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Chapter 9: Inheritance - Mendelian Traits

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Mendelian Inheritance

Web Lecture

Mendelian Inheritance

This chapter has some problems that resemble combination mathematics; don't let that through you, but do read through the problems carefully.

A personal saga: when Sarah McMenomy was born, her mother looked at her and predicted that her eyes would stay blue and her hair would stay blond, or maybe turn red. Both blue-eyed grandfathers were overjoyed. Sarah, now graduated from college, has blue eyes and (in its natural state) blond hair.

The problem of inheritance

Humans have known for a long time that some traits were inherited. Hair color, or facial shape, or skin color, for example, seemed to be passed from parent to child to grandchild. Sometimes the trait would disappear for a generation or two, only to reappear in the great-grandchild.

The ability to pass traits made many people interested in breeding for desirable traits, especially in livestock and crops. Some people thought that traits acquired during a person's lifetime could be passed to succeeding generations as well: for example, if a parent wanted a child who was healthy and strong, he or she should do body building exercises. The fact that healthy parents tend to have healthier children contributed to the persistence of this idea.

Mendel: the almost-perfect experiment

It wasn't until the nineteenth century, however, that anyone undertook a serious, long term, and thorough study of inherited traits. Gregor Mendel sensed that there was a pattern to inheritance, and set up a rigorous experiment to learn as much as he could about the way plants passed various characteristics to their offspring. In many ways, his experiments are a shining example of doing it right.

He chose an optimal organism to study: garden peas. Their fertilization (pollination) can be controlled by removing the organ (the stamen) which produces pollen (the male gamete) before it matures, and dusting the female organ (the carpel) with pollen from plants with specific identified characteristics. Peas reproduce rapidly, so that he could study several generations each season. They also have a number of distinct and easily differentiated traits, which allowed him to determine whether the pattern of inheritance was a general one, or unique to specific traits.


Besides setting up a good experiment, Mendel had to come up with a way of identifying what he was observing and recording: he had to invent a new kind of notation, and a whole new set of terms. Learning the terms now will help you keep the various concepts straight as you read the chapter.

the chemical code for a particular trait
different forms of the same gene, governing the same function or trait, but expressing it differently
an organism which, when bred with another true-breeding organism of the same type, will reproduce offspring with the same characteristics as the parents.
an offspring of two dissimilar parents.
parent, f1 filial, and f2 filial generations
parent, child, and grandchild generations.
dominant and recessive characteristics
traits which occur in more than one allele, and in which the dominant allele, if expressed in the parent, is always expressed in the offspring of the first filial generation. The recessive allele is expressed only if the allele form is not present. The two forms of the alleles are noted with capital and small letters for the dominant and recessive forms: B for Brown eyes, b for blue eyes.
the principle of segregation
the idea that multiple alleles of a trait are passed independently, and that the expression of the dominant allele does not preclude the existence of the recessive allele in the same organism.
phenotype and genotype
the phenotype is the type of allele expressed in the appearance of an individual. The genotype is the actual genetic trait constitution. In Mendel's pea plants experiment (see fig. 10-2), three of the plants are tall; their phenotype is "tall". But only one of these has two "tall" alleles and is TT; the other two have mixed alleles and are Tt. The three plants have the same phenotype, but two of them have a different genotype from the third.
homozygous and heterozygous
carrying the same (homo) or different (hetero) alleles. BB is homozygous dominant, bb is homozygous recessive and Bb is heterozygous dominant. What would it mean to say something is "heterozygous recessive"?
test cross
crossing a homozygous recessive individual (bb, bbzz) with a dominant phenotype (expressing B, BZ) to determine the genotype from the offspring. If there are any homozygous recessive offspring from the mating, then the parent with the dominant phenotype must also be carrying the recessive allele (Bb, BbZz). If only offspring with dominant phenotypes are observed, then the parent with the dominant phenotype is mostly likely homozygous dominant (BB, BBZZ); but this is an inference, and not conclusive. Why?

Monohybrid crosses

When we look at one isolated trait which can be expressed by different alleles, we can see how crossing two true-breeding parents can produce several different offspring expressions of the trait.

Dr. Christe is brown-eyed. So are her mother and brother. Dr. Bruce his brown-eyed, and so is his mother. But both Dr. Christe's and Dr. Bruce's fathers are blue-eyed. Their two older children are brown-eyed as well. Can you figure out why the grandpas were happy about Sarah's blue eyes?

We can chart the possibilities for eye color genes (this is simplified, eye color is actually controlled by multiple genes as well as multiple alleles) in either Dr. B or Dr. C with a Punnett chart. This one shows the expression of the dominant and recessive alleles in the parent, and the possible outcomes for the first generation:


Our first example is simple: the mother has only one type of allele available for gene B (eye color): the dominant B gene. The father has only one color available: the recessive b gene. In any given child, the mother will give one copy of her gene, and the father will give one copy of his gene. There are four possibilities, each possibility matching one gene B from the mother with one gene b from the father. Since the only two contributor genes are the same in all four cases, there is only one possible result, which we can summarize below:

Mother: BB
Father:bb Bb

In other words, both Dr. B and Dr. C are carrying alleles for blue and brown eyes. [Actually, we don't really know the genotype of either grandmother: it could have been either BB or Bb. But the Drs. have Bb genotypes, in spite of B phenotypes: Sarah's blue eyes prove that! Now the possibilities for their offspring:

Mother donates B Mother donates b
Father donates B BB Bb
Father donates b Bb bb

Given the table, and the fact that B, if present, will be expressed in the organism, for any given child, there is a 3:1 chance that the eye color will be brown, or a 1 in 4 chance that it will be blue. (Don't let the way I expressed this throw you. You need to be used to both ways of expressing probability).

Web site of the week: The Biology Project Mendelian Genetics Site

Take a look at this site. There are several tutorials on Monohybrid and Dihybrid Crosses and Sex-Linked Inheritance that will help you explore and apply these concepts.

Statistics 101A

For our purposes, you need only keep two principles in mind.