Chapter 10 Biology-McGraw Hill Notes

Chapter 10 Textbook Notes

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2 Mendelian Genetics

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How Genetics Began

Mendel rigorously followed various traits in the pea plants he bred. He analyzed the results of his experiments and formed hypotheses concerning how the traits were inherited.
The study of genetics, which is the science of heredity, began with Mendel, who is regarded as the father of genetics.

The Inheritance of Traits

Mendel noticed that certain varieties of garden pea plants produced specific forms of a trait, generation after generation.
For instance, he noticed that some varieties always produced green seeds and others always produced yellow seeds.
In order to understand how these traits are inherited, Mendel performed cross-pollination by transferring male gametes from the flower of a true-breeding green-seed plant to the female organ of a flower from a true-breeding yellow-seed plant.
To prevent self-fertilization, Mendel removed the male organs from the flower of the yellow-seed plant.
Mendel called the green-seed plant and the yellow-seed plant the parent generation—also known as the P generation.

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Genes in Pairs

An allele is defined as an alternative form of a single gene passed from generation to generation.
He called the form of the trait that appeared in the F1 generation dominant and the form of the trait that was masked in the F1 generation recessive.

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Dominance

An organism with two of the same alleles for a particular trait is homozygous for that trait.
An organism with two different alleles for a particular trait is heterozygous (heh tuh roh ZY gus) for that trait, in this case Yy.

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Genotype and Phenotype

A yellow-seed plant could be homozygous or heterozygous for the trait form.
The outward appearance of an organism does not always indicate which pair of alleles is present.
The organism’s allele pairs are called its genotype.
In the case of plants with yellow seeds, their genotypes could be YY or Yy.
The observable characteristic or outward expression of an allele pair is called the phenotype.
The phenotype of pea plants with the genotype yy will be green seeds.

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Mendel’s Law of Segregation

Mendel’s law of segregation states that the two alleles for each trait separate during meiosis.
These heterozygous organisms are called hybrids.

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Monohybrid Cross

Mendel continued his experiments by allowing the Yy plants to self-fertilize.
A cross such as this one that involves hybrids for a single trait is called a monohybrid cross.
The Yy plants produce two types of gametes—male and female—each with either the Y or y allele. The combining of these gametes is a random event.
This random fertilization of male and female gametes results in the following genotypes—YY, Yy, Yy, or yy. Notice that the dominant Y allele is written first, whether it came from the male or female gamete.
In Mendel’s cross, there are three possible genotypes: YY, Yy, and yy; and the genotypic ratio is 1:2:1. The phenotypic ratio is 3:1—yellow seeds to green seeds.

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Dihybrid Cross

Once Mendel established inheritance patterns of a single trait, he began to examine simultaneous inheritance of two or more traits in the same plant. In garden peas, round seeds (R) are dominant to wrinkled seeds (r), and yellow seeds (Y) are dominant to green seeds (y). If Mendel crossed homozygous yellow, round-seed pea plants with homo-zygous green, wrinkle-seed pea plants, the P cross could be represented by YYRR × yyrr. The generation genotype would be YyRr—yellow, round-seed plants. These -generation plants are called dihybrids because they are heterozygous for both traits.

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Law of Independent assortment

From these results, he developed the law of independent assortment, which states that a random distribution of alleles occurs during gamete formation.

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Punnet Square

In the early 1900s, Dr. Reginald Punnett developed what is known as a Punnett square to predict the possible offspring of a cross between two known genotypes. Punnett squares make it easier to keep track of the possible genotypes involved in a cross.

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Punnett square—monohybrid cross

Can you roll your tongue like the person pictured in Figure 12? Tongue-rolling ability is a dominant trait, which can be represented by T. Suppose both parents can roll their tongues and are heterozygous (Tt) for the trait. What possible phenotypes could their children have? The number of squares is determined by the number of different types of alleles—T or t—produced by each parent. In this case, the square is 2 squares × 2 squares because each parent produces two different types of gametes. Notice that the male gametes are written across the horizontal side and the female gametes are written on the vertical side of the Punnett square. The possible combinations of each male and female gamete are written on the inside of each corresponding square.

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Punnett square—dihybrid cross

Now examine the Punnett square in Figure 13. Notice that in the P cross, only two types of alleles are produced. However, in the dihybrid cross—when the generation is crossed—four types of alleles from the male gametes and four types of alleles from the female gametes can be produced. The resulting phenotypic ratio is 9:3:3:1—9 yellow round to 3 green round to 3 yellow wrinkled to 1 green wrinkled. Mendel’s data closely matched the outcome predicted by the Punnett square.

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3 Gene Linkage and Polyploidy

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Gene Recombination

The new combination of genes produced by crossing over and independent assortment is called genetic recombination.
The possible combinations of genes due to independent assortment can be calculated using the formula 2n, where n is the number of chromosome pairs.
For example, pea plants have seven pairs of chromosomes. For seven pairs of chromosomes, the possible combinations are 27, or 128 combinations.
Because any possible male gamete can fertilize any possible female gamete, the number of possible combinations after fertilization is 16,384 (128×128).
In humans, the possible number of combinations after fertilization would be 223×223, or more than 70 trillion. This number does not include the amount of genetic recombination produced by crossing over.

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Gene Linkage

Chromosomes contain multiple genes that code for proteins. Genes that are located close to each other on the same chromosome are said to be linked and usually travel together during gamete formation. Follow closely related genes A and B in Figure 14 through the process of meiosis. The linkage of genes on a chromosome results in an exception to Mendel’s law of independent assortment because linked genes usually do not segregate independently.

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Polyploidy

Polyploidy is the occurrence of one or more extra sets of all chromosomes in an organism.
Roughly one in three species of known flowering plants are polyploid. Polyploid plants are selected by plant growers for their desirable characteristics. Commercially grown bread wheat (6n), oats (6n), and sugar cane (8n) are polyploid crop plants. Polyploid plants, such as the ones shown in Figure 16, often have increased vigor and size.

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