Chapter 14

Mendel and the Gene Idea

A.  Gregor Mendel’s Discoveries             Mendel brought an scientific and mathematical approach to studying heredity this is the field of Genetics.             He studied peas.  Why?            -Peas have a variety of characters that were easily studied.  Characters are heritable features (eg. flower color).  Each variant of a character is called a trait (eg. purple or white flower).

-Some selected traits used by Mendel were (See Table 14.1 for complete list): flower color, seed color, seed shape, and stem length.

Mendel was extremely lucky in choosing the pea plant with which to work. This is because, the pea plant traits that he studied are all discontinuous traits.

This means that they are either one way or the other, there is no in between. For example, pea plants have either purple or white flowers; smooth or wrinkled seeds.  These traits have no gradations.

This is important, because it allowed Mendel to discern how traits are passed from one generation to the next. There are many traits that have gradations and we will see some of these later in the Chapter.

One example is the carnation flower’s colors (See next)

There were other reasons that Mendel used pea plants:

- Stamens (male reproductive organs) could be removed to control mating.  (There would be no self-fertilization.) Thus, he could mate male and female gametes as he chose and could control his experiments.

 

 - This was be done by taking pollen (sperm) from one plant, and adding it to the carpel (female organ) of another plant that had its stamen removed (Figure 14.1)

 

 - By fertilizing plants by hand, the parents of each pea seed would be known.

            - In addition, Mendel used only true-breeding plants. With these plants, the traits remain constant after self-fertilization. (This means that the plants contain two identical genes both genes encode the same trait.) For example, because a pea plant has only genes for white flowers, if it self-fertilizes, all the offspring will only have genes for white flowers. Thus, the trait is constant in each generation.           

For his breeding experiments, Mendel did the following in which he tracked heritable characteristics for three generations (Figure 14.3):

- Produced offspring by hybridization.  Hybridization is the mating of two (2) true-breeding individuals.

- True-breeding parents are called the P generation.

- Hybrid offspring are called the F1 generation.

-He then allowed the F1 generation to self-pollinate, the

offspring of this group are called the F2 generation.

Note the ratio of three purple to one white flower!!

By observing those three generations, Mendel laid the foundation for two important principles:            1.  Law of Segregation2.  Law of Independent Assortment

Let’s look at the tenets of the Law of Segregation in detail:

a.  There are multiple versions of the same gene (each version is a different allele; See Figure 14.4).b.  Each organism inherits two (2) alleles for each character; one allele from each parent. c.  If the two alleles are different, then the dominant allele is fully expressed; the recessive allele has no noticeable effect on the organism’s appearance.d.  The two alleles for each character separate during gamete production (Occurs during meiosis) Segregation

A Punnett Square is a device for predicting the results of a genetic cross between two individuals of known genotypes.  It is used to illustrate the 3:1 ratio that Mendel observed in the F2 generation.

 The Punnett Square and how to use it is described in Figure 14.5 (p. 255) – Mendel’s law of segregation.

Remember the following vocabulary words and apply them to Fig. 14.5 and 14.6:

- Homozygous:  contains identical alleles for a character- Heterozygous:  contains two different alleles for a character- Phenotype:  an organism’s traits- Genotype:  an organism’s genetic makeup

         One of the things we can do with a Punnett Square is to devise a way to reveal the genotype of an unknown organism. This is done by doing a Testcross                                    - By breeding an organism of unknown genotype with an organism with a homozygous recessive individual, we can determine the genotype of the unknown individual.  The ratio of phenotypes in the offspring is used to determine unknown genotype. For example:                         Figure 14.7 (p. 256) – A testcross.

            2.  Law of Independent Assortment                        Mendel did his experiments by following only a single character at a time. Instead, one can follow two characters at a time, to demonstrate the Law of Independent Assortment

Each allele pair segregates independently from other allele pairs during gamete formation.

These experiments use what’s called a dihybrid cross. Figure 14.8 (p. 257) – Testing two hypotheses for segregation in a dihybrid cross. Note that the combination of two traits gives a 9:3:3:1 ratio!

B.  Extending Mendelian Genetics There are many factors that make genetics not as straight forward as Mendel saw. These include:             1.  Incomplete dominance                        - F1 hybrids have a phenotype somewhere in between the

phenotypes of the two parents. Figure 14.10 (p. 261) – Incomplete dominance in snapdragon color.

Can you determine how one can see if it’s incomplete dominance?

2.  Dominance vs. co-dominance                        - Both alleles are equally expressed.  Example:  Tay-Sachs disease Homozygous recessive do not produce an enzyme to metabolize lipids that accumulate in brain cells, which causes the cells to die.

Heterozygotes produce the enzyme but only at half the amount produced in homozygous dominants. You don’t see the symptom of the disease, because half the normal amount of enzyme is sufficient.           

3.  Multiple alleles

- Most genes have more than two (2) alleles.  Example:  Blood type = A; B; AB; O

4.  Pleiotropy                        - Most genes affect an organism in many ways; they don’t affect just one phenotypic character. Example The many effects of sickle cell anemia 

5.  Epistasis

                       

- One gene affects the expression of another gene.

                       

 Figure 14.11 (p. 263) – An example of epistasis. In this case, the gene for color is B where BB = black, and bb = brown. But a second gene, C, determines whether pigment can be produced. C allows for pigment to be produced, c does not allow pigment to be produced (albino).

            6.  Polygenic inheritance                        - Two or more genes affect one phenotypic character; the opposite of pleiotropy.  These are called quantitative characters. Example, skin color, where three genes impart color.                     Figure 14.12 (p. 263) – A simplified model for polygenic inheritance of skin color.

7.  Environmental impact on gene expression                        - Environmental factors/conditions may alter gene expression. Figure 14.13 (p. 264) – The effect of environment on phenotype.

C.  Mendelian Inheritance in Humans                         - We are unable to manipulate mating patterns of humans for experimentation. For this reason, - Traits are studied by gathering information and placing it into a family tree. - The interrelationships between parents and children are called the family pedigree.                         Figure 14.14 (p. 265) – Pedigree analysis.

Many Human Disorders Follow Mendelian Patterns of Inheritance 1.  Recessively Inherited Disorders

- Heterozygous individuals exhibit normal phenotype because one copy of the normal allele is typically sufficient. - Heterozygotes, who are phenotypically normal, are called carriers.  They may transmit the recessive allele to their offspring.                                                - Cystic fibrosis                                                - Tay-Sachs disease                                                - Sickle-cell disease

2.  Dominantly Inherited Disorders                                    - Lethal dominant alleles are uncommon because, if they cause death before maturity, then the allele will not be passed to future generations. - Huntington’s disease (nervous disorder) is caused by a late-acting allele and is sometimes passed to future generations. 3.  Multifactorial Disorders                                    - Most diseases are influenced not only by genetics, but also by environmental factors                                                - Heart disease, diabetes, cancer, alcoholism, mental illnesses

            Technology  for Genetic Testing and Counseling  1.  Carrier Recognition                                    - Determine whether prospective parents are heterozygous carriers of a recessive trait                        - Identify carriers of diseases such as Tay-Sachs, sickle cell, or cystic fibrosis - Ethical issues?

2.  Fetal Testing                                    a.  By inserting a needle into the uterus, physicians can extract amniotic fluid.  In some cases, the fluid can be used to detect genetic disorders.  The technique is known as amniocentesis. ** In rare cases, amniocentesis can result in complications or fetal death.  Therefore, it is reserved for cases in which the risk for defect is greatest. b.  An alternative technique is called chorionic villus sampling (CVS).  A tube is inserted through the cervix and fetal tissue from the placenta is extracted. c.  Other techniques, such as ultrasound, can be used to examine the fetus directly for physical abnormalities.  Figure 14.17 (p. 270) – Testing a fetus for genetic disorders.

  3.  Newborn Screening                                    - Some genetic disorders can be detected by simple tests perfomed soon after birth.                                                  - Phenylketonuria (PKU): inability to break down phenylalanine

Chapter 15

You should review and understand Figure 15.1