Lesson 9.04 – Dihybrid Crosses & Law of Independent Assortment

STANDARDS: B2.c and B2.g; B3.a and B3.b; B7.a and B7.b

INTRODUCTION

After showing that alleles segregate during the formation of gametes, Mendel wondered if they did so independently. In other words, does the segregation of one pair of alleles affect the segregation of another pair of alleles? For example, does the gene that determines whether a seed is round or wrinkled in shape have anything to do with the gene for seed color? Must a round seed also be yellow? 

INSTRUCTION 

Independent Assortment 

To answer these questions, Mendel performed an experiment to follow two different genes as they passed from one generation to the next. Mendel's experiment is known as a two-factor cross or Dihybrid Cross. 

Click here to watch part #4 about Dihybrid Crosses.

The Two-Factor Cross: F₁ 

First, Mendel crossed true-breeding plants that produced only round yellow peas (genotype RRYY) with plants that produced wrinkled green peas (genotype rryy). All of the F₁ offspring produced round yellow peas. This shows that the alleles for yellow and round peas are dominant over the alleles for green and wrinkled peas. A Punnett square for this cross, shown below, shows that the genotype of each of these F₁ plants is RrYy.

Two-Factor Cross: F₁  

Mendel crossed plants that were homozygous dominant for round yellow peas with plants that were homozygous recessive for wrinkled green peas. All of the F₁ offspring were heterozygous dominant for round yellow peas.

This cross does not indicate whether genes assort, or segregate, independently. However, it provides the hybrid plants needed for the next cross—the cross of F₁ plants to produce the F₂ generation.

The Two-Factor Cross: F₂  

Mendel knew that the F₁ plants had genotypes of RrYy. In other words, the F₁ plants were all heterozygous for both the seed shape and seed color genes. How would the alleles segregate when the F₁ plants were crossed to each other to produce an F₂ generation? Remember that each plant in the F₁ generation was formed by the fusion of a gamete carrying the dominant RY alleles with another gamete carrying the recessive ry alleles. Did this mean that the two dominant alleles would always stay together? Or would they “segregate independently,” so that any combination of alleles was possible?

In Mendel's experiment, the F₂ plants produced 556 seeds. Mendel compared the variation in the seeds. He observed that 315 seeds were round and yellow and another 32 were wrinkled and green, the two parental phenotypes. However, 209 of the seeds had combinations of phenotypes—and therefore combinations of alleles—not found in either parent. This clearly meant that the alleles for seed shape segregated independently of those for seed color—a principle known as independent assortment.

Put another way, genes that segregate independently—such as the genes for seed shape and seed color in pea plants—do not influence each other's inheritance. Mendel's experimental results were very close to the 9 : 3 : 3 : 1 ratio that the Punnett square shown below predicts. Mendel had discovered the principle of independent assortment.  

The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes. Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms.

Two-Factor Cross: F₂  

When Mendel crossed plants that were heterozygous dominant for round yellow peas, he found that the alleles segregated independently to produce the F₂ generation.

Mendel's principles form the basis of the modern science of genetics. These principles can be summarized as follows:

• The inheritance of biological characteristics is determined by individual units known as genes. Genes are passed from parents to their offspring.
• In cases in which two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.
• In most sexually reproducing organisms, each adult has two copies of each gene—one from each parent. These genes are segregated from each other when gametes are formed.
• The alleles for different genes usually segregate independently of one another.

Despite the importance of Mendel's work, there are important exceptions to most of his principles. For example, not all genes show simple patterns of dominant and recessive alleles. In most organisms, genetics is more complicated, because the majority of genes have more than two alleles. In addition, many important traits are controlled by more than one gene.  Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes. 

Incomplete Dominance 

A cross between two four o'clock (Mirabilis) plants shows one of these complications. The F₁ generation produced by a cross between red-flowered (RR) and white-flowered (WW) plants consists of pink-colored flowers (RW), as shown in the Punnett square at right. Which allele is dominant in this case? Neither one. Cases in which one allele is not completely dominant over another are called incomplete dominance. In incomplete dominance, the heterozygous phenotype is somewhere in between the two homozygous phenotypes. 

Co dominance 

A similar situation is co dominance, in which both alleles contribute to the phenotype. For example, in certain varieties of chicken, the allele for black feathers is co dominant with the allele for white feathers. Heterozygous chickens have a color described as “erminette,” speckled with black and white feathers. Unlike the blending of red and white colors in heterozygous four o'clocks, black and white colors appear separately. Many human genes show codominance, too, including one for a protein that controls cholesterol levels in the blood. People with the heterozygous form of the gene produce two different forms of the protein, each with a different effect on cholesterol levels.

Multiple Alleles  

Many genes have more than two alleles and are therefore said to have multiple alleles. This does not mean that an individual can have more than two alleles. It only means that more than two possible alleles exist in a population. One of the best-known examples is coat color in rabbits. A rabbit's coat color is determined by a single gene that has at least four different alleles. The four known alleles display a pattern of simple dominance that can produce four possible coat colors, as shown in the figure at right. Many other genes have multiple alleles, including the human genes for blood type.

Polygenic Traits

Many traits are produced by the interaction of several genes. Traits controlled by two or more genes are said to be polygenic traits, which means “having many genes.” For example, at least three genes are involved in making the reddish-brown pigment in the eyes of fruit flies. Different combinations of alleles for these genes produce very different eye colors. Polygenic traits often show a wide range of phenotypes. For example, the wide range of skin color in humans comes about partly because more than four different genes probably control this trait.

Mendel's principles don't apply only to plants. At the beginning of the 1900s, the American geneticist Thomas Hunt Morgan decided to look for a model organism to advance the study of genetics. He wanted an animal that was small, easy to keep in the laboratory, and able to produce large numbers of offspring in a short period of time. He decided to work on a tiny insect that kept showing up, uninvited, in his laboratory. The insect was the common fruit fly, Drosophila melanogaster.

Morgan grew the flies in small milk bottles stoppered with cotton gauze. Drosophila was an ideal organism for genetics because it could produce plenty of offspring, and it did so quickly. A single pair of flies could produce as many as 100 offspring. Before long, Morgan and other biologists had tested every one of Mendel's principles and learned that they applied not just to pea plants but to other organisms as well.

The characteristics of any organism, whether bacterium, fruit fly, or human being, are not determined solely by the genes it inherits. Rather, characteristics are determined by interaction between genes and the environment. For example, genes may affect a sunflower plant's height and the color of its flowers. However, these same characteristics are also influenced by climate, soil conditions, and the availability of water. Genes provide a plan for development, but how that plan unfolds also depends on the environment.

PRACTICE 

1. Take notes on the above information.
2. Click here (http://www.biology.arizona.edu/mendelian_genetics/problem_sets/monohybrid_cross/monohybrid_cross.html) to solve problems 8-12.
3. Click here to watch #5; a summary of Mendelian Genetics and the vocabulary.

ASSESSMENT

1. Turn in your notes.
2. Turn in your answers (at the bottom of your notes) to the practice genetics problems from #2 above.
3. Take the 9.04 Quiz.