Genetics Ch.2

Transmission Geneticss

Who was Gregor Mendel?

Studied plant physiology and plant biology with Professor Franz Unger and physics with Prof. Christian Doppler and Prof Andreas von Ettingshausen. Learned to think critically about prevailing theories of plant reproduction and hybridization.

In 1856, he began his work on the heredity of traits in the edible pea plant Pisum sativum (34 different varieties of peas). Settled on 14 strains after two years that represented seven individual traits working with them for the next 5 years.

He successfully identified principles of hereditary transmission, testing the blending theory of heredity, which was the predominant hereditary theory at the time (1800s)

Dichotomous

Two easily distinguished forms of expression in a seed or plant

What were the key understandings that caused Mendel’s superior insight?

Came principally from his familiarity with quantitative thinking and his understanding of the particulate nature of matter. Central was counting the number of progeny with specific phenotypes. The logical and component data gathering was key to Mendel’s ability to formulate the hypotheses that explained his results.

Self-fertilization in plants

Having both male (anther) and female (ovule) reproductive structures. Reproduction by organisms that contain both male and female reproductive structures; the natural reproductive mechanism on many plants, including pea plants.

Artificial cross-fertilization in plants

A controlled cross between plants made by an investigator who transfers pollen from one plant to fertilize the other plant.

In pea plant experiment: they cut away the anthers to prevent the flower from producing pollen, and then they use a small paintbrush to manually fertilize the emasculated flower with pollen from a different plant.

Blending theory of heredity

An obsolete theory of heredity proposing the traits of offspring are the average of parental traits.

For example, the blending theory would predict that crossing a black cat and a white cat would produce gray kittens, and that the original black or white colors would never reappear if the gray kittens were bred to one another.

What was Mendel’s new experimental design that paved the way for Mendel’s success at correctly describing two fundamental laws of heredity?

Following an initial observation, he devised a hypothesis to explain the observation and then carried out an independent experiment to test the hypothesis. In fact, the experimental design Mendel constructed is an example of the hypothesis-driven experimental approach scientists use today, known as the scientific method.

Scientific Method

1. Make initial observations about a phenomenon or process.

2. Formulate a testable hypothesis to explain the observations.

3. Design a controlled experiment to test the hypothesis.

4. Collect data from the controlled experiment.

5. Interpret the experimental results, comparing the observed results with those expected under the assumptions of the hypothesis.

6. Draw reasonable conclusions, reformulating or retesting the hypothesis if necessary.

What were Mendel’s five specific features of the breeding experiments?

(1) controlled crosses between plants

(2) use of pure-breeding strains to begin the experimental controlled crosses

(3) selection of dichotomous traits

(4) quantification of results

(5) use of replicate, reciprocal, and test crosses.

Cross-pollination

The transfer of pollen from the anther of one flower to the stigma of a flower on a different plant of the same species.

In Mendel’s experiments: carry out artificial cross-fertilization, employing carefully selected plants as pollen and egg donors to ensure that the progeny could be used to test a hereditary hypothesis.

Controlled Genetic Crosses

Genetic Crosses by an investigator who usually knows the genotypes and/or phenotypes of the organisms being crossed

Pure-Breeding strains/True-Breeding strains

A group of genetically identical homozygous organisms that, when self-fertilized or intercrossed, only produce offspring that have a phenotype identical to the parents. Consistently produce the same phenotype.

Parental Generation (P Generation)

The parents of F1 progeny. In controlled genetic crosses, the parents are pure-breeding.

First Filial Generation (F1 generation)

The first generation of offspring. In genetic experiments, usually the offspring produced by crossing pure-breeding parents.

Second Filial Generation

The offspring produced by mating F1 organisms to one another.

Third Filial Generation

(third filial generation) The third generation, produced by crossing F2 organisms.

What were the 7 traits Mendel studied?

The 14 pure-breeding strains were bred for:

(1) seed color (yellow or green)

(2) seed shape (round or wrinkled)

(3) pod color (green or yellow)

(4) pod shape (inflated or constricted)

(5) flower color (purple or white)

(6) flower position (axial or terminal)

(7) plant height (tall or short)

What were the three genetic-cross strategies that have become approaches to genetic analysis?

Replicate crosses, reciprocal crosses, test crosses

Replicate Crosses

Repeated crosses involving parents with the same genotypes and phenotypes.

Reciprocal Crosses

Paired crosses involving distinct parental phenotypes in which the sexes are switched (i.e., if one cross is ♂ phenotype A × ♀ phenotype B, the reciprocal cross is B × ♀ phenotype phenotype A).

Test Crosses

The cross of an organism with the dominant phenotype that may be heterozygous with an organism that is homozygous for a recessive allele. Also known as test-cross analysis.

Dominant phenotype

The phenotype observed in a heterozygous organism that is identical to the phenotype observed in a homozygote. The phenotype produced when an organism is homozygous for the dominant allele or carries a single copy of the dominant allele in the heterozygous genotype. Compare with recessive phenotype.

Recessive Phenotype

The phenotype observed in an organism that is homozygous for the recessive allele. Compare with dominant phenotype.

Homozygous genotype

A diploid genotype characterized by the presence of two identical alleles of a gene.

pure-breeding organisms; If a homozygous plant is self-fertilized, or if two pure-breeding plants expressing the same trait are crossed, the progeny have the same phenotype for the trait and the same homozygous genotype as the parents

Heterozygous Genotype

A diploid genotype characterized by the presence of two different alleles of a gene.

in a genetic cross between pure-breeding parents with different traits

What were Mendel’s results that revealed consistent features?

(1) dominance of one phenotype over the other in the F1 generation

(2) reemergence of the recessive phenotype in the F2 generation

(3) a ratio of approximately 3:1 (dominant : recessive) among F2 phenotypes.

What is Mendel’s new hereditary hypothesis after rejecting the blending theory?

each trait is determined by two “particles of heredity”—what today we call “alleles.”

Particulate inheritance

Mendel’s theory that genetic information is transmitted from one generation to the next as discrete units or elements of heredity.

Dominant Allele

An allele that is expressed in the phenotype if the genotype contains one or more copies of the allele.

Recessive Allele

An allele that can only be expressed in a phenotype if two copies of the allele make up the genotype.

The recessive allele only produces the recessive phenotype when it is in a homozygous genotype.

Monohybrid Cross

A genetic cross between organisms that are heterozygous for one gene.

Phenotypic ratio

• A ratio or set of relative proportions between organisms with different phenotypes—for example, the ratio of progeny produced by a monohybrid cross (3:1) or a dihybrid cross (9:3:3:1).

Genotypic ratio

(1) A ratio or set of relative proportions between organisms with different genotype

(2) The ratio of ¼ : ½ : ¼ observed among the homozygous and heterozygous F2 progeny of a monohybrid cross.

Punnet Square

• Named in honor of early 20th-century geneticist Reginald Punnett, a checkerboard-like diagram that predicts the genotypes and genotype frequencies of progeny from a genetic cross.

The Punnett square separates the two alleles carried by each reproducing organism, placing the reproductive cells, or gametes, from one parent along the vertical margin of the diagram, and those from the other parent along the horizontal margin.

Law of Segregation

Mendel’s 1st Hypothesis/Mendel’s 1st Law

describes the particulate nature of inheritance; The separation of alleles of a gene during gamete formation.

The two alleles for each trait will separate (segregate) from one another during gamete formation, and each allele will have an equal probability (1/2) of inclusion in a gamete. Random union of gametes at fertilization will unite one gamete from each parent to produce progeny in ratios that are determined by chance.