Principles Of Inheritance And Variation Class 12 Biology Chapter 4 Notes

Principles of Inheritance And Variation Class 12 Biology Chapter 4 Notes

Gregor Mendel’s experiments on inheritance in garden peas

Gregor Mendel and Inheritance

Mendel made significant progress in the understanding of inheritance during the mid-nineteenth century.
He conducted hybridization experiments on garden peas from 1856 to 1863.
Mendel’s work laid the foundation for the laws of inheritance in living organisms.
He applied statistical analysis and mathematical logic to biological problems, marking a pioneering approach.
His experiments had a large sample size, which enhanced the credibility of his data.
Mendel’s inferences were confirmed through experiments on successive generations of test plants, establishing general rules of inheritance.

Characteristics Investigated

Mendel examined traits in garden pea plants that exhibited two opposing traits, such as tall or dwarf plants, yellow or green seeds.
This allowed him to establish fundamental rules governing inheritance, which later scientists expanded upon to account for diverse natural observations.

True-Breeding Lines

Mendel conducted artificial pollination/cross-pollination experiments using several true-breeding pea lines.
True-breeding lines show stable trait inheritance and expression over multiple generations due to continuous self-pollination.

Pairs of Contrasting Traits

Mendel selected 14 pairs of true-breeding pea plant varieties, each pair being similar except for one character with contrasting traits.
Some of the selected contrasting traits included smooth or wrinkled seeds, yellow or green seeds, inflated or constricted green or yellow pods, and tall or dwarf plants.

Contrasting Traits Studied by Mendel in Pea Plants

Stem height: Tall / Dwarf
Flower colour: Violet / White
Flower position: Axial / Terminal
Pod shape: Inflated / Constricted
Pod colour: Green / Yellow
Seed shape: Round / Wrinkled
Seed colour: Yellow / Green

Mendel’s Experiments and the Laws of Inheritance

I. Mendel’s Hybridization Experiment

Mendel’s experiment: Gregor Mendel conducted groundbreaking hybridization experiments with pea plants to study the inheritance of specific traits.
Experimental setup: He crossed tall and dwarf pea plants to observe the transmission of a single gene.
First hybrid generation: Known as the F1 progeny, all plants were tall, resembling one of the parent plants.
Trait expression in F1 generation: Mendel noticed that only one of the parental traits was expressed in the F1 generation, and there was no blending of traits.
Second hybrid generation: Called the F2 generation, Mendel observed that some of the offspring were dwarf, which was not seen in the F1 generation.
Phenotypic ratio in F2 generation: The proportion of dwarf plants in the F2 generation was 1/4th, while 3/4th of the plants were tall. There was no intermediate height observed.

II. The Concept of Genes

Mendel’s proposal: Mendel hypothesized that specific elements were stably passed down from one generation to the next through gametes, maintaining their characteristics over successive generations. These elements were termed “factors,” which we now refer to as genes.
Genes as units of inheritance: Genes contain the information required to express a particular trait in an organism.
Alleles: Genes responsible for a pair of contrasting traits are known as alleles. Alleles are slightly different forms of the same gene. For example, for the trait of height, T represents tallness, and t represents dwarfness. T and t are alleles of each other.
Genotypes and phenotypes: Genotypes are the genetic composition of an organism (e.g., TT, Tt, or tt for height), while phenotypes are the observable traits (e.g., tall or dwarf).

III. Dominance and Recessiveness

Dominance and recessiveness: Mendel observed that in pairs of dissimilar factors, one factor dominates the other, as seen in the F1 generation. The dominant factor masks the expression of the recessive factor. In the case of height, T (tallness) is dominant over t (dwarfness).
Notation: To represent dominance and recessiveness, capital letters (e.g., T) are used for the dominant trait, and lowercase letters (e.g., t) are used for the recessive trait.
Heterozygotes: Organisms with dissimilar alleles (e.g., Tt) are heterozygous for a specific trait.
Monohybrid cross: A cross between individuals with different alleles for a single character, such as the cross between TT and tt.

IV. Punnett Square

Developed by Reginald C. Punnett: A graphical tool to calculate the probabilities of different genotypes in the offspring resulting from a genetic cross.
Configuration: Possible gametes are written on the top row and left columns of the square, with all possible combinations represented in the boxes below.

V. Phenotypic and Genotypic Ratios

Phenotypic ratios: In the F2 generation, the phenotypic ratio was 3/4 tall (either TT or Tt) to 1/4 dwarf (tt).
Genotypic ratios: In the F2 generation, the genotypic ratio was 1/4 TT, 1/2 Tt, and 1/4 tt.

VI. Self-Pollination and Test Cross

Mendel’s self-pollination experiments: Mendel found that dwarf F2 plants continued to produce dwarf offspring in subsequent generations, confirming their homozygous genotype (tt).
Test Cross: To determine the genotype of a tall plant with an unknown genotype, Mendel performed a test cross. In a test cross, an organism with a dominant phenotype is crossed with a recessive parent to reveal the genotype of the dominant individual.

VII. Mendel’s Laws of Inheritance

First Law (Law of Dominance): The dominant allele (e.g., T) masks the expression of the recessive allele (e.g., t) in heterozygotes.
Second Law (Law of Segregation): During gamete formation through meiosis, alleles segregate or separate randomly, with a 50% chance of each allele being transmitted to the offspring.

Mendel’s Law of Dominance

I. Law of Dominance

Characters are controlled by discrete units called factors, which we now call genes.
Factors occur in pairs, with each individual inheriting one member of each pair from each parent.
In a dissimilar pair of factors, one member of the pair dominates or masks the expression of the other, and it is termed the dominant factor, while the other is referred to as the recessive factor.

II. Application of the Law of Dominance

Monohybrid Cross in the F1 Generation: When Mendel performed a monohybrid cross, he observed that only one of the parental characters was expressed in the F1 generation.
Expression of Both Characters in the F2 Generation: In contrast, during the F2 generation, both parental characters were expressed.
Phenotypic Proportion in the F2 Generation: The law of dominance explains the 3:1 phenotypic proportion observed in the F2 generation, where 3/4 of the offspring exhibit the dominant trait, and 1/4 exhibit the recessive trait.

Law of Segregation

Mendel’s Law of Dominance is firmly grounded in the principle that alleles do not exhibit blending of traits. This law explains why both parental characters reappear in the F2 generation, even though only one of them is visible in the F1 stage. It is important to understand the underlying mechanisms:

1. No Blending of Alleles:

The law is based on the observation that alleles for a specific trait do not blend together in a way that creates an intermediate or mixed phenotype.
Instead, one allele, typically the dominant one, fully expresses its trait in the presence of the recessive allele.

2. Expression in F2 Generation:

Despite the dominance of one allele in the F1 generation, both alleles for a particular trait are recovered in their original form in the F2 generation.
This phenomenon accounts for the reappearance of the recessive trait in the F2 generation.

3. Allele Segregation During Gamete Formation:

During the formation of gametes (sperm and egg cells), the two alleles of a gene segregate from each other.
This means that each gamete receives only one of the two alleles carried by the parent.
For homozygous parents (having two identical alleles), all gametes produced are the same.
For heterozygous parents (having two different alleles), they produce two types of gametes, each carrying one allele.

Incomplete Dominance and Explanation of Dominance

I. Incomplete Dominance

Mendel’s experiments with pea plants revealed complete dominance.
However, in some cases, the F1 generation doesn’t resemble either parent and displays an intermediate phenotype, known as incomplete dominance.
Example: Flower color inheritance in the dog flower (snapdragon or Antirrhinum sp.).
Cross between red-flowered (RR) and white-flowered (rr) true-breeding plants results in pink-flowered F1 generation (Rr).
When the F1 generation is self-pollinated, the F2 generation ratio is 1 (RR) Red: 2 (Rr) Pink: 1 (rr) White.
Genotype ratios in F2 follow Mendelian monohybrid cross rules, but phenotype ratios differ from the typical 3:1 dominant : recessive ratio.
Explanation needed for why R is not completely dominant over r in this case.

II. Explanation of the Concept of Dominance

Dominance refers to the interaction between alleles of a gene that determines the expression of a particular trait.
In a diploid organism, each gene exists in two copies, forming a pair of alleles, which may not always be identical in a heterozygote.
Allele changes: Alleles can differ due to genetic mutations or variations that modify the information they contain.
Example: Consider a gene responsible for producing an enzyme, with two allelic forms.
The normal allele produces a functional enzyme required for a specific biochemical reaction.
The modified allele can result in different outcomes:
- (i) Production of a normal but less efficient enzyme.
- (ii) Production of a non-functional enzyme.
- (iii) No enzyme production at all.
Dominance determination: The functioning of the phenotype/trait depends on the unmodified (functioning) allele.
Dominant allele: The allele that produces the original phenotype or trait, typically because it codes for a functional protein.
Recessive allele: The allele that results in a non-functional protein or no protein production, leading to a different phenotype.
Incomplete dominance: Occurs when neither allele is completely dominant over the other, resulting in an intermediate phenotype.
In the snapdragon example, pink flowers appear because neither the red nor white allele is completely dominant, and the resulting phenotype is a blend of both.

Co-dominance and Multiple Alleles

I. Co-dominance

In co-dominance, the F1 generation exhibits a phenotype that resembles both parents simultaneously.
Example: ABO blood groups in humans, controlled by the I gene with three alleles: IA, IB, and i.
IA and IB produce slightly different sugars on the surface of red blood cells, while i doesn’t produce any sugar.
Dominance relationships: IA and IB are both completely dominant over i.
Phenotypic expression:
- When IA and i are present, only IA is expressed (A type of sugar).
- When IB and i are present, only IB is expressed (B type of sugar).
- When IA and IB are present together, they both express their sugars (AB type of sugar).
Co-dominance example: Red blood cells display both A and B types of sugars.

II. Multiple Alleles

ABO blood grouping illustrates multiple alleles, where more than two alleles govern the same trait.
In humans, each person has two of the three I gene alleles (IA, IB, or i).
Three possible genotypes determine the ABO blood type (e.g., IAIA, IBIB, IAIB).
Multiple alleles’ complexity: Six different genotypes are possible from these three alleles.
Phenotypes: The number of possible phenotypes depends on the combinations of alleles.
Multiple alleles in populations: While individuals possess only two alleles, multiple alleles can be observed in populations through extensive studies.

III. Multiple Effects of a Single Gene

Some genes can have multiple effects or produce more than one outcome.
Example: Starch synthesis in pea seeds, controlled by one gene with two alleles (B and b).
B is associated with efficient starch synthesis, resulting in large starch grains in BB homozygotes.
b is linked to less efficient starch synthesis, yielding smaller starch grains in bb homozygotes.
Seed appearance: BB seeds are round, while bb seeds are wrinkled, suggesting B’s dominance.
However, in Bb heterozygotes, starch grain size is intermediate, showing incomplete dominance.
Dominance not intrinsic: Dominance is not solely based on a gene or its product but depends on the specific phenotype under consideration.

Mendel’s Dihybrid Cross and the Law of Independent Assortment

Mendel extended his experiments to study pea plants that differed in two characters simultaneously, leading to dihybrid crosses. One of the dihybrid crosses involved pea plants with seeds exhibiting two traits: color and shape.

I. Dominance in Dihybrid Cross

Mendel’s dihybrid cross involved pea plants with yellow and round seeds (RRYY) and pea plants with green and wrinkled seeds (rryy).
F1 Generation: The offspring from this cross had yellow and round seeds.
Determining Dominance:
- Yellow color was dominant over green.
- Round shape was dominant over wrinkled.
These results mirrored the outcomes of separate monohybrid crosses involving seed color and seed shape.

II. Genotypic Symbols

Mendel used genotypic symbols to represent the alleles for each trait:
- Y for dominant yellow seed color.
- y for recessive green seed color.
- R for dominant round seed shape.
- r for recessive wrinkled seed shape.
Genotype of the Parent Plants:
- Yellow and round seed plant: RRYY.
- Green and wrinkled seed plant: rryy.

III. Dihybrid Cross

The dihybrid cross between these parent plants can be represented as follows:
- Parent Plant Genotypes: RRYY (Yellow and round) x rryy (Green and wrinkled).
- Gametes: The parent plants produce gametes, RY and ry.
- F1 Hybrid: The F1 hybrid resulting from fertilization is RrYy.

IV. F2 Generation

When Mendel self-hybridized the F1 plants, he observed the outcomes in the F2 generation.
Phenotypic Segregation:
- Yellow seeds segregated in a 3:1 ratio, similar to a monohybrid cross.
- Green seeds appeared in 1/4th of the F2 plants.
Shape Segregation:
- Round and wrinkled seed shapes also segregated in a 3:1 ratio, as in a monohybrid cross.

V. Law of Independent Assortment

Mendel’s dihybrid cross results demonstrated that traits for seed color and seed shape independently sorted during gamete formation and inheritance.
Law of Independent Assortment: Alleles for different traits assort independently during gamete formation, leading to a variety of possible combinations in the offspring.

Mendel’s Law of Independent Assortment

I. Law of Independent Assortment

In a dihybrid cross, Mendel observed a phenotypic ratio of 9:3:3:1, which represented the combinations of two pairs of traits: round, yellow; wrinkled, yellow; round, green; and wrinkled, green.
Derivation of the Ratio: This ratio can be derived as a combination of two independent ratios: 3 yellow : 1 green and 3 round : 1 wrinkled.
- (3 Round : 1 Wrinkled) x (3 Yellow : 1 Green) = 9 Round, Yellow : 3 Wrinkled, Yellow : 3 Round, Green : 1 Wrinkled, Green.
Mendel proposed Mendel’s Law of Independent Assortment based on these observations.
Law of Independent Assortment: When two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair of characters.

II. Application of the Law of Independent Assortment

Understanding Independent Assortment: The Punnett square can help visualize the independent segregation of genes during meiosis and gamete formation in an F1 RrYy plant.
- Segregation of R and r: 50% of gametes carry R, and 50% carry r.
- Segregation of Y and y: 50% of gametes carry Y, and 50% carry y.
- Independent Assortment: The segregation of R and r is independent of the segregation of Y and y, resulting in four different genotypes of gametes: RY, Ry, rY, and ry, each with a frequency of 25% (1/4th).
Punnett Square Application: Using a Punnett square, one can determine the composition of zygotes and the genotypes of the F2 generation.

III. Genotype and Phenotype Combinations

In the Punnett square data, different genotypic and phenotypic combinations can be derived.
Genotypes: The four possible genotypes of offspring are RY, Ry, rY, and ry.
Phenotypes: The associated phenotypes depend on the traits governed by these genotypes.

IV. Genotypic Ratio in F2 Stage

Using the Punnett square data, one can calculate the genotypic ratio in the F2 stage.
The genotypic ratio can be filled in the given format.

V. Verification of Genotypic Ratio

It’s essential to determine if the genotypic ratio matches the expected 9:3:3:1 ratio as per Mendel’s observations.

Chromosomal Theory of Inheritance

Background:

Gregor Mendel published groundbreaking work on inheritance in 1865.
However, it remained unrecognized until 1900 due to various challenges.

Challenges:

Limited communication in Mendel’s time hindered the spread of his work.
Mendel’s concept of genes as stable and discrete units was initially met with skepticism.
His mathematical approach to biology was unconventional.
Lack of physical proof for the existence and composition of genes.

Rediscovery and Chromosome Revelation:

In 1900, three independent scientists (de Vries, Correns, and von Tschermak) rediscovered Mendel’s findings.
Advancements in microscopy allowed the observation of cell division and the discovery of chromosomes.
Chromosomes were visualized as structures in the nucleus, doubling and dividing before cell division.
By 1902, scientists understood the movement of chromosomes during meiosis.

Chromosome-Gene Linkage:

Walter Sutton and Theodore Boveri linked chromosome behavior to Mendel’s laws.
Chromosomes and genes both exist in pairs, with alleles located on homologous chromosomes.
Chromosomes segregate during gamete formation, paralleling gene segregation.
Anaphase of meiosis I showed independent alignment of chromosome pairs.
Sutton and Boveri proposed the chromosomal theory of inheritance, where chromosome behavior mirrors gene behavior.

Experimental Verification:

Thomas Hunt Morgan and his colleagues experimentally verified the chromosomal theory.
They used Drosophila melanogaster (fruit flies) due to their suitability for genetic studies.
Drosophila offered rapid life cycles, easy sex differentiation, and observable hereditary traits.
It generated numerous progeny from a single mating, making it ideal for genetic research.
Drosophila studies revealed diverse hereditary variations, confirming and advancing the chromosomal theory.

Linkage and Recombination in Drosophila Genetics

Dihybrid Crosses in Drosophila:

Thomas Hunt Morgan conducted dihybrid crosses in Drosophila to study sex-linked genes.
Similar to Mendel’s pea experiments, he hybridized different traits in Drosophila.
One example involved yellow-bodied, white-eyed females and brown-bodied, red-eyed males.
When intercrossing their F1 progeny, Morgan noticed that these two genes did not segregate independently.
The observed F2 ratio significantly deviated from the expected 9:3:3:1 ratio for independent gene segregation.

Discovery of Linkage and Recombination:

Morgan and his team determined that the genes were located on the X chromosome.
They quickly realized that when two genes in a dihybrid cross were on the same chromosome, there was a higher proportion of parental gene combinations than non-parental ones.
This phenomenon was attributed to the physical association of genes on a chromosome, leading to the term “linkage.”
“Recombination” was coined to describe the generation of non-parental gene combinations.

Variability in Linkage:

Morgan and his group found that genes on the same chromosome could be tightly or loosely linked.
Some genes showed very low recombination (tight linkage), while others exhibited higher recombination (loose linkage).
For instance, the genes white and yellow were tightly linked with only 1.3% recombination.
In contrast, white and miniature wing showed 37.2% recombination.

Genetic Mapping:

Alfred Sturtevant, a student of Morgan, used the frequency of recombination between gene pairs on the same chromosome to measure the distance between genes.
This allowed them to create genetic maps that specified the relative positions of genes on a chromosome.
Genetic maps have become vital tools in genome research and sequencing.
They were employed in projects like the Human Genome Sequencing Project to chart the arrangement of genes in the human genome.

Polygenic Inheritance: Understanding Complex Traits

Mendel’s experiments primarily focused on traits with distinct alternate forms, such as purple or white flower color.
However, many traits in nature do not exhibit such clear-cut distinctions and instead vary across a gradient.
Traits like human height or skin color are examples of such continuous traits.
These traits are controlled by multiple genes and are referred to as polygenic traits.
Polygenic inheritance also considers the influence of the environment on the phenotype.

Polygenic Trait Basics:

In polygenic inheritance, the phenotype results from the combined contributions of three or more genes.
Each gene has multiple alleles that contribute to the trait.
The effect of each allele is additive, meaning they contribute incrementally to the final phenotype.

Example – Human Skin Color:

Consider the example of human skin color, which is influenced by multiple genes.
Let’s assume three genes, A, B, and C, control skin color, with dominant forms (A, B, C) responsible for dark skin and recessive forms (a, b, c) for light skin.
Individuals with all dominant alleles (AABBCC) will have the darkest skin color.
Those with all recessive alleles (aabbcc) will have the lightest skin color.
Individuals with a mix of dominant and recessive alleles will have intermediate skin tones.
The specific combination and number of each type of allele in an individual’s genotype determine the darkness or lightness of their skin.

Key Points:

Polygenic traits involve multiple genes, each with multiple alleles.
Phenotype results from the additive effects of these alleles.
Continuous traits, like human height or skin color, are classic examples of polygenic inheritance.
Environmental factors can also influence the expression of polygenic traits.
Understanding polygenic inheritance helps explain the diversity of traits in populations and the variability seen in continuous traits.

Pleiotropy: A Single Gene, Multiple Phenotypic Effects

In genetics, we often observe that a single gene can have an impact on more than one phenotypic trait.
When a single gene influences multiple, seemingly unrelated phenotypes, it is referred to as a pleiotropic gene.
Pleiotropy is common and can result from a gene’s influence on various metabolic pathways, leading to different phenotypic expressions.

Example – Phenylketonuria (PKU):

PKU is a classic example of pleiotropy in humans.
This disease is caused by a mutation in a single gene responsible for coding the enzyme phenylalanine hydroxylase.
The pleiotropic effects of this mutation manifest in multiple phenotypic expressions:

Mental Retardation: Individuals with PKU typically experience intellectual disabilities due to the inability to metabolize phenylalanine properly.
Reduced Hair and Skin Pigmentation: Another effect of the same gene mutation is a reduction in the production of melanin, leading to lighter hair and skin pigmentation.

In this case, a single gene’s mutation affects both cognitive development and pigmentation, demonstrating pleiotropy.

Mechanism of Pleiotropy:

Pleiotropy often occurs when a gene plays a pivotal role in multiple biochemical pathways.
Changes in the gene’s function can have cascading effects on different biological processes, resulting in diverse phenotypic outcomes.
The underlying mechanism of pleiotropy is the gene’s influence on various metabolic pathways, each contributing to a different phenotype.

Significance:

Understanding pleiotropy is crucial for comprehending the complexity of genetic inheritance.
It underscores the interconnectedness of biological processes and how genetic mutations can have far-reaching consequences on an organism’s traits.
Studying pleiotropic genes is essential in fields like medical genetics, where mutations can lead to various health issues.

Sex Determination: A Genetic Puzzle Unraveled

Historical Perspective:

Understanding the mechanism of sex determination has long intrigued geneticists.
Early insights into the genetic and chromosomal basis of sex determination emerged from studies in insects.

Henking’s Discovery:

In 1891, scientist Henking observed a specific nuclear structure consistently present during spermatogenesis in certain insects.
He noted that 50% of sperm received this structure, later identified as a chromosome.
Henking referred to this structure as the “X body,” although its significance remained unclear.

X-Chromosome and XO Type:

Further investigations revealed that Henking’s “X body” was indeed a chromosome, now known as the X-chromosome.
In many insects, sex determination followed the XO type mechanism.
In this mechanism, all eggs carried an additional X-chromosome besides the autosomes, while sperm could carry either an X-chromosome or not.
Fertilization of eggs by sperm with an X-chromosome led to the development of females, while those fertilized by sperm without an X-chromosome became males.
This mechanism results in an unequal number of chromosomes between males and females.

XY Type of Sex Determination:

In contrast, XY type of sex determination was observed in various insects and mammals, including humans.
In this mechanism, both males and females have the same number of chromosomes.
Males possess an X-chromosome and a distinct Y-chromosome, while females have a pair of X-chromosomes in addition to the autosomes.
The presence of the Y-chromosome in males differentiates them from females.
This mechanism results in an equal number of chromosomes between males and females.

Male Heterogamety vs. Female Heterogamety:

In both XO and XY mechanisms, males produce two types of gametes: with or without the X-chromosome (for XO) or with X or Y-chromosomes (for XY).
This dual-gamete production is referred to as male heterogamety.
In some organisms like birds, a different mechanism known as female heterogamety is observed.
In female heterogamety, both males and females have the same total number of chromosomes.
Females produce two different types of gametes concerning sex chromosomes: Z and W.
Females have one Z and one W chromosome, while males possess a pair of Z-chromosomes alongside autosomes.

Conclusion:

The study of different mechanisms of sex determination, including XO, XY, and ZW systems, has enriched our understanding of genetics and sex determination in various species.
These mechanisms highlight the fascinating diversity of life’s genetic systems and their impact on the determination of an individual’s sex.

Sex Determination in Humans: The XY Type Mechanism

Chromosomal Basis:

In humans, the mechanism of sex determination follows the XY type.
Of the 23 pairs of chromosomes present in humans, 22 pairs are identical in both males and females, known as autosomes.
The remaining pair of chromosomes determines an individual’s sex.
In females, this pair consists of two X-chromosomes (XX), while in males, it consists of one X-chromosome and one Y-chromosome (XY).

Gamete Production:

During spermatogenesis (the process of sperm formation) in males, two types of gametes are produced.
Approximately 50% of the total sperm generated carry the X-chromosome, while the other 50% carry the Y-chromosome, in addition to autosomes.
In contrast, females produce only one type of ovum (egg), which contains an X-chromosome.

Fertilization Determines Sex:

When fertilization occurs, there is an equal probability of the ovum being fertilized by either an X-chromosome-carrying sperm or a Y-chromosome-carrying sperm.
The genetic makeup of the sperm that fertilizes the ovum ultimately determines the sex of the offspring.
If the ovum is fertilized by a sperm carrying an X-chromosome, the resulting zygote will develop into a female (XX).
Conversely, if the ovum is fertilized by a sperm carrying a Y-chromosome, the offspring will be male (XY).

Equal Probability of Sex:

This mechanism ensures that in each pregnancy, there is always a 50% probability of either a male or a female child.
Unfortunately, societal misconceptions and biases have led to the unfair blaming and mistreatment of women in some cultures for giving birth to female children, despite the fact that the sex of the child is determined by the genetic makeup of the sperm.

Sex Determination in Honey Bees: Haplodiploid System

Chromosomal Basis:

In honey bees, sex determination operates on a unique system known as haplodiploidy.
This system is based on the number of sets of chromosomes an individual inherits.
Honey bee offspring can develop into either females (queens or workers) or males (drones) based on their chromosome content.

Development of Females:

When a honey bee egg is formed through fertilization, resulting from the union of a sperm and an egg, it develops into a female bee (queen or worker).
These females are diploid, meaning they have two sets of chromosomes, for a total of 32 chromosomes.

Development of Males:

In contrast, unfertilized eggs in honey bees develop into males (drones) via a process called parthenogenesis.
Males are haploid, meaning they have only one set of chromosomes, totaling 16 chromosomes.

Haplodiploid System:

The unique haplodiploid sex-determination system in honey bees has distinctive characteristics:
- Males are haploid (1n), with half the number of chromosomes compared to females.
- Females are diploid (2n), with a complete set of chromosomes.
- Males produce sperms through mitosis, not meiosis.
- Males do not have fathers (since they develop from unfertilized eggs) and cannot have sons.
- Males have grandfathers (since they are produced from eggs laid by queens) and can have grandsons.

Comparison with Birds:

In birds, sex determination operates differently.
It is the female (the mother) who determines the sex of the chicks.
Birds have a ZW sex-determination system. Females have a pair of sex chromosomes (ZW), while males have two different sex chromosomes (ZZ).
The type of sex chromosome an egg carries determines whether the resulting chick will be male (ZZ) or female (ZW).
Thus, in birds, the egg’s genetic makeup (ZW or ZZ) is responsible for determining the sex of the offspring.

Mutation: Genetic Alteration and Variation

Mutation is a phenomenon that leads to changes in the DNA sequences of an organism.
These changes in DNA can result in alterations to both the genotype (genetic makeup) and phenotype (observable traits) of an organism.
In addition to genetic recombination, mutation is a crucial factor contributing to genetic diversity.

Types of Mutations:

Chromosomal Aberrations: Mutation can involve alterations in the structure or number of chromosomes. These changes are known as chromosomal aberrations and are commonly observed in cancer cells. They can include deletions (loss) or insertions/duplications (gain) of DNA segments.
Point Mutation: Point mutations involve changes in a single base pair of DNA. These mutations can have significant consequences. One classic example is sickle cell anemia.
Frame-Shift Mutations: Deletions or insertions of base pairs in DNA can cause frame-shift mutations, which can alter the reading frame of a gene and lead to non-functional proteins.

Causes of Mutations:

Various chemical and physical factors can induce mutations, and these agents are referred to as mutagens.
UV radiation, for example, is a mutagen that can cause mutations in organisms.

Note on Mechanism:

While the mechanisms underlying mutations are complex and beyond the scope of this discussion, it’s important to recognize that mutations can occur through various processes, including DNA replication errors, exposure to mutagenic chemicals, or radiation damage.

Significance of Mutations:

Mutations play a vital role in evolution by introducing new genetic variations into populations.
Some mutations may lead to genetic disorders or diseases, while others may confer advantageous traits, driving natural selection.

GENETIC DISORDERS

1). Pedigree Analysis:

Tracing Inheritance Patterns in Human Genetics

The concept that certain characteristics and disorders are inherited within families has been recognized for a long time in human society.
After the rediscovery of Mendel’s work on inheritance, the systematic analysis of the inheritance patterns of traits in humans began.
Since controlled crosses like those in pea plants are not possible in humans, the study of family history concerning the inheritance of specific traits became an alternative approach.
This analysis of traits over several generations of a family is known as pedigree analysis.

Purpose of Pedigree Analysis:

Pedigree analysis is a valuable tool in human genetics.
It is used to trace the inheritance of particular traits, abnormalities, or diseases within families.
The analysis is represented in the form of a family tree that spans multiple generations.

Standard Symbols in Pedigree Analysis:

Pedigree analysis employs standard symbols to represent individuals and their traits within a family tree.
These symbols help geneticists and researchers to visualize the inheritance pattern.

Genetic Basis:

As studied in this chapter, all features in any organism are controlled by genes located on DNA within chromosomes.
Genetic information carried by DNA is transmitted from one generation to the next without any change or alteration.
However, occasional changes or alterations in the genetic material, known as mutations, can occur.

Association with Disorders:

Many disorders in humans have been found to be associated with the inheritance of altered or mutated genes or chromosomes.
Pedigree analysis is a critical tool in identifying patterns of inheritance for these genetic disorders.
It helps geneticists understand how genetic traits or diseases are passed down through generations within families.

2). Mendelian Disorders in Humans

Genetic disorders in humans can be broadly categorized into two groups: Mendelian disorders and chromosomal disorders.
Mendelian disorders are primarily determined by alterations or mutations in a single gene.
The inheritance patterns of Mendelian disorders can be understood using principles of inheritance and analyzed through pedigree analysis.

Common Mendelian Disorders:

Haemophilia: Haemophilia is a sex-linked recessive disorder. It affects the blood’s ability to clot and is characterized by prolonged bleeding after injury.

Transmission: Unaffected carrier females can pass the disease to their sons.
Example: Queen Victoria’s family pedigree shows haemophilic descendants.

Sickle-Cell Anemia: Sickle-cell anemia is an autosomal recessive disorder caused by a mutation in the HbA and HbS genes.

Transmission: Both parents must be carriers (heterozygous) for their offspring to have the disease.
The mutation results in the substitution of Glutamic acid (Glu) with Valine (Val) in the beta globin chain of hemoglobin, causing RBCs to take on a sickle-like shape.

Colour Blindness: Colour blindness is a sex-linked recessive disorder related to defects in red or green cone cells in the eyes. It primarily affects males.

Transmission: Carrier mothers can pass the gene to their sons.

Phenylketonuria (PKU): PKU is an autosomal recessive metabolic disorder that leads to the accumulation of phenylalanine in the body, causing mental retardation.

Transmission: Both parents must be carriers (heterozygous) for their offspring to have the disease.

Thalassemia: Thalassemia is an autosomal recessive blood disorder that results in reduced synthesis of either alpha or beta globin chains of hemoglobin.

Transmission: Both parents must be unaffected carriers (heterozygous) for their offspring to have the disease.
It can be classified into alpha and beta thalassemia based on which globin chain is affected.

Understanding Mendelian Inheritance:

Mendelian disorders may be either dominant or recessive, and their inheritance can be determined through pedigree analysis.
Analysis of family trees helps in identifying patterns of inheritance for specific traits.
Sex-linked recessive traits, like haemophilia and colour blindness, show distinct transmission patterns from carrier females to male offspring.

3. Chromosomal Disorders in Humans

Chromosomal disorders are genetic disorders caused by abnormalities in the number or arrangement of chromosomes.
These disorders often result from errors during cell division, leading to conditions such as aneuploidy (gain or loss of chromosomes) and polyploidy (multiple sets of chromosomes).
Chromosomal disorders can have significant consequences and are typically associated with developmental and health issues.

Types of Chromosomal Disorders:

Aneuploidy: Aneuploidy occurs when there is an abnormal number of chromosomes due to errors in chromosome segregation during cell division.

Example: Down’s syndrome (trisomy 21), which results from the presence of an extra copy of chromosome 21.

Polyploidy: Polyploidy involves having more than two sets of chromosomes in an organism. This condition is commonly observed in plants.

Polyploidy results from the failure of cytokinesis after telophase during cell division, leading to the formation of extra sets of chromosomes.

Common Chromosomal Disorders:

Down’s Syndrome: Down’s syndrome is caused by trisomy 21, where an individual has three copies of chromosome 21 instead of the usual two.

Characteristics: Individuals with Down’s syndrome may have short stature, a small round head, a furrowed tongue, and developmental delays.
Genetic Basis: The presence of an extra copy of chromosome 21 leads to physical and mental developmental challenges.

Klinefelter’s Syndrome: Klinefelter’s syndrome is caused by the presence of an additional X chromosome, resulting in a karyotype of 47, XXY.

Characteristics: Affected individuals typically have overall masculine development but may also exhibit feminine characteristics, such as breast development (gynecomastia).
Genetic Basis: The extra X chromosome can result in infertility.

Turner’s Syndrome: Turner’s syndrome is caused by the absence of one X chromosome, resulting in a karyotype of 45, X0.

Characteristics: Affected females may have rudimentary ovaries and lack secondary sexual characteristics.
Genetic Basis: The absence of one X chromosome leads to sterility and various physical features associated with Turner’s syndrome.

Consequences of Chromosomal Disorders:

Chromosomal disorders can have serious consequences for individuals, affecting physical, psychomotor, and mental development.
Each disorder is associated with distinct characteristics and health challenges, and they are typically caused by errors in chromosome number or structure.