Ch. 14:Mendel and the Gene Idea

I. IntroductionA. Heritable traits (brown eyes, green eyes, blue eyes) are passed down from parents to offspring. B. Blending hypothesis: Offspring should

have a blend of parental traits. (Yellow + Blue = Green) This blending hypothesis is incorrect.

C. Particulate inheritance: parental genes retain their separate identities, and are sorted and then passed on to future generations.

D. Gregor Mendel documented particulate inheritance by studying sweet pea plants.

II. Gregor MendelA. An Augustinian monk who studied the sciences and math. His studies

influenced his experimentation in genetics.B. In 1857, Mendel began breeding garden

peas to study inheritance.1. Peas were an ideal subject to study:

a. Many heritable traits (color, height, pod shape, etc.)

b. Easy to control mating

- Self-pollination- Cross-pollination

C. Mendel’s experiment:1. Mendel cross-pollinated to hybridize

two contrasting, true-breeding pea varieties. - True-breeding: P generation- Offspring: F1 generation

2. Mendel then allowed for the F1 generation to self-pollinate to producethe F2 generation.

3. Mendel quantitative analysis of the F2 generation enabled him to come upwith two important principles of heredity:

a. The law of segregationb. The law of independent assortment

D. Law of segregation: two alleles for a trait are packaged into separate gametes.

1. When the P generation cross-pollinated,the F1 generation were all purple.

2. When the F1 generation self-pollinated,the F2 generation hadboth purple and white.

The ratio between purple and white flowers in the F2 generation were 3:1.Mendel reasoned that though the F1 generation had no white flowers, they must have carried the heritable trait for white flower color.Mendel said that the purple color gene was “dominant” and the white color gene was “recessive.”

3. Mendel found similar 3:1 ratios of traitsin the F2 generation when he conductedcrosses for these traits:

Example: Round v. Wrinkled pea seedsF1 = 100% RoundF2 = 75% Round, 25% Wrinkled

4. Mendel stated that different genes(different alleles) account for variationsin inherited characteristics.

Ex. Purple andwhite color geneshave different DNA sequences.

5. Mendel also stated that an organisminherits two alleles for each trait, one from each parent.-Diploid-Homologous chromosomes

6. If two alleles inherited are different, thenthe dominant allele will be expressed.The recessive allele will have no effect.

7. The two alleles segregate during gameteproduction; homologues will separate when gametes are made. This is the Law of Segregation

8. Mendel’s Law of Segregation accounts for his observation of 3:1 in the F2 generation:a.The F1 generation will create two kinds of gametes = half will have the white allele, half will have the purple allele. b. During self-pollination, the gametes

will combine randomly to form four combinations.

c. A Punnett Square can be made to predict the results of a genetic cross:

9. An organism with two identical alleles for a character is homozygous for thattrait.

10.An organism with two different allelesfor a character is heterozygous for thattrait.11.An organisms genetic makeup is called

its genotype.12.An organisms physical traits is called

its phenotype. Two organisms can have the same phenotype but different genotypes.

The only wayto produce a white offspringis to have tworecessive traits (homozygous recessive).

13.You cannot predict correctly the genotype of an organism with a dominant phenotype. A purple flower could be PP, or Pp.

a. The only way to know the genotype ofan organism with a dominant traits is by doing a “test-cross.”

E. Law of Independent Assortment: Each pair of alleles segregates into gametesindependently. 1. Mendel did other experiments that

followed the inheritance of two differentcharacters in a dihybrid cross (v. monohybrid). 2. Mendel studied seed shape and color:

Y = Yellow, y = Green R = Round, r = wrinkled

3. Mendel crossed true-breeding plants that had yellow, round seeds (YYRR) with true-breeding plants that has green, wrinkled seeds (yyrr).

4. One possibility is that the two characteristics could be transmitted as a package. F1 = Yellow, Round F2 = 3 Yellow, Round:

1 Green, Wrinkled** However, this was not the results of Mendel’s experiment.

5. Instead, the two characteristics segregate independently of one another.

F1 = Yellow, Round Their gametes can have four combinations: YR, Yr, yR, yr

Therefore, the ratios for the F2generation is:F2 = 9 Yellow, Round (Yy, Rr):

3 Yellow, Wrinkled (Yy, rr): 3 Green, Round (yy, Rr): 1 Green, Wrinkled (yy, rr)

** Whenever Mendel did a dihybrid cross, he always got the 9:3:3:1 ratio. This can be explained as the result of the “Law of Independent Assortment.”

III. Extending Mendelian GeneticsA. Mendel chose to study pea characteristics

that had simple genetic; each phenotypiccharacteristic was determined by one gene.

B. However, in most cases, the relationshipbetween phenotype and genotype is rarely simple.

C. Incomplete Dominance: F1 hybrids (heterozygotes) show a intermediate phenotype:

Example: SnapdragonsP = Red, WhiteF1 = 100% PinkF2 = 25% Red

50% Pink25% White

Mendel’s peas showed that heterozygous and homozygous dominant plants had the same phenotype. This is complete dominance.

D. Codominance: two alleles affect the phenotype in distinguishable ways. Example: Blood types A, B, AB, and OTay-Sach’s Disease

E. Because an allele is dominant does not necessarily mean that it is more common in a population than the recessive allele.Example: Polydactyly is a dominant gene.However, the recessive allele is far moreprevalent.

F. Multiple Alleles: Most genes have more than one allele in a population.1. Example: Blood alleles A, B, and O

with four possible phenotypes for blood type.

a. A and B alleles are codominant.b. Both A and B alleles are dominant to

O.c. A type blood = genotype AA or AO.

A type blood has oligosaccharides onthe surface of the RBCs. A type blood also produces antibodies against B type blood.

d. B type blood = genotype BB or BO.B type blood has oligosaccharides onthe surface of RBCs. B type blood also produces antibodies against Atype blood.

e. Individuals with O type blood haveneither A or B type oligosaccharides on their RBCs. They do however produce antibodies against both Aand B blood types.

f. Matching compatibility for blood typeis crucial for transfusions. - O type blood = universal donor- AB type blood = universal recipient

G. Pleiotropy: Genes have multiple effects.Example: Sickle Cell gene has multiple ofeffects.

H. Epistasis: a gene on one locus can alterthe effects of another gene on a different locus.Example: Coat color in mice is determinedby two genes.1. The epistatic gene, determines whether

or not pigment will be deposited into hair. Presence of pigment ( C ) is dominant to no presence ( c ). 2. Pigment color black (B) is dominant to

brown pigment (b).

3. A mouse with cc will be an albino regardless of genotypes for hair colorblack or brown.

4. A cross between two mice that are heterozygous for both genes (Cc, Bb) follows the law of independent assortment:

However, theratio will be 9 black:3 brown:4 white

I. Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character.1. Example: Human skin color is due to more than 3 separate genes. For simplicity sake, however, we will consider just 3 genes for determining skin color.

a. A, B, and C are the 3 different genes, all incompletely dominant to the alleles (a, b, and c).

b. An individual with AABBCC genotype will be very dark. An individual with AaBbCc genotype will have an intermediate shade.

c. Because the alleles have a cumulativeeffect, an individual with genotype AaBbCc will have the same skin coloras an individual with genotype AABbcc.

d. A cross betweentwo individualswith AaBbCcgenotypes would resultin a bell-shapedcurve, called a normal distribution.

J. The environmental impact on phenotype:1. Example: Nutrition can influence height.

Exercise influences build. Sun exposureinfluences skin color. Practice improvesintelligence tests. Identical twins?

2. The products of a genotype can be a wide range of variation. This phenotypicrange due to the environment is called the norm of reaction.

Norm of reaction:colors of hydrangeaflowers, range from blue to pink, depending on the acidity of soil.

IV. Mendelian Inheritance in HumansA. While peas are an easy subject to study

genetics, humans are not.1. Human generation span is too long.2. Parents produce few offspring.3. Breeding experiments is socially

unacceptable. B. Pedigree analysis reveals Mendelian

patterns in human inheritance.1. Phenotypic information is gathered from

as many members of a family across generations.

2. The information can then be mapped onto a family tree.

3. Example: Widow’s peak (W) is dominant over a straight hairline (w). We can tryand predict the genotypes of individualsin a family tree.

a. If an individual has no widow’s peakbut both his/her parents have a widow’s peak, we can predict that bothparents are heterozygous.

4. A pedigree can help us understand thepast and predict the future.

C. Many human disorders follow Mendelian patterns of inheritance. 1. Thousands of genetic disorders can be

inherited as recessive traits. 2. Genetic disorders can range from mild

to life-threatening.3. Heterozygous individuals are

phenotypically normal but are “carriers”of the disorder.

4. Most individuals that are born with a disorder are born to carriers with normal phenotypes.a. Two carriers have ¼ probability of

having a child with the disorder, ½ probability of having carriers, and ¼ free.

5. Some genetic disorders are found morecommonly among people of certain ethnic backgrounds.

a. Cystic fibrosis: 1 in 2,500 caucasiansof european descent. 1 in 25 caucasians are carriers. CF is caused by a defective Cl- membrane protein that causes build up of mucus in the lungs, pancreas, & digestive system.)

b. Tay-Sachs: 1 in 3,600 births in Ashkenazic Jews. T-S is caused by a defective enzyme that cannot breakdown certain lipids in the brain. Thiscauses brain degeneration.

c. Sickle-Cell Anemia: 1 in 400 AfricanAmericans. It is caused by a substitution in one amino acid of thehemoglobin protein. When oxygen levels are low in blood, red blood cells deform into a sickle shape.

This sickling can cause a number of results.This sickling has pleiotropic effects.

The non-sickling allele is incompletelydominant to the sickle-cell allele. Heterozygous individuals are carriersand can suffer some symptoms of the disease under blood oxygen stress. Both normal and abnormalhemoglobins are synthesized. Individuals that are heterozygous are also resistant to malaria, a parasite that spends part of its lifecycle inside RBCs.

5. Dominant Inherited Disorders:a. Achondroplasia, a form of dwarfism,

has an incidence of one case in 10,000 people.

c. A lethal dominant allele can be passeddown to generations if the onset ofthe disease is later in life. Example: Huntington’s Disease, a degenerative brain disorder; onsetbt. 35-45 years of age.

b. Lethal dominant alleles are much less common than lethal recessives because an offspring with a lethal dominant will die before passing theallele on to future generations.

-Offspring born to a parent who has the allele for Huntington’s disease has a 50% chance of inheriting the disease and the disorder. -Molecule geneticists have recentlydiscovered that the gene for HD isfound on the tip of chromosome #4.

d.Polydactyly: a rare dominant phenotype

6. Multifactorial Diseases: Diseases causedby genetics and the environment.Example: Heart disease, diabetes, cancer, alcoholism, schizophrenia, andmanic-depressive disorder.

7. Albinism is a rare condition that is inherited as a recessive phenotype in many animals, including humans.

D. Technology is providing new tools for genetic testing and counseling.

1. Many hospitals have genetic counselors that can provide information to prospective parents who are concernedabout a family history of a specific disease.

2. Using Mendelian probability (Punnett Squares), one can determine the risks of passing on lethal genes.

3. Tests determine in utero if a child has adisorder. One technique is called an amniocentesis.

-can be done after 14-16 weeks of pregnancy.-fetal cells are extracted and karyotyped.-other disorders can be detected from chemicals in the amniotic fluid.

4. Chorionic villus sampling (CVS) can allow faster karyotyping and can be performed as early as the eighth to tenth week of pregnancy.

-Fetal tissue is extracted from the chrionic villi of the placenta.

5. Ultrasound and fetoscopy, allow fetal health to be assessed visually in utero.

-Both fetoscopy and amniocentesis cause complications in about 1% of cases. They can cause maternal bleeding or fetal death.-These techniques are usually reserved for cases in which the risk of a genetic disorder or other type of birth defect is relatively great.

After the results of a test are revealed, the parents must face the difficult decision of terminating the pregnancy or preparing to care for a child with a genetic disorder.

6. Some genetics tests can be done afterthe child is born. -Example: PKU - phenyketonuria-1 in 10,000-15,000 births-causes a build-up of the amino acid phenylalanine, and its derivative phenypyruvate in the blood to toxic levels.-this build-up can cause mental retardation. -if the genetic test is given at birth, a child can be given a special diet low in phenylalanine, which usually promotes normal growth.