Chapter 12: The Basic Principles of Heredity: Comprehensive Study Guide

Mendel’s Seven Characters of Study in Pea Plants

• Background: Gregor Mendel utilized the garden pea plant (Pisum sativum) to study heredity, focusing on seven distinct characters, each with two contrasting traits.

• Character 1: Flower Color     * Dominant trait: Purple     * Recessive trait: White

• Character 2: Seed Color     * Dominant trait: Yellow     * Recessive trait: Green

• Character 3: Seed Shape     * Dominant trait: Smooth     * Recessive trait: Wrinkled

• Character 4: Pod Color     * Dominant trait: Green     * Recessive trait: Yellow

• Character 5: Pod Shape     * Dominant trait: Inflated     * Recessive trait: Pinched

• Character 6: Stem Height     * Dominant trait: Tall     * Recessive trait: Short

• Character 7: Flower Position     * Dominant trait: Axial (along the stem)     * Recessive trait: Terminal (at the tip of the stem)

Fundamental Hereditary Terminology

• Gene: A discrete unit of hereditary information containing instructions for a specific trait (e.g., flower color).

• Allele: Alternative forms of a gene; for instance, the gene for flower color has an allele for purple and an allele for white.

• Locus: The specific physical location of a gene on a chromosome.

• Phenotype: The observable physical or physiological traits of an organism (e.g., being a "Tall plant").

• Genotype: The genetic makeup or set of alleles of an organism (e.g., $TT$ or $Tt$).

• Dominant Allele: An allele that is fully expressed in the phenotype of a heterozygote, masking the recessive allele.

• Recessive Allele: An allele whose phenotypic effect is not observed in a heterozygote; it is only expressed when the individual is homozygous for that allele.

• Homozygous: Having two identical alleles for a given gene locus (e.g., $TT$ or $tt$).

• Heterozygous: Having two different alleles for a given gene locus (e.g., $Tt$).

• Homologous Pair of Chromosomes: Chromosomes that carry genes for the same traits at the same loci, though they may carry different alleles.

Mendel’s Principles of Inheritance

• Principle of Segregation:     * During the process of meiosis, the alleles for each locus segregate (separate) so that each gamete receives only one allele.     * This ensures that offspring inherit one allele from each parent.

• Principle of Independent Assortment:     * Alleles of different loci are distributed randomly into gametes.     * This results in recombination, which is the production of new gene combinations in offspring that were not found in either parent.     * Chromosomal Basis: This occurs due to the orientation of homologous chromosomes on the metaphase plate during meiosis.

Types of Genetic Crosses

• Monohybrid Cross:     * A cross between two homozygous parents that differ at only one locus (e.g., $BB imes bb$).     * F1 generation results: All offspring are heterozygous (e.g., $Bb$) and show the dominant phenotype.     * F2 generation results: A phenotypic ratio of $3:1$ (Dominant to Recessive) and a genotypic ratio of $1:2:1$ ($1/4$ homozygous dominant, $1/2$ heterozygous, $1/4$ homozygous recessive).

• Dihybrid Cross:     * A cross between two homozygous parents that differ at two loci (e.g., $BBSS imes bbss$).     * F1 generation results: All offspring are dihybrids ($BbSs$).     * F2 generation results: A phenotypic ratio of $9:3:3:1$.         * $9/16$: Both dominant traits (e.g., Black, short-haired).         * $3/16$: First dominant, second recessive (e.g., Black, long-haired).         * $3/16$: First recessive, second dominant (e.g., Brown, short-haired).         * $1/16$: Both recessive traits (e.g., Brown, long-haired).

• Test Cross:     * Used to determine the genotype of an individual with a dominant phenotype but unknown genotype.     * The unknown individual is crossed with a homozygous recessive individual.     * If any offspring show the recessive phenotype, the unknown parent must be heterozygous.     * If all offspring show the dominant phenotype, the unknown parent is likely homozygous dominant.

Rules of Probability in Genetics

• Genetic Ratios as Probabilities: Probability values range from $0$ to $1$.

• Product Rule ("AND" Rule):     * Predicts the combined probability of independent events occurring together.     * Multiply the individual probabilities.     * Example: Probability of tossing two heads is $rac{1}{2} imes rac{1}{2} = rac{1}{4}$.

• Sum Rule ("OR" Rule):     * Predicts the combined probability of mutually exclusive events.     * Add the individual probabilities.     * Example: Probability of tossing one head and one tail (Head-Tail OR Tail-Head) is $rac{1}{4} + rac{1}{4} = rac{1}{2}$.

Linkage and Recombination

• Linkage:     * The tendency for a group of genes located on the same chromosome to be inherited together.     * Linked genes do not independently assort.

• Recombination of Linked Genes:     * Results from crossing-over during prophase I of meiosis.     * Crossing-over involve the exchange of genetic material between homologous chromatids.     * Frequency of recombination is used to construct linkage maps of chromosomes.

• Fruit Fly Two-Point Test Cross Example (Linkage Detection):     * Parents: Grey, normal wings ($BbVv$) and Black, vestigial wings ($bbvv$).     * Expected results (if independent assortment): $575 : 575 : 575 : 575$.     * Actual results: $965$ Grey, normal; $944$ Black, vestigial; $206$ Grey, vestigial; $185$ Black, normal.     * The higher frequency of parental phenotypes confirms linkage.

Sex Determination and X-Linked Inheritance

• Mammalian Sex Determination:     * Females: XX chromosomes.     * Males: XY chromosomes (Y is smaller and determines maleness).     * Egg cells always contain an X chromosome.     * Sperm cells contain either an X or a Y chromosome, thus determining the sex of the zygote.

• Hemizygosity: Males have only one X chromosome, meaning they are hemizygous for X-linked genes. A single recessive X-linked allele will be expressed in males.

• X-Linked Red-Green Colorblindness:     * $X^C$ = Normal vision; $X^c$ = Colorblindness.     * A carrier female ($X^C X^c$) and a normal male ($X^C Y$) have a $rac{1}{4}$ chance of having a colorblind son.     * A colorblind male ($X^c Y$) and a homozygous normal female ($X^C X^C$) will have daughters who are all carriers and sons who are all normal.

• Dosage Compensation: Mechanisms in female mammals to compensate for the double dose of X-chromosomes (e.g., evidenced by calico/tortoiseshell hair patterns in cats involving orange and black alleles).

Extensions of Mendelian Genetics

• Incomplete Dominance:     * The heterozygote shows an intermediate phenotype between the two homozygotes.     * Example: Four o'clocks. Red ($R^1 R^1$) crossed with White ($R^2 R^2$) produces Pink ($R^1 R^2$). F2 ratio is $1 ext{ Red} : 2 ext{ Pink} : 1 ext{ White}$.

• Codominance:     * The heterozygote simultaneously expresses the phenotypes of both homozygotes.     * Example: ABO blood types (Type AB individuals express both A and B antigens).

• Multiple Alleles:     * When more than two alleles exist for a single locus.     * Rabbit Coat Color Hierarchy: $C$ (Dark gray) > $c^{ch}$ (Chinchilla) > $c^{h}$ (Himalayan) > $c$ (Albino).     * ABO Blood Types: Alleles $I^A$, $I^B$, and $i$.

• Pleiotropy:     * A single allele has multiple phenotypic effects.     * Example: Sickle-cell anemia. Two copies of the sickle-cell allele result in abnormal hemoglobin, which causes red blood cells to sickle. This leads to a cascade of effects including heart failure, physical weakness, anemia, brain damage, spleen damage, kidney failure, rheumatism, and pneumonia.

• Gene Interaction:     * Two or more genes work together to produce a single phenotype.     * Example: Chicken comb shapes. Walnut ($P-R-$), Pea ($P-rr$), Rose ($ppR-$), and Single ($pprr$).

• Epistasis:     * A gene at one locus alters the phenotypic expression of a gene at a second locus.     * Example: Labrador retriever coat color. Black ($B-E-$), Chocolate ($bbE-$), and Yellow ($--ee$). The "e" gene is epistatic to the "b" gene; if $ee$ is present, the lab is yellow regardless of the $B$ or $b$ alleles.

• Polygenic Inheritance:     * The additive effect of two or more genes on a single phenotypic character.     * Example: Human skin pigmentation shows a continuous distribution from very light to very dark.

Genes and the Environment

• Norm of Reaction:     * The range of phenotypic possibilities that can arise from a single genotype under different environmental conditions.     * Example: Human height. Genes set a potential range (norm of reaction), but environmental factors like diet and nutrition determine the actual phenotype within that range.