Mendelian Traits in Humans

Mendelian Traits in Humans

Mendelian traits in humans follow patterns of inheritance as described by Gregor Mendel. These traits are controlled by a single gene and exhibit either dominant or recessive inheritance.

 Some examples of Mendelian traits in humans include:

 1. Widow's Peak: The presence of a V-shaped hairline, controlled by a single gene with two variants: dominant (W) for a widow's peak and recessive (w) for a straight hairline.

 2. Rolling Tongue: The ability to roll the tongue into a U-shape, controlled by a single gene with a dominant allele (R) for rolling and a recessive allele (r) for non-rolling.

 3. Attached vs. Free Earlobes: The attachment of earlobes to the side of the head is determined by a single gene with a dominant allele (A) for attached earlobes and a recessive allele (a) for free earlobes.

 These traits demonstrate the principles of Mendelian inheritance, with offspring inheriting alleles from both parents that determine their phenotype.

 Mendelian disorders

Mendelian disorders are genetic diseases caused by mutations in single genes. They can be classified into two main types: dominant and recessive.

Common Mendelian disorders include:

1. Haemophilia:

- Haemophilia is a genetic disorder that affects blood clotting.

- It is an X-linked recessive disorder, which means it is more common in males.

- Individuals with hemophilia lack a protein needed for blood clotting, resulting in prolonged bleeding even from minor injuries.

- Haemophilia is transmitted from unaffected carrier females to male offspring. Carrier females have a 50% chance of passing on the defective gene to their sons.

- Queen Victoria is known to have carried the gene for haemophilia, leading to its presence in European royal families.

 2. Sickle-cell Anaemia:

- Sickle-cell anemia is an autosomal recessive disorder caused by a mutation in a gene that codes for haemoglobin.

- Individuals with two copies of the mutated gene (HbS) have the disease (HbSHbS) and exhibit symptoms like pain, fatigue, and anaemia.

- Heterozygous individuals (HbAHbS) are carriers, often referred to as having sickle-cell trait, and are typically asymptomatic.

- The disease results in the abnormal shape of red blood cells (RBCs) under low oxygen conditions, leading to blockages in blood vessels.

 3. Colour Blindness:

- Colour blindness is a sex-linked recessive disorder, primarily affecting males.

- It results from mutations in genes located on the X chromosome.

- Red-green color blindness is the most common type, where individuals have difficulty distinguishing between red and green colors.

- Males, who have only one X chromosome, are more susceptible to color blindness.

- Carrier females have one normal and one mutated X chromosome, making them carriers but not usually affected.

 4. Phenylketonuria (PKU):

- Phenylketonuria is an autosomal recessive metabolic disorder.

- Affected individuals lack an enzyme that converts phenylalanine into tyrosine, leading to the accumulation of phenylalanine and its derivatives.

- This accumulation can result in intellectual disabilities and is detected through urine tests.

- PKU can be managed with dietary restrictions that limit phenylalanine intake.

 5. Thalassemia:

- Thalassemia is another autosomal recessive blood disorder.

- It results from mutations that reduce the synthesis of alpha or beta globin chains in haemoglobin.

- Thalassemia can be classified into alpha and beta thalassemia, depending on which globin chain is affected.

- The severity of thalassemia varies, with homozygous individuals often experiencing more severe symptoms, including anaemia.

- Carrier parents (heterozygous) can pass on the mutated gene to their offspring, potentially leading to the disease.

 Chromosomal disorders

Chromosomal disorders are genetic conditions resulting from anomalies in the number or structure of chromosomes. These disorders are categorized into two major types: aneuploidy and polyploidy, each characterized by distinct chromosomal abnormalities.


Aneuploidy refers to a condition where an organism has an abnormal number of chromosomes, typically resulting from nondisjunction during meiosis. Two common examples of aneuploidy are:

 1. Down's Syndrome: This chromosomal disorder, also known as trisomy 21, occurs when an individual has an extra copy of chromosome 21. Down's syndrome leads to intellectual disabilities and distinctive physical features, such as a flattened face and slanted eyes.

 2. Turner's Syndrome: Turner's syndrome results from the absence of one X chromosome in females (45, X0). Affected individuals typically have underdeveloped ovaries and are sterile. They may lack certain secondary sexual characteristics.

 3. Klinefelter's Syndrome: Klinefelter's syndrome is caused by the presence of an additional X chromosome in males (47, XXY). Such individuals have overall masculine development but may also exhibit some feminine characteristics, such as breast development (gynecomastia). They are typically sterile.


Polyploidy involves an abnormal number of complete sets of chromosomes. While it is more common in plants, it can occur in animals as well. Two main types of polyploidy are:

 1. Triploidy: Triploidy results from the presence of three complete sets of chromosomes (3n) instead of the usual two sets (2n). It often leads to developmental abnormalities and is not compatible with life in humans.

 2. Tetraploidy: Tetraploidy occurs when an organism has four complete sets of chromosomes (4n). It's typically lethal in humans but can be observed in some types of cancer cells.