Lecture 5

Cummings 3: pp 49-66)

TRANSMISSION OF GENES

I. Divisions of genetics.

II. Mendelian analysis of heredity.

III. Law of Genetic Segregation

IV. Independent Assortment

V. Variations

Terms

Errata Page 63, Fig. 3.12: "Meiosis I" should read "Metaphase I." Page 69, item 4 of left column, should read, "...of the F2 generation."

I. Divisions of Genetics

A. Transmission genetics is concerned with the mechanisms and rules of transmission of genes from one generation to the next.

B. Molecular genetics deals with the chemical structure of genes and the chemical details of gene function.

C. Population genetics is concerned with the distribution of genes in populations.

A. Mendel was successful in part because he studied discontinuous traits, with simple alternative forms, rather than continuous variables.

B. Based on analysis of monohybrid crosses, Mendel realized that hereditary units may be present without being expressed.

1. An allele that is dominant is expressed in the F₁ generation.

2. The recessive trait can be recovered in the F₂ generation.

3. Alleles are not altered because of combinations with other alleles, e.g. a recessive allele is not altered in the F₁ because it happens not to be expressed. Genes are stable, and the variations observed are due to various combinations of genes.

A. A brief way of stating the law is that a gamete receives only one of the two alleles present in the parent, the choice being entirely random.

B. The following observations led to and are consistent with this law:

1. Reciprocal matings give the same results; therefore parental contributions are equal.

2. An allele is dominant if it is expressed in the F₁ generation; the alternative allele is recessive. The dominant allele is usually written in upper case letters and the recessive allele in lower case.

3. The genotypic ratio in the F₂ is 1AA:2Aa:1aa, or 1:2:1. The phenotypic ratio in the F2 is 3(AA + Aa):1aa.

4. The cross Aa × aa is a backcross. Offspring occur in the ratio 1Aa:1aa.

C. Combinations of alleles can be predicted by the laws of probability.
1. Prob MN = prob M × prob N, where M and N are independent events. The joint probability of a particular combination is the product of the probabilities of the individual events, since the probabilities of particular maternal and paternal gametes are independent of each other.

2. Genotypic combinations can be predicted by a Punnett Square, in which the kinds and proportions of gametes from one parent are placed along one axis and the same for the other parent along the other axis.

A. The phenotypic ratio in the F₂ of a dihybrid cross is 9A– B– : 3A– bb : 3aaB– : 1aabb. [A– means either AA or Aa, etc.]

B. Parental combinations of alleles are not maintained in the gametes. This is called genetic recombination.

C. Independent assortment occurs for all pairs of loci except those located very close together on the same chromosome.

D. In 1900, Mendel's laws were "rediscovered" by other scientists. In 1902, it was recognized that Mendel's hereditary units (genes) and chromosomes follow the same rules of transmission. It was proposed that chromosomes are the physical location of genes (the chromosome theory of inheritance).

V. There are variations in phenotype/genotype relationships that were not observed by Mendel in his early studies.

A. The number of possible alleles at any one locus can be very large. At some human loci, hundreds of alleles are known.

B. Incomplete dominance describes the situation in which the F₁ phenotype is intermediate between the parental phenotypes.

C. Codominance is used to describe pairs of alleles, both of which are expressed in heterozygous combination.

D. The terms dominant and recessive refer to the phenotypic expression of combinations of alleles. A particular allele may be dominant, codominant, incompletely dominant, or recessive, depending on which allele is on the homologous chromosome.

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