Mendel's Second Law

1. What is Mendel's second law?
   • Mendel showed that traits that are unrelated assort or segregate independently of one another. Thus an individual who is hybrid for two different genes Aa Bb would produce four types of haploid gametes: AB, ab, Ab, and aB.
   • When he used two such traits in peas, yellow versus green peas and wrinkled versus smooth peas, he got a complex ratio from mating the dihybrids to one another. Thus Aa Bb x Aa Bb yields a complex phenotypic ratio of 9 (AB): 3(Ab): 3(aB):1(ab). Don't worry, I won't ask you to work it out. You'll do that when you take a genetics course.
   • You'll never see this ratio for human traits. To get it you'd need two monogenic traits at once in a family. Since most of those traits in humans are rare genetic disorders, forget about seeing a family with both an albino and cystic fibrosis. Also who has 16 or more kids to check this out? You'd need a different approach to verify this in humans.

2. Why does the dihybrid yield four types of gametes?
   • In 1903 Walter Sutton related Mendel's laws to the events taking place during sperm and egg production. He argued that chromosomes pair during meiosis and then undergo a reduction from diploid to haploid. He also argued that the pairing was a chance event, sometimes a paternal (sperm derived) chromosome being on one plane and the maternal below it; and sometimes the reverse could happen. This is called the chromosome theory of heredity.
   • Imagine a pair of homologues (look-alike chromosomes with the same sequences of genes) chromosomes that carry the alleles A and a. Let us say the first alignment is A on top and a below or
     A
     a
     Now imagine a second pair of homologous chromosomes, ones with a different sequence of genes that happened to carry the alleles B and b. Let us ask how they can align. If it is a matter of chance, half will align like this:
     B
     b
     and half will align like this:
     b
     B
     Thus the first pair of homologues could just as easily end up as:

     A	B
     a	b

     just as likely they could ending up as:

     A	b
     a	B

3. How does Mendel's second law apply to humans?
   • Many traits require the participation of more than one pair of genes.
   • These include traits like human skin color. Skin color is quantitative. We come in many shades of color from darkly melanized skin color to a very pale pink skin color. At least two major pigment producing genes and several minor pigment enhancing genes are involved in human skin color. Skin color is additive. There is no simple dominance; the melanizing genes are additive in their effect. A child of a West African with black-looking skin and a North European with pale pink skin is likely to be brown in skin color. Skin color is a quantitative trait. Mendel's second law interprets the number of skin color varieties among the offspring of interracial (miscegenated) matings.
   • Mendel's second law (also called the law of independent assortment) also applies to continuous traits.
   • A continuous trait shows no clear categories. Human height is an example. We can line up people and their heights will form a bell shaped curve of distribution from smallest (rare), average (commonest), to tall (rare). Between any two people side by side along the curve they may be another person who is of a height that can fit between them. The number of genes involved in continuous traits is large and cannot yet be determined by breeding analysis. These are called polygenic traits because there are many genes involved in the trait.
   • Mendel's second law also accounts for modifier effects that alter the expression or intensity of monogenic traits. Some children with cystic fibrosis are sick the day that they are born. Some do not get diagnosed for a year or more because their symptoms don't emerge at birth. Modifier genes can intensify or diminish the expression of monogenic traits. This modification of traits is also called variability.

4. How much variation does Mendel's law allow?
   • Humans have 23 pairs of chromosomes (they have a diploid chromosome number of 46). If your father's 23 chromosomes align independently of your mother's 23 chromosomes that formed you at fertilization, you would have 2 to the 23rd power or about 9 million different ways you could distribute them in packets of 23 per sperm or egg.
   • That is just a minimum amount of the variation that actually exists. Each pair of chromosomes is also shuffled during the meiotic process. The number of combinations of our 70,000 to 100,000 genes is astronomically large (trillions or quadrillions of possible combinations among our offspring).

5. Why is there so much variation?
   • You will learn later this semester in this course that evolution occurs in all forms of life. Living things change over time because their variations are tested in the world in which they live. Some combinations of genes confer better opportunities to live, mate, and raise offspring. Some combinations are not so lucky. Evolution is the study of the fate of our genes.

Back to Main Page