Principles Of Inheritance And Variation

1. Turner Syndrome:
- Individuals have a total of 45 chromosomes.
- Karyotype: (missing one sex chromosome).
2. Klinefelter Syndrome:
- Individuals have a total of 47 chromosomes.
- Karyotype: (one extra X chromosome).
Final Answer:

Mendel's Law of Segregation, also known as the First Law of Inheritance, states that:

Step 1: Explanation. - Each individual has two alleles for a trait (one from each parent). - During gamete formation (meiosis), these alleles segregate randomly. - Hence, each gamete carries only one allele for the given trait. - Fertilization restores the pair of alleles in the offspring.

Step 2: Example (Monohybrid Cross). Consider Mendel's experiment with pea plants: - Cross between a homozygous tall plant (TT) and a homozygous dwarf plant (tt). - F generation: All plants are heterozygous (Tt), showing tallness. - F generation (selfing of Tt): The ratio obtained is This shows that alleles separate during gamete formation and recombine at fertilization.

Final Answer:

In humans, the sex chromosomes (X and Y) are responsible for sex determination.
- Females have two X chromosomes .
- Males have one X and one Y chromosome .
The sperm determines the sex of the offspring:
- If the sperm carries an chromosome the child will be female .
- If the sperm carries a chromosome the child will be male .
Final Answer:

Step 1: Understanding Mendel's dihybrid cross.
In a dihybrid cross, Mendel studied the inheritance of two traits simultaneously. He crossed pea plants with two contrasting traits (e.g., round yellow seeds × wrinkled green seeds).

Step 2: Law of independent assortment.
According to Mendel's law of independent assortment, alleles of different genes assort independently during gamete formation, producing new combinations of traits.

Step 3: Phenotypic ratio.
When the F₁ heterozygotes are self-crossed, the F₂ generation shows four phenotypes in the ratio: - 9 with both dominant traits, - 3 with first dominant and second recessive, - 3 with first recessive and second dominant, - 1 with both recessive traits. Thus, the phenotypic ratio is .

Step 4: Conclusion.
The correct answer is (A) 9 : 3 : 3 : 1.

Codominance is a form of inheritance in which both alleles of a gene pair in a heterozygote are fully expressed, resulting in offspring with a phenotype that shows both traits simultaneously, without blending. Key Features: Example:
- In humans, the ABO blood group system is a classic example. The alleles and are codominant. - A person with genotype has both A and B antigens expressed on red blood cells, leading to AB blood group.

Step 1: Understanding congenital metabolic disorders.
Congenital metabolic disorders are inherited conditions that affect enzyme function and biochemical pathways from birth, leading to improper metabolism of certain substances.

Step 2: Analysis of options.
- (A) Phenylketonuria: A congenital metabolic disorder caused by a mutation leading to deficiency of the enzyme phenylalanine hydroxylase. This prevents metabolism of phenylalanine, causing accumulation and severe neurological issues.
- (B) Colour blindness: A genetic disorder but not metabolic; it is due to defects in cone cells of the retina.
- (C) Haemophilia: A genetic bleeding disorder caused by deficiency of clotting factors, not metabolic.
- (D) Anaemia: May result from nutritional deficiency or other causes, but it is not categorized as a congenital metabolic disorder.

Step 3: Conclusion.
The correct answer is (A) Phenylketonuria, which is a congenital metabolic disorder.

Sex-linked inheritance:
Sex-linked inheritance refers to the inheritance pattern of genes located on the sex chromosomes, typically the X chromosome. In humans, males have one X and one Y chromosome (XY), whereas females have two X chromosomes (XX). This difference results in different inheritance patterns for males and females. Males are more often affected by sex-linked disorders because they have only one X chromosome, while females have a second X chromosome to potentially compensate for the defective gene.
Haemophilia:
Haemophilia is a sex-linked recessive disorder. The gene responsible for haemophilia is located on the X chromosome. Males with the defective gene on their single X chromosome will express the disease because they have no second X chromosome to mask the defective gene. Females, on the other hand, would need to inherit the defective gene on both X chromosomes to exhibit the disease. If a female inherits the defective gene on only one X chromosome, she becomes a carrier and does not show symptoms of the disease.
Colourblindness:
Colourblindness is another example of a sex-linked recessive disorder. The gene for colour vision is located on the X chromosome. Like haemophilia, males with the defective gene on their single X chromosome will be colourblind. Females need to inherit two copies of the defective gene (one from each parent) to be colourblind. A female with only one defective copy will be a carrier but not affected by the condition.

Mendel's Law of Dominance is one of the three fundamental principles of inheritance that were established by Gregor Mendel in the 19th century. According to Mendel's law of dominance, when an organism inherits two alleles for a particular trait, one allele may be dominant and the other recessive. The dominant allele will mask the expression of the recessive allele in the organism's phenotype, meaning that the trait controlled by the dominant allele will be expressed, while the trait controlled by the recessive allele will not. ### Key Terms: \begin{enumerate} \item Dominant Allele: The allele that expresses its trait even in the presence of a different allele. It is usually represented by a capital letter (e.g., T for tall in pea plants). \item Recessive Allele: The allele that is masked in the presence of a dominant allele. It only expresses its trait when an organism has two copies of the recessive allele. It is represented by a lowercase letter (e.g., t for short in pea plants). \item Homozygous: When an organism inherits two identical alleles for a trait (e.g., TT or tt). \item Heterozygous: When an organism inherits two different alleles for a trait (e.g., Tt). \end{enumerate} ### Explanation with Example: Consider the example of pea plants, which were used by Mendel in his experiments. In this case, the height of the plant is governed by a single gene with two alleles: - The dominant allele (T) causes tall plants. - The recessive allele (t) causes short plants. Mendel crossbred plants with different genotypes and observed the following: - **Homozygous Dominant (TT):** These plants were tall because the dominant allele T was expressed. - **Homozygous Recessive (tt):** These plants were short because both alleles were recessive, and no dominant T allele was present to mask the effect. - **Heterozygous (Tt):** These plants were also tall because the dominant allele T masked the effect of the recessive allele t. ### Mendel's First Experiment (Monohybrid Cross): Mendel crossed a homozygous tall plant (TT) with a homozygous short plant (tt). All the F1 offspring in this cross had the genotype Tt and exhibited the dominant tall phenotype. This demonstrated that the dominant allele (T) masks the recessive allele (t) in the heterozygous state. When Mendel crossed two F1 plants (Tt), the resulting F2 generation had a 3:1 phenotypic ratio, with 75% of the plants being tall (dominant phenotype) and 25% being short (recessive phenotype). This result confirmed the principle of dominance and suggested that the alleles segregate independently during gamete formation.

Polygenic Inheritance:
Polygenic inheritance refers to the type of inheritance in which a characteristic is determined by the combined effects of two or more genes, and these genes are often located at different loci on different chromosomes. Unlike Mendelian inheritance, where a single gene controls a trait with distinct dominant and recessive alleles, polygenic traits exhibit a continuous range of phenotypes. These traits are often quantitative and show a gradual change, rather than the discrete classes seen in Mendel's experiments. In polygenic inheritance, each gene contributes a small, additive effect to the overall phenotype. As a result, the phenotype does not follow the simple dominant/recessive pattern, and the expression of the trait can vary along a spectrum. This continuous variation leads to a wide range of phenotypic expressions, such as height, skin color, and weight. ### Characteristics of Polygenic Traits: 1. **Multiple Genes Involved:** More than one gene controls the trait. 2. **Continuous Variation:** The trait shows a range of phenotypes, rather than discrete categories. 3. **Additive Effect:** Each gene contributes additively to the overall phenotype, so the more dominant alleles present, the more pronounced the trait. ### Example of Polygenic Inheritance: Human Skin Color: Human skin color is a classic example of polygenic inheritance. It is controlled by the interaction of several genes, each contributing to the overall pigmentation of the skin. The primary genes involved are located on different chromosomes, and each gene has multiple alleles that contribute to skin pigmentation. - Individuals with darker skin have more of the dominant alleles for pigmentation, leading to higher amounts of melanin. - Individuals with lighter skin have a greater number of recessive alleles for pigmentation, resulting in less melanin production. The combination of alleles from each gene creates a continuous spectrum of skin tones, from very light to very dark. ### Other Examples of Polygenic Inheritance: - **Human Height:** Height is determined by the interaction of multiple genes. While each gene has a small effect on an individual's height, the overall combination of alleles determines whether a person is short, average, or tall. - **Intelligence:** Like height and skin color, intelligence is influenced by multiple genes, although environmental factors also play a significant role. ### Mendel's Model vs. Polygenic Inheritance: Unlike Mendel's experiments, where traits were controlled by single genes with clear dominant and recessive alleles, polygenic inheritance involves multiple genes that interact to produce a range of phenotypes. Therefore, polygenic inheritance patterns do not follow the simple Mendelian ratios of 3:1 or 1:2:1 but rather produce bell-shaped curves when the phenotypic distribution is plotted.

Codominance:
Codominance is a genetic phenomenon in which two different alleles of a gene are both expressed in the phenotype of an organism. Unlike incomplete dominance, where one allele partially masks the expression of the other, in codominance, both alleles contribute equally to the organism's traits. This results in a heterozygous individual displaying both traits simultaneously. Example of Codominance:
A well-known example of codominance is the inheritance of the AB blood group in humans. The A and B alleles are codominant, meaning that if an individual inherits both the A allele from one parent and the B allele from the other parent, the resulting phenotype will express both A and B antigens on the surface of the red blood cells. This results in the AB blood type. Genotype and Phenotype:
- Genotype: IAIB (heterozygous)
- Phenotype: AB blood type
In this case, both the A and B alleles are fully expressed, making it a clear example of codominance.

Human beings have a total of 46 chromosomes, which are divided into 23 pairs. These chromosomes contain the genetic material that determines the characteristics of an individual. The pairs are divided into two categories: autosomes and sex chromosomes.
1. Autosomes:
Out of the 23 pairs, 22 are autosomes. These are the non-sex chromosomes and are numbered from 1 to 22. They carry genes that are responsible for the majority of an individual's traits, such as eye color, height, and blood type. The autosomes are the same in both males and females.
2. Sex Chromosomes:
The remaining pair consists of sex chromosomes, which determine the biological sex of the individual. There are two types of sex chromosomes: X and Y.
- In females, the sex chromosomes are XX.
- In males, the sex chromosomes are XY.
The presence of two X chromosomes (XX) typically results in a female, while one X and one Y chromosome (XY) results in a male. The sex chromosomes not only determine sex but also carry genes that influence sexual development and fertility.
3. Chromosomal Disorders:
The number of chromosomes is usually fixed at 46 (23 pairs). However, in some cases, there can be chromosomal abnormalities, such as Down syndrome, where an individual has 47 chromosomes (an extra copy of chromosome 21). Other disorders like Turner syndrome (45 chromosomes, missing one X in females) and Klinefelter syndrome (47 chromosomes, with an extra X in males) can also occur due to chromosomal abnormalities.
Thus, the total number of pairs of chromosomes in humans is 23.

Step 1: Background.
Colourblindness is an X-linked recessive disorder.

Step 2: Cross explanation.

Step 3: Punnett square.

Step 4: Results.

Step 5: Answer.
Probability of her son being a carrier = 0, because males cannot be carriers (they have only one X chromosome). A son will either be normal (50%) or colourblind (50%).

The type of cross used to determine the genotype of a tall pea plant is called a Test Cross. Explanation: - A tall pea plant may have either of the two genotypes: - To find out whether the tall plant is TT or Tt, it is crossed with a homozygous recessive dwarf plant (tt). Case 1: If the tall plant is TT (homozygous) All offspring will be tall. Case 2: If the tall plant is Tt (heterozygous) Conclusion: - If all progeny are tall → the parent tall plant is homozygous (TT). - If progeny are 50% tall and 50% dwarf → the parent tall plant is heterozygous (Tt).

Gregor Johann Mendel, the father of genetics, selected pea plants (Pisum sativum) for his hybridization experiments due to the following reasons: \begin{enumerate} \item Distinct contrasting traits: Pea plants show easily observable contrasting traits such as tall/dwarf plants, round/wrinkled seeds, green/yellow pods, etc. \item Short life cycle: They have a short life span, which allowed Mendel to study several generations in a short period. \item Ease of cultivation: Pea plants are easy to grow and require simple care. \item Natural self-pollination: Ensures genetic stability. \item Artificial cross-pollination possible: Flowers are large and bisexual, making artificial hybridization (manual crossing) easy. \end{enumerate} Conclusion: Because of these advantages, Mendel obtained reliable and reproducible results, which helped him formulate the Laws of Inheritance.

Step 1: Background.
Mendel published his work on pea plant experiments in 1865, but it remained unnoticed until 1900, when rediscovered independently by De Vries, Correns, and Tschermak.

Step 2: Reasons for unrecognition. \begin{enumerate} \item Ahead of time: Mendel's work was conceptually advanced, and the scientific community of his time could not appreciate the significance of statistical analysis and probability in biology. \item Lack of knowledge of chromosomes: Cytological studies of chromosomes and their role in heredity were not understood then, so Mendel's "factors" (genes) could not be correlated with physical structures. \end{enumerate}

Step 3: Conclusion.
Only after cytology and genetics advanced, Mendel's laws were recognized as the foundation of heredity.

Step 1: Definition.
Sex-linked recessive traits are those traits whose genes are located on the X chromosome and express themselves only when present in a homozygous condition in females or a hemizygous condition in males.

Step 2: Characteristics of inheritance.

Step 3: Pedigree pattern.

Step 4: Examples of sex-linked recessive disorders. \begin{enumerate} \item Haemophilia: Blood fails to clot normally due to absence of clotting factors. \item Colour blindness: Inability to distinguish between red and green colours. \item Duchenne muscular dystrophy: Severe muscle degeneration. \end{enumerate}

Step 5: Conclusion.
Sex-linked recessive inheritance demonstrates how X-chromosome plays a key role in the transmission of genetic disorders, with males being more vulnerable than females.

Step 1: Background.
Gregor Mendel, the father of genetics, conducted experiments on garden pea plants (Pisum sativum) to study the inheritance of traits. From his experiments, he proposed three laws of inheritance: \begin{enumerate} \item Law of Dominance \item Law of Segregation \item Law of Independent Assortment \end{enumerate} Here, we focus on the Law of Dominance.

Step 2: Statement of the law.
The Law of Dominance states that when two different alleles of a character are present in an organism (heterozygous condition), only one allele expresses itself (dominant), while the other allele remains masked (recessive).

Step 3: Example of Mendel's experiment.
Mendel crossed pure tall plants ( ) with pure dwarf plants ( ): All plants in the F₁ generation were tall, showing that tallness (T) is dominant over dwarfness (t).

Step 4: Punnett Square.
All F₁ hybrids are tall (Tt), proving that the dominant trait (Tallness) masks the recessive trait (Dwarfness).

Step 5: Importance of the law.

Step 6: Limitation.
The law of dominance does not explain incomplete dominance or codominance, where both alleles may express partially or equally (e.g., flower colour in snapdragon, ABO blood groups in humans).

Step 1: Introduction.
Gregor Johann Mendel, known as the "Father of Genetics," performed hybridization experiments on pea plants (Pisum sativum). His work explained how traits are passed from one generation to the next.

Step 2: Experimental Work.
- Mendel chose pea plants because they had distinct, contrasting traits (like tall/dwarf, round/wrinkled).
- He conducted monohybrid and dihybrid crosses.
- By analyzing the offspring, he discovered patterns of inheritance.

Step 3: Contributions (Laws).
1. Law of Segregation: Each individual carries two alleles for a trait, which segregate during gamete formation, ensuring each gamete receives one allele.
2. Law of Independent Assortment: Alleles of different traits assort independently during gamete formation.
3. Law of Dominance: When two contrasting alleles are present, one dominates (dominant trait) while the other is masked (recessive).

Step 4: Impact.
Although Mendel's work was initially ignored, it was rediscovered in 1900 by Hugo de Vries, Correns, and von Tschermak. His principles became the foundation of classical genetics.

Final Answer: