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Natural Science - Year II

Unit 45: Mendel and Inheritance

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Science Web Assignment for Unit 45

This Unit's Homework Page History Lecture Science Lecture Lab Parents' Notes

Science Lecture for Unit 45: The Laws of Inheritance

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Genetic Inheritance

Inheritance is the mechanism by which parents pass physical characteristics to their offspring. All organisms do this: bacteria, mushrooms, roses, toads, trees, and people carry specific information abou t how each trait is expressed. We can easily see resemblances between parents and children in traits like eye, skin, and hair color, or the shape of fingers or ears, or whether a child has freckles or a widow's peak (the pointy shape of the hairline on the forehead).

It had long puzzled people why some traits seemed to show up in every generation, while others some disappeared to reappear later. Some traits seemed to "win" or dominate over others: if a brown-eyed parent married a blue-eyed parent, more children would have brown eyes than blue eyes....some of the time!

Mendel's experiments provided the first serious study of inheritance phenomena. You can simulate his studies of simple traits with computer models. A simple trait is one where there are only two possible expressions: the dominant expression or the recessive one. The rule is that if a dominant trait is present in the genetic makeup of the individual, the genotype, it will be expressed in the outward appearance or behavior of the cell or organism, the phenotype. For example, an individual with two copies of alleles for each gene could have the genotype YyWw, but only the dominant traits of the phenotype YW will be expressed.

Let's assume that we have two different traits, pea color (Yellow dominant, represented by Y, and green recessive, represented by y, and wrinkled dominant, represented by W, along with smooth recessive, represented by w. In genetic terms, we have a color gene with two forms or alleles, and a skin type gene, with two textures or alleles.

Each plant has two copies of each allele for a gene. If the plant has two copies of the same allele, we say that it is homozygous for that allele: YY or yy for color, or WW or ww for skin type. If the copies are different, we say that it is heterozygous for the allele: Yy for color, Ww for skin type.

A parent plant can only pass one copy or allele of each gene to its offspring, so that the offspring winds up with two copies, one from each parent. If the parent is homozygous for a gene (WW), it doesn't matter which physical copy of the gene (W) it passes to the offspring: there is only one type of allele available. But if the parent is heterozygous (Ww), it does matter: the difference between the W and w alleles may determine the phenotype or expressed trait of the offspring, depending on what it inherits from the other parent.

We can represent the possibilities of inheritance in a table called a Punnet Square. We write the possible contributions of one parent across the top, and the possible contributions of the other parent down the side, and the resulting possible combinations in the cells of the table.

Suppose that we have two parents, each YyWw, and each can give only two of the four alleles to the offspring. The possible combinations from each parent are YW, Yw, yW, and yw, and there are sixteen possible resulting combinations:

Yw YWYw YwYw yWYw ywYw
yW YWyW YwyW yWyW ywyW
yw YWyw Ywyw yWyw ywyw

Now, the order of appearance doesn't really matter, so we can summarize this information by the number of each genotype combination:

Genotypes: Genetic alleles carried by the individual Phenotypes: Traits actually expressed or seen
Total YW Phenotypes:9
Total Yw Phenotypes:3
Total yW Phenotypes:3
Total yw Phenotypes:1

What this means is that if we start with parents who carry both alleles for both genes, we have a 1 in 16 chance (1:16) of having offspring with both recessive traits expressed, while there are nine chances out of 16 (9:16 or better than half) that a given offspring will express both dominant traits, and only a 3:16 chance that a given offspring will express one of the dominant traits combined with the other recessive trait.

Read about Mendel's discoveries at the Pea Soup site.

  • What is a simple trait? Why is it a good subject for experimentation?
  • Why did Mendel insist that plants be protected from all foreign pollen?
  • What is a masked trait?
  • What is a purebred plant? How can you tell a plant is purebred?
  • What is a dominant variation? A recessive variation?
  • How would you label a pea plant which always breeds true for green and wrinkled characteristics?
  • How would you label a plant which always breeds true for yellow and smooth characteristics?
  • How would you label a green, smooth plant which, when bred with itself, produces some green, some yellow, some smooth, and some wrinkled children?
  • What is the difference between the genotype of a plant and its phenotype?
  • How is information which determines these characteristics passed from generation to generation?

Study/Discussion Questions

Further Study On your Own (Optional)