Heredity Class 10 Notes

A trait or character is a feature or characteristic of an organism, such as eye colour, height, skin colour or the ability to roll the tongue. Heredity is the process by which such traits pass from parents to offspring.Offspring of the same parents are never identical in every respect. The differences among them are called variations, and they arise both from the mixing of parental traits and from the influence of the environment.

• Trait or character: a physical feature that can be inherited
• Heredity: the passing of traits from one generation to the next
• Variation: the differences among individuals of the same species
• Gene: the unit of inheritance carried on the chromosomes of the cell
• Variations are of two kinds, genetic variations and environmental variations
• The environment can modify a trait such as height or muscle bulk, but it cannot change genes

Gregor Mendel, an Austrian monk, worked in a monastery garden from 1856 and is called the father of genetics. He chose the garden pea because it is easy to grow, has a short life cycle and shows clear contrasting traits.He selected pure-breeding plants, crossed them by hand, collected the seeds and counted the offspring generation after generation, so that his conclusions rested on counting rather than on opinion.

• Pure-breeding or true-breeding: a plant that always shows the same trait generation after generation
• He chose contrasting pairs: tall and dwarf, round and wrinkled, purple and white
• He pollinated by hand and removed the anthers to prevent unwanted pollen
• He harvested, counted and classified the seeds of each generation
• He published his results after nearly eight years of patient work
• His work was ignored for about thirty-five years and was rediscovered later

Mendel proposed that inheritance is carried by factors that pass from parent to offspring. These factors occur in pairs, and each parent contributes one factor to the pair present in the offspring.These factors are the genes, which lie on the chromosomes inside the nucleus of the cell. The two forms of the same gene are called alleles, and whether a characteristic appears depends on which alleles are present.

• Every characteristic is controlled by a pair of factors, one from each parent
• The factors of a pair separate or segregate during gamete formation
• The factors are located on chromosomes, and each factor is called a gene
• The two alleles of a gene occupy the same position on homologous chromosomes
• Dominance is decided by expression, and not by size, strength or usefulness
• Genes control both structural traits and physiological traits

In a monohybrid cross of two true-breeding plants, one with a dominant trait and one with a recessive trait, every first-generation offspring shows only the dominant trait.This is Mendel's law of dominance: a dominant allele expresses itself in the presence of a recessive allele, so the recessive character appears masked and makes no visible contribution to the offspring.

• Dominant trait: appears in the F1 even when only one copy of the allele is present
• Recessive trait: appears only when both copies of the allele are present
• Examples: tall over dwarf, round over wrinkled, purple over white
• The recessive allele is not destroyed, it is only masked in the F1
• Dominance is not about size, strength or how useful a trait is
• The same law applies to both monohybrid and dihybrid crosses

During the formation of gametes the two factors carried by an organism separate, so that each gamete receives only one factor of the pair. When two gametes fuse, the pair is restored in the offspring. This is Mendel's law of segregation.The law applies to every characteristic and explains why a trait that was hidden in the F1 generation reappears in the F2 generation in a definite proportion.

• Each gamete receives one allele of each gene pair
• The pair is restored at fertilisation, producing a diploid zygote
• In a Tt plant the two alleles separate, giving equal numbers of T and t gametes
• Crossing Tt with Tt gives TT, Tt and tt in the ratio 1 : 2 : 1
• The phenotypic ratio for the same cross is tall : dwarf = 3 : 1
• The law was later restated and confirmed through work on fruit flies

A monohybrid cross is one in which only one pair of contrasting traits is studied. Crossing Tt with Tt gives a genotypic ratio of 1 TT : 2 Tt : 1 tt and a phenotypic ratio of 3 tall : 1 dwarf.Dwarf plants reappear in the F2 generation because the recessive allele hidden in the Tt plants segregates during gamete formation.

• Parents: true-breeding TT crossed with true-breeding tt
• F1 generation: all Tt and all tall
• F2 generation: TT, Tt, Tt and tt from a cross between two Tt plants
• Genotypic ratio: 1 : 2 : 1
• Phenotypic ratio: 3 tall : 1 dwarf
• The recessive trait is masked in the F1 and reappears in the F2

A dihybrid cross studies two pairs of contrasting traits at the same time, such as seed shape and seed colour. Mendel crossed round yellow seeds with wrinkled green seeds and counted the four possible classes of offspring.The board expects the F2 ratio of 9 : 3 : 3 : 1, with the four classes being round yellow, round green, wrinkled yellow and wrinkled green, in that order.

• Two gene pairs are followed together, for example Rr for shape and Yy for colour
• Parents: round yellow RRYY crossed with wrinkled green rryy
• F1 generation: all round and yellow, carrying RrYy
• F2 generation: 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green
• Mendel did not fully work this ratio out himself, but the board expects it
• This ratio is the strongest experimental evidence for Mendel's laws

In humans the mother has two X chromosomes, XX, and the father has one X and one Y, XY. The ovum always carries an X chromosome, so the sperm decides whether the child is a girl or a boy.Mendel reached his conclusions through controlled experiments and counting, while Lamarck had earlier proposed the inheritance of acquired characters, an idea that experiment does not support.

• Human female: XX, human male: XY
• All ova carry an X chromosome, while the sperms carry either X or Y
• XX gives a daughter and XY gives a son
• Birds show the reverse pattern, with ZZ for the male and ZW for the female
• In many plants sex is determined by the distribution of chromosomes
• Mendel experimented and counted, while Lamarck proposed inheritance of acquired characters