Question

║⊕QUESTION⊕║
'' Biology gives you a brain, Life turns it into a mind.''
↓
CLASS 12
PRINCIPLES OF INHERITANCE AND VARIATION
↑
With a monohybrid cross, explain the law of dominance and segregation.

Answer 1

$\underline{\underline{\sf Law \ Of \ Dominance \colon}}$

• The dominant trait gets expressed in the first filial generation and the recessive trait appears in 1/4th of the population of second filial generation.

Monohybrid Cross : Cross considering one trait

Considering the pod shape, Inflated pod is dominant (RR) while constricted pod is recessive (rr) in nature. Chromosome 4 carries the gene that determines pod shape.

Parental Generation : RR rr

F1 Generation : Rr

On self fertilizing the offspring produced in F1,

$\begin{array}{|c|c|c|}\cline{1-3} & \sf R & \sf r \\ \cline{1-3} \sf R & \sf RR & \sf Rr \\ \cline{1-3} \sf r & \sf Rr & \sf rr \\ \cline{1-3} \end{array}$

Here,

Phenotype = 3 : 1

Genotype : 1 : 2 : 1

$\underline{\underline{\sf Law \ Of \ Segregation \colon}}$

• We notice that the recessive trait that was lost in the F1 generation has resurfaced in F2 generation without any blending.

• Generally, a chromosome bears the alleles of a gene. During gamete formation, the chromosomes and thus the genes get seperated but the 2n condition is restored during the formation of zygote.

Answer 2

$\mathcal{\pink{\underline{\underline{\purple{Law\:Of\:Dominance\:And\:Segregation\:}}}}}$

☃️ Inheritance is the acquiring of genetic characteristics or traits from parents by their offspring .

☃️ Mendel started his research with monohybrid cross .

☃️ Mendel observed that traits which were absent in the $\rm\red{F_1}$ generation had reappeared in the $\rm\red{F_2}$ generation .

☃️ These observations led to the formulation of the Law of Dominance and the Law of Segregation .

LAW OF DOMINANCE :-

It states that,

$\rm\green{\bigstar}$ “ When parents with pure, contrasting traits are crossed together only one form of trait appears in the next generation . The hybrid offsprings will exhibit only the dominant trait in the phenotype ” .

• Law of dominance is known as the first law of inheritance .

• In this law, each character is controlled by distinct units called factors, which occur in pairs .

• If the pairs are heterozygous, one will always dominate from the other .

✍️ Law of dominance explains that in a monohybrid cross between a pair of contrasting traits, only one parental character will be expressed in the $\rm\red{F_1}$ generation and both parental characters will be expressed in the $\rm\red{F_2}$ generation in the ratio 3:1 .

✍️ The one which is expressed in the $\rm\red{F_1}$ generation is called the dominant trait and the one which is suppressed is called a recessive trait .

✍️ In simple words, the law of dominance states that recessive traits are always dominated or masked by the dominant trait . This law can be described by Mendel’s experiment .

✍️ A monohybrid cross is a cross between the two monohybrid traits [TT and tt] . Here plants which have the same characters, but differ in only one character were crossed .

• The cross-pollination of tall and dwarf plants resulted in tall plants and the offspring were called $\rm\red{F_1}$ progeny .

✯ He then continued his experiment with self-pollination of $\rm\red{F_1}$ progeny plants . This resulted in both tall and short plants in the ratio of 3:1 which gave rise to the law of segregation .

LAW OF SEGREGATION :-

It states that,

$\rm\green{\bigstar}$ “ During the formation of gamete, each gene separates from each other so that each gamete carries only one allele for each gene ” .

• Law of segregation is the second law of inheritance .

✨ This law explains that the pair of alleles segregate from each other during meiosis cell division (gamete formation), so that only one allele will be present in each gamete .

✨ In a monohybrid cross, both the alleles are expressed in the $\rm\red{F_2}$ generation without any blending .

✨ Thus, the law of segregation is based on the fact that each gamete contains only one allele .

☞ This law is based on four basic concepts :-

1. A gene exists in more than one form of an allele .
2. When gametes are produced by meiosis, the allelic pairs separate, leaving each gamete with a single allele .
3. Every organism inherits two alleles for each trait .
4. The two alleles of a pair are different, i.e. one is dominant and one is recessive .