Mendel interpreted his results intelligently. He imagined that contrasting form of a trait, tall or short was determined by heredity factors. Each individual had a pair of these factors. One factor of this pair had come in him from his male parent, the other from his female parent. Both of these factors together controlled expression of a trait in him. Mendel assigned symbols to factors. He designated dominant factor with a capital letter and recessive factor with a small letter, e.g. ‘T’ for tallness factor and ‘t’ for shortness factor.
“What are Mendel’s factors? For him these were discrete physical units of inheritance. Johanson coined the term ‘gene’, and people started calling them genes. Today for us these factors are parts of DNA, the base sequences that carry the biological information to determine a trait. Mendel’s factors are alleles of genes.”
Alleles or Allelomorphs
Alternative forms of a single gene are called alleles or allelomorphs. e.g. ‘T’ and ‘t’ are alleles of each other. They control alternative phenotypes of a trait. They are members of same gene pair. They occupy the same gene locus.
In P1 generation the true breeding tall plant carried ‘TT’ alleles while short plant carried ‘tt’ alleles. Each of them was homozygous.
When both the alleles of a gene pair in an organism are same, the organism’s genotype is homozygous for that gene pair.
An individual with a homozygous genotype is a homozygote.
Mendel imagined that during formation of gametes, the factors of a pair (alleles) separated from each other so that each gamete received only one factor (allele) for each trait. Each gamete was “pure” for only one allele.
“A homozygote forms all gametes of one type.”
Distribution of alleles into gametes was at random. Every gamete had equal chance of getting one allele or the other of a pair. So half the gametes had one allele, and the other half carried the other allele. At fertilization male gamete with one factor (T) united at random with female gamete carrying its factor (t) to restore the complete set of two factors (Tt) for the trait in zygote (Fig 17.16a).
Doesn’t the mode of transmission of Mendelian factors look like behavior of chromosomes (Fig 17.16b)?
F1 offspring developing from the zygote was heterozygous ‘Tt’, because the two alleles of its gene pair were different.
When the alleles of a gene pair are different in an organism, the organism’s genotype is heterozygous of its gene pair. An individual with a heterozygous genotype is a heterozygote.
F1 offspring was a monohybrid for the trait. It was tall in appearance like its one parent, same phenotype; but actually very different in genetic make up or genotype. Its parent was homozygous tall, it was heterozygous tall.
17.6.5 How did contrasting factors behave when present together in F1?
The two different factors (alleles) in F1 stayed together and coexisted side by side without interfering or intermingling each other’s nature. One (T) became dominant, the other (t) recessive, but their trait determinative natures were not modified.
“Dominance only suppresses a recessive gene’s expression. It does not affect its nature.”
But at he time of gamete formation those two alleles ‘T’ and ‘t’ separated or segregated from each other in a manner that only one allele went into one gamete. Each gamete got only one of the two alleles, not both. Their distribution was again at random. Half the gametes got ‘T’, the other half got‘t’ (Fig 17.17).
Gametes had a random chance of fertilization. Every O gamete had an equal chance to fertilize every O gamete. There was no choice.
17.6.6 Punnett square
Let us visualize self – fertilization of F1 with the help of punnett square (Fig 17.18). it is also called checkerboard. Punnett was a British mathematician who introduced it. It is a simple method for tracking the kind of gametes produced as well as all possible combinations at fertilization. Gametes of one parent are arranged on the top, and the other parent’s gametes on the side of the square. Each block of the square represents a possible combination of alleles at fertilization, i.e. the genotype of the offspring.
Punnett square indicates that half the pollens produced by F1 had ‘T’, and half had ‘t’. Similarly half the eggs produced by F1 had ‘T’, and half the eggs had’t’.
At fertilization ¼ of F2 progeny would have been ‘TT’ (homozygous tall), ¼ + ¼ = ½ ‘Tt’ (heterozygous tall), and ¼ ‘tt’ (short).
What Mendel really observed in F2 was phenotypic ratio 3 : 1. By carefully looking at genotypic ratio 1 : 2 : 1 we can easily understand and explain Mendel’s data of F2(Fig 17.19).
Mendel’s law of segregation
“The two coexisting alleles of an individual for each trait segregate (separate) during gamete formation so that each gamete gets only one of the two alleles. Alleles again unite at random fertilization of gametes.”
17.6.7 Test cross
Mendel devised test cross to find out genotype of an individual who had dominant phenotype, e.g. tall.
Test cross is a mating in which an individual showing a dominant phenotype is crossed with an individual showing its recessive phenotype (Fig 17.20).
A phenotypically tall individual could be homozygous (TT) or heterozygous (Tt).
17.6.8 Inheritance of two traits simultaneously
After thoroughly studying single trait inheritance Mendel decided to go for two traits at a time, e.g. seed shape and seed colour. Seed shape had two distinct phenotypes: seed could be round or wrinkled. Seed colour was another trait. It also had two phenotypes, yellow or green.
Mendel took true breeding round and yellow seed plants and crossed them to true breeding wrinkled and green seed plants. All the F1 offspring were round and yellow seeded due to dominance. He allowed these F1 dihybrids to self – fertilize. The result of this dihybrid cross was surprising for him. F2 progeny consisted of not only the two parental combinations (round green and wrinkled yellow). He got 9 : 3 : 3 : 1 ratio among F2 (Fig 17.21).
Appearance of recombinant phenotypes in F2 indicated that some “shuffling” of alleles had occurred. Mendel inferred the mechanism of this shuffling as independent assortment of alleles into gametes.
Alleles for seed colour and seed shape were not fixed or tied in parental combinations i.e., ‘R’ with ‘Y’ and ‘r’ with ‘y’; rather they were free to assort independently, ‘R’ could go with ‘Y’ or ‘y’ in any gamete with equal probability. Similarly ‘r’ could go with ‘Y’ or ‘y’ in any gamete with equal chance. Four kinds of gametes were produced in equal number, with a perfect ratio 1 : 1 : 1 : 1 (Fig 17.22).
Random fertilization of these gametes would produce 9 : 3 : 3 : 1 ratio (Fig 17.23 a,b)
17.6.9 Mendel’s law of independent assortment:
“When two contrasting pairs of traits are followed in the same cross, their alleles assort independently into gametes.
“Alleles in one pair inherit independently of alleles of the other pair. The distribution of alleles of one trait into gametes does not influence the distribution of alleles of the other trait. It means that chance for a plant to be round or wrinkled is independent of its chance to be yellow or green.”
Conditions for independent assortment:
Only those contrasting pairs of traits can show independent assortment whose alleles are riding nonhomologous chromosomes.
Mendel was lucky. The allelic pair for each of the seven characters he studied was on separate homologous pair of chromosomes. Had he studied an eighth trait, its alleles would have been linked to alleles of any other trait on the same homologous pair, and could have never assorted independently. Can you guess why? Recall pea has how many homologous pairs of chromosomes (Fig 17.24).
Independent assortment of genes depends upon independent assort – ment of their chromosomes.