Observing Patterns in Inherited Traits

By the late nineteenth century… Charles Darwin’s theory of natural selection suggested that a population could evolve if members show variation in heritable traits Variations that improved an individual’s chances of surviving would be more common in each generation Over time, a population____________________ Prevailing view:“____________ _______________” Hereditary material must be fluid Fluids from both parents blend at fertilization, causing ___________________________

What’s Wrong with This Theory? Blending inheritance doesn’t explain the observable variations in a population that allows for evolution Would produce uniform populations Many observations did not support blending Example: The theory of natural selection does not fit with this view of inheritance

Gregor Mendel Called: Used experiments in plant breeding and his knowledge of mathematics to form his own hypotheses about inheritance

_________ _____ Pisum sativum: the garden pea plant This plant can fertilize itself Its flowers produce both male and female gametes, which fertilize and give rise to new plants Mendel could use true-breeding varieties Successive generations will be ____________ in a certain trait Offspring grown from seeds of white-flowered parent plants also have white flowers Peas can also be cross-fertilized by human manipulation of pollen

Mendel’s Experimental Approach Mendel cross- fertilized true- breeding garden pea plants having clearly contrasting traits For example, white flowers vs. purple flowers Figure 10.3

Producing Hybrids True-breeding lineage occurs when offspring inherit __________ _________ in each generation ___________ are the offspring of a cross between two individuals that breed true for different forms of a trait Each inherits ______________ ___________ for a trait being studied

Producing _________ Figure 10.5

Terms Used in Modern Genetics _______ : heritable units of information about traits Each is located at its own particular ________ on the chromosome _________ : different molecular forms of the same gene ____________ : permanent change in a gene’s information that alters a gene’s molecular structure

Heritable Units of Information Figure 10.4

Alleles When both alleles are the same, the condition is the _______________ If the alleles differ, then it is the ______________ condition An allele is ___________ when its effect on a trait masks the effect of any __________ allele paired with it Dominant alleles are signified by capitalized letters Lowercase letters signify recessive ones

Alleles A homozygous dominant individual has a pair of dominant alleles: ______ A heterozygous individual has a pair of nonidentical alleles: ______ Homozygous recessive: pair of recessive alleles _______

Gene Expression Dominant allele may mask effect of recessive allele on the homologous chromosome Gene expression is the process in which the gene’s information is converted to its function. Expressed genes _________________________. _____________ : an individual’s alleles at any or all gene loci (their genetic make up) ______________ : an individual’s observable traits; how their genes are expressed

Mendel’s Experiments Mendel suspected that every plant inherits two “units” of information for a trait, one from each parent Mendel’s first experiments were _____________ __________ Monohybrid crosses have two parents that are true- breeding for contrasting forms of a trait For example, pea plants with white flowers and pea plants with purple flowers ____ = parent generation ____ = first-generation offspring ____ = second-generation offspring

Monohybrid Crosses In the first-generation offspring (F1): One form of the trait (white flowers) disappears All offspring have purple flowers When Mendel crossed those offspring (F1 x F1), the white flowers reappeared! What is going on? The parent pea plants must have been: __________________________________________________

Monohybrid Cross The parent pea plants must have been: F1: The offspring, therefore, must ALL be: F2: The offspring of the second cross (F1 x F1) will:

Monohybrid Cross We now know that all members of the F1 offspring are heterozygous (Aa) because one parent could produce only an A gamete and the other could produce only an a gamete Mendel assumed that each sperm has an equal probability of fertilizing an egg

Monohybrid Cross The numerical ratios of crosses suggested that genes do not blend For example, the F2 offspring showed a 3:1 phenotypic ratio of purple to white Thus, each new plant has three chances in four of having at least one dominant allele

Testcross ___________ : method of determining genotype One individual of unknown genotype is crossed with another that is homozygous recessive The results will show if the individual is homozygous or heterozygous for a dominant trait To support his concept of segregation, Mendel crossed F1 plants with homozygous recessive individuals A ratio of ______ of recessive and dominant phenotypes supported his hypothesis

Theory of Segregation Mendel’s Theory of Segregation: diploid organisms have pairs of genes, on pairs of homologous chromosomes States that diploid organisms inherit two genes per trait on pairs of chromosomes. Each gene segregates from the other during meiosis such that each gamete will receive only one gene per trait.

Remember… there are always exceptions to the rules!

F2 Offspring: Dominant and Recessive Traits Figure 10.6

Mendel’s Dihybrid Experiments _______________ are the offspring of parents that breed true for different versions of two traits In addition to his monohybrid crosses, Mendel also performed experiments involving two traits: a ___________ _______________ True breeding parents: AABB x aabb F1 hybrid offspring: Gametes

Dihybrid Crosses Mendel correctly predicted that all F1 plants would show both of the dominant alleles Example: He wondered if the genes for flower color and plant height would always travel together when two F1 plants were crossed

Dihybrid Cross The F2 results: tall and purple-flowered: dwarf and white-flowered: dwarf and purple-flowered: tall and white-flowered:

Punnett Square How did Mendel get these ratios? *Reference Figure 10.9 in your textbook

Mendel’s Theory of ______________ ______________ The Mendelian theory of independent assortment states that as meiosis ends, genes on pairs of homologous chromosomes have been sorted out for distribution into one gamete or another, independently of gene pairs of other chromosomes Requires qualification, because gene pairs do not always assort independently

Why was Mendel lucky? Beyond simple dominance patterns… 1)3) 2)4)

Codominance In _______________, nonidentical alleles for a gene are fully expressed in heterozygotes May occur in ___________ _________ __________, in which three or more alleles of a gene persist among individuals Example: ______________________________ Blood type is determined by markers produced by three genes (a multiple allele system) Red blood cells have membrane glycolipids which helps the body identify its own blood cells The ABO gene encodes an enzyme which determines the form of the glycolipid I A and I B are each dominant to i, but are codominant to each other

ABO Blood Types Blood type is determined by the alleles carried for the ABO gene: _________________ A and B are codominant when paired; this means that some people can express both genes and have _____ blood The O allele is __________ when paired with A or B Figure 10.10

Incomplete Dominance In ___________ __________, one dominant allele cannot completely mask the expression of another One allele of a pair is not fully dominant over its partner Example: ______________________________ A true-breeding red-flowered snapdragon crossed with a white-flowered snapdragon will produce _________ ____________ This pink color is because there is not enough red pigment (produced by the dominant) allele to completely mask the effects of the white allele

Epistasis Traits are expressed through ____________ : interactions among products of two or more gene pairs Two alleles can mask the expression of another gene’s alleles because of these interactions Examples: variations in chicken combs and Labrador retriever coat colors

Single Genes with a Wide Reach Sometimes the expression of alleles at one location can have effects on two or more traits, which is called ______________ Marfan syndrome: genetic disorder which arises by mutations in the fibrillin gene Fibrillin protein allows elasticity in many tissues, including the heart, skin, blood vessels, skeleton, and tendons Characterized by these effects: lanky skeleton, leaky heart valves, weakened blood vessels, deformed air sacs in lungs, pain, and lens displacement in the eyes