7.3 Mendelian Genetics & Inheritance Patterns

Foundations of Mendelian Genetics

Modern genetics began in the 1860s with the work of Gregor Mendel, an Austrian monk who conducted systematic breeding experiments using garden pea plants (Pisum sativum). Mendel demonstrated that traits are inherited as discrete physical units (which we now call genes) that remain unchanged across generations, rather than blending together. To analyze genetic patterns effectively on the TEAS 7 exam, candidates must master basic genetic terminology:

• Gene: A specific sequence of DNA on a chromosome that encodes instructions for building a functional protein or determining a trait.
• Allele: An alternative form or variant of a gene. For example, a gene for seed color might have a yellow allele and a green allele.
• Locus (plural, loci): The specific physical location of a gene on a chromosome.
• Dominant Allele: An allele that fully expresses its phenotypic effect even when only one copy is present (represented by an uppercase letter, e.g., A).
• Recessive Allele: An allele whose phenotypic effect is masked in the presence of a dominant allele, expressing its phenotype only when two copies are present (represented by a lowercase letter, e.g., a).
• Homozygous: An individual possessing two identical alleles for a given gene (e.g., AA for homozygous dominant or aa for homozygous recessive).
• Heterozygous: An individual possessing two different alleles for a given gene (e.g., Aa).
• Genotype: The underlying genetic composition or combination of alleles in an organism (e.g., AA, Aa, or aa).
• Phenotype: The observable physical, physiological, or anatomical expression of a genotype (e.g., purple flowers vs. white flowers).

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Mendel's Fundamental Laws

Mendel formulated two core principles of inheritance based on his mathematical analysis of pea plant crosses:

1. Law of Segregation (Mendel's First Law)

Every diploid individual possesses two alleles for each gene. During gamete formation (meiosis), these two alleles segregate (separate) from each other so that each gamete carries only one allele for each gene. Cytologically, the Law of Segregation occurs during Anaphase I of meiosis, when homologous chromosome pairs split and move to opposite poles.

2. Law of Independent Assortment (Mendel's Second Law)

Alleles of two or more different genes segregate independently of one another during gamete formation, provided the genes are located on different chromosomes or far apart on the same chromosome. The inheritance of an allele for one trait (such as seed color) does not influence the inheritance of an allele for another trait (such as plant height). Cytologically, Independent Assortment occurs during Metaphase I of meiosis, when maternal and paternal tetrads align randomly along the metaphase plate.

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Punnett Squares and Probability Ratios

A Punnett square is a visual diagram used to predict the genotypes and phenotypes resulting from a genetic cross between individuals of known genotypes.

Monohybrid Cross

A monohybrid cross tracks the inheritance of a single trait. Consider a cross between two heterozygous purple-flowered pea plants ($Pp \times Pp$), where purple color ($P$) is dominant over white color ($p$):

                  MONOHYBRID CROSS (Pp x Pp)

                        Female Gametes
                         P        p
                     +--------+--------+
                   P |   PP   |   Pp   |
        Male         | (Purp) | (Purp) |
       Gametes       +--------+--------+
                   p |   Pp   |   pp   |
                     | (Purp) | (White)|
                     +--------+--------+

• Offspring Genotypic Ratio: $1\ PP : 2\ Pp : 1\ pp$ (or 1:2:1; $25%$ homozygous dominant, $50%$ heterozygous, $25%$ homozygous recessive).
• Offspring Phenotypic Ratio: $3\ \text{Purple} : 1\ \text{White}$ (or 3:1; $75%$ dominant phenotype, $25%$ recessive phenotype).

Dihybrid Cross

A dihybrid cross tracks the inheritance of two unlinked traits simultaneously. Consider a cross between two individuals heterozygous for both seed shape ($R$ = round, $r$ = wrinkled) and seed color ($Y$ = yellow, $y$ = green): $RrYy \times RrYy$.

Each parent produces four distinct gamete types in equal proportions: $RY$, $Ry$, $rY$, and $ry$. Combining these gametes in a $4 \times 4$ Punnett square yields 16 total offspring boxes, resulting in a classic 9:3:3:1 phenotypic ratio:

• 9/16 Round, Yellow (dominant for both traits)
• 3/16 Round, Green (dominant for shape, recessive for color)
• 3/16 Wrinkled, Yellow (recessive for shape, dominant for color)
• 1/16 Wrinkled, Green (recessive for both traits)

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Non-Mendelian Inheritance Patterns

While Mendelian principles explain simple dominant-recessive inheritance, many traits exhibit more complex interactions where alleles do not display simple dominance or reside on sex chromosomes.

                  PATTERNS OF INHERITANCE SUMMARY

  +--------------------+--------------------------------------------------+
  | Pattern            | Phenotypic Outcome in Heterozygotes (Aa)         |
  +--------------------+--------------------------------------------------+
  | Complete Dominance | Expresses dominant allele (A) exclusively         |
  | Incomplete Dom.    | Expresses intermediate blend between AA and aa   |
  | Codominance        | Expresses both A and a alleles fully & equally   |
  | X-Linked Recessive | Males (X^a Y) express trait; Females need X^aX^a |
  +--------------------+--------------------------------------------------+

1. Incomplete Dominance

In incomplete dominance, neither allele is completely dominant over the other. The heterozygous phenotype is an intermediate blend between the two homozygous phenotypes. A classic example is flower color in snapdragons (Antirrhinum majus):

• Red flowers ($C^R C^R$) crossed with White flowers ($C^W C^W$) yield 100% Pink flowers ($C^R C^W$) in the $F_1$ generation.
• Crossing two pink snapdragons ($C^R C^W \times C^R C^W$) yields a phenotypic and genotypic ratio of 1 Red ($C^R C^R$) : 2 Pink ($C^R C^W$) : 1 White ($C^W C^W$).

2. Codominance

In codominance, both alleles for a gene are fully and simultaneously expressed in heterozygous individuals without blending. An important human example is the ABO blood group system, governed by a single gene ($I$) with three alleles: $I^A$, $I^B$, and $i$.

• $I^A$ codes for A-antigen surface glycoproteins on red blood cells.
• $I^B$ codes for B-antigen surface glycoproteins.
• $i$ codes for no surface antigen and is recessive to both $I^A$ and $I^B$.

Alleles $I^A$ and $I^B$ display codominance with respect to each other. An individual inheriting genotype $I^A I^B$ has Type AB blood, expressing both A antigens and B antigens on the membranes of their red blood cells.

3. Sex-Linked (X-Linked) Inheritance

Sex-linked traits are governed by genes located on sex chromosomes, primarily the X chromosome (X-linked traits). Females possess two X chromosomes ($XX$), whereas males possess one X chromosome and one Y chromosome ($XY$).

Because males have only a single X chromosome, they are hemizygous for X-linked genes. If a male inherits a mutated X-linked recessive allele from his mother, he lacks a second X chromosome to mask it and will express the condition. Females must inherit two copies of the recessive allele (homozygous recessive) to display the phenotype.

Consequently, X-linked recessive disorders (such as hemophilia A, red-green color blindness, and Duchenne muscular dystrophy) occur far more frequently in males than in females.

              X-LINKED RECESSIVE CROSS: CARRIER MOTHER & NORMAL FATHER

                 Mother Genotype: X^N X^n (Carrier)
                 Father Genotype: X^N Y   (Normal)

                             Mother's Ova
                              X^N         X^n
                          +----------+----------+
                      X^N |  X^N X^N |  X^N X^n |
        Father's          | (Normal  | (Carrier |
        Sperm             |  Female) |  Female) |
                          +----------+----------+
                      Y   |  X^N Y   |  X^n Y   |
                          | (Normal  | (Affected|
                          |   Male)  |   Male)  |
                          +----------+----------+

Key rules for X-linked recessive inheritance on the TEAS 7:

• Affected males inherit the mutant X allele from their mothers.
• An affected father cannot pass an X-linked recessive disorder to his sons (he passes his Y chromosome to sons), but passes the mutant X allele to 100% of his daughters (making them obligate carriers).
• A carrier female ($X^N X^n$) has a $50%$ chance of passing the mutant allele to any son, who will be affected ($X^n Y$).