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Genetics: Introduction and Analyzing Crosses

Introduction to Mendelian Genetics

The Storage and Transmission of Hereditary Information

Author:Pamela Peters, from "Biotechnology: A Guide To Genetic Engineering." Wm. C. Brown Publishers, Inc., 1993.
Excerpted from Genentech's Access Excellence

In the mid-1800s, a monk named Gregor Mendel, working in Brno in the Czech Republic, carried out an amazing piece of scientific detective work. Mendel observed that the offspring of certain plants had physical characteristics similar to the physical characteristics of the plants' parents or ancestors. Gregor Mendel wondered why related organisms, both plant and animal, tended to resemble one another and how familial resemblances might be explained. Gregor Mendel reasoned that close observation of inheritance might provide him with the answer for which he searched. He therefore set out to examine and quantify the physical traits in pea plants (because of their speedy reproductive cycles) in an attempt to predict the traits that would occur in future generations.

During years of painstaking work, Mendel counted many thousands of instances of seven different traits, including plant height, flower color and position, seed color and shape, and pod color and shape. Mendel concluded that certain particles or "factors" were being transmitted from parent to offspring and so on, thus providing a connection from one generation to the next. Mendel suggested that these factors were directly responsible for physical traits. His interpretation of the experimental data further suggested that each individual had not one, but two factors for each trait, and that these factors interacted to produce the final physical characteristics of the individual. Both the location and the identity of Mendel's factors remained unknown for years.

Mendel suspected that heredity depends on contributions from both parents, and that specific characteristics from each parent are passed on rather than being blended together in the offspring. The drawing illustrates his experiment with peas in which he demonstrated his concept. A parent homozygous for the allele for spherical seeds is crossed with a parent homozygous for the allele for wrinkled seeds. Each parent makes gametes of only one kind, either S or s, and these combine at fertilization to form plants that all have the genotype Ss and the spherical seed phenotype. When the F1 plants self-pollinate they produce two kinds of eggs, S and s, and the same two types of male sex cells. These combine randomly in four different ways to form F2 plants. Three of the four possible combinations produce genotypes that determine the spherical seed phenotype, and the fourth produces the genotype for the wrinkled seed phenotype, so that the observed ratio is 3:1. The illustration at the bottom of the page, called a Punnett Square, is a handy device for keeping track of the ways gametes can combine at fertilization.

GENETICS: Introduction and Analyzing Crosses

Written by Brian White, MIT. Copyright 1995

Since we take a problem-solving approach to genetics in this course, we've included this section to supplement the reading in Purves, Oriens, and Heller with more detail and problem solving strategies.

The best way to learn genetics is to practice the type of logical-puzzle-solving types of problems that we give.

One good general method for solving genetics problems is:

1) Look at the data and make a reasonable model to explain it. (i.e. it seems reasonable that the trait is recessive and sex-linked)

2) Pick appropriate symbols for the genotypes in the model. This is important, well-chosen symbols can make a problem much easier to solve. (i.e. XX and XY are affected, but XX, XX, and XY are not.)

Tip: It is usually much less confusing to use one letter for each gene than to use different letters for each allele. For example: if color is controlled by one gene, and it can be green or white, use G and g for green and white alleles - if you use g and w, you may get confused later and think that it is two different genes.

3) Work out what this model would predict from the experiments described. (i.e. all the sons of an affected mother will be affected.)

4) See if your prediction matches the given results. (i.e. are all the sons of an affected mother affected) If so, you are done. If not, revise your model and try again.

With practice, your initial guess models will be more and more likely to be correct.

Interpreting Cross Data

First, some definitions:

genotype - The genetic constitution of an organism. For example: Aa, or heterozygous for A and a. Not directly observable in a living organism.

phenotype - The observable properties of an organism. For example: red hair. To determine the phenotype from a given genotype, one must know which alleles are dominant or recessive.

wild-type (wt) - the 'normal' allele, the allele found in the majority of a wild population. Usually the normally-functioning allele, and often (but not always) the dominant allele. For example: for a soil bacterium, the wild-type organism can make all the amino acids it needs for survival; for human beings, the wild-type allele for skin pigmentation results in the production of some melanin pigment.

mutant - an organism carrying a mutation; an altered allele with a different function than wild-type. Often (but not always) recessive. In the above examples: a soil bacterium that requires the amino acid proline because of a genetic defect is considered a proline-requiring mutant; an albino human presumably has a mutation affecting the skin pigmentation pathway.

dominant - refers to the relationship of a particular pair of alleles (i.e.. "brown hair is dominant to blond hair", not "brown hair is dominant".). The dominant allele is the allele whose phenotype appears in the heterozygote (dominance and recessivity are only meaningful in diploids).

recessive - refers to the relationship of a particular pair of alleles (i.e.. "blonde hair is recessive to brown hair", not"blonde hair is recessive"). The recessive allele is the allele whose phenotype is masked in the heterozygote.
Note that a mutant is defined relative to wild-type and a dominant allele is defined relative to a recessive allele. These terms are not absolute.

pure-breeding - when two phenotypically identical pure-breeding individuals are crossed, the progeny are identical to the parents. This almost always means that pure-breeding individuals are homozygous.

Common Genotype Symbols:

Complicating Factors

*Multiple alleles with Co-dominance - In the above cases there were two alleles for each gene - resulting in three possible genotypes. Sometimes, there are more. Blood types are an example, one gene locus can have an i, i, or i allele. An individual' s genotype could then be one of six possibilities: ii, ii, ii, ii, ii, ii.

*Semi-dominance - In the definition of dominance, we assumed that there were only two possible phenotypes; that of the dominant allele and that of the recessive allele. Sometimes, the heterozygote (Aa) has a third phenotype, usually intermediate between recessive and dominant. For example, if AA produces red flowers, and aa white flowers: if A is simply dominant, Aa will produce red flowers; if A is semi-dominant, Aa could produce pink flowers.

Tip: If you see more than two phenotypes for a given locus, you should suspect that multiple alleles or semi-dominance is involved.

*Recessive Lethal Alleles - Sometimes, a gene is essential for the viability of an organism. If 'a' is a recessive allele which results in the production of an inactive protein which is normally required for an essential function, individuals with the genotype aa will die. This will result in unusual progeny ratios and missing classes of progeny. For example: normally, Aa X Aa would be expected to produce 25% AA, 50% Aa, 25% aa - or 75% 'A' phenotype, 25% 'a' phenotype; if aa individuals are inviable, you would expect 33% AA, 67%Aa - or 100% 'A' phenotype (the 'a' phenotype would never be observed).

*Lethal Allele Combinations - Sometimes in multiple allele systems, certain combinations of alleles are also lethal. This will also produce skewed progeny ratios and missing classes of progeny.

Tip: If you see unusual ratios and you do not suspect linkage (see later) or expected progeny classes are missing, you should suspect that some form of lethality may be involved.

Solving Problems: Practicing Crosses

For example: A wild population contains red-eyed and white-eyed flies. A scientist crosses two white-eyed flies and gets all white eyed progeny (cross 1). She crosses two red eyed flies and gets all red-eyed progeny (cross 2). When she crosses a different pair of red-eyed flies, she gets 22 white eyed progeny and 78 red-eyed progeny (cross 3). Explain her observations, giving the most probable genotypes of the parents and progeny of each cross.

- try making a model where white is dominant to red. Appropriate symbols would be:

W - dominant white allele

w - recessive red allele

White eyed flies are either Ww or WW, red eyed flies must be ww.

- cross 1: to get all white progeny, the parents must be WW and WW. The data are compatible with this model.

- cross 2: ww X ww would give all ww - red eyed. This is also compatible with the data.

- cross 3: ww X ww would give no white eyed progeny

This is not compatible with the data. Therefore white cannot be dominant.

- try making a model where red is dominant to white. Appropriate symbols would be:

R - dominant red allele

r - recessive white allele

White eyed flies are rr, red flies are RR or Rr.

- cross 1: to get all white progeny, the parents must be rr and rr. OK.

- cross 2: as above, if the parents were RR and RR, all the progeny would be RR - red. This is also OK.

- cross 3: RR X RR would give all red progeny

RR X Rr would give all red progeny

Rr X Rr would give 75% red, 25% white - this prediction matches the data.

Therefore, the model that red is dominant to white fits the cross data best.

cross 1: rr X rr gives only rr progeny

cross 2: RR X RR gives only RR progeny

cross 3: Rr X Rr gives 75% R_ (RR or Rr), 25% rr

For problems with more than one trait (if the problem involved red and white eyes and short and long wings, for instance), treat each trait independently (work with eye color alone by counting red-eyed/short-winged and red-eyed/long-winged as simply red-eyed) to break the problem into two smaller problems.

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