The pedigree chart given below shows a particular trait which is absent in parents but present in the next generation irrespective of sexes. Draw your conclusion on the basis of the pedigree.
The pedigree chart shows that the trait is autosome linked and recessive in nature. But, the parents are carriers (i.e., heterozygous) hence, among the offsprings only few show the trait irrespective of sex. The other offsprings are either normal or carrier.
In order to obtain the $F_1$-generation Mendel pollinated a pure-breeding tall plant with a pure-breeding dwarf plant. But for getting the $\mathrm{F}_2$-generation, he simply self-pollinated the tall $\mathrm{F}_1$ plants. Why?
Characters segregate during gamete formation. Pure-breeding parents give rise to $F_1$ with heterozygous conditions. Only self-pollination of heterozygotes can result in all possible recombinations of characters in progeny as mating is random.
'Genes contain the information that is required to express a particular trait.' Explain.
Genes contain the information required to express a particular trait can be explained by the following experiment.
G Beadle and E Tatum set an experiment to prove that one gene possess a particular trait and is responsible for the production of one enzyme or protein. They performed their experiment on Neurospora crassa which were nutritionally mutant.
It was proved that a single protein contains several polypeptide and each polypeptide is controlled by separate gene. Thus, each gene expresses a particular trait. This theory was called one-gane-one enzyme or one gene-one polypeptide hypothesis.
But after the discovery of cistron (the functional unit of gene), the theory was named as one-cistron-one polypeptide hypothesis.
How are alleles of particular gene differ from each other? Explain its significance.
Alleles are polymorphs that differ in their nucleotide sequence resulting in contrasting phenotype expression. Alleles are the alternative forms of a same gene for, e.g., genes for height have two allele, one for dwarfness ( t ) and one for tallness ( T ).
Significance
(i) A character may have two or more contrasting phenotypic expression, thus resulting variation in the population.
(ii) These are used in the studies of inheritance and in understanding their behaviour.
In a monohybrid cross of plants with red and white flowered plants. Mendel got only red flowered plants. On self-pollinating these $F_1$ plants got both red and white flowered plants in 3:1 ratio. Explain the basis of using RR and rr symbols to represent the genotype of plants of parental generation.
Phenotypic ratio Red flower : White flower $3: 1$
Genotype ratio RR : Rr : rr 1:2:1
Generally, upper case letters are used as symbols for dominant and lower case for recessive traits of the same gene (alleles). Experiment shows that it is a monohybrid cross with $3: 1$ ratio in $\mathrm{F}_2$-generation.
This shows parents must be true-breeds. As parents are diploid and homologous chromosomes carry alleles with similar type they are represented with RR and rr.