Mendelian genetics: Patterns of Inheritance – Gregor Johann Mendel (1822 – 1884) is called “father of genetics”. He was a priest by profession, but an amateur scientist by genius. In 1854 he began his famous breeding experiments on garden pea pisum sativum in his monastery garden. Mendel performed series of experiments and brilliantly analysed his data. He formulated two laws of heredity; law of segregation, and law of independent assortment. These laws laid the foundation of classical genetics in 1866.
His work lay neglected for 34 years. In 1900, 16 years after his death, his work was rediscovered and acknowledged.
Mendel was a genius. He did differently form other workers of his time. He wisely chose pisum sativum as his experimental organism because he knew its advantages:
1. Peas grew well. It is hermaphrodite ( ) because both of its male and female sex organs are on the same flower. It is normally self – fertilizing because its stamens and carpel are completely enclosed by two petals, which fuse to form a keel around them (Fig 17.10 a,b).
There is very remote chance for foreign pollen to enter into keel. But its pollination can be easily controlled. The hermaphrodite flower is surgically changed into a female parent by removing its anthers before the pollens are mature. It can now be cross – fertilized by transferring pollens with a paintbrush form anthers of another pea plant taken as the male parent (Fig. 17.10c)
2. Pea produces a large number of fertile seeds within a relatively short growing season. As the time gap between generations is short, many generations can be easily raised within a short time.
3. Pea has many sharply distinct traits. Each trait has two clear – cut alternative forms or varieties. Mendel called such alternative varieties as antagonistic pairs or contrasting pairs. He settled on seven such pairs for his study.
Mendel’s experimental approach was systematic. He first isolated and perpetuated lines of pea that bred true. Self – fertilization within such pure – breeding lines produced offspring like the parents. After establishing 14 true – breeding lines or varieties of seven characters, he did hybridization or cross – fertilization.
Hybrid is an organism that arises form mating between two genetically different parents. He worked with patience. First he did hybridization for just one character at a time. He cross – fertilization plants differing in one contrasting pair only. The offspring of such a cross are called monohybrids. Then he made monohybrid cross by allowing self –n fertilization among monohybrids.
He worked with large number of plants to produce fairly large number of offspring. He worked on each of the even pairs of traits separately. His large data increased uniformity of the result and reduced the chance of individual variation. He counted the progeny and did statistical analysis. It provided him clear – cut ratios. He compared his results and formulated law of segregation.
Then he studied inheritance of two traits at a time. He made Dihybrids by cross – fertilizing plants differing in two contrasting pairs. Then he made dihybrid cross by allowing self – fertilization among dihybrids. He counted their progeny and did analysis of his results. He got clear – cut ratio even now. He formulated Law of independent assortment.
17.6.2 Seven contrasting pairs of traits
Mendel chose to work on seven characters or traits in pea(Figure 17.11). Each trait had two sharply distinct varieties e.g.; seed shape had a round and a wrinkled phenotype. Plant height was either tall or short. Seed color could be yellow or green. Mendel called them contrasting pairs of traits. Each of this variety bred true generation after generation.
A true breeding variety upon self – fertilization always produces offspring identical to the parents. e.g. a true breeding “tall” plant produces only “tall” offspring (Fig 17.12). Similarly a true breeding “short” produces only “short” offspring. He established 14 true breeding varieties of seven traits.
17.6.3 Cross – fertilization
Mendel cross – fertilization a true breeding “tall” male plant with a true breeding “short” female (Fig. 17.13a). He called this first parental generation (P1). Their offspring were called F1 or first filial generation. Filial means progeny. All the offspring of F1 were tall like one of the parents. Short phenotype could not appear. Tall dominated short. Its dominance was complete because no offspring intermediate in size between two parents was found. There was no mixing or blending.
Mendel called the trait that appeared in F1 as the dominant trait; while the trait, which was hidden, suppressed or masked as recessive trait.
A question arose in Mendel’s mind. “Did the appearance of a trait depend in any way upon the sex of the parent?” Many people in those days believed in Aristotle’s misconception that only the male parent contributed most to an offspring’s inherited characters. The female parent just served to nourish. In order to test it, Mendel made a reciprocal cross by reversing sexes of P1 (Fig 17.13b).
Mendel concluded:
“Both parents have equal genetic contribution. Capability of expression in F1 is a trait’s own characteristic. It has nothing to do with sex of the contributor.”
The result was same. Again all F1 monohybrids were tall. It happened for all the seven contrasting pairs of traits (Fig 17.14)
Monohybrid cross: It is a cross between two monohybrids. Mendel self – fertilized F1 tall plants to raise F2 progeny. Surprisingly ¾ of F2 were tall and ¼ short. A clear cut 3:1 ratio was found. The reappearance of short in F2 indicated that this phenotype though not expressed in F1, did exist in some masked form (Fig 17.15). The dominant trait might have temporarily masked its expression, but it could not change or modify its determining nature.
“Reappearance of recessive trait in F2 disproves any blending in F1.”
F1 tall was actually very different from P1 tall. How? Self – fertilization of P1 tall produced 100 %tall offspring, but self – fertilization of F1 tall gave 75 % tall and 25 % short progeny. F1 tall was not true breeding rather it was a monohybrid.
Mendel got the same results and similar
3 : 1 ratio in offspring of monohybrid crosses for all the seven contrasting pairs of trait.
F2 x F2
\
F3
Mendel also raised F3 generation by self – fertilizing F2 plants. He found that 1/3 of F2 tall produced only tall, while 2/3 of F2 tall produced both tall and short in 3 : 1 ratio. But F2 short produced only short.
Can you say that 1/3 of F2 tall were like P1 tall and 2/3 of F2 tall were like F1 tall?
