Gene Linkage and Pedigrees

Overview of Gene Linkage and Linkage Groups

• Definition of Linkage Groups: A linkage group refers to a group of genes whose loci (locations) are situated on the same chromosome.

• Inheritance Pattern: Any two genes located on the same chromosome are considered linked. Because they stay together during meiosis (unless crossing over occurs), they are usually passed on to the next generation together in the same combination found in the parent.

• Relationship with Independent Assortment: Linked genes do not follow Mendel’s Law of Independent Assortment because they are physically tied to the same structure (the chromosome).

• Example of Non-Linkage: In garden peas, seed color and seed shape are not linked. The locus for the color gene is on a completely different chromosome than the locus for the shape gene.

• Genomic Examples of Linkage Groups:  - Sweet Peas (Lathyrus adoratus): Genes for flower color and pollen grain shape are linked.  - Corn (Zea mays): Genes for kernel color and kernel "waxiness" are linked.

Linkage and Crossing Over Mechanics

• Crossing Over in Meiosis: To achieve a new combination of alleles from linked genes, DNA must be swapped between non-sister chromatids during Prophase I of meiosis. This occurs at points called chiasmata.

• Proximity and Recombination Frequency:  - The distance between two alleles on a chromosome determines the likelihood of crossing over.  - The further apart a pair of alleles are on a chromosome, the more likely it is that crossing over will occur between them, leading to recombination.  - Specific Example: The gene $SCN5A$ is more likely to cross over with $PDCD10$ than $PDCD10$ is to cross over with $SOX2$. This suggests that the physical distance between $SCN5A$ and $PDCD10$ is greater than the distance between $PDCD10$ and $SOX2$.

• Gamete Production:  - Without crossing over, gametes only carry the parental combinations (e.g., $PL$ or $pl$).  - With crossing over, recombinant gametes are produced (e.g., $PL, Pl, pL, pl$).  - In a typical cross of linked genes, parental gametes appear in much higher frequencies (e.g., $80\%$) compared to recombinant gametes (e.g., $20\%$).

Sex Linkage (X-Linkage)

• Definition: Sex linkage is the association of a specific characteristic with gender. This occurs because the gene controlling the characteristic is located on a sex chromosome (specifically the $X$ chromosome).

• Chromosomal Differences:  - Females possess two copies of the $X$ chromosome ($XX$).  - Males possess only one copy of the $X$ chromosome and one $Y$ chromosome ($XY$).

• X-Linked Alleles: Alleles for sex-linked traits are found only on the $X$ chromosome. The $Y$ chromosome is significantly smaller and usually does not carry a corresponding allele for these traits.

• Examples of Sex-Linked Traits:  - Color Blindness: Produced by a recessive allele on the $X$ chromosome ($X^b$). The dominant allele for normal vision is $X^B$.  - Hemophilia: A blood-clotting disorder produced by a recessive allele ($X^h$). The dominant allele for normal clotting is $X^H$.  - Nystagmus: Involuntary eye movement.  - Muscular Dystrophy: Weakening of the muscles.

• Phenotypes and Carriers:  - Females have 3 possible genotypes for recessive X-linked traits: homozygous dominant (normal), heterozygous (carrier), or homozygous recessive (affected). Because females have a second $X$ chromosome, a dominant allele can mask a recessive disease allele.  - Males have only 2 possible genotypes: dominant (normal) or recessive (affected). Males cannot be "carriers" of X-linked recessive traits; if they inherit the allele, they will express the trait.

Genetic Notation and Problem Solving

• General Allele Symbols:  - Simple Dominance: Use a single letter. Dominant = Uppercase (e.g., $A$); Recessive = Lowercase (e.g., $a$).  - Co-dominant/Multiple Alleles: Use a base letter with a superscript for each allele (e.g., $H^k$ and $H^w$). For human blood types, the base letter is always "$I$".  - Sex-Linked Traits: Use the letter $X$ to represent the chromosome. The allele is shown as a superscript (e.g., $X^H$ for normal clotting, $X^h$ for hemophilia).

• AP Representation of Linked Genes: Linked genes are represented by two horizontal lines between the alleles to signify they are on a homologous pair of chromosomes.

• Dihybrid Crosses: Use the FOIL method (First, Outer, Inner, Last) to determine gamete combinations for unlinked genes.

• Practice Cross Examples:  - Cross 1: A color-blind man ($X^bY$) and a female with normal vision whose father was color-blind (carrier: $X^BX^b$).  - Cross 2: A man with normal clotting and nystagmus cross with a female normal for both traits (where the female's father had hemophilia and her mother had nystagmus).  - Drosophila Cross: In Drosophila, gray body ($G$) is dominant to black body ($g$) and long wings ($L$) are dominant to short wings ($l$). These are linked. If you cross a double heterozygote and a homozygous recessive individual, the recombinants must be identified.

Recombination Concepts

• Definition: Recombination is the re-assortment of alleles into combinations that are different from those found in either parent.

• Contributors to Recombination:  - Independent Assortment: The random orientation of homologous pairs during Metaphase I.  - Crossing Over: The physical exchange of segments between non-sister chromatids in Prophase I.  - Fertilization: The random combination of two different gametes.

• Identifying Recombinants: If a parent is $Aabb$ and another is $aaBb$, the parental offspring types are $Aabb$ and $aaBb$. Any offspring with types $AaBb$ or $aabb$ are considered recombinants.

Pedigree Analysis

• Purpose: A pedigree is a chart showing the genetic history of a family over generations. It allows scientists and genetic counselors to analyze biological relationships, track trait inheritance, and predict outcomes for future generations.

• Standard Symbols:  - Square: Male.  - Circle: Female.  - Shaded/Filled: Affected individual.  - Unshaded/Empty: Unaffected/Normal individual.  - Horizontal Line connecting Square and Circle: Marriage/Mating.  - Vertical Line descending from Mating: Offspring.  - Roman Numerals (I, II, III): Label generations.  - Arabic Numerals (1, 2, 3): Label individuals within a generation (numbered from left to right).

• Interpreting Inheritance Models:  - Autosomal: Approximately equal distribution (1:1 ratio) between affected males and females.  - Autosomal Dominant: At least one parent must have the disorder for an offspring to be affected.  - Autosomal Recessive: Neither parent has to have the disorder (parents can be carriers).  - Sex-Linked (X-Linked) Recessive: Significantly more males are affected than females. Affected fathers pass the allele to all daughters (making them carriers), but never to sons.  - Sex-Linked (X-Linked) Dominant: An affected father will pass the trait to all of his daughters and none of his sons.

Special Pedigree Cases and Practice

• Huntington's Disease: Caused by a dominant allele ($H$). An affected person can be $HH$ or $Hh$.

• Incomplete Dominance: Creates more than two colors (e.g., Red x White = Pink). In pedigree diagrams, gray shading may be used to represent the intermediate phenotype.

• Codominance (Sickle Cell): Individuals can have normal blood ($Hb^AHb^A$), sickle cell trait/carrier state ($Hb^AHb^S$), or severe sickle cell anemia ($Hb^SHb^S$).

• Case Study: Queen Victoria: She was a carrier of the X-linked recessive hemophilia allele. She passed this trait to various European royal families (including the Russian and Spanish royals). Important figures in this pedigree include Leopold (hemophiliac male), Alice (carrier), and Beatrice (carrier).

• Pedigree Practice Problems:  - Example 1: Determining the genotypes of individuals such as Leopold, Alice, and Bob based on a family tree related to hemophilia.  - Example 2: Analyzing a "mystery disorder" to determine if it is dominant or recessive, and autosomal or sex-linked.  - Example 3: Distinguishing whether an affected father passing a trait to all daughters signifies an X-linked dominant trait.  - Example 4: Calculating the probability (e.g., $0\%$, $12.5\%$, $25\%$, or $50\%$) of a specific offspring in a hemophilia pedigree being affected.