Genes and Meiosis

• Heredity: the transmission of traits from one generation to the next

• Variation: demonstrated by the differences in appearance that offspring show from parents and siblings

• Genetics: the scientific study of heredity and variation

• Genes: the units of heredity and are made up of segments of DNA

• Gametes (sperm and eggs): are the reproductive cells that pass genes on to the next generation

• Each gene has a specific position, or locus, on a certain chromosome

• A clone is a group of genetically identical individuals from the same parent (from asexual reproduction)

• Karyotype: an ordered display of the pairs of chromosomes from a cell

• The two chromosomes in each pair are called homologous chromosomes, or homologs

Meiosis

• Meiosis: cell division that creates gametes

  • Meiosis takes place in two sets of cell divisions, called meiosis Iand meiosis II resulting in four (haploid - 23 chromosomes) daughter cells, rather than the two (diploid - 46 chromosomes) daughter cells in mitosis

• A gamete (sperm or egg) contains a single set of chromosomes and is haploid (n) (product of meiosis)

• Our cells contain 23 homologous pairs of chromosomes (one from each parent) totalling 46

• During S-phase the chromosomes are replicated resulting in sister chromatids

• Homologs may have different versions of genes, each called an allele

• In crossing over, nonsister chromatids exchange DNA segments

• Each homologous pair has one or more X-shaped regions called chiasmata — this is where crossing over occurs.

• A zipper-like structure called the synaptonemal complex forms during this attachment (synapsis)

• The DNA breaks are closed so that a paternal chromatid is joined to a piece of maternal chromatid, and vice versa

Mendelian Genetics

• true-breeding - plants that produce offspring of the same variety when they self-pollinate

• hybridization - crossing two different, true-breeding varieties (resulting in a hybrid)

• Law of Segregation: when an organism makes gametes, each gamete receives just one gene copy, which is selected randomly

• An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character (ex. AA or aa)

• An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character (ex: Qq)

• Heterozygotes are not true-breeding, unlike homozygotes

• Phenotype - physical appearance

• Genotype - genetic makeup (alleles ex: Bb)

Dihybrid Crosses

• Law of independent assortment: states that each pair of alleles segregates independently of any other pair during gamete formation

• *Genes located near each other on the same chromosome tend to be inherited together (gene linkage)

Inheritance outside of complete dominance

• Incomplete dominance: the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

• Codominance: two dominant alleles affect the phenotype in separate, distinguishable ways

• Multiple Alleles (common in many populations): when genes have more than two allelic forms

  • For example, the four phenotypes of the ABO blood group in humans are determined by three alleles of the gene IA, IB, and i.

• Sex-linked gene: A gene that is located on either sex chromosome

• X-linked genes: What genes on the X chromosome are called

  • For a recessive X-linked trait to be expressed

    • A female needs two copies of the allele (homozygous

    • A male only needs one copy of the allele (hemizygous)

  • X-linked recessive disorders are much more common in males than in females.

• Most genes have multiple phenotypic effects, a property called pleiotropy

• Quantitative characters are those that vary in population along a continuum/spectrum

• Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype.

  • eg. skin color

• Epistasis: when a gene at one locus alters the phenotypic expression of a gene at a second locus

• A pedigree is a family tree that describes the interrelationships of parents and children across generations