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 traitsVariations 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 fluidFluids 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 evolutionWould produce uniform populationsMany observations did not support blending

Example:

The theory of natural selection does not fit with this view of inheritance

Gregor MendelCalled:

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

_________ _____Pisum sativum: the garden pea plantThis 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 varietiesSuccessive generations will be ____________

in a certain traitOffspring grown from seeds of white-flowered

parent plants also have white flowersPeas 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 HybridsTrue-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 traitsEach 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

AllelesWhen both alleles are the same, the

condition is the _______________If the alleles differ, then it is the

______________ conditionAn allele is ___________ when its effect on a

trait masks the effect of any __________ allele paired with itDominant alleles are signified by

capitalized lettersLowercase letters signify recessive ones

AllelesA 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 ExpressionDominant 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 ExperimentsMendel 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 CrossesIn the first-generation offspring (F1):

One form of the trait (white flowers) disappears

All offspring have purple flowersWhen Mendel crossed those offspring (F1 x

F1), the white flowers reappeared!What is going on?

The parent pea plants must have been:

__________________________________________________

Monohybrid CrossThe 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 CrossWe 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 CrossThe numerical ratios of crosses suggested

that genes do not blendFor example, the F2 offspring showed a

3:1 phenotypic ratio of purple to whiteThus, 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 SegregationMendel’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 CrossesMendel correctly predicted that all F1

plants would show both of the dominant allelesExample:

He wondered if the genes for flower color and plant height would always travel together when two F1 plants were crossed

Dihybrid CrossThe F2 results:

tall and purple-flowered:dwarf and white-flowered:

dwarf and purple-flowered:tall and white-flowered:

Punnett SquareHow 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)

CodominanceIn _______________ , nonidentical alleles for a gene are

fully expressed in heterozygotesMay 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 cellsThe ABO gene encodes an enzyme which

determines the form of the glycolipidIA and IB are each dominant to i, but are

codominant to each other

ABO Blood TypesBlood 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 DominanceIn ___________ __________, one dominant allele

cannot completely mask the expression of anotherOne allele of a pair is not fully dominant over its

partnerExample: ______________________________

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

EpistasisTraits 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 ReachSometimes 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 geneFibrillin 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