2 Inheritance Patterns and Genetic Analysis

Genes, Alleles, and Mendel’s Laws

A gene is associated with a trait and can occur in different forms called alleles. In a diploid organism, an individual typically inherits two alleles for each gene, one from each parent. The combination of alleles is the genotype; the observable trait is the phenotype. An individual with two identical alleles is homozygous, while one with two different alleles is heterozygous.

Mendel’s law of segregation explains how allele pairs are passed on: the two alleles separate during gamete formation, and each gamete receives one. At fertilization, alleles from two gametes combine. Independent assortment means that alleles of different genes assort independently into gametes when those genes are unlinked. This is usually true for genes on different chromosomes, while nearby genes on the same chromosome may be inherited together.

In complete dominance, a heterozygote shows the dominant phenotype, while the recessive phenotype appears only in a recessive homozygote. Dominance describes how alleles relate for a particular trait; it does not mean that an allele is more common or more beneficial.

Takeaway: Alleles separate into gametes and combine at fertilization to form an offspring’s genotype.

Predicting Outcomes of Genetic Crosses

A Punnett square lays out possible gametes from each parent and combines them to show possible offspring genotypes. Consider two heterozygotes, represented by $Yy \times Yy$. Each parent can produce gametes carrying either $Y$ or $y$, so the four equally likely combinations are $YY$, $Yy$, $Yy$, and $yy$.

The expected genotype ratio is $1\,YY : 2\,Yy : 1\,yy$. If $Y$ is completely dominant to $y$, the expected phenotype ratio is $3$ dominant to $1$ recessive. These are probabilities, not guaranteed counts among a particular group of offspring.

A monohybrid cross follows one gene. A test cross pairs an individual with a dominant phenotype but unknown genotype with a homozygous recessive individual. If any offspring show the recessive phenotype, the unknown parent carried a recessive allele. However, a small number of offspring may not reveal the parent’s genotype with certainty.

A dihybrid cross follows two genes. For $AaBb \times AaBb$, if both genes show complete dominance and assort independently, each parent can produce the gametes $AB$, $Ab$, $aB$, and $ab$. The expected phenotype ratio is $9:3:3:1$: nine offspring show both dominant traits, three show the first dominant and second recessive trait, three show the first recessive and second dominant trait, and one shows both recessive traits. Linkage or interactions between genes can change this outcome.

Takeaway: Cross ratios are expected probabilities based on stated inheritance assumptions, not predictions of exact family sizes.

Inferring Inheritance from Pedigrees

A pedigree is a family diagram used to trace a trait across generations. Conventionally, squares represent males, circles represent females, shaded symbols indicate individuals with the trait, horizontal lines connect partners, and vertical lines lead to offspring. A pedigree can suggest inheritance patterns and constrain possible genotypes, but it may not establish a single explanation uniquely.

Common pattern clues include:

• Autosomal dominant: The trait often appears in successive generations, and an affected person commonly has an affected parent. If one parent is heterozygous and affected while the other is unaffected, each child has a $\frac{1}{2}$ chance of inheriting the allele.

• Autosomal recessive: The trait can appear among children of unaffected carrier parents and may skip generations. For two carriers, $Aa \times Aa$, each child has a $\frac{1}{4}$ chance of being $aa$ and affected, assuming complete penetrance.

• X-linked recessive: This pattern often affects more males because males typically have one X chromosome. An X-linked allele is not passed from father to son; an affected father passes his X chromosome to his daughters, not his sons.

• X-linked dominant: There is also no father-to-son transmission. Assuming the usual $XX/XY$ inheritance pattern, an affected father passes the X-linked allele to all daughters and none of his sons.

These clues are not absolute. Small families, new variants, incomplete penetrance, or inaccurate family histories can obscure an inheritance pattern.

Takeaway: Consider several family relationships together; a pedigree can support more than one possible explanation.

Inheritance Beyond Complete Dominance

Mendelian segregation can still apply when a heterozygote does not resemble a dominant homozygote. In incomplete dominance, the heterozygote has an intermediate phenotype. For example, crossing red-flowered and white-flowered snapdragons can produce pink heterozygotes. Crossing two such heterozygotes gives a $1:2:1$ phenotype ratio.

In codominance, both alleles are detectably expressed in the heterozygote. In the ABO blood group, $I^A$ and $I^B$ are codominant, and each is dominant to $i$. Thus, $I^AI^B$ gives type AB, while $ii$ gives type O. The ABO system also demonstrates multiple alleles: a gene can have more than two alleles in a population, although a diploid individual carries at most two copies of that gene.

In polygenic inheritance, several genes contribute to a trait, often producing a range of phenotypes rather than a few distinct categories. Epistasis occurs when alleles of one gene affect or mask the expression of another gene, changing phenotype ratios expected from independent single-gene traits.

These patterns explain why observed traits may not match simple dominant–recessive ratios even when allele transmission follows the law of segregation.

Takeaway: How alleles are transmitted and how their effects appear in a phenotype are related but distinct questions.