NIS - BIOLOGY

Lecture 40 – Lecture 41 – Lecture 42Mendelian Genetics

Ozgur Unal

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Gregor Mendel

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What do you think this person’s occupation was?

Gregor Mendel (1833-1884) was an Austrian monk and plant breeder.He studied the traits of garden pea plants.He published his findings on the method and the mathematics of inheritance.The study of genetics (science of heredity) began with Mendel.

Pea Plants

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Pea plants are easy to grow.Pea plants usually reproduce by self-fertilization.Self-fertilization occurs when a male gamete within a flower combines with a female gamete in the same flower (sexual or asexual?).

Mendel discovered that pea plants can easily be cross pollinated by hand.He transferred a male gamete from the flower of one pea plant to the female reproductive organ in a flower of another pea plant.He analyzed the results of his experiments and formed hypothesis concerning how the traits were inherited.

The Inheritance of Traits

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The Inheritance of Traits

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Some pea plants always produce yellow seeds.Some pea plants always produce green seeds.Mendel cross pollinated these two plants and got a new generation of pea plants.Mendel called the green-seed plant and the yellow-seed plant the parent generation P generation.

The Inheritance of Traits

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The new generation of plants from the parent generation has all yellow seeds.This new generation is called first filial (F1) generation.The green-seed trait seem to have disappeared in F1 generation.Mendel planted the F1 generation of yellow seeds, allow the plants grow and self fertilize Second filial (F2) generationOf the seeds he collected, 6022 were yellow and 2001 were green 3 : 1 ratio.

Genes in Pairs

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Mendel concluded that there must be two forms of the seed trait in the pea plants: Yellow-seed and green-seed.Each form is controlled by a factor called an allele.An allele is defined as an alternative form of a single gene passed from generation to generation.The gene for yellow seeds and the gene for green seeds are each different forms of a single gene.

Mendel called the form of the trait that appeared in the F1 generation dominant (yellow seed) and the form of the trait that was masked in the F1 generation recessive (green seed).

Dominance

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The allele for the yellow-seed form of the trait is represented by Y (dominant).The allele for the green-seed form of the trait is represented by y (recessive).An organism with two of the same alleles for a particular trait is homozygous for that trait.Homozygous yellow-seed plants YYHomozygous green-seed plants yyAn organism with two different alleles for a particular trait is heterozygous for that trait.Heterozygous yellow seed plants YyWhen aleles are present in the heterozygous state, the dominant trait will be observes.

Dominance

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Dominance

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Genotype and Phenotype

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A yellow-seed plant can be heterozygous or homozygous for the trait form YY or YyThe outward appearence of an organism does not always indicate which pair of alleles is present.The organism’s allele pairs are called its genotype.The observable characteristic or outward expression of an allele pair is called the phenotype.

Mendel’s Law of Segregation

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Check out Figure 10.9Mendel’s law of segregation states that the two alleles for each trait separate during meiosis.During fertilization, two alleles for that trait unite.All resulting F1 generation plants will have the genotype Yy and will have yellow seeds because yellow is dominant to green.These heterozygous organisms are called hybrids.

Monohybrid Cross

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A cross that involved hybrids for a single trait is called a monohybrid cross.The Yy plants produce two types of gametes.The combining of these gametes is a random event.In Mendel’s F1 cross, there are three possible genotypes: YY, Yy and yy genotypic ratio is 1 : 2 : 1.The phenotypic ratio is 3 : 1.

Dihybrid Cross

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Once Mendel established inheritance pattern of a single trait, he began to examine simultaneous inheritance of two ro more traits in the same plant.Round seeds (R) are dominant ro wrinkled seeds (r)Yellow seeds (Y) are dominant to green seeds (y)Homozygous yellow-round seed pea plants YYRRHomozygous green-wrinkled seed pea plants yyrrP cross YYRR x yyrrF1 generation genotype from this P cross YyRrF1 generation phenotype from this P cross Yellow roundThe F1 generation plants are called dihybrids because they are heterozygous for both traits.

Dihybrid Cross

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Law of Independent Assortment

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Mendel allowed F1 pea plants with the genotype YyRr to self fertilize in a dihybrid cross.He calculated the genotypic and phenotypic ratios of the offspring in both F1 and F2 generations.Law of independent assortment states that a random distribution of alleles occurs during gamete formation.Genes of separate chromosomes sort independently during meiosis.Check out Figrue 10.11

Punnett Squares

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In the early 1900s, Reginald Punnett developed a Punnett square.A punnett square predicts the possible offspring of a cross between two known genotypes.

Punnett Square – Monohybrid Cross:Yy Y and y gametesPunnett square shows the possible combinations of eachmale and female gamete.

Punnett Squares

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Punnett Square – Dihybrid Cross:YyRr YR, Yr, yR or yrThe Punnett square shows the possible combinations of these fours gametes from male and female.

Punnett Squares

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Example: Can you roll your tongue?Ability to roll tongue (T) is a dominant trait over the inability to roll tongue (t).Draw the Punnett Square for the possible combinations of the children for a given genotype of the parents.http://www.zerobio.com/drag_gr11/mono.htmhttp://wps.prenhall.com/wps/media/objects/487/498795/CDA10_1/CDA10_1b/CDA10_1b.htm