Unit 4: Gene Expression and Inheritance Study Guide 2025 Part B - Meiosis & Genetics

Chapter 8 The Cellular Basis of Reproduction and Inheritance

Meiosis and Crossing Over

• Describe the number and organization of human chromosomes in a typical somatic cell. 

  • Somatic Cell: 46 chromosomes (23 pairs)

    • They are condensed enough to be viewed with a microscope and arranged into matching pairs

• Distinguish between autosomes and sex chromosomes. (8.11, Khan Academy 1 and Khan Academy 2)

  • Autosomes: chromosomes other than sex chromosomes (44 of them in humans) are organized with a twin that resembles it in length, centromere position, and staining pattern.

  • Sex Chromosomes: they determine an individual’s sex and are always in the 23rd chromosome spot; human females have a homologous pair of X chromosomes (XX), but males have one X and one Y which are partly homologous.

• Distinguish between somatic cells and gametes and between diploid cells and haploid cells. (8.12)

  • Somatic Cells: most animals and plants contain pairs of homologous chromosomes (diploid = 2n)

    • Ex. for humans the diploid number is 46 (2n=46)

  • Gametes: egg and sperm cells each have a single set of chromosomes (haploid = n)

    • Ex. for humans the haploid number is 23 (n=23)

• Explain why sexual reproduction requires meiosis. (8.13)

  • It is is required to maintain a constant number of chromosomes in each species from one generation to the next

• List the phases of meiosis I and meiosis II and describe the events characteristic of each phase. Recognize the phases of meiosis from diagrams and micrographs. (8.13)

  • Meiosis I

    • Prophase I - homologous chromosomes form tetrads, nonsister chromatids of each homologous pair of chromosomes exchange segments (crossing over), the chromosomes condense, the nuclear membrane disintegrates, centrioles move to opposite poles

    • Metaphase I - homologous pairs line up at the metaphase plate, spindles are attached to the centromeres

    • Anaphase I - homologous chromosomes separate through the shortening of spindle fibers attached to the centromere and lengthening to separate the poles

    • Telophase I and Cytokinesis - chromosomes decondense, nuclear membrane reforms around each set of chrosomes, the cytoplasm divides splitting each cell into 2 cells

RESULTS OF MEIOSIS I = 2 Haploid Cells

• Meiosis II (essentially the same as mitosis but starts with a haploid cell))

  • Prophase II - the chromosomes condense, the nuclear membrane disintegrates, centrioles move to opposite poles

  • Metaphase II - individual chromosomes line up at the metaphase plate, spindles are attached to the centromeres

  • Anaphase II - sister chromatids separate and become individual chromosomes through the the shortening of spindle fibers attached to the centromere and lengthening to separate the poles

  • Telophase II and Cytokinesis - chromosomes decondense, nuclear membrane reforms around each set of chrosomes, the cytoplasm divides splitting each cell into 2 cells

• Describe the similarities and differences between mitosis and meiosis. Explain how the result of meiosis differs from the result of mitosis. (8.14)

  • Similarities

    • Prophase

      • Nuclear envelope breaks down

      • Centrioles move to opposite poles

      • Chromosomes condense

    • Metaphase

      • Spindle has formed (chromosomes are attached)

    • Anaphase

      • Spindle fibers shorten (some lengthen to separate poles)

    • Telophase

      • Nuclear membrane reforms around each set of chromosomes

      • Chromosomes decondense

    • Cytokinesis

      • Cytoplasm divides splitting each cell into 2 cells

  • Differences

    • Prophase - each duplicated chromosome remains separate

    • Prophase I - homologous chromosomes form tetrads and cross over

    • Metaphase - duplicated chromosomes line up singly

    • Metaphase I - duplicated homologous chromosomes line up in pairs

    • Anaphase - sister chromatids separate

    • Anaphase I - homologous chromosomes separate

Others:

• Mitosis involves one division of the nucleus and cytoplasm while meiosis involves two divisions.

• Mitosis starts with a diploid cell, meiosis II starts with haploid cells

• Mitosis produces 2 daughter cells, meiosis produces 4 daughter cells

• Explain how independent orientation of chromosomes at metaphase I, random fertilization, and crossing over contribute to genetic variation in sexually reproducing organisms. (8.15-8.17)

  • Independent orientation: Chromosome pairs line up randomly in meiosis, creating different combos of mom’s and dad’s genes.

  • Crossing over: Chromosomes swap pieces, mixing genes and making each chromosome unique.

  • Random fertilization: Any sperm can meet any egg, making each baby genetically different.

Alterations of Chromosome Number and Structure

• Define nondisjunction, explain how it can occur, and describe what can result (8.18).

  • Nondisjunction is when chromosomes don’t separate properly during meiosis.

  • It can happen in meiosis I (homologous chromosomes fail to separate) or meiosis II (sister chromatids fail to separate).

  • This can result in gametes with too many or too few chromosomes, leading to disorders like Down syndrome (extra chromosome 21).

• Explain how and why karyotyping is performed (8.19).

  • Karyotyping is a process where scientists take a picture of a person's chromosomes, arrange them in pairs, and check for abnormalities.

  • It’s done by collecting cells (like from blood), stopping them during cell division, and staining the chromosomes so they can be seen under a microscope.

  • Karyotyping is used to diagnose genetic disorders (like Down syndrome) and to check for extra, missing, or damaged chromosomes.

• Describe the main types of chromosomal alterations (8.23).

  • Deletion - a segment of a chromosome is removed

  • Duplication - a segment of a chromosome is copied and inserted into the homologous chromosome

  • Inversion - a segment of a chromosome is removed and then reinserted opposite to its original orientation

  • Reciprocal translocation - segments of two nonhomologous chromosomes swap locations with each other

Chapter 9 Patterns of Inheritance

Mendel’s Laws

1. Explain why Mendel’s decision to work with peas was a good choice. Define and distinguish between true-breeding organisms, hybrids, the P generation, the F1 generation, and the F2 generation. (9.2, Khan Academy)

   • Peas: Short generation times, produced large numbers of offspring from each mating, cam in many readily distinguishable varieties, matings could be strictly controlled (the petals almost completely enclose the reproductive organs–the stamens and carpel, and they are usually able to self-fertilize in nature.

   • True-breeding organisms: Varieties of homozygous organisms that consistently produce offspring with the same traits as the parent over many generations of self-pollination. This means that the traits are stable and predictable, making them ideal for genetic studies.

   • Hybrids: Offspring that result from the mating of individuals from two different species or from two true-breeding varieties of the same species; an offspring of two parents that differ in one or more inherited traits; an individual that is heterozygous for one or more pairs of genes.

   • P generation: The true-breeding parents from which offspring are derived in studies of inheritance; P stands for parental.

   • F1 generation: The offspring of two parental (P generation) individuals; F1 stands for first filial (“son”).

   • F2 generation: The offspring of the F1 generation; F2 stands for second filial.

2. Define and distinguish between the following pairs of terms: homozygous and heterozygous; dominant allele and recessive allele; genotype and phenotype. Also, define a monohybrid cross and a Punnett square. (9.3)

   • Homozygous is having two identical alleles for a given gene, and heterozygous is having two different alleles for a given gene.

   • Dominant allele is the allele that determines the phenotype of a gene when the individual is heterozygous for that gene (uppercase letter), and the recessive allele is the allele that has no noticeable effect on the phenotype of a gene when the individual is heterozygous for that gene.

   • Genotype is the genetic makeup of an organism (ex. PP), and phenotype is the expressed traits of an organism (purple flower).

   • Monohybrid cross: A genetic cross between two individuals focusing on a single trait. This type of cross helps to understand how alleles for a specific trait are inherited from one generation to the next.

   • Punnett square: A diagram used in genetics to predict the possible genotypes of offspring from a particular cross or breeding experiment. It visually represents the combinations of parental alleles and helps determine the probability of inheriting specific traits.

3. Explain how Mendel’s law of segregation describes the inheritance of a single characteristic. (9.3, Khan Academy)

   • Mendel's Law of Segregation: states that each individual has two alleles for a trait, which separate during gamete formation, so that each gamete carries only one allele. This ensures that offspring inherit one allele from each parent, resulting in a combination of alleles for the trait.

4. Describe the genetic relationships between homologous chromosomes. (9.4)

   • Homologous chromosomes are pairs of chromosomes in a diploid cell that carry genes for the same characteristics at corresponding loci. Each pair consists of one chromosome from the female parent and one from the male parent. Although they carry the same genes, the versions of these genes, known as alleles, can differ. For example, one chromosome might have an allele for brown coat color (C), while its homologous partner might have an allele for white coat color (c).

5. Explain how Mendel’s law of independent assortment applies to a dihybrid cross. Illustrate this law with examples from Labrador retrievers and Mendel’s work with peas. (9.5, Khan Academy)

Mendel’s Law of Independent Assortment explains that when an organism forms gametes (egg or sperm), the alleles for one trait do not influence how alleles for another trait are passed on — they assort into gametes independently.

• Peas: When Mendel crossed plants that differed in seed shape (round vs. wrinkled) and seed color (yellow vs. green), he saw that the traits appeared in new combinations in the F2 generation (like round green or wrinkled yellow), which wouldn’t happen if the traits were inherited together. The 9:3:3:1 ratio from RrYy × RrYy shows that seed shape and color genes are inherited separately.

• Labrador Retrievers: When two BbNn Labs are crossed, coat color and vision traits appear in all combinations (e.g., black with PRA, chocolate with normal vision), proving that the inheritance of one trait doesn't affect the other — they sort independently during reproduction.

6. Explain how and when the rule of multiplication and the rule of addition can be used to determine the probability of an event. Explain why Mendel was wise to use large sample sizes in his studies. (9.7, Khan Academy)

   • Rule of Multiplication: Used to find the probability of two independent events both happening—e.g., the chance of getting aabbcc from AaBbCc parents is 1/4 × 1/4 × 1/4 = 1/64.

   • Rule of Addition: Used when an outcome can happen in multiple ways—e.g., the probability of getting Aa or AA is the sum of both individual probabilities.

   • Mendel’s Sample Sizes: Mendel used large sample sizes to reduce random error and clearly observe inheritance patterns, making his conclusions more reliable.

7. Explain how family pedigrees can help determine the inheritance of many human traits. (9.8)

Family pedigrees help determine the inheritance of human traits by showing how traits are passed from one generation to the next, allowing geneticists to apply Mendel’s laws and predict genotypes.

• Pedigrees act like family trees that record traits over time.

• By analyzing patterns, geneticists can see if a trait is dominant or recessive.

• For example, if a child has a straight hairline and one parent has a widow’s peak, it suggests that parent is a carrier (Hh).

• Pedigrees are especially useful for studying genetic disorders and traits controlled by single genes.

8. Explain how recessive and dominant disorders are inherited. Provide examples of each. (9.9)

Recessive and dominant disorders are inherited differently.

• Recessive Disorders: These occur when a person inherits two recessive alleles (aa). Carriers (Aa) don't show symptoms but can pass the disorder on. Examples: Cystic fibrosis and sickle cell anemia. If both parents are carriers, each child has a 25% chance of inheriting the disorder.

• Dominant Disorders: These occur when a person has one dominant allele (Aa). Only one copy is needed for the disorder to show. Example: Huntington’s disease. There's a 50% chance of passing it to children.

Variations on Mendel’s Laws

1. Describe the inheritance patterns of incomplete dominance, multiple alleles,
   codominance, and polygenic inheritance. Provide an example of each. (9.11-9.15, Single gene, Multiple genes)

• Incomplete Dominance: This occurs when the heterozygote shows an intermediate phenotype, like crossing red and white snapdragons to produce pink flowers. The red pigment is produced less in the heterozygote than in the red homozygote.

• Multiple Alleles: Some traits have more than two alleles. For example, the ABO blood group has three alleles (A, B, O), leading to four blood types (A, B, AB, O).

• Codominance: Both alleles in a heterozygote are fully expressed. In the ABO blood group, both the A and B alleles are expressed in individuals with type AB blood.

• Polygenic Inheritance: Multiple genes contribute to a single trait, such as human height, which is influenced by many genes, each adding a little to the final height.

Sex Chromosomes and Sex-Linked Genes

1. Describe patterns of sex-linked inheritance (9.21-9.22)

Sex-Linked Genes: These are genes on the X or Y chromosomes, with most being X-linked.

Inheritance Patterns:

• Males (XY): Only need one recessive allele on the X to show the trait. E.g., XrY = color blindness.

• Females (XX): Need two recessive alleles to express the trait. E.g., XrXr = color blindness.

Example:

• Fruit flies: XRY = red eyes, XrY = white eyes for males. XRXR or XRXr = red eyes for females, XrXr = white eyes.

Human Disorders: X-linked disorders (e.g., color blindness) are more common in males because they have only one X chromosome.

2. Explain why sex-linked disorders are expressed more frequently in men than in women. (9.22, Khan Academy)

Sex-linked disorders are more common in men because they have only one X chromosome. If they inherit a recessive allele for a disorder, they will express it. Women need two copies of the recessive allele to show the disorder, making them less likely to be affected.

Key Terms

Word Roots

-centesis = a puncture (amniocentesis: a technique for determining genetic abnormalities in a fetus based on the presence of certain chemicals or defective fetal cells in the amniotic fluid, obtained by aspiration from a needle inserted into the uterus)

co- = together (codominance: an inheritance pattern in which a heterozygote expresses the distinct trait of both alleles)

di- = two (dihybrid cross: an experimental mating of individuals differing at two genetic loci) 

gen- = produce (genotype: the genetic makeup of an organism)

hetero- = different (heterozygous: having two different alleles for a given gene)

homo- = alike (homozygous: having two identical alleles for a given gene)

mono- = one (monohybrid cross: an experimental mating of individuals differing at one genetic locus)

pheno- = appear (phenotype: the expressed traits of an organism)

pleio- = more (pleiotropy: the control of multiple phenotypic characteristics by a single gene) 

poly- = many; gene- = produce (polygenic inheritance: the additive effect of two or more gene loci on a single phenotypic character)

re- = again; com- = together; bin- = two at a time (recombinant: an offspring carrying
combinations of alleles different from those in either of its parents as a result of independent
assortment or crossing over)