Chapter 10 Section 2: Mendelian Genetics Ms. Diana and Ms. Su’ad

How Genetics began Gregor Mendel is considered the father of the science of genetics. In 1866, he published his findings on his experiments with pea plants. Mendel observed that certain traits were inherited following specific patterns. Inheritance (heredity) – the passing of traits to the next generation.

How Genetics Began Mendel was successful because he chose to use pea plants in his studies. Pea plants are true-breeding, meaning they consistently produce offspring with only one form of a trait. Pea plants usually reproduce by self-fertilization. This happens when a male gamete within a flower combines with a female gamete in the same flower. Mendel soon discovered that pea plants could be cross-pollinated with other pea plants. He could easily transfer a male gamete from one plant to a female gamete in another plant.

The Inheritance of Traits Mendel noticed that certain types of pea plants produced specific forms of a trait, generation after generation. He also noticed that certain types always produced green seeds and others always produced yellow seeds. To understand why this was happening, he performed cross-pollination. To prevent self-fertilization, he removed the male organs from the yellow-seed pea plant. By doing this, he was able to call the green-seed plant and the yellow-seed plant the parent generation (P generation).

F1 and F2 Generations When Mendel grew the seeds from the cross between green-seed and yellow-seed plants, all of the resulting offspring had yellow-seeds. The offspring of the P cross is called F1 generation. The green-seed trait seemed to have disappeared in the F1 generation. Mendel then planted the F1 yellow seeds, allowed the plants to grow and self-fertilize. The results of the F2 cross resulted in Mendel collecting 6,022 yellow seeds and 2,001 green seeds, almost a perfect 3:1 ratio of yellow to green. Mendel studied 7 different traits (seed color, flower color, seed shape, seed texture, stem length, see pod shape, and flower position) and found that the F2 generation plants from these crosses all showed a 3:1 ratio.

Genes in Pairs Mendel concluded that there must be two forms of the seed trait (color) in the pea plants – and that each is controlled by a factor called an allele. Allele – an alternative form of a single gene passed from generation to generation. Therefore, the gene for yellow seeds and green seeds are just different forms of the same gene. Mendel concluded that the alleles existed in pairs in each plant. Dominant – the form of the trait that appears in the F1 generation. Recessive – the form of the trait that was hidden in the F1 generation and sometimes reappears in the F2 generation.

Dominance Mendel was able to show that the recessive allele for green seeds had not disappeared, instead it was hidden by the dominant allele. Dominant alleles are shown by using a capital letter. Ex. Y Recessive alleles are shown by using a lower case letter. Ex. y Homozygous – two of the same alleles for a trait. Ex. YY or yy Heterozygous – two different alleles for a trait. Ex. Yy

Genotype and Phenotype The way an organism looks on the outside doesn’t always indicate which pair of alleles is present. Genotype – the organism’s alleles pairs. Phenotype – the observable characteristics or outward appearance of an allele pair.

Mendel’s Law of Segregation Law of Segregation – states that two alleles for each trait separate during meiosis and during fertilization, two alleles for that trait unite. * All F1 generation plants will have the genotype Yy with yellow seeds because yellow is dominant to green. These heterozygous organisms are called hybrids.

Monohybrid Cross Mendel continued his experiments by allowing the Yy plants to self-fertilize. Monohybrid cross – involves hybrids for a single trait. The Yy plants produce 2 types of gametes– male and female, each with either Y or y allele. How the gametes combine is random, but the results were the following genotypes: YY, Yy, Yy, or yy.

Monohybrid Cross The genotypic ratio is 1:2:1. The phenotypic ratio is 3:1 – yellow seeds to green seeds.

Dihybrid Cross Once Mendel figured out the patterns for a single trait, he started to examine inheritance of two or more traits in the same plant.

Law of Independent Assortment Mendel allowed F1 pea plants with the genotype YyRr to self-fertilize in a dihybrid cross. Law of Independent Assortment – states that a random distribution of alleles occurs during gamete formation. Genes on separate chromosomes sort independently during meiosis.

Punnett Squares Punnett Squares – used to predict the possible offspring of a cross between two known genotypes.

Probability The inheritance of genes can be compared to the probability of flipping a coin. The probability of the coin landing on heads is 1 out of 2. If you flip the coin twice, then the probability becomes ¼ (½ x ½) both times. The results were not exact with Mendel, but the larger the number of offspring involved in a cross, the more likely it will match the results predicted by a Punnett square.

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