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Review the notes for meiosis and gametes, if necessary.  Below are the results of Gregor Mendel's breeding experiments with garden peas. It is important to know that each of the seven characteristics studied occurred on different chromosomes, of which there are 7 pairs in peas (2N = 14). At the time these experiments were done, neither mitosis, meiosis, nor chromosomes had been discovered, nor was DNA known to carry the genetic material. 

You must first learn the terminology of genetics before you can apply it to solving genetics problems. The original individuals in a breeding experiment are the P1 (parental) generation. Their offspring are the F1 (First filial) generation. When these individuals are allowed to breed at random, they produce the F2 generation. In each experiment below, the characteristic on the left side is Dominant, that is it shows up in all F1 plants, while the Recessive trait is hidden. The examples in the next several illustrations show specific applications of some of these crosses.  A hybrid is the result of breeding two different variations of a trait. If only one trait is studied, this is a monohybrid cross. If two traits are studied simultaneously, this is a dihybrid cross. Many seeds or plants resulted from each of Mendel's crosses, but the approximate ratio obtained was always 3/4 dominant to 1/4 recessive characteristic.

In this series of crosses between true-breeding purple and white flowered plants (the P1 generation), the offspring (F1) all have purple flowers. When those are allowed to breed, flowers of their offspring (the F2 generation) will be 75% purple and 25% white. Of these purple flowered plants, 50% of them will not be true breeding, because they are heterozygous, that is they have one allele for purple flowers (P) and one for white flowers (p). The true-breeding purple flowered plants are homozygous dominant (PP), while the white flowered plants are homozygous recessive (pp).

In the above example, let P = Purple (the dominant trait), and p = white (the recessive trait). Since we have two of each chromosome, true-breeding purple peas will have two alleles (one portion of DNA) for the dominant trait. Thus, PP will represent the genotype for the purple (phenotype = physical characteristic) trait, and pp represents the genotype of the white true-breeding plants.

All F1 hybrids, will have the genotype Pp, and will be purple. However, the F2 generation will possess approximately 3/4 purple phenotype (of which 1/4 = PP, and 2/4 = Pp genotypes) and 1/4 white (pp genotype).  The Punnett square is one of the most popular ways of representing this experiment. Note an error in the top large square: the 'P' under the word "Gametes" should be a 'p.'

In the offspring, 2/3 of the purple-flowered plants will be expected to have the genotype Pp, since they inherit a 'P' allele from the purple-flowered parent and a 'p' allele from the white-flowered parent.

To determine the genotype of a dominant phenotype, always cross it back to the recessive phenotype. If all offspring are purple, you have a homozygous (same alleles) PP plant being tested. If the dominant plant is heterozygous (different alleles) Pp plant, 1/2 of the offspring should be purple (Pp), and 1/2 should be white (pp).

A cross showing incomplete dominance, in which heterozygous individuals show intermediate characteristics.

Here we look at round (R), yellow (Y) seeds crossed with a plant producing wrinkled (r), green (y) seeds.

Epistasis: The purple pigment in corn requires that two enzymes (controlled by two dominant alleles) must be active for the pigment to form. Thus, two white varieties of corn showing the genotypes AAbb and aaBB, will produce a ratio of 9/16 purple and 7/16 white ears, depending upon the nine different possible arrangements of the chromosomes (and alleles) for these characteristics.

An example of epistatic effects in coat color of Labrador retrievers, resulting in differences in both coat color and nose color.

Multiple alleles: the ABO Blood Groups. Blood type O is recessive, requiring two alleles for type O. Both types A and B are dominant over type O, so they can be homozygous (AA; BB) or heterozygous (AO; BO), and still produce types A and B, respectively. However, types A and B are co-dominant to one another, and the combination of both alleles gives blood type AB.

Inherited characteristics can often be deduced by examining pedigrees. Hemophilia, a sex-linked characteristic (carried only on the X chromosome) in the royal family of England. Males are indicated by squares, females by circles; Affected individuals are shown in black, and female "carriers" are half-shaded. The mutation was traced back to Queen Victoria.

One well known example of a mutation in a gene is shown below: one slight error in the gene for the beta chain of hemoglobin substitutes valine for glutamic acid, which drastically changes the shape and function of the hemoglobin (Hb) molecule. The resulting mutation produces an abnormal shape in the Hb molecule that causes sickle cell anemia when both the sperm and egg carry the mutation. The child will carry the two recessive alleles (ss), rather than Ss (sickle cell trait), or SS (normal hemoglobin). For more on DNA and mutations see DNA structure & function.

The sickle cell mutation causes hemoglobin molecules to clump together. The valine in position 6 adheres to a notch on the opposite side of another molecule of Hb, causing in long chains to form. 

Sickle cell anemia, in which the normal hemoglobin molecule mutates by exchanging the 6th amino acid on the beta chain from glutamic acid to valine. Normal Hb has the genotype SS. Sickle cell anemia occurs when an individual inherits two recessive alleles (ss). Sickle cell trait exists when one inherits the heterozygous condition (Ss). The malaria parasite (Plasmodium falciparum) does not survive in these individuals; they may have a slight anemia, but they survive better than either normal individuals (SS- who often die of malaria), or those who die of sickle cell disease (ss). For more information, click on blood cells.

Correlation of the range of P. falciparum in Africa, and the sickle cell allele.

Nondisjunction sometimes occurs during meiosis, and at least one pair of homologous chromosomes fails to divide. When this occurs in sex chromosomes, the possible outcomes of fertilization of an egg are shown below.

Gene map of a human X chromosome. Over 59 defects have been traced to this chromosome, which will appear more often in males if they are recessive mutations.

A listing of the location of additional mutations on the X chromosome.

Inheritance patterns of sex-linked genes. In this example, inheritance of red eyes and white eyes are shown in fruit flies, Drosophila melanogaster, the geneticist's "guinea pig."

Nondisjunction of an autosome (in this case, chromosome 21) results in Down syndrome (also called trisomy 21). This chromosome has also been known to attach to another autosome and result in trisomy 21. Since the human brain is so complex, almost any deletion or duplication of genetic material will result in some degree of mental retardation.

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