Heredity

  • Heredity, commonly referred to as inheritance or biological inheritance, is the process by which DNA-based genetic information is transmitted from parents to offspring.
  • The differences in characters of parents and offspring are known as variations.
  • This can occur through either asexual reproduction or sexual reproduction.
  •  It involves the transmission of traits and characteristics from one generation to the next through the transfer of genetic material known as genes.

Genes

  • Gene is the functional unit of heredity.
  • These are small segments of DNA that code for particular RNA or protein molecules. This supports the maintenance of generational variety and the process of species evolution.

Traits               

  • It refers to any observable characteristic of an organism. There are two types of traits:  Acquired and inherited.
  • Inherited traits are passed down from parents to offspring through genetic material in the form of genes. For example, if both parents have brown or black hair, for example, their offspring may inherit either colour or a combination of the two.
  • Acquired Traits are the traits that arise as a result of environmental effects. These do not contain DNA code.

Mendel’s Work

  • Gregor Johann Mendel, known as the ‘Father of Genetics’, was an Austrian Monk who worked on pea plants to understand the concept of heredity.
  • His work laid the foundation of modern genetics.
  • He made three basic laws of inheritance – The Law of Dominance, The Law of Segregation and The Law of Independent Assortment.

Dominant and Recessive Traits

  • A dominant trait is one that appears when an organism's genotype has at least one copy of the dominant allele. For example In Mendel’s experiment height is the dominant trait.                                         
  • A recessive trait- A trait which is not expressed in the presence of a dominant allele is known as recessive.
  • So, a recessive character/trait is present in an organism but cannot be seen if a dominant allele exists.

Monohybrid Cross

  • A cross between two organisms that involves only one character is referred to as a monohybrid cross.         
  • The ratio of characters arising out of this cross at F2 generation is called the monohybrid ratio.

Example

  • Mendel first crossed two pea plants that were true-breeding for different heights, one with tall height and the other with short height.
  • The height of the plant offspring was all tall. He called this generation as F1 generation.
  • After that, he crossed two of the F1 plants, observing that the F2 generation's offspring had a 3:1 ratio of tall to short plants.       
  • So, 3:1 is a monohybrid ratio.
  • This meant that the trait for tall height was dominant over the trait for short height.
  • Here, the height of the plant is considered at a time.

Dihybrid Cross        

  • When two characters are considered while crossing two organisms, then such a cross is known as a dihybrid cross.
  • The ratio of characters arising out of this cross at F2 generation is called the dihybrid ratio.

Example

  • Mendel chose two pea plant characteristics, seed color, and seed shape, and crossed two true-breeding plants that had different alleles of both traits.
  • One parent had a round, yellow seeds (RRYY), and the other had wrinkled, green seeds (rryy).
  • The offspring, called the F1 generation, all had a round, and yellow seeds (RrYy), which showed that one allele of each trait was dominant over the other.   
  • Mendel then crossed two of the F1 plants with each other to create the F2 generation.
  • He observed that the offspring had a 9:3:3:1 ratio of seed types, with 9 plants having round and yellow seeds, 3 plants having round and green seeds, 3 plants having wrinkled and yellow seeds, and 1 plant having wrinkled and green seeds.    
  • Thus, 9:3:3:1 is the dihybrid ratio.

Mendel Laws

  • Law of Dominance: If the two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance.
  • Law of Segregation: The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes.
  • Law of Independent Assortment: Each pair of alleles segregates independently of other pairs of alleles during gamete formation.

Sex Determination

  • Sex determination is the process of figuring out the gender of an individual based on the genetic material of that individual.
  • Different factors affect an embryo's sex in different animals.
  • In humans, sex determination happens on the basis of the presence or absence of the Y chromosome.
  • Human chromosomes are present in pairs, with one copy inherited from each parent. 
  • 22 pairs of such chromosomes are called autosomal chromosomes. One pair of chromosomes that determines the sex of an individual is known Sex chromosome.
  • Women have XX chromosomes and Men have XY chromosomes (The Y chromosome is smaller than the X chromosome).
  • An ovum always contains an X chromosome.
  • An ovum, upon fusion with the Y-containing sperm, gives rise to a male child and upon fusion with the X-containing sperm, gives rise to a girl child.
  • The sex of a child depends on the chromosome they receive from their father.