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Chapter 6 Heredity
The Big Idea Heredity is the passing of the instructions for traits
from one ________to the next.
Section 1 Mendel and His Peas
Key Concept The work of Gregor Mendel explains the rules of
heredity and is the foundation of modern genetics.
Before Mendel
• The passing of traits from parents to offspring is called________.
• People once believed that traits from both parents are simply mixed
together in the offspring. This is called ________inheritance.
• While offspring often look like a combination of both parents, the traits
are not actually blended together. But the rules of heredity were not
__________until a scientist named Gregor Mendel did some
experiments with ____plants.
Gregor Mendel’s Work
• The work of Gregor Mendel explains the rules of heredity and is the
___________of modern genetics.
• Mendel’s experiments showed that blending inheritance is incorrect for
many traits. His work changed the way people think about__________.
• As a _______in Austria, Mendel chose to put most of his time into
research.
Self-Pollinating Peas
• Mendel chose to study the garden pea. They worked well for his
experiments because they grow quickly and there are many__________.
• Pea plants were ideal also because they can self-pollinate. This means
sperm from a flower can __________the eggs of a flower on the same plant.
• Self-pollinated plants can be_____ _______, which means the offspring
always have the same characteristics as the parent.
Cross-Pollinating Peas
• In ____ ________, sperm from one plant fertilize the eggs on a
different plant. Many plants can be fertilized only through cross
pollination.
• In cross-pollination, sperm are carried in pollen by_______, birds, bats,
or other organisms. In some plants, the pollen is carried by the wind.
• Pea plants can also be fertilized through cross-pollination.
Mendel’s Work with Pea Plants
• Mendel used only true-breeding plants to see what happened if plants
with different ______for the same characteristic were crossed.
• To ensure cross-fertilization, he removed the pollen-producing _______
from one plant.
• He then used _______from a different plant to fertilize the anther-less
plant. This way he could control which parents crossed.
• Mendel studied one characteristic at a time. A ____________is a feature
that has different forms in a population, such as hair color.
• The different forms of a characteristic are called______, such as brown
hair, red hair, or black hair.
• For his experiments, Mendel chose the characteristics of pea plants,
including flower color, seed_______, pod color, and plant height.
Mendel’s First Experiments
• In his crosses, ________found the offspring always had the
characteristics of one parent, but not the other.
• The offspring of such a cross are called _____ _________plants.
• In the cross between purple flowered plants and white-flowered plants,
the offspring ______had purple flowers.
• Mendel chose to call the trait that appeared in the first generation the
_________trait.
• He chose to call the trait that disappeared in the first generation the
_________trait.
Mendel’s Second Experiments
• In his ________set of experiments, Mendel allowed the first generation
plants to self-pollinate.
• In each case, _______of the offspring had the dominant trait, but some of
them had the recessive trait.
• The offspring of the first generation plants are called the _________
generation.
• Mendel counted the number of plants that had the dominant trait and
the number of plants that had the recessive trait for each characteristic.
• Mendel then computed the ______of dominant to recessive plants. A ratio
is an expression of a relationship between two numbers.
• Mendel found a ratio of about _______dominant traits for every one
recessive trait. (The ratio can be written as 3:1 or 3/1).
Mendel’s Conclusions
• Mendel realized that his results could only be explained if each plant
had _____sets of instructions for each characteristic.
• He concluded that each parent gives one set of instructions to the
_________, and that the dominant instructions determine the trait.
• Mendel published his findings in_______, but 30 years passed before the
significance of his work was realized.
ANSWER BOX
generation, second, blending, pea, 1865, all, dominant, characteristic, fertilize,
insects, shape, inheritance, traits, pollen, three, foundation, two, most, variations,
discovered, recessive Mendel, traits, second, monk, heredity, anthers, truebreeding, cross-pollination, first-generation, ratio, offspring,
Section 2 Traits and Inheritance
Key Concept Sudden _______along breaks in Earth’s crust can
release energy in the form of seismic waves.
A Great Idea
• Mendel knew that each parent passed on ___________for each trait.
Scientists now call these instructions for an inherited trait genes.
• Offspring end up with two versions of each gene for every
characteristic—_____from each parent.
• Different versions of the same gene are called________.
• For the offspring to show a _________trait, it must inherit the recessive
allele from both parents. Offspring show the ________trait if they inherit
the dominant allele form even just one parent.
Phenotype Genotype
• The appearance of an organism is called its__________.
• The particular alleles of an organism are called its_________.
• Dominant alleles are shown with ________letters. Recessive alleles are
shown with lowercase letters.
• If purple flowers are dominant, and the letter “P” stands for the alleles
for flower color, than the genotype of a purple flowered plant could be
PP or____.
• The genotype of a plant with a white phenotype (white flowers) has to
be____.
• Organisms with two of the same alleles (PP or pp) are __________for
that trait.
• Organisms with one of each allele (Pp) are ____________for that trait.
• The genotype of a true breeding purple pea is____. A true breeding white
plant is pp. All of their offspring would be Pp, and would be purple.
Punnett Squares
• A _________square is used to predict the possible genotypes of offspring
in a particular cross.
• Once you know the possible genotypes, you can also determine the
phenotypes (_________) of the offspring.
• A Punnett _______can help explain the results of Mendel’s second set of
experiments.
• A ______of Pp with Pp gives the following results: PP, Pp, pP, and pp.
• All except pp will have purple flowers, because all except pp have the
dominant allele.
What Are the Chances?
• The mathematical chance that something will happen is known as
___________.
• Probability is usually written as a percentage or a__________. When you
toss a coin, there is a 50% chance of getting heads.
• In a parent with two different alleles, such as Pp, there is a 50%
chance of offspring getting P and a 50% _________of getting p.
Calculating Probabilities
• To find the probability of several events happening, _________the
probability of the first event by the probability of the second event.
• The probability of tossing heads _______in a row is 1/2 X 1/2, or 1/4.
• The probability of getting white flowers (pp) in the cross Pp X Pp is 1/2
X 1/2, or ____(25%).
More About Traits
• In Mendel’s experiments, each trait was ___________by one gene. But
sometimes the inheritance of traits is more complicated.
• A gene may __________more than one trait, or a trait may be influenced
by more than one gene.
• For example, your eye color is the result of ________genes acting
together.
• The____________, both internal and external, can also affect the
phenotype, or appearance, of an organism.
• For example, height is influenced by__________. In some organisms,
coloration can be determined by diet.
• In humans, skin color is determined by several genes. It is also affected
by exposure to the_____.
Genetic Variation
• Scientists estimate than ________have about 30,000 genes.
• While humans may have the ______genes, each individual has a unique
set of alleles.
• The differences in the sets of alleles between individuals in a population
is called genetic_________.
• Genes affect more than your appearance. Genes affect the ________
functioning of your body as well.
• Every hormone, enzyme, protein, and other __________that your body
makes is influenced by genes.
ANSWER BOX
motions, one, nutrition, same, environment, sun, alleles, pp, PP, recessive,
several, Punnett, dominant, probability, Pp, capital, fraction, phenotype,
chance, genotype, controlled, internal, twice, 1/4 , variation, instructions, cross,
heterozygous, square, influence, appearance, multiply, homozygous, chemical
Section 3 Meiosis
Key Concept Meiosis and sexual reproduction allow for the
combination of genetic material from _____different cells.
Chromosome Numbers
• Meiosis and sexual reproduction increase the number of possible
genetic combinations in a____________.
• There is a carefully controlled process that ensures sexual reproduction
leads to the proper number of ___________in the offspring.
• Each _______has a normal number of chromosomes in their body cells.
• ________body cells have 46 chromosomes, corn cells have 20, and dogs
cells have 78.
• Each chromosome in a body cell has a matching pair with the same
genes but different_______. Chromosomes that carry the same sets of
genes are called homologous ______________.
Chromosomes in Reproduction
• Cells that have homologous pairs of chromosomes are called_________.
Body cells are diploid.
• Before sexual reproduction can occur, an organism must make sex
cells. The male sex cells are_______, and the female sex cells are eggs.
• Sex cells do not have homologous pairs and are called__________. Human
sex cells have 23 chromosomes.
• Sex cells are haploid so that when they combine during fertilization,
they produce an individual with the __________number of chromosomes.
• Human body cells have _____ chromosomes.
Meiosis
• Sex cells are made during a process called_________.
• Meiosis is a copying process that produces cells that have ______the
usual number of chromosomes.
• When a cell undergoes meiosis, it divides twice. This produces ______
cells, each with half the number of chromosomes as the parent cell.
Meiosis and Mendel
• Meiosis can help explain the results first observed by Mendel.
• If the true breeding plant for wrinkled seeds provided the sperm, all of
the sperm cells would have the same recessive allele, r.
• If the true breeding plant for smooth seeds provided the eggs, all of the
eggs would have the dominant allele R.
• The only __________possible in the offspring in this cross is Rr.
• The only ___________possible is smooth seeds, since all of the offspring
would have the dominant allele.
Meiosis and Inheritance
• So in sexual reproduction, half of the offspring’s genetic material comes
from the mother’s egg, and half comes from the ________ sperm.
• Mitochondria, which have their own DNA, come from the mother’s egg.
So all of the ____________in your body can be traced back to your
mother.
answer box
two, chromosomes, father’s, four, genotype, meiosis, chromosomes, phenotype, half,
species, 46, normal, diploid, Human, sperm, haploid, alleles, population, mitochondria