation of a substrate '$S$'.
The modified allele could be responsible for: $(i)$ a normal or less efficient enzyme,$(ii)$ a non-functional enzyme,or $(iii)$ the absence of the enzyme.
In the first case,the modified allele is equivalent to the unmodified allele,resulting in the same phenotype. Thus,the transformation of substrate '$S$' occurs normally.
However,if the modified allele produces a non-functional enzyme or no enzyme at all,the phenotype may be affected. The phenotype depends on the functioning of the unmodified allele.
The allele that produces the functional enzyme,which determines the original phenotype,is considered 'dominant',while the modified allele is considered 'recessive'.

(N/A)

Test Cross	Back Cross
$1$. It is a cross between an $F_1$ hybrid and a homozygous recessive parent.	$1$. It is a cross between an $F_1$ hybrid and any of its parents (either dominant or recessive).
$2$. It is used to determine the genotype of an organism showing a dominant phenotype.	$2$. It is used to improve the traits of a specific variety by introducing genes from a parent.
$3$. All test crosses are back crosses.	$3$. All back crosses are not necessarily test crosses.

(A) Mendel's success was primarily due to his choice of the garden pea plant ($Pisum$ $sativum$) for his experiments.
Key reasons include:
- The pea plant has several clearly defined,contrasting traits.
- It is easy to grow and has a short life cycle.
- It typically undergoes self-pollination,which allows for the production of pure-breeding lines.
- Cross-pollination can be easily performed artificially.
- It produces a large number of offspring,which makes statistical analysis reliable.

(N/A) Genetics is the branch of biology that deals with the study of inheritance and variation. It scientifically addresses questions regarding why offspring resemble their parents or show differences in their characteristics,and why siblings may look similar or different.
Inheritance is the process by which characters are passed on from parent to progeny. It is the basis of heredity,which refers to the transmission of genetic traits from parents to offspring.

(N/A) $1.$ Offspring: The organisms produced as a result of sexual reproduction are known as offspring.
$2.$ Phenotype: The observable physical or biochemical characteristics of an organism,as determined by both genetic makeup and environmental influences,are known as phenotype.

(N/A) $1$. Dominant trait: It is the trait that expresses itself in the $F_1$ generation even in the presence of a contrasting allele. It masks the expression of the recessive allele.
$2$. Recessive trait: It is the trait that remains hidden or suppressed in the $F_1$ generation and only expresses itself in the homozygous condition (e.g.,$tt$ or $aa$).

(N/A) $1$. Homozygous: An individual is said to be homozygous for a trait if it possesses two identical alleles for that specific gene (e.g.,$TT$ or $tt$). These individuals are often referred to as true-breeding or pure-breeding lines.
$2$. Heterozygous: An individual is said to be heterozygous for a trait if it possesses two different alleles for that specific gene (e.g.,$Tt$). These individuals are often referred to as hybrids.
$3$. In summary,homozygous organisms produce gametes with identical alleles,whereas heterozygous organisms produce gametes with different alleles.

(N/A) Gregor Mendel: He formulated the laws of inheritance through various experiments on pea plants.
Reginald $C$. Punnett: He developed the Punnett square,a graphical representation used to predict the genotypes and phenotypes of offspring in a genetic cross.

(B) In $1900$,Hugo de Vries,Carl Correns,and Erich von Tschermak independently rediscovered Mendel's laws of inheritance. They conducted experiments on plant hybridization and realized that Mendel's work,which had been overlooked for decades,provided the fundamental principles for understanding the transmission of traits from parents to offspring.

(B) The criteria for selecting an organism for genetic studies are as follows:
$(i)$ Shorter life span of the organism,which allows for studying multiple generations in a short time.
$(ii)$ They should produce a large number of progeny to facilitate statistical analysis.
$(iii)$ Clear differentiation of sex and traits,making them easily identifiable.
$(iv)$ Presence of many types of hereditary variations.
$(v)$ Availability of pure breeding lines.

(B) Mendel performed self-pollination (selfing) among the $F_1$ tall hybrids $(Tt)$ to allow the segregation of alleles.
According to the Law of Segregation,the alleles of a gene pair separate from each other during gamete formation.
By self-pollinating the $F_1$ plants,Mendel allowed the recessive alleles to pair up again,which resulted in the reappearance of the dwarf phenotype in the $F_2$ generation in a $3:1$ phenotypic ratio.

(A) When crossing red and white flowered plants,only red-flowered plants appeared in the $F_1$ generation. However,white-flowered plants reappeared in the $F_2$ generation,which was derived from self-pollinating the $F_1$ individuals.
Mendel reasoned that there is a 'factor' (now called a gene) for every character. Accordingly,there must be one factor $(R)$ for the red flower trait and another factor $(r)$ for the white flower trait.
If an organism possessed only one copy of the gene,the reappearance of the white flower trait in the $F_2$ generation would be impossible. Furthermore,the observed phenotypic ratio of $3:1$ (red to white) indicates that each organism must possess two copies of a particular gene,which segregate during gamete formation. This observation forms the basis of Mendel's Law of Segregation and the Law of Dominance.

(N/A) The wrinkled phenotype of pea seeds is determined by the recessive allele $(w)$.
In the homozygous recessive condition $(ww)$,the starch branching enzyme is defective.
This leads to the production of less branched starch,which results in a lower osmotic pressure within the developing seed.
Consequently,the seed loses water during maturation and shrinks,resulting in the wrinkled appearance.

(C) true-breeding line for a trait is one that has undergone continuous self-pollination or brother-sister mating.
It shows stability in the inheritance of the trait for several generations.
This means that the offspring produced are identical to the parents for that specific trait.

(D) Mendel's success was due to several factors:
$(i)$ He chose the garden pea plant ($Pisum$ $sativum$),which has a short life cycle and distinct contrasting traits.
$(ii)$ He maintained accurate records of all observations and used statistical analysis and mathematical logic to explain his results.
$(iii)$ He focused on one or two traits at a time,which simplified the analysis of inheritance patterns.
$(iv)$ He performed cross-pollination experiments carefully to ensure pure-breeding lines.
$(v)$ He studied multiple generations $(F_1, F_2, F_3)$ to confirm the consistency of his findings.

(D) Gregor Mendel selected $14$ true-breeding pea plant varieties.
These varieties were chosen as pairs that were similar in all aspects except for one character with contrasting traits.
These $14$ varieties represented $7$ pairs of contrasting traits (e.g.,tall/dwarf,violet/white flowers,etc.).

(C) Gregor Mendel selected $14$ true-breeding pea plant varieties (lines) as pairs which were similar except for one character with contrasting traits.
These $14$ varieties represented $7$ pairs of contrasting traits (e.g.,tall/dwarf,violet/white flowers,etc.).
Therefore,the total number of true-breeding varieties selected was $14$.

(N/A) Mendelian experiments involve controlled breeding and the study of multiple generations to observe inheritance patterns. These experiments cannot be performed on humans for the following reasons:
$1$. The generation time in humans is very long,making it difficult to study multiple generations within a reasonable timeframe.
$2$. The number of offspring produced by a human couple is very small,which is insufficient for statistical analysis of Mendelian ratios.
$3$. Controlled breeding or experimental crosses are ethically and socially unacceptable in human populations.

(N/A) $True \ breeding \ line$ is one that has undergone continuous self-pollination,showing stable trait inheritance and expression for several generations.

(N/A) The offspring produced by the self-pollination or self-crossing of $F_1$ individuals are known as $F_2$ individuals. This generation represents the second filial generation in a genetic cross.

(N/A) Mendel proposed that something was being stably passed down unchanged from parent to offspring through the gametes over successive generations. He called these units of inheritance 'factors'. In modern genetics,these 'factors' are known as 'genes'.

(N/A) $Genes$ are the fundamental units of inheritance. They consist of specific segments of $DNA$ that contain the genetic information required to express a particular trait in an organism.
$Alleles$ are slightly different forms of the same gene. They arise due to mutations and are responsible for the variations in a specific trait,such as tall or dwarf height in pea plants.

(N/A) $1$. Dominant gene: The allele that expresses its phenotypic effect in both homozygous and heterozygous conditions is called a dominant gene.
$2$. Recessive gene: The allele that expresses its phenotypic effect only in the homozygous condition and remains masked in the presence of a dominant allele is called a recessive gene.

(N/A) Phenotype refers to the observable physical or biochemical characteristics of an organism,as determined by both genetic makeup and environmental influences. It is the apparent expression seen due to the combination of genes. For example,in pea plants,'tall' and 'dwarf' are phenotypic expressions of the underlying genotypes.

(N/A) Genotype refers to the genetic constitution of an organism,which determines the specific traits or characters expressed. It represents the set of alleles present in an individual for a particular trait. For example,in pea plants,the genotypes for height can be $TT$ (homozygous dominant),$Tt$ (heterozygous),or $tt$ (homozygous recessive).

(N/A) Homozygous refers to a condition in an organism where both alleles for a specific gene are identical.
For example,in a diploid organism,the genotypes $TT$ (homozygous dominant) or $tt$ (homozygous recessive) represent the homozygous condition.

(N/A) Heterozygous is a genetic condition in which an individual possesses two different alleles for a specific gene or character.
For example,in a pea plant,the genotype $Tt$ represents a heterozygous condition for height,where $T$ is the allele for tallness and $t$ is the allele for dwarfness.

(N/A) Punnett square is a graphical representation used to calculate the probability of all possible genotypes of offspring resulting from a genetic cross. It was developed by the British geneticist Reginald $C$. Punnett. It helps in visualizing the segregation of alleles and the random fertilization of gametes.

(N/A) test cross is a genetic cross between an individual with a dominant phenotype (whose genotype is unknown) and an individual that is homozygous recessive for the trait.
This cross is performed to determine the genotype of the organism showing the dominant phenotype.
If the offspring show a $1:1$ ratio of dominant to recessive phenotypes,the parent is heterozygous.
If all offspring show the dominant phenotype,the parent is homozygous dominant.

(N/A) The cross between the progeny of $F_{1}$ and the homozygous recessive parent is called a $Test$ $cross$.
It is useful in determining the genotype of an organism showing a dominant phenotype.
By performing this cross,we can identify whether the individual is homozygous dominant or heterozygous for a particular trait.

(B) The term 'Genetics' was coined by William Bateson in $1905$.
Gregor Mendel is known as the 'Father of Genetics' for his experiments on pea plants,but he did not coin the term itself.
$T. H.$ Morgan is famous for his work on linkage and crossing over in Drosophila.
Therefore,the correct answer is Bateson.

(B) Gregor Mendel conducted his hybridization experiments on garden peas $(Pisum \text{ } sativum)$.
He selected $7$ pairs of contrasting traits for his study.
These traits include stem height (tall/dwarf), flower color (violet/white), flower position (axial/terminal), pod shape (inflated/constricted), pod color (green/yellow), seed shape (round/wrinkled), and seed color (yellow/green).
Therefore, the correct answer is $7$ pairs.

(D) Mendel studied seven pairs of contrasting traits in pea plants.
In these traits,one is dominant and the other is recessive.
$1$. Height: Tall (dominant) vs. Dwarf (recessive).
$2$. Pod color: Green (dominant) vs. Yellow (recessive).
$3$. Seed color: Yellow (dominant) vs. Green (recessive).
$4$. Flower position: Axial (dominant) vs. Terminal (recessive).
Comparing the options:
- Tall plant is dominant.
- Green pod is dominant.
- Yellow seed is dominant.
- Terminal flower is recessive.
Therefore,the correct option is $D$.

(C) Gregor Mendel,in his experiments on pea plants,proposed that something was being stably passed down,unchanged,from parent to offspring through the gametes over successive generations. He called these entities 'factors'. In modern genetics,these factors are now referred to as 'genes'. Therefore,the correct term used by Mendel for these units of inheritance is 'factors'.

(A) The genes that code for a pair of contrasting traits are known as $Alleles$ (or $Allelomorphs$).
$Alleles$ are slightly different forms of the same gene that occupy the same locus on homologous chromosomes.
For example,in pea plants,the gene for height has two $Alleles$: one for tallness $(T)$ and one for dwarfness $(t)$.

(B) test cross is a genetic cross between an individual with a dominant phenotype (but unknown genotype) and a homozygous recessive individual.
Its primary purpose is to determine the genotype of the individual displaying the dominant trait.
If the offspring show a $1:1$ ratio of dominant to recessive phenotypes,the parent is heterozygous.
If all offspring show the dominant phenotype,the parent is homozygous dominant.

(A) In pea plants,the gene $B$ controls starch synthesis and seed shape.
$BB$ homozygotes produce large starch grains and round seeds.
$bb$ homozygotes produce small starch grains and wrinkled seeds.
In the heterozygous condition $(Bb)$,the starch grains are of intermediate size,which suggests incomplete dominance for starch grain size.
However,the phenotype of the seed shape in $Bb$ individuals is round,which is identical to the $BB$ phenotype.
Therefore,with respect to the seed shape,the $B$ allele shows complete dominance over the $b$ allele.

(B) The number of different types of gametes produced by an organism with a given genotype can be calculated using the formula $2^n$,where $n$ is the number of heterozygous gene pairs.
In the genotype $AaBbCc$,there are $3$ heterozygous gene pairs ($Aa$,$Bb$,and $Cc$).
Therefore,$n = 3$.
Number of gametes = $2^n = 2^3 = 2 \times 2 \times 2 = 8$.
Thus,the organism will produce $8$ different types of gametes.

(D) Gregor Mendel's work on the laws of inheritance remained unrecognized for many years. In $1900$,three scientists independently rediscovered Mendel's results: Hugo de Vries (from the Netherlands),Carl Correns (from Germany),and Erich von Tschermak (from Austria). These scientists confirmed Mendel's findings on the inheritance of traits in plants.

(D) Mendel's work remained unrecognized for several years due to the following reasons:
$1$. Communication was not easy in those days,and his work could not be widely publicized.
$2$. His concept of 'factors' (genes) as stable and discrete units that control the expression of traits and do not 'blend' with each other was not accepted by his contemporaries,as they were looking for continuous variation.
$3$. Mendel's approach of using mathematics and statistics to explain biological phenomena was entirely new and unacceptable to the biologists of that time.
$4$. He could not provide any physical proof for the existence of factors (i.e.,what they were made of or where they were located).

(B) test cross is defined as a cross between an individual of unknown genotype (usually $F_1$ generation) and a homozygous recessive individual.
This cross is used to determine whether the dominant phenotype is homozygous or heterozygous.
If the offspring show a $1:1$ phenotypic ratio,the $F_1$ individual is heterozygous.
If all offspring show the dominant trait,the $F_1$ individual is homozygous dominant.

(C) According to Mendel's law of dominance,when a homozygous tall plant $(TT)$ is crossed with a homozygous dwarf plant $(tt)$,all the offspring in the $F_1$ generation will be heterozygous tall $(Tt)$.
Since the allele for tallness $(T)$ is dominant over the allele for dwarfness $(t)$,the phenotype of all $F_1$ individuals will be tall.
Therefore,the percentage of dwarf progeny in the $F_1$ generation is $0 \%$.