Chapter 7
Mendelian Inheritance
You are NOT responsible for sections 11, 12,14, & 16…these
sections have been crossed out on the outline.
Chapter Outline
1 Why do offspring resemble their parents?
7.1 Family resemblance: Your mother and father each contribute to your genetic makeup.
TAKE-HOME MESSAGE 7.1: Offspring resemble their parents because they inherit
genes—instruction sets for biochemical, physical, and behavioral traits, some of which
are responsible for diseases—from their parents.
7.2 Some traits are controlled by a single gene.
TAKE-HOME MESSAGE 7.2: More than 9,000 human traits are determined by the
instructions a person carries on a single gene, and the traits exhibit straightforward
patterns of inheritance.
7.3 Mendel learned about heredity by conducting experiments.
TAKE-HOME MESSAGE 7.3: In the mid-1800s, Gregor Mendel conducted studies
that helped us understand heredity. He focused on easily observed and categorized traits
in garden peas and applied methodical experimentation and rigorous hypothesis testing to
determine how traits are inherited.
7.4 Segregation: You’ve got two copies of each gene but put only one copy in each sperm
or egg.
TAKE-HOME MESSAGE 7.4: Each parent puts a single set of instructions for building
a particular trait into every sperm or egg it makes. This instruction set is called a gene.
The trait observed in an individual depends on the two copies (alleles) of the gene it
inherits from its parents.
7.5 Observing an individual’s phenotype is not sufficient for determining its genotype.
TAKE-HOME MESSAGE 7.5: It is not always possible to determine an individual’s
genetic makeup, known as a genotype, by observation of the organism’s outward
appearance, known as its phenotype. For a particular trait, an individual may carry a
recessive allele whose phenotypic effect is masked by the presence of a dominant allele.
Much genetic analysis therefore makes use of clever experiments and careful recordkeeping, using Punnett squares, to determine organisms’ genotypes.
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Probability and chance play central roles in genetics.
7.6 Chance is important in genetics.
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TAKE-HOME MESSAGE 7.6: Probability plays a central role in genetics. In
segregation, each gamete that an individual produces receives only one of the two copies
of each gene the individual carries in its other cells, but it is impossible to know which
allele goes into the gamete. Chance plays an important role in fertilization, too: All of the
sperm or eggs produced by an individual are different from one another, and any one of
those gametes may be the gamete involved in fertilization.
7.7 A test-cross enables us to figure out which alleles an individual carries.
TAKE-HOME MESSAGE 7.7: In a test-cross, an individual that exhibits a dominant
trait but has an unknown genotype is mated with an individual that is homozygous
recessive. The phenotypes of the offspring reveal whether the unknown-genotype
individual is homozygous dominant (all of the offspring exhibit the dominant trait) or
heterozygous (half of the offspring show the dominant trait and half show the recessive
trait).
7.8 We use pedigrees to decipher and predict the inheritance patterns of genes.
TAKE-HOME MESSAGE 7.8: Pedigrees help scientists, doctors, animal and plant
breeders, and prospective parents determine the genes that individuals carry and the
likelihood that the offspring of two individuals will exhibit a given trait.
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The translation of genotypes into phenotypes is not a
black box.
7.9 Incomplete dominance and codominance: The effects of both alleles in a genotype
can show up in the phenotype.
TAKE-HOME MESSAGE 7.9: Sometimes the effects of both alleles in a heterozygous
genotype are evident in the phenotype. With incomplete dominance, the phenotype of a
heterozygote appears to be an intermediate blend of the phenotypes of the two
homozygotes. With codominance, a heterozygote has a phenotype that exhibits
characteristics of both homozygotes.
7.10 What’s your blood type? Some genes have more than two alleles.
TAKE-HOME MESSAGE 7.10: In multiple allelism, a single gene has more than two
alleles. Each individual still carries only two alleles, but in the population, more than just
two alleles exist. This is the case for the ABO blood groups in humans.
7.11 Multigene traits: How are continuously varying traits such as height influenced by
genes?
TAKE-HOME MESSAGE 7.11: Many traits, including continuously varying traits such
as height, eye color, and skin color, are influenced by multiple genes.
7.12 Sometimes one gene influences multiple traits.
TAKE-HOME MESSAGE 7.12: In pleiotropy, one gene influences multiple, unrelated
traits. Most, if not all, genes may be pleiotropic.
7.13 Why are more men than women colorblind? Sex-linked traits differ in their patterns
of expression in males and females.
TAKE-HOME MESSAGE 7.13: The patterns of inheritance of most traits do not differ
between males and females. However, when a trait is coded for by a gene on a sex
chromosome, such as color vision on the X chromosome, the pattern of expression differs
for males and females.
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7.14 Environmental effects: Identical twins are not identical.
TAKE-HOME MESSAGE 7.14: Genotypes are not like blueprints that specify
phenotypes. Phenotypes are generally a product of the genotype in combination with the
environment.
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Some genes are linked together.
7.15 Most traits are passed on as independent features: Mendel’s law of independent
assortment.
TAKE-HOME MESSAGE 7.15: Genes tend to behave independently, such that the
inheritance pattern of one trait doesn’t usually influence the inheritance of any other trait.
7.16 Red hair and freckles: Genes on the same chromosome are sometimes inherited
together.
TAKE-HOME MESSAGE 7.16: Sometimes, having one trait influences the presence of
another trait. This is because the alleles for two genes are inherited and expressed almost
as a package deal when the genes are located close together on the same chromosome.
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