Unit 10_Meiosis and Genetics Fundamentals

Meiosis and Gamete Production

• Texas Expected Knowledge and Skills (TEKS) 6G: Recognize the significance of meiosis to sexual reproduction.

• Conceptual Mnemonic:

  • Mitosis is how your Toes got made (somatic/body cells).

  • Meiosis is how your folks made You (gametes/sex cells).

• Structural Overview of Meiosis:

  • Parent Cell: Starts as a diploid cell ($2n$) before chromosome duplication. For example, a cell with $2n = 6$.

  • Prophase I: Homologous chromosome pairs held together by chiasma (the site of crossing over) and sister chromatid cohesion.

  • Metaphase I: Pairs of homologous chromosomes line up at the metaphase plate.

  • Anaphase I: Homologs separate, but sister chromatids remain attached at the centromere.

  • Telophase I: Results in two daughter cells of meiosis I.

  • Meiosis II:

    • Metaphase II: Individual chromosomes line up at the metaphase plate.

    • Anaphase II: Sister chromatids separate.

    • Daughter Cells of Meiosis II: Results in four haploid ($n$) daughter cells.

The Purpose and Results of Meiosis

• Primary Purpose: To create gametes (sex cells) such as eggs and sperm.

• Comparison to Mitosis: Unlike mitosis, meiosis creates daughter cells that contain HALF the number of chromosomes of the parent cell.

• Chromosomal Count Transformation:

  • Starts with Diploid ($2n$) cells: Contains the full set of chromosomes.

  • Ends with Haploid ($1n$) cells: Contains a half set of chromosomes.

• Human Specifics:

  • Human Diploid sex cell: $2n = 46$.

  • Meiosis I reduces this to two cells with $n = 23$.

  • Meiosis II results in four total sperm or egg cells, each with $n = 23$.

• End Results Summary:

  • Creation of four daughter cells.

  • All four cells are Haploid ($1n$).

  • Segregation: This process reduces the number of chromosomes in each daughter cell.

  • Genetic Variation: Every daughter cell is genetically DIFFERENT, which is the basis for genetic variation in populations.

Genetic Variation and Fertilization

• Crossing Over:

  • Occurs during meiosis when chromosomes pair up and exchange segments of genetic material.

  • This is the primary mechanism leading to genetic variation.

  • Explains why siblings with the same parents can look significantly different.

• Sexual Reproduction:

  • Involves two parents contributing DNA.

  • Produces gametes such as sperm, eggs, and pollen.

• Fertilization Process:

  • A Sperm ($n$ haploid nucleus) and an Egg ($n$ haploid nucleus) unite.

  • Zygote: The cell formed by the union of sperm and egg.

  • Post-fertilization, mitosis takes over to allow the cell to divide and grow into a unique individual.

• Twin Formation Categories:

  • Identical Twins: Formed from one egg and one sperm. A clump of cells splits off before differentiation, resulting in two individuals with the same genetic makeup.

  • Fraternal Twins: Formed from two separate eggs and two separate sperm (mother releases two eggs at once). They are non-identical siblings born at the identical time.

Genetics Vocabulary and Mendelian Principles

• Genetics: The scientific study of heredity.

• Heredity: The passing on of characteristics (traits) from parents to offspring through sexual reproduction.

• Trait: A particular characteristic that varies between individuals (e.g., hair color, eye color).

• Genes: Sectors of a chromosome that determine the inherited trait; offspring receive one from the mother and one from the father.

• Alleles: Different forms of a single gene (e.g., brown, red, and blond hair are alleles for the hair color gene).

• Gregor Mendel:

  • An Austrian monk known as the Father of Genetics.

  • Studied inheritance by cross-breeding common pea plants.

• Law of Segregation: During meiosis, gene pairs separate so each gamete receives only one gene for each trait.

• Principal of Dominance:

  • Dominant Genes: Always expressed if the allele is present. Represented by capital letters ($B$). Organisms can be $BB$ or $Bb$ to show the trait.

  • Recessive Genes: Only expressed if two alleles are present ($bb$). Represented by lowercase letters ($b$).

Genotype, Phenotype, and Zygosity

• Homozygous: Having two identical alleles for a trait.

  • Homozygous Dominant: $BB$.

  • Homozygous Recessive: $bb$.

• Heterozygous: Having two different alleles for a trait (e.g., $Bb$).

• Genotype: The genetic makeup or combination of two alleles (the letters, e.g., $BB$, $Bb$, $bb$).

  • Always write the dominant capital letter first (e.g., $Ss$).

• Phenotype: The physical expression or characteristics of a trait (e.g., brown eyes, straight hair).

Punnett Square Methodology

• Definition: A diagram used to predict the outcome of a particular cross.

• Function: Determines the probability (chance) of genotypes and phenotypes in offspring. It does not determine the exact number of offspring produced.

• The 5-Step Process:

  1. Key: List the trait, both alleles, and corresponding phenotypes.

  2. Parents: Identify parent genotypes.

  3. Draw: Create the square and perform the cross.

  4. Genotype: Determine the ratio and fraction.

  5. Phenotype: Determine the ratio and fraction.

• Monohybrid Example (Guinea Pigs):

  • Trait: Hair ($A$ is dominant) vs. Hairless ($a$ is recessive).

  • Cross: Heterozygous mother ($Aa$) x Heterozygous father ($Aa$).

  • Results:

    • Genotypes: $1/4 AA$, $2/4 Aa$, $1/4 aa$.

    • Genotype Ratio: $1:2:1$.

    • Phenotypes: $75\%$ with hair, $25\%$ hairless.

Advanced Inheritance: Dihybrid Crosses and Independent Assortment

• Law of Independent Assortment: During gamete formation, segregating pairs of alleles on different chromosomes assort independently. Two traits (e.g., eye color and hair color) are inherited independently of each other.

• Dihybrid Punnett Square: Study of crossing two pairs of contrasting traits simultaneously.

  • Example: Fur Color ($B$: Black, $b$: White) and Coat Texture ($R$: Rough, $r$: Smooth).

  • Parental Genotype Example: $BbRr \times BbRr$.

  • Box Method for Gametes:

    1. Determine parent genotypes (e.g., $AaBb$).

    2. Use a $4$-box square to find possible gametes ($AB$, $Ab$, $aB$, $ab$).

    3. Place gametes on the sides of a $16$-square Punnett box.

• Dihybrid Practice Ratios:

  • A goat cross ($Bbhh \times BbHh$) involves tracking brown ($B$)/white ($b$) fur and smooth ($H$)/course ($h$) fur across $16$ possible offspring combinations.

Non-Mendelian Inheritance Patterns

• Incomplete Dominance:

  • No trait is dominant over another; a blending effect occurs.

  • Example: Red flowered snapdragon ($RR$) x White flowered snapdragon ($WW$) = $100\%$ Pink flowers ($RW$).

  • Practice: $RB \times RB$ (Purple flowers) results in a Genotype Ratio of $1:2:1$ ($25\%$ Red, $50\%$ Purple, $25\%$ Blue).

• Codominance:

  • Multiple alleles are dominant; both traits are displayed simultaneously.

  • Example: Speckled Chickens—White ($WW$) and Black ($BB$) alleles produce speckled ($WB$) offspring.

  • Sickle-Cell Anemia in Humans:

    • Normal RBCs are disk-shaped ($A$); abnormal are sickle-shaped ($B$).

    • Heterozygotes ($AB$) express BOTH normal and abnormal cells.

    • Normal allele ($A$), Sickle allele ($B$). Cross of $AB \times AA$ results in $50\%$ chance of offspring having sickle cell anemia trait.

Human Blood Typing and Sex-Linked Traits

• Multiple Alleles in Blood Typing:

  • Allele $A$: Co-dominant (Antigen A).

  • Allele $B$: Co-dominant (Antigen B).

  • Allele $O$: Recessive (No antigen).

• Genotypes and Types:

  • Type A: $AA$ or $AO$.

  • Type B: $BB$ or $BO$.

  • Type AB: $AB$.

  • Type O: $OO$.

• Transfusion Rules:

  • $O^-$ (O Negative): The Universal Donor; any blood type can receive it.

• Sex Determination:

  • Humans have $23$ pairs of chromosomes ($46$ total).

  • Pairs $1$-$22$: Autosomes.

  • Pair $23$: Sex chromosomes ($XY$ = Male, $XX$ = Female).

  • The Male determines the sex of the baby based on whether the sperm carries an $X$ or $Y$.

• Sex-Linked Inheritance:

  • Traits are carried on the $X$ allele; the $Y$ is too small.

  • Males have a higher chance of expressing recessive sex-linked traits because they lack a second $X$ to mask the gene.

  • Carriers: Females with one dominant and one recessive trait ($X^D X^d$).

Specific Sex-Linked Disorders

• Hemophilia:

  • Blood clotting disorder caused by a recessive allele on the $X$ chromosome.

  • Male prevalence: $1/10,000$.

  • Female prevalence: $1/10,000,000$ (requires affected father and carrier/affected mother).

• Red-Green Color Blindness:

  • Recessive allele on the $X$ chromosome.

  • Global rates: $1$ in $12$ men; $1$ in $200$ women.

  • Types:

    1. Protanopia: Difficulty distinguishing greens, yellows, oranges, reds, and browns.

    2. Deuteranomalia: Red-green blindness.

    3. Tritanopia: Blue-yellow blindness.

Pedigree Analysis

• Symbols:

  • Square: Male.

  • Circle: Female.

  • Shaded: Phenotypically affected.

  • Un-shaded: Phenotypically unaffected.

  • Half-filled: Carrier (only females for sex-linked traits).

• Lines:

  • Horizontal connecting male and female: Marriage/Mating.

  • Vertical line from marriage line: Offspring.

• Reading Generations: Generations are numbered (I, II, III). I-1 and I-2 refer to individuals in the first generation.