Cracking the AP Biology Exam

8

Heredity

BEYOND MENDELIAN GENETICS

Not all patterns of inheritance obey the principles of Mendelian genetics. In fact, many traits we observe are due to a combined expression of alleles. Here are a couple of examples of non-Mendelian forms of inheritance:

  • Incomplete dominance: In some cases, the traits will blend. For example, if you cross a white snapdragon plant (dominant) and a red snapdragon plant (recessive), the resulting progeny will be pink.
  • Codominance: Sometimes you’ll see an equal expression of both alleles. For example, an individual can have an AB blood type. In this case, each allele is equally expressed. That is, both the A allele and the B allele are expressed (IAIB). That’s why the person is said to have AB blood.
  • Polygenetic inheritance: In some cases, a trait results from the interaction of many genes. Each gene will have a small effect on a particular trait. Height, skin color, and weight are all examples of polygenetic traits.
  • Multiple alleles: Some traits are the product of many different alleles that occupy a specific gene locus. The best example is the ABO blood group system in which three alleles (IA, IB, and i) determine blood type.
  • Epistasis: In some cases, the genes at one locus may influence the expression of genes at another locus. For example, two gene loci affect the coat color of mice. In one case, black (B) is dominant to brown (b). Yet, at another gene locus a pair of alleles (C) and (c) also affect coat color. When an albino mouse from a true-breeding white strain (cc) and a mouse from a true-breeding brown strain (bb) reproduce, the offspring are all black (CcBb). In this example, the recessive albino genotype is epistatic to the brown/black genotype.
  • Pleiotropy: Sometimes an allele can affect a number of characteristics of an organism. For example, in sickle-cell anemia, multiple symptoms (such as pneumonia, heart failure, and limited mental functioning) are caused by a single pair of alleles.
  • Linked genes: Sometimes genes on the same chromosome stay together during assortment and move as a group. The group of genes is considered linked and tends to be inherited together. For example, the genes for flower color and pollen shape are linked on the samechromosomes and show up together. This pattern has led to methods for mapping human chromosomes.

Since linked genes are found on the same chromosome, they cannot segregate independently. That means they do not follow the probability rule and the expected results from a dihybrid cross.

In addition, the frequency of crossing-over between any two linked alleles is proportional to the distance between them. That is, the farther apart two linked alleles are on a chromosome, the more often the chromosome will break between them. This finding led to recombination mapping—mapping of linkage groups with each map unit being equal to 1 percent recombination. For example, if two linked genes, A and B, recombine with a frequency of 15 percent, and B and C recombine with a frequency of 9 percent, and A and C recombine with a frequency of 24 percent, what is the sequence and the distance between them?

The sequence and the distance of A-B-C is:

If the recombination frequency between A and C had been 6 percent instead of 24 percent, the sequence and distance of A-B-C would instead be: