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 (Figure 4.7). 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. These results were identical to those that he got when he made separate monohybrid crosses between yellow and green seeded plants and between round and wrinkled seeded plants.

Let us use the genotypic symbols \(\mathbf{Y}\) for dominant yellow seed colour and \(\mathbf{y}\) for recessive green seed colour, \(\mathbf{R}\) for round shaped seeds and \(\mathbf{r}\) for wrinkled seed shape. The genotype of the parents can then be written as \(\mathbf{RRYY}\) and \(\mathbf{rryy}\). The cross between the two plants can be written down as in Figure 4.7 showing the genotypes of the parent plants. The gametes \(\mathbf{RY}\) and \(\mathbf{ry}\) unite on fertilisation to produce the \(\mathrm{F}_1\) hybrid RrYy. When Mendel self hybridised the \(\mathrm{F}_1\) plants he found that \(3 / 4^{\text {th }}\) of \(\mathrm{F}_2\) plants had yellow seeds and \(1 / 4^{\text {th }}\) had green. The yellow and green colour segregated in a \(3: 1\) ratio. Round and wrinkled seed shape also segregated in a \(3: 1\) ratio; just like in a monohybrid cross.

Law of Independent Assortment

In the dihybrid cross (Figure 4.7), 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

Based upon such observations on dihybrid crosses (crosses between plants differing in two traits) Mendel proposed a second set of generalisations that we call Mendel’s Law of Independent Assortment. The law states that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters’.

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 \(\mathrm{F}_1\) \(\mathbf{RrYy}\) plant. Consider the segregation of one pair of genes \(\mathbf{R}\) and \(\mathbf{r}\). Fifty per cent of the gametes have the gene \(\mathbf{R}\) and the other 50 per cent have \(\mathbf{r}\). Now besides each gamete having either \(\mathbf{R}\) or \(\mathbf{r}\), it should also have the allele \(\mathbf{Y}\) or \(\mathbf{y}\). The important thing to remember here is that segregation of 50 per cent \(\mathbf{R}\) and 50 per cent \(\boldsymbol{r}\) is independent from the segregation of 50 per cent \(\mathbf{Y}\) and 50 per cent \(\mathbf{y}\). Therefore, 50 per cent of the \(\mathbf{r}\) bearing gametes has \(\mathbf{Y}\) and the other 50 per cent has \(\mathbf{y}\). Similarly, 50 per cent of the \(\mathbf{R}\) bearing gametes has \(\mathbf{Y}\) and the other 50 per cent has \(\mathbf{y}\). Thus there are four genotypes of gametes (four types of pollen and four types of eggs). The four types are \(\mathbf{RY, Ry, rY}\) and \(\mathbf{ry}\) each with a frequency of 25 per cent or \(1 / 4^{\text {th }}\) of the total gametes produced. When you write down the four types of eggs and pollen on the two sides of a Punnett square it is very easy to derive the composition of the zygotes that give rise to the \(\mathrm{F}_2\) plants (Figure 4.7). Although there are 16 squares how many different types of genotypes and phenotypes are formed? Note them down in the format given.

Can you, using the Punnett square data work out the genotypic ratio at the \(\mathrm{F}_2\) stage and fill in the format given? Is the genotypic ratio also 9:3:3:1?

\(
\begin{array}{|c|c|c|}
\hline \text { S.No. } & \text { Genotypes found in } \mathbf{F}_2 & \text { Their expected Phenotypes } \\
\hline & & \\
\hline
\end{array}
\)

Chromosomal Theory of Inheritance

Mendel published his work on inheritance of characters in 1865 but for several reasons, it remained unrecognised till 1900. Firstly, communication was not easy (as it is now) in those days and his work could not be widely publicised. Secondly, his concept of genes (or factors, in Mendel’s words) as stable and discrete units that controlled the expression of traits and, of the pair of alleles which did not ‘blend’ with each other, was not accepted by his contemporaries as an explanation for the apparently continuous variation seen in nature. Thirdly, Mendel’s approach of using mathematics to explain biological phenomena was totally new and unacceptable to many of the biologists of his time. Finally, though Mendel’s work suggested that factors (genes) were discrete units, he could not provide any physical proof for the existence of factors or say what they were made of.

In 1900, three Scientists (de Vries, Correns and von Tschermak) independently rediscovered Mendel’s results on the inheritance of characters. Also, by this time due to advancements in microscopy that were taking place, 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 (colored bodies, as they were visualised by staining). By 1902, the chromosome movement during meiosis had been worked out. Walter Sutton and Theodore Boveri noted that the behaviour of chromosomes was parallel to the behaviour of genes and used chromosome movement (Figure 4.8) to explain Mendel’s laws (Table 4.3). Recall that you have studied the behaviour of chromosomes during mitosis (equational division) and during meiosis (reduction division). The important things to remember are that chromosomes as well as genes occur in pairs. The two alleles of a gene pair are located on homologous sites on homologous chromosomes.

During Anaphase of meiosis I, the two chromosome pairs can align at the metaphase plate independently of each other (Figure 4.9). To understand this, compare the chromosomes of four different colour in the left and right columns. In the left column (Possibility I) orange and green is segregating together. But in the right hand column (Possibility II) the orange chromosome is segregating with the red chromosomes.

Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carried. Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance.

Following this synthesis of ideas, experimental verification of the chromosomal theory of inheritance by Thomas Hunt Morgan and his colleagues, led to discovering the basis for the variation that sexual reproduction produced. Morgan worked with the tiny fruit flies, Drosophila melanogaster (Figure 4.10), which were found very suitable for such studies. They could be grown on simple synthetic medium in the laboratory. They complete their life cycle in about two weeks, and a single mating could produce a large number of progeny flies. Also, there was a clear differentiation of the sexes – the male and female flies are easily distinguishable. Also, it has many types of hereditary variations that can be seen with low power microscopes.

Linkage and Recombination

Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked. The crosses were similar to the dihybrid crosses carried out by Mendel in peas. For example Morgan hybridised yellow-bodied, white-eyed females to brown-bodied, red-eyed males and intercrossed their \(\mathrm{F}_1\) progeny. He observed that the two genes did not segregate independently of each other and the \(\mathrm{F}_2\) ratio deviated very significantly from the 9:3:3:1 ratio (expected when the two genes are independent).

Morgan and his group knew that the genes were located on the X chromosome (Section 4.4) and saw quickly that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type. Morgan attributed this due to the physical association or linkage of the two genes and coined the term linkage to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non-parental gene combinations (Figure 4.11). Morgan and his group also found that even when genes were grouped on the same chromosome, some genes were very tightly linked (showed very low recombination) (Figure 4.11, Cross A) while others were loosely linked (showed higher recombination) (Figure 4.11, Cross B). For example he found that the genes white and yellow were very tightly linked and showed only 1.3 per cent recombination while white and miniature wing showed 37.2 per cent recombination. His student Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and ‘mapped’ their position on the chromosome. Today genetic maps are extensively used as a starting point in the sequencing of whole genomes as was done in the case of the Human Genome Sequencing Project, described later.