Overview: Heredity | Science Class 10 PDF Download

This chapter on "Heredity" explores the mechanisms by which traits and characteristics are inherited from one generation to the next. It delves into the concepts of genetic variation, inheritance, and the principles that govern these processes, as laid out by Gregor Mendel. The chapter also discusses how these inherited traits can lead to diversity within a species and the role of heredity in evolution. The chapter is crucial for understanding the basic principles of genetics and the biological processes that underlie inheritance.

Accumulation of Variation of Reproduction

• If one bacterium divides, and then the resultant two bacteria divide again, the four individual bacteria generated would be very similar. There would be only very minor differences between them.
•  However, if sexual reproduction is involved, even greater diversity will be generated. 
• In sexual reproduction, the parental generation provides information for basic body design for the next generation, which is accompanied by small changes.
• Different variations would provide different advantages to different individuals.
• The selection of variants by environmental factors forms the basis for the evolutionary process.

Heredity

Inherited traits 

• The traits that are inherited from the parents are called inherited characters. These traits always get transferred to the next generation but depending on the dominance or recessiveness, they may or may not be expressed.

Genes 

• Gene is the functional unit of heredity.
• Every gene controls one or several particular characteristic features in living organisms.

Rules for Inheritance of Traits: Mendel's Contributions

• Gregor Johann Mendel, through consistent studies on Garden Pea, arrived at laws of inheritance.
• He used plants that were pure breeding for a trait and considered contrasting characters like - tall and short plant size, round and wrinkled seeds, white and violet flowers etc. for his studies.
  

Dominant Gene 

The traits that express themselves in an organism in every possible combination and can be seen are called Dominant traits. In Mendel’s experiment, we see that the tall trait in pea plants tends to express more than the short trait. Therefore, the tall trait of the plant is said to be dominant over the short trait.

Recessive Gene 

A trait which is not expressed in the presence of a dominant gene is known as recessive. So, a recessive character/trait is present in an organism but cannot be seen if a dominant gene exists.

Monohybrid Cross

• In his experiments, he crossed plants with contrasting characters, studied the progeny of first-generation (F₁) and second-generation (F₂) and calculated ratios of plants with contrasting characters that were an original parental type or different.
• For example, he crossed a pure tall pea plant with a dwarf pea plant. He found that in the first generation (F₁) all the plants produced were tall. This led Mendel to propose that two copies of factors (now we know them as genes) that control a trait are present in each sexually reproducing organism
• But when plants of F₁ were crossed to get the second generation, he found that 3/4 of plants were tall while 1/4  were short.
• Mendel concluded that an offspring might receives half its genetic material from either parent and if we represent tallness as 'T’ and dwarfness as ‘t’ then -
  Tall parent can be represented as TT and dwarf parent can be represented as tt. This is genotypic representation of pure plants for their respective traits. Hence, both the chromosomes are same making the organism homozygous for that trait.
• When progeny of F₁ were self-pollinated the resultant progeny F₂ showed a ratio of 3 :1, i.e., three tall and one dwarf pea plant.
  

Dihybrid Cross

• A single copy of 'T' is enough to make the plant tall while both copies of T for the plant to be dwarf.
• The trait which is expressed in F₁ i.e., in heterozygous condition, is called dominant the one which remains hidden is termed recessive. The F₁ generation is the result of cross between two pure individuals for contrasting characters. Hence, it will have one chromosome from either parent (Ex: Tt), making it heterozygous.
• Similarly, when two pairs of contrasting characters were studied simultaneously, it was found that two traits are inherited independent of each other.
• For example, a plant with round seeds of yellow colour (RRYY) was crossed with a plant with wrinkled seeds of green colour (rryy).
• In F₁ generation, all plants produced resembled the dominant parent.
• However, on selfing F₁ x F₁ to get F₂ progeny, the result showed that the two traits i.e., round/wrinkled seeds and yellow/green colour were inherited independently.
• It also shows that new combinations of traits are formed in F₂.
  This shows that the two traits are inherited independently of each other.

Expression of genes 

• Genes control the expression of a trait or a character in an organism. 
• Genes produces proteins. 
• The proteins act as enzymes which can directly control a character or help in the formation of a hormone which can control the expression of a particular character or the proteins become a part of various structural components.

Sex Determination

• Humans have 23 pairs of chromosomes.
•  Out of these 23 pairs, 22 pairs are Autosomes and only one pair is the ‘Sex Chromosome’, which actively takes part in the process of sex determination.
• Male has one X and one Y (XY) sex chromosome. 
• Female has both X (XX) sex chromosome 
• All children will inherit an X chromosome from their mother, despite whether they are a boy or girl. Thus, the sex of the children will be determined by the type of chromosome inherited from their father. 
• A child who inherits Y chromosome will be a boy and who inherits X chromosome will be a girl.