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What happens when pea plants showing two different characteristics, rather than just one, are bred with each other? What do the progeny of a tall plant with round seeds and a short plant with wrinkled-seeds look like? They are all tall and have round seeds. Tallness and round seeds are thus dominant...

What happens when pea plants showing two different characteristics, rather than just one, are bred with each other? What do the progeny of a tall plant with round seeds and a short plant with wrinkled-seeds look like? They are all tall and have round seeds. Tallness and round seeds are thus dominant traits. But what happens when these F1 progeny are used to generate F2 progeny by self-pollination? A Mendelian experiment will find that some F2 progeny are tall plants with round seeds, and some were short plants with wrinkled seeds. However, there would also be some F2 progeny that showed new combinations. Some of them would be tall, but have wrinkled seeds, while others would be short, but have round seeds. You can see as to how new combinations of Figure 8.4 traits are formed in F2 offspring when factors controlling for seed shape and seed colour recombine to form zygote leading to form x F2 offspring (Fig. 8.5). Thus, the tall/short trait and the round RR yy rr YY (round, green) (wrinkled, yellow) seed/wrinkled seed trait are independently inherited. Ry rY 8.2.3 How do these Traits get Expressed? How does the mechanism of heredity work? Cellular DNA is F1 the information source for making proteins in the cell. A section Rr Yy (round, yellow) of DNA that provides information for one protein is called the gene for that protein. How do proteins control the x characteristics that we are discussing here? Let us take the Rr Yy F1 Rr Yy F1 example of tallness as a characteristic. We know that plants have hormones that can trigger growth. Plant height can thus RY Ry rY ry depend on the amount of a particular plant hormone. The F2 amount of the plant hormone made will depend on the RY efficiency of the process for making it. Consider now an enzyme RRYY RRYy RrYY RrYy that is important for this process. If this enzyme works Ry efficiently, a lot of hormone will be made, and the plant will be RRYy RRyy RrYy Rryy tall. If the gene for that enzyme has an alteration that makes rY the enzyme less efficient, the amount of hormone will be less, RrYY RrYy rrYY rrYy and the plant will be short. Thus, genes control characteristics, ry or traits. If the interpretations of Mendelian experiments we have been RrYy Rryy rrYy rryy discussing are correct, then both parents must be contributing 315 round, yellow 9 equally to the DNA of the progeny during sexual reproduction. We have disscussed this issue in the previous Chapter. If both 108 round, green 3 parents can help determine the trait in the progeny, both parents 101 wrinkled, yellow 3 must be contributing a copy of the same gene. This means that 32 wrinkled, green 1 each pea plant must have two sets of all genes, one inherited from 556 seeds 16 each parent. For this mechanism to work, each germ cell must Figure 9.5 Independent inheritance of two have only one gene set. separate traits, shape and colour of seeds Figure 8.5 How do germ-cells make a single set of genes from the normal two Independent inheritance copies that all other cells in the body have? If progeny plants inherited a of two separate traits, single whole gene set from each parent, then the experiment explained shape and colour of seeds in Fig. 8.5 cannot work. This is because the two characteristics ‘R’ and ‘y’ would then be linked to each other and cannot be independently Heredity 131 2024-25