fundamentals of genetics chapter 9 table of contents section 1 mendel’s legacy section 2 genetic...

Fundamentals of GeneticsChapter 9

Table of Contents

Section 1 Mendel’s Legacy

Section 2 Genetic Crosses

Section 1 Mendel’s LegacyChapter 9

Gregor Mendel• The study of how characteristics are transmitted from parents to

offspring is called genetics.

• Known as the “father of genetics” but not the first person to study genetics.

• A fair number of practical breeding experiments were done in ancient times.

• People knew there was a connection between sex and reproduction.


Gregor Mendel, continued• Mendel’s Garden Peas

– Mendel observed characteristics of pea plants. – Traits are genetically determined variants of a

characteristic.• Every day we observe heritable variations (eyes of brown,

green, blue, or gray) among individuals in a population.• These traits are transmitted from parents to offspring.

– Each characteristic occurred in two contrasting traits.– Traits create breeds.

• Another example: dogs were domesticated by 12,000 years ago; selective crossing has given us all the many breeds we have today


Gregor Mendel, continued

• Mendel’s Methods– Mendel used cross-

pollination techniques in which pollen is transferred between flowers of two different plants.

– Studied the traits that came about.


Mendel’s Experiments

• Mendel bred plants for several generations that were true-breeding for specific traits and called these the P generation.

• Offspring of the P generation were called the F1 generation.

• Offspring of the F1 generation were called the F2 generation.

Chapter 9

Three Steps of Mendel’s Experiments


Chapter 9

Click below to watch the Visual Concept.

Visual Concept

Mendel’s Experiments



Mendel’s Results and Conclusions• Recessive and Dominant Traits

– Mendel concluded that inherited characteristics are controlled by factors that occur in pairs.

– In his experiments on pea plants, one factor in a pair masked the other. The trait that masked the other was called the dominant trait. The trait that was masked was called the recessive trait.

• Mendel developed a hypothesis to explain these results that consisted of four related ideas.1. Alternative versions of genes (different alleles) account for variations in inherited characters.

• The purple-flower allele and white-flower allele are two DNA variations at the flower-color locus.

• 2. For each character, an organism inherits two alleles, one from each parent.

• In the flower-color example, the F1 plants inherited a purple-flower allele from one parent and a white-flower allele from the other

3.If two alleles differ, then one, the dominant allele, is fully expressed in the the organism’s appearance.

• The other, the recessive allele, has no noticeable effect on the organism’s appearance.– Mendel’s F1 plants had purple flowers because the purple-flower

allele is dominant and the white-flower allele is recessive.4. The two alleles for each character segregate (separate) during gamete

production.• The separation of alleles into separate gametes is known as Mendel’s

law of segregation.– If an organism has 2 identical alleles for a particular character, then

that allele exists as a single copy in all gametes.– If 2 different alleles are present, then 50% (1) of the gametes will

receive one allele and 50% (1) will receive the other.


Mendel’s Results and Conclusions, continued

• The Law of Segregation– The law of segregation states that a pair of

factors is segregated, or separated, during the formation of gametes.


Mendel’s Results and Conclusions, continued

• The Law of Independent Assortment– The law of independent assortment states that

factors for individual characteristics are distributed to gametes independent of one another.

– The law of independent assortment is observed only for genes that are located on separate chromosomes or are far apart on the same chromosome.


Support for Mendel’s Conclusions

• We now know that the factors that Mendel studied are alleles, or alternative forms of a gene.

• One allele for each trait is passed from each parent to the offspring.

Chapter 9


Visual Concept

Mendel’s Conclusions


Section 2 Genetic CrossesChapter 9

Genotype and Phenotype

• The genotype is the genetic makeup of an organism.

• The phenotype is the appearance of an organism.


Probability

• Probability is the likelihood that a specific event will occur.

• A probability may be expressed as a decimal, a percentage, or a fraction.

Chapter 9


Visual Concept

Calculating Probability



Predicting Results of Monohybrid Crosses

• A Punnett square can be used to predict the outcome of genetic crosses.

• A cross in which one characteristic is tracked is a monohybrid cross.

Chapter 9


Visual Concept

Punnett Square with Homozygous Cross


Chapter 9

Monohybrid Cross of Heterozygous Plants



Predicting Results of Monohybrid Crosses, continued

• A testcross, in which an individual of unknown genotype is crossed with a homozygous recessive individual, can be used to determine the genotype of an individual whose phenotype expresses the dominant trait.

Chapter 9


Visual Concept

Testcross




• Complete dominance occurs when heterozygous individuals and dominant homozygous individuals are indistinguishable in phenotype.

• This is what we have been crossing the last couple days, traits that had complete dominance.



• Incomplete dominance occurs when two or more alleles influence the phenotype and results in a phenotype intermediate between the dominant trait and the recessive trait.

• This occurs in some flowers. If you cross a red rose with a white rose, the offspring will all come out PINK!



• Codominance occurs when both alleles for a gene are expressed in a heterozygous offspring.

• This occurs in bulls. When a Roan bull (red colored fur is crossed with a White bull, all the offspring have red and white fur.


Predicting Results of Dihybrid Crosses

• A cross in which two characteristics are tracked is a dihybrid cross.

Chapter 12• Chromosome

– Genes reside on Chromosomes– Sex chromosomes contain genes that determine

an organism’s sex (gender). • In mammals, an individual carrying two X

chromosomes is female. • An individual carrying an X and a Y

chromosome is male.– The remaining chromosomes that are not directly

involved in determining the sex of an individual are called autosomes.

Karyotypes: Male and Female

Effects of Gene Location• Sex-Linked Genes and Traits

– Genes found on the X chromosome are X-linked genes. – A sex-linked trait is a trait whose allele is located on a sex

chromosome.– Because males have only one X chromosome, a male who

carries a recessive allele on the X chromosome will exhibit the sex-linked trait.

• Linked Genes– Pairs of genes that tend to be inherited together are called

linked genes. • Chromosome Mapping

– The farther apart two genes are located on a chromosome, the more likely a cross-over will occur

Mutations

• Germ-cell mutations occur in gametes and can bepassed on to offspring.

• Somatic-cell mutations occur in bodycells and affect only the individual organism.

• Chromosome mutations are changes in the structure of a chromosome or the loss or gain of an entire chromosome.

• Gene mutations are changes in one or more of the nucleotides in a gene.

Chromosome Mutation Gene Mutation

Inheritance of Traits

• Pedigrees – Geneticists use pedigrees to trace diseases or

traits through families. – Pedigrees are diagrams that reveal inheritance

patterns of genes.

Ch 12-2

Pedigree for Cystic Fibrosis

Genetic Traits and Disorders

• Polygenic Inheritance– Polygenic characters, such as skin color, are

controlled by two or more genes.• Complex Characters

– Complex characters, such as polygenic traits, are influenced by both genes and environment.

• Multiple Alleles– Multiple-allele characters, such as ABO blood

groups, are controlled by three or more alleles of a gene.

– (2 visual concepts)

Genetic Traits and Disorders

• X-Linked Traits– The gene for colorblindness, an X-linked recessive

gene, is found on the X chromosome.• Sex-influenced Trait

– A sex-influenced trait, such as pattern baldness, is expressed differently in men than in women even if it is on an autosome and both sexes have the same genotype.

– (Visual concepts)

Treating Genetic Disorders

• Among the treatments are symptom-relieving treatments and symptom-prevention measures, such as insulin injections for diabetes.

• Gene Therapy– In gene therapy, a defective gene is replaced with

a copy of a healthy gene.– Somatic cell gene therapy alters only body cells. – Germ cell gene therapy attempts to alter eggs or

sperm.

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