2.2 Genetics, Inheritance Patterns & Punnett Squares

2.2 Genetics, Inheritance Patterns & Punnett Squares

GED Exam Focus: Genetics questions focus heavily on predicting offspring traits using Punnett squares, distinguishing between dominant and recessive alleles, analyzing non-Mendelian traits like blood types, and interpreting pedigree diagrams to trace inherited diseases through generations.

Principles of Mendelian Genetics

Modern genetics began with the experiments of Gregor Mendel on pea plants. Mendel established fundamental laws governing how traits pass from parents to offspring.

Core Terminology

• Gene: A unit of heredity occupying a specific location (locus) on a chromosome that encodes a specific trait.
• Allele: Alternative versions of a gene. For example, a flower color gene may have a purple allele ($P$) and a white allele ($p$).
• Dominant Allele: An allele that masks the expression of another allele when present. Represented by an uppercase letter (e.g., $A$).
• Recessive Allele: An allele whose phenotypic expression is masked by a dominant allele. It is expressed only when two copies are present. Represented by a lowercase letter (e.g., $a$).
• Genotype: The genetic allele combination of an organism (e.g., $AA$, $Aa$, or $aa$).
  • Homozygous Dominant ($AA$): Possesses two copies of the dominant allele.
  • Heterozygous ($Aa$): Possesses one dominant allele and one recessive allele.
  • Homozygous Recessive ($aa$): Possesses two copies of the recessive allele.
• Phenotype: The physical, physiological, or behavioral expression of a genotype (e.g., Tall vs. Short, Purple vs. White).

Mendel's Fundamental Laws

1. Law of Segregation: During gamete formation (meiosis), the two alleles for a gene separate so that each egg or sperm carries only one allele for each gene.
2. Law of Independent Assortment: Alleles of different genes on non-homologous chromosomes assort independently of one another during gamete formation.

Monohybrid Crosses & Punnett Squares

A Punnett Square is a visual grid used to calculate the mathematical probabilities of offspring inheriting specific genotypes and phenotypes from a cross between two parents.

Step-by-Step Monohybrid Cross Method

Worked Example: In pea plants, tall height ($T$) is dominant over short height ($t$). Two heterozygous tall pea plants ($Tt \times Tt$) are crossed. Calculate the expected genotypic and phenotypic ratios.

Step 1: Identify parental genotypes. Parent 1: $Tt$ | Parent 2: $Tt$

Step 2: Set up the 2x2 Punnett Square grid. Place the alleles of Parent 1 across the top, and Parent 2 along the left side.

Step 3: Analyze the offspring genotypes.

• 1 out of 4 ($25%$) = $TT$ (Homozygous Dominant)
• 2 out of 4 ($50%$) = $Tt$ (Heterozygous)
• 1 out of 4 ($25%$) = $tt$ (Homozygous Recessive)
• Genotypic Ratio: 1 TT : 2 Tt : 1 tt (or 1:2:1)

Step 4: Analyze the offspring phenotypes.

• $TT$ ($25%$) and $Tt$ ($50%$) display the dominant Tall phenotype $\rightarrow 75%$ Tall.
• $tt$ ($25%$) displays the recessive Short phenotype $\rightarrow 25%$ Short.
• Phenotypic Ratio: 3 Tall : 1 Short (or 3:1)

Probability Summary for Heterozygous Monohybrid Cross (Aa x Aa):
+---------------------+-------------------+------------------+
| Offspring Genotype  | Genotypic Chance  | Phenotype        |
+---------------------+-------------------+------------------+
| AA                  | 25% (1/4)         | Dominant         |
| Aa                  | 50% (2/4)         | Dominant         |
| aa                  | 25% (1/4)         | Recessive        |
+---------------------+-------------------+------------------+
Total Dominant Phenotype: 75% | Total Recessive Phenotype: 25%

Non-Mendelian Inheritance Patterns

Not all genetic traits follow simple dominant/recessive rules. Complex inheritance patterns include:

1. Incomplete Dominance

In incomplete dominance, neither allele is completely dominant. The heterozygous phenotype is an intermediate blend between the two homozygous phenotypes.

• Example: Snapdragon flower color. Crossing homozygous Red ($RR$) with homozygous White ($WW$) produces $100%$ Pink ($RW$) offspring.

2. Codominance

In codominance, both alleles are fully and simultaneously expressed in the heterozygote without blending.

• Example: ABO Blood Group $AB$. An individual with genotype $I^A I^B$ expresses both A and B antigens on their red blood cells.
• Example: Roan coat color in cattle ($R^1 R^2$), exhibiting both red and white hairs.

3. Multiple Alleles

When a gene has more than two possible alleles in a population.

• Example: Human ABO Blood Types involve three alleles: $I^A$, $I^B$, and $i$.
  • $I^A$ and $I^B$ are codominant with each other.
  • $i$ is recessive to both $I^A$ and $I^B$.

4. Polygenic Traits

Traits controlled by the additive effects of two or more independent genes. Polygenic traits produce a continuous spectrum of variation (bell-shaped distribution curve) rather than discrete categories. Examples include human height, skin color, and eye color.

Sex-Linked Inheritance & Pedigree Analysis

Humans possess 22 pairs of autosomes and 1 pair of sex chromosomes ($XX$ for biological females; $XY$ for biological males).

X-Linked Recessive Inheritance

Genes located on the X chromosome are called sex-linked (X-linked) traits. Because males inherit only one X chromosome (from their mother), they are hemizygous for X-linked genes.

• If a male inherits an X chromosome carrying a recessive mutant allele ($X^b Y$), he will express the condition because he lacks a second X chromosome to mask it.
• Females ($XX$) express the condition only if they inherit two recessive alleles ($X^b X^b$). Heterozygous females ($X^B X^b$) are asymptomatic carriers.
• Examples: Red-green colorblindness, Hemophilia, Duchenne muscular dystrophy.

Cross: Carrier Female (X^B X^b) x Normal Male (X^B Y)

          X^B           X^b
    +-------------+-------------+
    |   X^B X^B   |   X^B X^b   |
X^B | Normal      | Carrier     |
    | Female      | Female      |
    +-------------+-------------+
    |   X^B Y     |   X^b Y     |
 Y  | Normal      | Colorblind  |
    | Male        | Male        |
    +-------------+-------------+
Result for Sons: 50% Normal (X^B Y), 50% Colorblind (X^b Y)
Result for Daughters: 50% Normal (X^B X^B), 50% Carrier (X^B X^b)

Reading Pedigree Charts

A pedigree is a family tree diagram showing the inheritance of a trait across multiple generations.

• Symbols: Squares represent biological males; circles represent biological females. Filled/shaded symbols represent affected individuals; unshaded symbols represent unaffected individuals.
• Determining Inheritance Patterns on the GED:
  • Autosomal Recessive: Trait often skips generations. Unaffected parents can have affected children ($Aa \times Aa \rightarrow aa$). Affects males and females equally.
  • Autosomal Dominant: Trait appears in every generation. Every affected child has at least one affected parent. Affects males and females equally.
  • X-Linked Recessive: Far more males are affected than females. Affected mothers pass the trait to all of their sons ($X^b X^b \rightarrow X^b Y$).