The Crosses Are Symbolized As Follows

GENETICS

Gregor Mendel performed the first genetic studies in the mid- 1800s using the garden pea as his experimental organism. In one experiment, he crossed yellow-seeded plants with green-seeded plants and obtained only yellow progeny seeds. He then planted these seeds, allowed the resulting plants to self-fertilize (Pea plants have both male and female parts.), and analyzed the seeds they produced. Each plant produced 3/4 yellow and 1/4 green seeds.

The crosses are symbolized as follows:

P: YY x yy (homozygous or pure yellow) (homozygous or pure green) 

F1: Yy (heterozygous or hybrid yellow)

 by self-fertilization

F2: 1/4 YY homozygous yellow 2/4 Yy heterozygous yellow 1/4 yy homozygous green

Mendel hypothesized that each seed carries two "factors", now called alleles, which determine the color. The possession of a dominant Y allele is sufficient to render the seed yellow; its absence (yy) yields green. Mendel further speculated that in the formation of gametes (ova and pollen) only one of the alleles is actually "packaged" per gamete. These notions explained the observations nicely, as shown below, and allowed Mendel to make predictions about the outcomes of other crosses as well.

88 P: YY "female" x yy "male"

(only Y ova)  (only y pollen)

F1: all Yy progeny

(½ Y ova , ½ y ova)  ( ½ Y pollen, ½ y pollen)

About 50 years after Mendel formulated his hypothesis about the transmission of alleles from parents to progeny, Sutton, an American student, suggested that the alleles were being moved on chromosomes. What features of chromosomal transmission from parents to progeny might have led him to this conclusion?

In a second series of experiments, Mendel examined the inheritance of two traits of the pea simultaneously. For example, he studied seed color and shape by crossing round yellow with wrinkled green. The F1 progeny were all round yellow. When these self-fertilized to produce the F2, the surprising outcome was 9/16 round yellow, 3/16 round green, 3/16 wrinkled yellow, and 1/16 wrinkled green. Mendel explained the outcome by stating that the transmission of the seed color alleles is independent of or unlinked to the transmission of the seed shape alleles. Thus, although yellow and round were originally transmitted together from one parent to the F1, they need not always be transmitted together. The same applies to wrinkled and green alleles.

89 P: round yellow x wrinkled green RR YY rr yy

(all RY ova)  (all ry pollen)

F1: all Rr Yy progeny round yellow

1/4 RY ova 1/4 RY pollen 1/4 Ry ova 1/4 Ry pollen 1/4 rY ova 1/4 rY pollen 1/4 ry ova 1/4 ry pollen

Examine the genotypes or descriptions of the alleles of the F2 above. If R confers roundness and Y yellow color, what fraction would be expected to be round yellow? round green? wrinkled yellow? wrinkled green?

Can you suggest a mechanism for the independent transmission of the color and shape alleles?

90 Mendel's laws of inheritance are applicable to most higher organisms, but there are some exceptions. For example, they apply to alleles carried on human autosomes, but need some modification to describe the inheritance of alleles on human sex chromosomes. Let's see if you can use Mendel's laws to describe the inheritance of seed color and shape in maize (corn).

Obtain an ear of corn labeled "A". With your partner(s), count the total numbers of purple and yellow seeds in a minimum of 5 rows of seeds. Record your data below.

# of purple seeds ______

# of yellow seeds ______

Total # counted ______

Calculate the percentage (and estimate the fraction) of seeds that are purple and yellow:

% purple seeds ______% yellow seeds ______

How do you think these ears of corn were produced? (If you think purple is dominant, use P for purple, p for yellow. If you think yellow is dominant, use Y for yellow, y for purple. Diagram the crosses that produced these ears.)

Obtain an ear labeled "B". This time, classify each seed as either:

91 (a) yellow shrunken, (b) yellow full, (c) purple shrunken, or (d) purple full. Count at least ten rows of seeds.

# yellow shrunken seeds ______

# yellow full seeds ______

# purple shrunken seeds ______

# purple full seeds ______

Total # counted ______

Again, calculate percentages and develop an explanation of how these ears were produced. Record the percentages and outline the crosses below.

% yellow shrunken seeds ______

% yellow full seeds ______

% purple shrunken seeds ______

% purple full seeds ______

(If you think shrunken is dominant, use S for shrunken, s for full. If you think full is dominant, use F for full, f for shrunken.)

92 Examine ears labeled "C" and "D" as above. Develop hypotheses to explain the production of these ears.

Human genetics

93 As stated earlier, the transmission of most human traits is Mendelian. The following are a few human phenotypes and their modes of inheritance.

PTC tasting: PTC is a chemical which tastes bitter to those who can taste it at all. Tasting is a dominant allele (T); non-tasting is the recessive allele (t). Thus, tasters are either TT or Tt, and non-tasters are tt. Taste a piece of PTC paper. If you don't taste it, you know your genotype. If you do taste it, do you know your genotype?

Suppose you could taste PTC, but your father could not. Would you now be able to figure out your genotype?

Thiourea tasting: The ability to taste this chemical is also determined by a dominant allele (A). Thus tasters are either AA or Aa, and non- tasters are aa. Test your ability to taste this chemical. In determining your genotype, would it be helpful to have knowledge of your parents or siblings ability to taste thiourea?

Sodium benzoate tasting: The ability to taste sodium benzoate is also determined by a dominant allele (B). Test yourself as above. Are there any individuals in your class who can taste one or two of the tested chemicals, but not the other(s)? What does that tell you?

Internet Resources

You can read Mendel's paper (translated to English) at http://www.netspace.org/MendelWeb/Mendel.html.

You can get information on any human genetic disorder by clicking on the OMIM (Online Mendelian Inheritance in Man) button at http://www.ncbi.nlm.nih.gov/.