4.3. Inheritance of Two Genes

Mendel also worked with and crossed pea plants that differed in two characters, as is seen in the cross between a pea plant that has seeds with yellow colour and round shape and one that had seeds of green colour and wrinkled shape. Mendel found that the seeds resulting from the crossing of the parents, had yellow coloured and round shaped seeds. Here can you tell which of the characters in the pairs yellow/green colour and round/wrinkled shape was dominant? Thus, yellow colour was dominant over green and round shape dominant over wrinkled.

Mendel's experiments included crossing pea plants that demonstrated two noticeable traits: seed color and seed shape. He discovered that when a yellow round-seeded plant was crossed with a green wrinkled-seeded plant, all offspring produced had yellow round seeds. This indicated that yellow color and round shape are the dominant traits in these plants. This means that for traits to appear dominant, they mask the effects of the recessive traits when both are present.

Think of this like a team sport. If you have one player who's very skilled (dominant trait) and another who's average (recessive trait), when they play together, the skilled player's abilities overshadow the average player's performance during matches, making it seem like the average player doesn't contribute much.

In the dihybrid cross, the phenotypes round,yellow; wrinkled, yellow; round, green and wrinkled, green appeared in the ratio 9:3:3:1. Such a ratio was observed for several pairs of characters that Mendel studied. The ratio of 9:3:3:1 can be derived as a combination series of 3 yellow: 1 green, with 3 round : 1 wrinkled. This derivation can be written as follows: (3 Round : 1 Wrinkled) (3 Yellow : 1 Green) = 9 Round, Yellow : 3 Wrinkled, Yellow: 3 Round, Green : 1 Wrinkled, Green.

This section explains Mendel's finding that certain traits can segregate independently during the formation of gametes. In simpler terms, the inheritance of one trait (like seed shape) does not affect the inheritance of another trait (like seed color). This leads to a typical phenotypic ratio of 9:3:3:1 when you cross two parent plants that are heterozygous for both traits. In essence, Mendel's experiments showed how traits from different characters can shuffle and assort during genetic inheritance.

You can think of this like picking teams for a game. If you have a bag of six red and six blue balls, and every time someone picks a ball, they put it back into the bag. The color of the balls they pick doesn't influence the next pick. Each pick is independent, demonstrating the concept of independent assortment, just like how pea plant traits segregate independently.

The Punnett square can be effectively used to understand the independent segregation of the two pairs of genes during meiosis and the production of eggs and pollen in the F1 RrYy plant. The four types are RY, Ry, rY and ry, each with a frequency of 25 per cent or 1/4th of the total gametes produced.

A Punnett square is a useful tool in genetics for predicting the possible genetic combinations of offspring from two parent organisms. For Mendel's dihybrid crosses, the Punnett square allows us to visualize how the gametes combine. Each combination represents a different genotype in the offspring, confirming that independent assortment leads to a variety of combinations.

Imagine a menu in a sandwich shop where you can choose a bread type and a filling. Each selection is independent; picking whole wheat bread doesn’t affect whether you can choose turkey or ham filling. Similarly, in a Punnett square, choosing a gene for seed color doesn't restrict the choice of gene for seed shape, allowing for a variety of combinations.

Mendel published his work on inheritance of characters in 1865 but for several reasons, it remained unrecognized till 1900...by which time scientists were able to carefully observe cell division. This led to the discovery of structures in the nucleus that appeared to double and divide just before each cell division. These were called chromosomes.

After Mendel's initial discoveries on genetic inheritance were largely overlooked, the Chromosomal Theory of Inheritance emerged. This theory proposed that genes (the factors Mendel discovered) are located on chromosomes. Observations during cell division showed that chromosomes also segregated and assorted independently, aligning perfectly with Mendel's principles. This theory helped to explain how the traits and variations observed by Mendel were physically transmitted during reproduction.

You might think of chromosomes as being similar to books on a shelf. Each book (chromosome) holds information (genes) on certain topics (traits). When you take a book off the shelf to study, you can only focus on the information contained in that one book, just as when chromosomes segregate, the traits carried on those chromosomes are separated and passed on independently.

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