Unit 5 AP Biology

Unit 5: Heredity

Prophase I of Meiosis

• Chromatin Condensation: Chromatin begins to condense into chromosomes.

• Alignment of Chromosomes: Sister chromatids and homologous chromosomes align.

• Crossing Over: Exchange of genetic material occurs between homologous chromosomes, increasing genetic diversity.

Metaphase I

• Alignment on Metaphase Plate: Homologous chromosomes align on the metaphase plate.

• Independent Assortment: Random orientation of chromosomes leads to genetic variation.

Meiosis I Overview

• Parent Cell: A diploid cell undergoes meiosis to produce haploid cells.

• Anaphase I: Homologous chromosomes separate to opposite poles.

• Telophase I: Nuclear envelope forms around the separated chromosomes.

• Ploidy Change: At the end of Meiosis I, cells are haploid.

Mitosis vs. Meiosis

• Mitosis: Produces 2 identical diploid daughter cells.

• Meiosis: Produces 4 genetically distinct haploid daughter cells.

Key Meiosis Stages

• Meiosis II: Similar to mitosis, where sister chromatids are separated.

• Prophase II and Metaphase II: Sister chromatids align on the metaphase plate.

• Anaphase II: Sister chromatids separate to opposite poles.

• Telophase II: Nuclear envelopes form around 4 haploid daughter cells.

Inheritance Patterns

• Monohybrid Cross: Examines inheritance of single traits.

  • Genotypes: AA (homozygous dominant), Aa (heterozygous).

  • Phenotypic Ratios: Complete dominance results in a 3:1 ratio.

• Codominance: Both traits are expressed in a heterozygote.

• Incomplete Dominance: The heterozygote exhibits a blend of both traits.

Dihybrid Cross

• Traits Studied: Two traits examined (e.g. round vs. wrinkled and yellow vs. green).

• Ratios: Complete dominance yields a 9:3:3:1 ratio.

Types of Inheritance

• Autosomal Inheritance: Genes located on autosomes.

• Sex-Linked Inheritance: Genes located on sex chromosomes (e.g., X-linked traits).

• Maternal Inheritance: Traits passed down from mitochondrial DNA.

Genetic Disorders

• Autosomal Recessive Disorders: e.g., Sickle Cell Disease, Tay Sachs Disease.

• Autosomal Dominant Disorders: e.g., Huntington's Disease.

• Chromosomal Disorders: e.g., Down Syndrome (Trisomy 21), Klinefelter Syndrome (XXY), Turner Syndrome (XO).

Phenotypic Variation

• Phenotype Plasticity: Environmental factors influence genetic expression leading to different phenotypes in similar genotypes; example: rabbit coat color influenced by temperature.

• Chromosomal Basis of Inheritance: Genes are located on chromosomes and transmitted during reproduction.

Law of Segregation

• Definition: Two alleles for each gene separate during gamete formation.

Law of Independent Assortment

• Definition: Genes on nonhomologous chromosomes assort independently during gamete formation.

Free Response Practice

• Example: Analysis of offspring ratios from F1 generation crosses.

Chi-Square Analysis**

• Purpose: Statistical test to compare observed vs. expected values.

• Example Calculation:

  • Calculate null hypothesis and determine if to reject or fail to reject.

• Conclusion: If the calculated Chi-square value is less than the critical value, do not reject the null hypothesis.

Unit 5: Heredity

Prophase I of Meiosis

• Chromatin Condensation: Chromatin, the complex of DNA and proteins, begins to condense into visible chromosomes that can be easily observed under a microscope. Each chromosome consists of two sister chromatids joined at a region called the centromere.

• Alignment of Chromosomes: Sister chromatids (identical copies of a chromosome) and homologous chromosomes (pairs of chromosomes from each parent) align closely together. This precise arrangement is essential for the subsequent processes of crossing over and segregation.

• Crossing Over: A critical event occurs where homologous chromosomes exchange segments of genetic material in a process known as recombination. This exchange generates new allele combinations, significantly increasing genetic diversity in the gametes produced later.

Metaphase I

• Alignment on Metaphase Plate: Homologous chromosomes line up across the metaphase plate (the equatorial plane of the cell), ensuring that they are properly orientated for separation. The spindle fibers extend from opposite poles of the cell to attach to the centromeres of each homologous pair.

• Independent Assortment: During this phase, the orientation of each homologous chromosome pair is random, resulting in a variety of possible combinations of maternal and paternal chromosomes in the resulting gametes. This randomness is a key factor in genetic variation among offspring.

Meiosis I Overview

• Parent Cell: A diploid cell (containing two complete sets of chromosomes, one from each parent) undergoes meiosis to eventually produce four haploid cells, each with half the genetic information.

• Anaphase I: The homologous chromosomes are pulled apart to opposite poles of the cell, ensuring that each resulting daughter cell will receive one chromosome from each homologous pair.

• Telophase I: Nuclear envelopes form around the two sets of separated chromosomes, which may decondense back into chromatin. Cytokinesis often follows, dividing the cell into two distinct haploid cells.

• Ploidy Change: At the conclusion of Meiosis I, each daughter cell is haploid (n), containing one copy of each chromosome, crucial for sexual reproduction.

Mitosis vs. Meiosis

• Mitosis: This process produces 2 identical diploid daughter cells, necessary for growth, repair, or asexual reproduction. Mitosis involves one round of division and maintains the chromosome number of the original cell.

• Meiosis: Conversely, meiosis produces 4 genetically distinct haploid daughter cells, integral for sexual reproduction. It involves two rounds of division (Meiosis I and Meiosis II) and introduces genetic variation through crossing over and independent assortment.

Key Meiosis Stages

• Meiosis II: This phase is similar to mitosis, where sister chromatids are separated, further ensuring genetic diversity.

• Prophase II and Metaphase II: Sister chromatids align along the metaphase plate again, and the spindle apparatus prepares for separation.

• Anaphase II: The sister chromatids are finally pulled apart to opposite poles, ensuring that each resulting cell will have unique genetic materials.

• Telophase II: Nuclear envelopes reform around each set of chromosomes, resulting in the formation of 4 distinct haploid daughter cells, which can become gametes (sperm or eggs).

Inheritance Patterns

• Monohybrid Cross: This method examines the inheritance of a single trait, focusing on the genetic ratio among offspring.

  • Genotypes: Possible genotypes include AA (homozygous dominant), Aa (heterozygous), and aa (homozygous recessive).

  • Phenotypic Ratios: When complete dominance is present, a typical phenotypic ratio of 3:1 emerges, showcasing the dominance of one trait over another.

• Codominance: In cases of codominance, both traits are fully expressed in the heterozygote phenotype, exemplified by blood types (e.g., AB blood type).

• Incomplete Dominance: This occurs when the heterozygote displays a phenotype that is a blend of both parental traits, as seen in flower color – red and white flowers producing pink offspring.

Dihybrid Cross

• Traits Studied: A dihybrid cross examines two different traits simultaneously (e.g., seed shape: round vs. wrinkled, and seed color: yellow vs. green).

• Ratios: In complete dominance, a phenotypic ratio of 9:3:3:1 is typically observed among the offspring, providing insights into how multiple traits assort independently.

Types of Inheritance

• Autosomal Inheritance: This involves genes located on non-sex chromosomes (autosomes) and affects both males and females equally.

• Sex-Linked Inheritance: Genes that are located on sex chromosomes exhibit different patterns of inheritance in males and females, especially concerning X-linked traits, which often manifest more prominently in males due to their single X chromosome.

• Maternal Inheritance: Traits passed down from maternal mitochondrial DNA, exclusively inherited from the mother, leading to conditions steeped in maternal genetic patterns.

Genetic Disorders

• Autosomal Recessive Disorders: Common examples include Sickle Cell Disease and Tay Sachs Disease, where individuals need two copies of the recessive allele for the disorder to manifest.

• Autosomal Dominant Disorders: These disorders, such as Huntington's Disease, require only one copy of the dominant allele to express the phenotype.

• Chromosomal Disorders: These include conditions like Down Syndrome (Trisomy 21) where individuals have an extra chromosome, Klinefelter Syndrome (XXY), and Turner Syndrome (XO), which arise from nondisjunction during meiosis.

Phenotypic Variation

• Phenotype Plasticity: Environmental factors can significantly influence genetic expression, leading to different observable traits (phenotypes) among individuals with the same genotype; an example is the varying coat colors of rabbits influenced by seasonal temperature changes.

• Chromosomal Basis of Inheritance: Genes are organized on chromosomes, which are vital units for the transmission of genetic traits during reproduction.

Law of Segregation

• Definition: This law states that during the formation of gametes, the two alleles for each gene separate, ensuring that each gamete carries only one allele for each gene.

Law of Independent Assortment

• Definition: This principle asserts that genes located on nonhomologous chromosomes assort independently during gamete formation, leading to genetic diversity.

Free Response Practice

• Example: Students should practice analyzing offspring ratios derived from F1 generation crosses to deepen their understanding of inheritance patterns and probabilities.

Chi-Square Analysis**

• Purpose: Chi-Square is a statistical test utilized to compare observed values against expected values in genetic data, assessing how well observed data fits expected ratios.

• Example Calculation:

  • Determine the null hypothesis, which posits that there is no significant difference between observed and expected ratios.

  • If