Mendelian Genetics and Meiosis

Meiosis and Genetic Diversity

• Meiosis generates genetic diversity through random orientation of bivalents and crossing over.
• Crossing over occurs when non-sister chromatids in a bivalent swap sections of DNA, resulting in an exchange of genes.
• This exchange creates new combinations of alleles on the chromosomes, increasing genetic diversity in offspring.

Random Orientation of Bivalents

• In meiosis I, bivalents line up in the center of the cell, and spindle fibers attach to each chromosome, but which pole they attach to is random (50% chance for either).
• The random orientation determines which alleles end up in each gamete (sperm or egg).
• The random orientation of multiple bivalents creates a vast number of possible combinations (2^n, where n is the number of bivalents).

Dihybrid Crosses

• Students should understand how Punnett grids predict genotypic and phenotypic ratios in dihybrid crosses involving pairs of unlinked autosomal genes.
• The 9:3:3:1 and 1:1:1:1 ratios are derived from Punnett grids.

Autosomal Gene Linkage

• In crosses involving linkage, alleles of linked genes can fail to assort independently.
• The symbols used to denote alleles should be shown alongside vertical lines representing homologous chromosomes.

Recombinants

• Students should understand how to determine the outcomes of crosses between an individual heterozygous for both genes and an individual homozygous recessive for both genes.
• Identify recombinants in gametes, genotypes of offspring, and phenotypes of offspring.

Chi-Squared Test

• Students should understand the concept of statistical significance, the p = 0.05 level, null/alternative hypothesis, and the idea of observed versus expected results.

Non-Disjunction and Chromosomal Abnormalities

• Errors can occur during meiosis where chromosomes don't separate properly, leading to cells with an extra or missing chromosome, usually lethal.
• Down syndrome is a rare exception where having an extra chromosome (number 21) isn't fatal, but causes developmental problems.
• Non-disjunction can also cause sex chromosome abnormalities like Klinefelter's syndrome (XXY) and Turner's syndrome (X).

Genotype and Alleles

• Genotype is the combination of alleles inherited by an organism.
• Individuals with two identical alleles (BB or bb) are homozygous, while those with different alleles (Bd) are heterozygous.
• Genes can have different versions, called alleles, which can arise from mutations.

Genetic Crosses in Flowering Plants

• Genetic crosses are widely used to breed new varieties of crop or ornamental plants.
• Pollination is needed to carry out a cross, and plants such as peas produce both male and female gametes on the same plant, allowing self-pollination and self-fertilization.
• Scientists study inheritance patterns by crossing flower varieties like peas, using Punnett squares to analyze the crosses.