1. Understand what chromosomes are, what their role is, and how organisms get them.

2. Explain what events in meiosis contribute to genetic variability, and how these events relate to Mendel's two laws.

3. Understand the relationships of the following genetic terms: homologous chromosomes, genes, dominant and recessive alleles, genotype, phenotype, homozygous (dominant and recessive), heterozygous, autosome, monohybrid, dihybrid, and pedigree.

4. Given the genotypes of two mating individuals (or of their offspring), predict the genotypes and phenotypes of offspring that they might produce (or of the parents giving rise to them).

5. Interpret pedigrees for human genetic conditions that are autosomal dominant or autosomal recessive.

Homologous chromosomes: Chromosome pairs within cells which have the same sequence of genes. One chromosome of each pair comes from each of the parents through its gamete.

*Gene: A sequence of DNA specifying the sequence of amino acids of a particular protein involved in the expression of a trait.

*Gene Locus (pl. loci): The specific location of a sequence of DNA on a particular chromosome that specifies the sequence of amino acids of a particular protein involved in the expression of a trait. For example, the segment of DNA located at position 393 on chromosome #7 is the gene locus of the Cystic Fibrosis membrane protein gene.

Autosomes: Chromosomes that do not determine sex. Humans have 44 of them (22 pairs). The two remaining chromosomes determine sex and are known as the sex chromosomes.

*Allele: An alternate form of a gene. (An alternate sequence of DNA at a particular Gene Locus). A dominant allele masks the expression of another allele (and is commonly symbolized by a capital letter). The masked allele is recessive (and is commonly symbolized by the corresponding small letter).
The normal Cystic Fibrosis membrane protein allele produces the normal membrane protein. Other slightly different DNA sequences at this location produce variations of the protein or no protein at all, resulting in the disease.

Genotype: The combination of alleles in an individual (an individual's genetic constitution).

Phenotype: The observable expression of an allele combination.

Homozygous: Possessing two identical alleles of a gene. An individual with two dominant alleles is homozygous dominant. If both alleles are recessive, the individual is homozygous recessive.

Heterozygous: Possessing two different alleles of a gene.

Monohybrid: An individual heterozygous for a particular gene. In a monohybrid cross, two heterozygous individuals are mated. The expected genotypic ratio is 1:2:1 (i.e., AA, Aa, aa), and the expected phenotypic ratio is 3:1. To help derive these ratios, a diagram called the Punnett Square can be used.

Dihybrid: An individual who is heterozygous for two particular genes.

* Probably the three most frequently confused terms in Biology

GeneralWeb resources:

• Meiosis Tutorial from the University of Arizona (still incomplete)

• Meiosis Animations From McGill University

• National Institute of Health Gene Testing Glossary

• The Dictionary of Cell Biology

• Mendelian Genetics Directory

• MendelWeb

• What is a chromosome?

• Electron micrograph image

• See pg. 317 in your text (Fig. 15.10) to see the relationship between DNA and chromosomal protein.

• Primate Cytogenetics Network (chromosome squashes, karyotypes, and ideograms)

• A gene Map of the Human Genome

• Human gene sequence collection

• The real Y chromosome

Review of Meiosis

• Meiosis and Sexual Reproduction

• Virtual Meiosis

• Meiosis: an illustration of the meiotic process provided by Access Excellence.
  There is something very wrong with this illustration. Be the first to tell me what it is in the "Talk to Ed" discussion in the Web Crossing Bio chat Folder.

• Mendel's Law of Segregation (see Fig. 13.5, text)

During meiosis, homologous pairs of chromosomes (and the genes that compose them) separate from one another and are packaged into separate gametes. At fertilization, gametes combine at random to form the individuals of a new generation.

By knowing the genotypes of the parents or the phenotypes of the offspring (assuming that there are enough to see all, or most of the possible phenotypes), it is possible to understand inheritance patterns, determine dominance of traits, and other genetic characteristics.

This sort of analysis was pioneered by Gregor Mendel (the "Father of Genetics") using peas. He scored different genetic lines for such characteristics as flower color, plant height, seed texture, and followed their inheritance patterns for many generations to determine the basic rules of inheritance.

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• Punnett Square diagram of a cross involving only ONE GENE (Monohybrid)

Mendel's Second Law - Independent Assortment
(The Inheritance of Two Genes)

A gene on one chromosome does not influence the inheritance of a gene on a different (nonhomologous) chromosome because meiosis packages chromosomes randomly into gametes.

• Independent assortment refers to the RANDOM arrangement of paired homologous chromosomes during the middle part of the first division of MEIOSIS.

• Mendel's Law of Independent Assortment (see Fig. 13.14, text)

• See last lecture's topic dealing with the independent assortment of chromosomes.
  Independent Assortment 1
  Independent Assortment 2

• Punnet square analysis of a cross involving TWO GENES. (Dihybrid)

Pedigree Analysis

• Characteristics determined by single genes are called Mendelian traits (see Table 13.2 for some unusual ones; see Table 13.3 for some Mendelian disorders in humans).

• Pedigrees depict family relationships and indicate which ones have a particular trait. They can reveal the mode of inheritance of a particular trait.

• Common pedigree symbols

• EXAMPLE: A pedigree of hemophilia in the Royal Families of Europe

• Autosomal dominant traits (see Fig. 13.11, text)
  • Dentinogenesis imperfecta and what it is.

    Can affect both sexes (because it is carried on a nonsex chromosome); AA or Aa have disorder; an affected individual must have an affected parent

• Autosomal recessive traits (see Fig. 13.10 text)
  • Cystic fibrosis

    Can affect both sexes; aa has disorder; can skip generations; carriers (Aa) occur and show no symptoms

  • Diagram of simple inheritance

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