Genetics and Plant Breeding

Mendel's law of segregation states that allele pairs separate or segregate during gamete formation and randomly unite at fertilization. Organisms inherit two alleles for each trait. When gametes are produced by meiosis, allele pairs separate leaving each cell with a single allele for each trait. When the two alleles of a pair are different, one is dominant and the other is recessive.

3:1 ratio was the important clue for Mendle to crack the law of inheritance. When he crossed the two pea plants in a scientific manner, the result obtained were in the ratio of 3:1 and he concluded that individual has two copies of a given gene that determines the trait of the offspring. He gave the law of segregation i.e., one of the two gene copies present in the individual is passed on to the gametes. Thus, the correct answer is option B. 

Mendel's law is valid only when two pure breeding contrasting characters are crossed. In that case the dominant trait will be expressed in both homozygous and heterozygous conditions. Mendel's second law is only valid for genes located in different chromosomes. For genes situated in the same chromosome, i.e., linked genes the law is not valid since the segregation of these genes is not independent.

Linkage vs. Independent Assortment:
Chromosomes contain thousands of genes. Genes that are on separate chromosomes are inherited independently of one another and are said to follow the principle of independent assortment. Remember that this principle states that an RrYy individual is expected to make equal numbers of RY, Ry, rY and ry gametes.
Genes located on the same chromosome are not free to participate in independent assortment and are called as linked genes. These genes tend to be inherited together as a unit, as you can see in the first part of this animation. In this case, the RrYy individual makes only RY and ry gametes because these alleles are present together on the parental homologs and are not separated during gamete formation.
When two genes are always inherited together in this manner, it is referred to as complete linkage. Complete linkage creates only parental gametes, meaning that the gametes that are produced are chromosome copies of the parent they came from. If all genes demonstrated complete linkage during gamete formation, then the result would be that each individual chromosome in a gamete would be either a maternal or paternal replica and the offspring would show little genetic variation from their parents. For this reason, complete linkage rarely occurs in nature and other modes of inheritance, such as crossing over, have evolved.

Genes that are on the same chromosome, or "linked", do not assort independently, but can be separated by recombination. Two genes close together on the same chromosome tend to be inherited together and are said to be linked.
Linked genes can be separated by recombination in which homologous chromosomes exchange genetic information during meiosis; this results in parental, or non-recombinant genotypes, as well as a smaller proportion of recombinant genotypes.
Geneticists can use the amount of recombination between genes to estimate the distance between them on a chromosome.

The linkage is the tendency of closely placed genes on a chromosome to stay together during inheritance; it produces more parental combination and less/no new combinations of a gene. 

According to the law of dominance, a trait is represented by two contrasting factors of a gene in a heterozygous individual but the only dominant trait is expressed and recessive one is masked by the presence of dominant allele. Incomplete dominance and co-dominance are the exception to the law of dominance. Law of segregation states that the two factors for a trait, present together in a heterozygous individual, do not get mixed and are separated during gametogenesis. Segregation of chromosomes of a homologous pair during meiosis followed by their distribution to gametes singly ensures its universal acceptance without any exception. Law of independent assortment tells about segregation and distribution of factors governing two different traits. The tendency of closely placed genes on a chromosome to stay together during inheritance i.e. linkage and therefore no independent assortment make the linked traits exception to the law of independent assortment. According to the principle of a unit factor, each character is governed by a pair of the discrete unit, called factor or genes. Multiple alleles serve as an exception to this rule. Thus, the correct answer is option B.

• According to the law of dominance, a trait is represented by two contrasting factors of a gene in a heterozygous individual; the allele/factor that can express itself in a heterozygous individual is called as a dominant trait. The other factor whose effect is masked by the presence of a dominant factor is called recessive factor. It does not tell about gametes.
•  The law of segregation states that the two factors for a trait, present together in a heterozygous individual, do not get mixed and are separated during gametogenesis. 
• Thus each gamete receives one allele for a trait and two types of gametes are formed; 50% gametes carry factor for dominance and 50% carry the recessive factor. 
•  Law of independent assortment tells about segregation and distribution of factors governing two different traits; not about segregation of factors of one trait. 
• The principle of unit factor tells about the presence of two alternative forms of a gene governing each character of organisms. 

• According to the law of dominance, a trait is represented by two contrasting factors of a gene in a heterozygous individual; only dominant trait is expressed, irrespective of the presence of both factors, in a heterozygous individual. It does not tell about the reappearance of the missing trait in $F _2$ generation. 
• The law of segregation states that the two factors for a trait, present together in a heterozygous individual, do not get mixed and are separated during gametogenesis. 
• Thus each gamete receives one allele for a trait and two types of gametes are formed; 50% gametes carry factor for dominance and 50% carry the recessive factor. 
• Random fusion of these gametes leads to the presence of both recessive and dominant individuals in $F _2$ generation. Law of independent assortment tells about segregation and distribution of factors governing two different traits; not about segregation of factors of one trait. 
• Incomplete dominance is the condition when none of the factors of a gene is dominant and the phenotype of a heterozygous dominant individual is a blend of dominant and recessive traits. It affects the phenotype of $F _1$ individuals, not that of $F _2$. Thus, the correct answer is option B.

The Principle of Independent Assortment states that two or more characters are inherited when the responsible gene separate independently from one another when reproductive cells develop. During meiosis, the pairs of homologous chromosome divide in half and form the haploid cell. This separation of chromosomes is random. After meiosis, each haploid cell of the offspring contain mixture of genes from the organism's mother and the father.

The principle of segregation is also called as the law of purity of gametes. A gamete may carry either the dominant or the recessive factor but not both as we find in F$ _1$ individuals. The gametes which are formed are always pure for a particular character. This is why it is called either as the principle of segregation or law of purity of gametes. Thus, the correct answer is option C.

DNA is the unit of heredity and determines the genetic traits of organisms. Inheritance of genetic traits from parent to offspring is ensured by processes of meiosis and syngamy. Meiosis is the process of cell division that reduces the number of chromosomes and thus, the DNA content to half in the resultant daughter cells i.e. gametes. Fusion of male and female gametes during syngamy restores the DNA content in the resultant offspring. Thus, each offspring consists of half paternal and half maternal DNA which in turn determines its traits. Thus, the correct answer is option C.

Mendel proposed the law of inheritance of traits from the first generation to the next generation. Law of inheritance is made up of three laws: Law of segregation, law of independent assortment and law of dominance. Therefore, (D) is the correct answer.

Mendel explained that one or more traits can be inherited from the parents individually and can be transmitted to the next generation. Mendels law of inheritance says that at least one trait is inherited by the progeny from the parent. Therefore, (a) is the correct answer.