3 - mendel and gene idea

8/13/2019 3 - Mendel and Gene Idea

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Chapter 14-Mendel and the

Gene Idea


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Gregor Mendels Discoveries

Mendel brought an experimental and quantitative approach

to studying heredity- this is the field of Genetics.

Discovered particulate ( v s . blending) nature of heredity.

Discovered alleles, and their dominant/recessive nature.

Discovered that alleles segregate (packaged into gametesindependently).

Discovered that pairs of alleles segregate independently.

These discoveries occurred before chromosomes or DNAknown.

Contemporary of Darwin.


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Mendel conducted his research using peas.

Why?

-Peas have a variety of characters and traitsthat were easily studied.

-Characters are heritable features ( e.g. ,flower color).-Each variant of a character is called atrait ( e.g. , purple or white flower).

-Some selected characters used by Mendelwere: flower color, seed color, seed shape,and stem length.


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Mendel chose to study only characters with either/ortraits.

The pea traits that he studied are discontinuous traits,

meaning the traits are either one way or the other (no in -between or gradations).

For example, pea plants had eitherpurple or white flowers; smooth or wrinkled seeds.

This allowed Mendel to discern how traits are passed fromone generation to the next.

Many traits have gradations instead. One example is thecarnation flowers colors.


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Sweet pea flowers


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Round andwrinkled

peas


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There were other reasons that Mendel used pea plants.

Stamens (male reproductive organs) could be removedto control mating- so no self-fertilization. Thus, he couldmate male and female gametes as he chose and couldcontrol his experiments.

This was be done by taking pollen (sperm) from oneplant, and adding it to the carpel (female organ) ofanother plant that had its stamen removed.

By fertilizing plants by hand, the parents of each peaseed would be known.


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In addition, Mendel used only true-breeding plants.

I.e. , the traits remain constant after self-fertilization.

This means that the plants contained two identicalversions (alleles) of the gene encoding the trait.

For example, if a pea plant has only genes for whiteflowers, if it self-fertilizes, all the offspring will only havegenes for white flowers.


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In his breeding experiments, Mendel did the following, andtracked heritable characteristics for three generations:

1) Produced offspring by hybridization.

Hybridization is the mating of two (2) true-breedingindividuals.

True-breeding parents are called the P generation.

Hybrid offspring are called the F 1 generation.

2) He then allowed the F 1 generation to self-pollinate.

The offspring of this group are called the F 2 generation.


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Mendel studies established two important principles ofgenetics:

1. Law of Segregation2. Law of Independent Assortment

1. Tenets of the Law of Segregation:

a. There are multiple versions of the same gene (eachversion is a different allele). b. Each organism inherits two (2) alleles for eachcharacter; one allele from each parent.

c. If the two alleles are different, then the dominant alleleis fully expressed; the recessive allele has no noticeableeffect on the organisms appearance. d. The two alleles for each character separate duringgamete production (occurs during meiosis) Segregation


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Punnett Square: used for predicting results of a geneticcross between two individuals of known genotypes.

It is used to illustrate the 3:1 ratio that Mendel observed inthe F 2 generation.

Homozygous: contains identical alleles for a character

Heterozygous: contains two different alleles for a character

Phenotype: an organisms traits

Genotype: an organisms genetic makeup


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Segregation ofalleles and

fertilization aschance events


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Using a Punnett Square to reveal genotype: by testcross

By breeding an organism of unknown genotype with an

organism with a homozygous recessive individual, we candetermine the genotype of the unknown individual.

The ratio of phenotypes in the offspring is used todetermine unknown genotype.

Example:


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2. Law of Independent Assortment

Mendel did most of his experiments by following only a

single character at a time.

Instead, one can follow two characters at a time, todemonstrate their Independent Assortment.

Each allele pair segregates independently during gameteformation.

These experiments use a dihybrid cross.

If independent, the combination of two traitsgives a 9:3:3:1 ratio!


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Beyond Simple Mendelian Genetics: many factors canmake genetics more complex than Mendel saw, including:

1. Incomplete dominance

F1 hybrids have a phenotype somewhere in between thephenotypes of the two parents.

Example: Incomplete dominance in snapdragon color.


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2. Dominance vs. co-dominance

Both alleles are equally expressed.

Example: MN blood grouping

-Two alleles for surface molecules of blood, M or N

-So, either MM, MN, or NN


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3. Multiple alleles

Most genes have more than two (2) alleles.

Example: Blood type = A; B; AB; O


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4. Pleiotropy

Most genes affect an organism in many ways; they dontaffect just one phenotypic character.

Example: The many effects of sickle cell anemia.


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5. Epistasis

One gene affects the expression of another gene.

Example:

In the following case with mice, the gene for color is Bwhere BB = black, and bb = brown.

But a second gene, C, determines whether pigment canbe produced.

C allows for pigment to be produced.

c does not allow pigment to be produced (albino).


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6. Polygenic inheritance

Two or more genes affect one phenotypic character; theopposite of pleiotropy.

These are called quantitative characters.

Example: skin color, where three genes impart color.


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Mendelian Inheritance in Humans

We are unable to manipulate mating patterns of humansfor experimentation. For this reason,

Traits are studied by gathering information and placing itinto a family tree.

The inter-relationships among parents and childrenacross generations are called the family pedigree.


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Many Human Disorders Follow Mendelian Patterns ofInheritance

1. Recessively Inherited Disorders

- Heterozygous individuals exhibit normal phenotypebecause one copy of the normal allele is typically

sufficient.- Heterozygotes, who are phenotypically normal, are calledcarriers. They may transmit the recessive allele to theiroffspring.

- Cystic fibrosis- Tay-Sachs disease- Sickle-cell disease

2 Dominantly Inherited Disorders


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2. Dominantly Inherited Disorders

Lethal dominant alleles are uncommon because, if theycause death before maturity, then the allele will not bepassed to future generations.

Example: Huntingtons disease (nervous disorder) is causedby a late-acting allele and is sometimes passed to future

generations.

3. Multifactorial Disorders

Most diseases are influenced not only by genetics, but alsoby environmental factors.

Example: Heart disease, diabetes, cancer, alcoholism,mental illnesses.


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Technology for Genetic Testing and Counseling

1. Carrier Recognition

Determine whether prospective parents are heterozygouscarriers of a recessive trait.

Identify carriers of diseases such as Tay-Sachs, sickle cell,or cystic fibrosis.

Ethical issues?

2 Fetal Testing


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2. Fetal Testing

a. Inserting a needle into the uterus, physicians can extractamniotic fluid. In some cases, the fluid can be used todetect genetic disorders. The technique is known asamniocentesis.

In rare cases, amniocentesis can cause complications or

fetal death, so reserved when greatest risk for defect.

b. An alternative technique is called chorionic villussampling (CVS). A tube is inserted through the cervix andfetal tissue from the placenta is extracted.

c. Other techniques, such as ultrasound, can be used toexamine the fetus directly for physical abnormalities.


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3. Newborn Screening

Some genetic disorders can be detected by simple tests

performed soon after birth.

Phenylketonuria (PKU): inability to break downphenylalanine.


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