Observable Patterns of Inheritance

Observable Patterns of Inheritance

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Probability True-breeding Hybrid Segregation Traits Genes Homozygous Heterozygous Phenotype Genotype Dominant Recessive Terms to Know

Genes • Chemical factors that determine traits (units of information) • Analogy: Genes are like a combination of ingredients in a recipe. They code for a specific food. • Passed from parents to offspring • Each has a specific location (locus) on a chromosome

Alleles • Different forms of a gene (back to analogy…replacing jiffy p.b. with skippy p.b.) • Dominant allele (Uppercase letter) overrules a recessive allele (lowercase letter) that it is paired with

Allele Combinations • Homozygous =purebred • having two identical alleles at a locus • AA (dominant expressed) or aa (recessive expressed) • Heterozygous =hybrid • having two different alleles at a locus • Aa (dominant expressed)

Genotype & Phenotype • Genotype refers to particular genes an individual carries • Phenotype refers to an individual’s observable traits • Cannot always determine genotype by observing phenotype

Tracking Generations • Parental generation P mates to produce • First-generation offspring F1 mate to produce • Second-generation offspring F2

Earlobe Variation • Whether a person is born with attached or detached earlobes depends on a single gene • Gene has two molecular forms (alleles)

Earlobe Variation • You inherited one allele for this gene from each parent • Dominant allele specifies detached earlobes (E) • Recessive allele specifies attached earlobes (e)

Dominant & Recessive Alleles • If you have attached earlobes, you inherited two copies of the recessive allele • If you have detached earlobes, you may have either one or two copies of the dominant allele

Early Ideas About Heredity • People knew that sperm and eggs transmitted information about traits • Blending theory • Problem: • Would expect variation to disappear • Variation in traits persists

Gregor Mendel Strong background in plant breeding and mathematics Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring

Mendel was born in1822 • Austrian monk • Studied at the Univ. of Vienna • Teacher (High School)

Figure 24–5 The Structure of a Flower Stamen Carpel Stigma Anther Style Filament Ovary Petal Ovule Sepal Section 24-1

The Garden Pea Plant • Self-pollinating • True breeding (different alleles not normally introduced) • Can be experimentally cross-pollinated

How did Mendel fertilize the plants?

F1 Results of One Monohybrid Cross

F M Dominant trait is expressed Recessive appears

Figure 11-3 Mendel’s Seven F1 Crosses on Pea Plants Seed Shape Seed Color Seed Coat Color Pod Shape Pod Color Flower Position Plant Height Round Yellow Gray Smooth Green Axial Tall Wrinkled Green White Constricted Yellow Terminal Short Round Yellow Gray Smooth Green Axial Tall

F1 Results of Mendel’s Dihybrid Crosses • All plants displayed the dominant form of both traits • We now know: • All plants inherited one allele for each trait from each parent • All plants were heterozygous (AaBb)

Principle of Dominance • Some alleles are dominant and others are recessive.

Mendel wanted to know if the recessive alleles disappeared or are they still in the f1,just hidden.

Principles of Dominance P Generation F1 Generation F2 Generation Tall Short Tall Tall Tall Tall Tall Short

Mendel’s Theory of Segregation • An individual inherits a unit of information (allele) about a trait from each parent • During gamete formation, the alleles segregate from each other

Independent Assortment • Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait • Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis

Independent Assortment Metaphase I OR A A a a A A a a B B b b b b B B Metaphase II: A A a a A A a a B B b b b b B B Gametes: B B b b b b B B A A a a A A a a 1/4 AB 1/4 ab 1/4 Ab 1/4 aB

F2 Results of Monohybrid Cross

The physical characteristic Type of alleles

Impact of Mendel’s Work • Mendel presented his results in 1865 • Paper received little notice • Mendel discontinued his experiments in 1871 • Paper rediscovered in 1900 and finally appreciated

Probability • The likelihood that a particular event will occur. • Flip a coin. • We use Punnett Squares

D 38- Deduce the probable mode of inheritance of traits (e.g.,

Homozygous recessive Homozygous recessive a a a a A A Aa Aa Aa Aa a A aa Aa aa Aa Punnett Squares of Test Crosses Two phenotypes All dominant phenotype

Female gametes A a A AA Aa Male gametes a Aa aa Punnett Square of a Monohybrid Cross Dominant phenotype can arise 3 ways, recessive only one

Test Cross • Individual that shows dominant phenotype is crossed with individual with recessive phenotype • Examining offspring allows you to determine the genotype of the dominant individual

Tt X Tt Cross

Genetics Practice Problem 1 • What occurs when a purple plant that is heterozygous is fertilized by a white plant? • Identify generations • Punnett Square • Genotypes % • Phenotype %

Principle of Independent Assortment • The genes for different traits separate independently of one another during the formation of gametes.

Figure 11-10 Independent Assortment in Peas

Yellow round 9/16 • Green round 3/16 • Yellow wrinkled 3/16 • Green wrinkled 1/16 • 9 : 3 : 3 : 1 Ratio

Dihybrid Cross Experimental cross between individuals that are homozygous for different versions of two traits

Straight Pinky (Dominant) Bent Pinky (Recessive) Straight Thumb (Dominant) Curved Thumb (Recessive)

More Dominant Traits Polydactylism Achondroplastic Dwarfism Tay-Sachs Disease - One Wrong Letter

Observable Patterns of Inheritance