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Chapter 9Patterns of

Genetic Inheritance

Gregor Mendel Deduced Laws of Inheritance

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9.1 A blending model of inheritance existed

prior to Mendel

Austrian monk Gregor Mendel developed the fundamental laws of heredity after performing a series of experiments with pea plants

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Figure 9.1 Gregor Mendel examining a pea plant

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9.2 Mendel designed his experiments well

Figure 9.2A Garden pea anatomy and the

cross-pollination procedure Mendel used

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Figure 9.2B Garden pea traits and crosses studied by Mendel

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Single-Trait Crosses Reveal Units of Inheritance and the Law

of Segregation

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9.3 Mendel’s law of segregation describes how gametes

pass on traits The law of segregation states:

Each individual has two factors for each trait The factors segregate (separate) during the

formation of the gametes Each gamete contains only one factor from each pair

of factors Fertilization gives each new individual two factors for

each trait

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Figure 9.3 Monohybrid cross done by Mendel

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9.4 The units of inheritance are alleles of genes

Traits are controlled by alleles - alternate forms of a gene Found on homologous chromosomes at a particular

gene locus The dominant allele masks the expression of

the other allele - the recessive allele Genotype refers to the alleles an individual

receives at fertilization Homozygous - an organism has two identical alleles

at a gene locus Heterozygous - an organism has two different alleles

at a gene locus Phenotype - the physical appearance of the

individual. 9-10

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Figure 9.4 Occurrence of alleles on homologous chromosomes

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Two-Trait Crosses Support the Lawof Independent Assortment

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9.5 Mendel’s law of independent assortment describes inheritance

of multiple traits

The law of independent assortment states the following: Each pair of factors separates (assorts)

independently (without regard to how the others separate)

All possible combinations of factors can occur in the gametes

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Figure 9.5 Dihybrid cross done by Mendel

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9.6 Mendel’s results are consistent with the laws of probability

Figure 9.6 Use of Punnett square to calculate probable events 9-16

9.7 Testcrosses support Mendel’s laws and indicate the genotype

Testcross - intentional breeding in order to determine underlying genotypes One-trait Testcross - When a heterozygous

individual is crossed with one that is homozygous recessive, the results are always a 1:1 phenotypic ratio

Two-trait Testcross - when an individual is heterozygous for two traits is crossed with one that is recessive for the traits, the offspring have a 1:1:1:1 phenotypic ratio

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Figure 9.7A One-trait testcross, when the individual with the dominant phenotype is heterozygous

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Figure 9.7B One-trait testcross when the individual with the dominant phenotype is homozygous

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Mendel’s Laws Apply to Humans

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9.8 Pedigrees can reveal the patterns of inheritance

Some genetic disorders are medical conditions inherited from parents

Some may be due to the inheritance of abnormal alleles on autosomal chromosomes - all the chromosomes except the sex chromosomes Carriers - those individuals that carry the abnormal

allele but do not express it

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Figure 9.8A Autosomal recessive pedigree

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Figure 9.8B Autosomal dominant pedigree

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9.9 Some human genetic disorders are autosomal recessive

Tay-Sachs Disease - uncontrollable seizures, and paralysis prior to dying Results from a lack of the enzyme Hex A

Cystic Fibrosis - most common lethal genetic disease of Caucasians in the U.S. Genetic testing for the recessive allele is possible

Phenylketonuria - most commonly inherited metabolic disorder affecting nervous system Many diet products have warnings that they contain

phenylalanine Sickle-cell Disease - genotype HbS HbS has

many symptoms from anemia to heart failure Individuals who are HbA HbS have sickle-cell trait

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9.10 Some human genetic disorders are autosomal dominant

Neurofibromatosis - many children with neurofibromatosis have learning disabilities and are hyperactive

Huntington disease - a neurological disorder that leads to progressive degeneration of brain cells

Achondroplasia - a common form of dwarfism associated with a defect in the growth of long bones

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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES

9.11 Genetic disorders may now be detected early on

Testing Fetal Cells Amniocentesis - long needle withdraws a small amount of the

fluid that surrounds the fetus and contains a few fetal cells Chorionic Villi Sampling (CVS) - tube is inserted through the

vagina into the uterus and fetal cells are obtained by suction Testing the Embryo

A single cell can be removed from the 8-celled embryo and subjected to preimplantation genetic diagnosis (PGD)

Testing the Egg Polar bodies (nonfunctional cells produced during egg

formation) receive a haploid number of chromosomes When a woman is heterozygous for a recessive genetic disorder,

about half the polar bodies have received the mutated allele, while the egg has received the normal allele

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Figure 9.11A Prepregnancy testing of an embryo

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Figure 9.11B Prepregnancy testing of an egg

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Complex Inheritance Patterns Extend the Range of Mendelian Analysis

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9.12 Incomplete dominance still follows the law of segregation

Incomplete dominance - heterozygote has an intermediate phenotype between that of either homozygote

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Figure 9.12 Incomplete dominance

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9.13 A gene may have more than two alleles

Multiple alleles - gene has several allelic forms Example: blood type is determined by multiple alleles

IA = A antigen on red blood cells IB = B antigen on red blood cells i = Neither A nor B antigen on red blood cells

Possible phenotypes and genotypes for blood type:

This is an example of codominance because both IA and IB are fully expressed 9-32

9.14 Several genes and the environment can influence a single

multifactorial characteristic

Polygenic inheritance occurs when a trait is governed by two or more genes Multifactorial traits - controlled by polygenes subject

to environmental influences

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Figure 9.14 Polygenic inheritance: Dark dots stand for dominant alleles; the shading stands for environmental influences

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9.15 One gene can influence several characteristics

Pleiotropy - when a single gene has more than one effect

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Figure 9.15A Marfan syndrome illustrates the multiple effects a single human gene can have

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Chromosomes Are the Carriers of Genes

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9.16 Traits transmitted via the X chromosome have a unique

pattern of inheritance

X-linked alleles have a different pattern of inheritance than autosomal alleles The Y chromosome cannot offset the inheritance of

an X-linked recessive allele

Affected males always receive their X-linked recessive mutant allele from the female parent

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Figure 9.16 X-linked inheritance

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9.17 Humans have X-linked disorders

Color Blindness - the alleles for the red- and green-sensitive proteins are on the X chromosome

Muscular Dystrophy - occurs in males but the recessive allele remains in the population through passage from mother to daughter

Hemophilia - 1 in 10,000 males is affected by both external and internal bleeding

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Figure 9.17 X-linked recessive pedigree

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9.18 The genes on one chromosome form a linkage group

Gene linkage - the existence of several genes on the same chromosome Genes on a single chromosome form a linkage

group because they tend to be inherited together

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Figure 9.18 A simplified map of the genes on chromosome 2 of Drosophila

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9.19 Frequency of recombinant gametes maps the chromosomes A linkage map can also be called a chromosome

map because it tells the order of gene loci on chromosomes

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Figure 9.19 Example of incomplete linkage

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APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES

9.20 Thomas Hunt Morgan is commonly called “the fruit fly guy” In 1908, Morgan began experimenting with the fruit

fly (Drosophila melanogaster)

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Connecting the Concepts:Chapter 9

The first of Mendel’s laws tells us that an individual has two alleles, but the gametes have only one allele for every trait The second law tells us that the gametes have all

possible combinations of alleles Polygenic inheritance and X-linked inheritance

extend the range of Mendelian analysis Males are more apt than females to display an X-

linked disorder Genes do have loci on the chromosomes, but

today we know that genes are composed of DNA

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