Tag: heredity: from parents to offsprings

Meiosis (a type of cell division) is the cytological basis for Mendel's laws of segregation and independent assortment.

• Mendelian factors or genes, as well as chromosomes, are present in pairs.
• According to this Mendelian concept, inheritance of a trait depends on the passing-on of units. 
• For any given trait, an individual inherits one gene from each parent so that the individual has a pairing of two genes.

• The trait is a distinguishable feature of a character and its detectable variant. For example, tallness and dwarfness. 
• The hybrid plant possesses a heterozygous condition in which genotype have both contrasting alleles such as Tt in the hybrid plant. T represents tallness and t represents dwarfness.
•  Dominant character tallness (T) can be expressed in both conditions such as homozygous (TT) and heterozygous condition (Tt) but recessive character dwarfness (t) can be expressed only in homozygous (tt) condition and thus unable to express in hybrid plants.

• Segregation literally means separation. In Mendelian genetics, in a given plant or animal each character is governed by two alleles, one coming from the father and one coming from the mother.
•  At the time of gamete formation both the alleles, of each gene, separate from each other and go to separate gametes. 
• Thus, each gamete will be having only one allele from each pair of the allele, e.g., for tallness if we denote two alleles in a plant as Tt then that plant can form two possible gametes one having T and the other having t allele.

In case of pea plant, two types of tall plants are possible- one homozygous and other heterozygous. In case a homozygous, tall pea plant is crossed with a homozygous dwarf pea plant, then all the off springs are tall, i.e., dwarf character masked and is not shown by any of the offspring but next when two of the progeny heterozygous tall pea plants are crossed the dwarf character reappears, even when this time none of the parent was dwarf. The characters are passed on from one generation to the other by gametes. Hence, it is clear that gametes remain pure between generation the gamete with alleles for dwarfness is not disturbed or polluted or altered between generations and even when the F$ _1$ plants were phenotypically tall the allele for dwarfness remained pure and were passed on to F$ _2$ plants.

• Segregation means separation. In classical Mendelian genetics, it is assumed that each individual has two alleles which code for each character. 
• One allele comes from the father and the other from the mother. These two alleles separate from each other during gamete formation. 
• Thus, each gamete contributes a single allele for each character during zygote formation.

The Mendelian principles which were discovered in 1865 and 1866 were discovered again by Hugo de Vries, Tsechermark and Correns independently.

Mendel's law of genetics is applicable only in plants which reproduce sexually. In Mendelian genetics he explains about the independent assortment which results in the formation of gametes which has the contrasting characters from both male and female parents. It will be expressed according to its zygosity and dominant traits. if the recessive trait is present in its homozygous form, the character will be expressed. At the same time, dominant character will be expressed in both homozygous and heterozgous forms. Pisum sativum which Mendel chose was a sexually reproducing plant.

Pea plant has 2n= 14. Mendel chose 7 characters the genes for which are present on 7 different chromosomes. The presence of different chromosomes allowed genes for two traits to assort independently during dihybrid cross and recombinant progeny, as well as parental progeny, is produced. If he had chosen eight characters, genes for any two characters must be present on the same chromosome and would exhibit linkage and the law of independent assortment is violated. Law of dominance explains the appearance of a dominant trait by heterozygous genotype and law of segregation tells about segregation of alleles of heterozygous genotype during gamete formation. Law of purity tells about the presence of a single allele of a gene in gametes. Thus, the correct answer is option C.