Ch.12 Mandelian Genetics and Inheritance

Gregor Mendel

Austrian monk and scientist known as the father of genetics who discovered the fundamental laws of inheritance through breeding experiments with pea plants.

Mendel's experimental method

Cross-pollinating true-breeding pea plants with contrasting traits, observing the resulting generations, and quantitatively tracking and counting trait distributions.

Monohybrid cross

A breeding experiment between two organisms that focuses on the inheritance of a single gene or trait.

$P$ generation

The parental generation in a genetic cross, typically consisting of true-breeding individuals.

$F_1$ generation

The first filial generation, consisting of the direct offspring produced by crossing individuals of the parental ($P$) generation.

$F_2$ generation

The second filial generation, produced by self-fertilizing or crossing individuals from the $F_1$ generation.

Dominant trait

A characteristic that is expressed in the phenotype whenever at least one copy of the corresponding dominant allele is present.

Recessive trait

A characteristic that is expressed only when an organism possesses two copies of the recessive allele, remaining masked in the presence of a dominant allele.

Genotype

The underlying genetic makeup or specific combination of alleles present in an organism.

Phenotype

The observable physical, physiological, or biochemical traits of an organism, shaped by its genotype and environment.

Difference between genotype and phenotype

Genotype describes the set of alleles an organism carries, whereas phenotype describes the visible, expressed physical traits.

Genotypic ratio

The relative proportion of different allele combinations present among the offspring of a genetic cross.

Phenotypic ratio

The relative proportion of different observable physical traits displayed by the offspring of a genetic cross.

Allele

An alternative structural form or sequence variant of a specific gene found at the same chromosomal locus.

Homozygous

Possessing two identical alleles for a specific gene (such as $AA$ or $aa$).

Heterozygous

Possessing two distinct, different alleles for a specific gene (such as $Aa$).

Dominant allele

An allele whose associated phenotype is fully expressed in a heterozygote under complete dominance.

Recessive allele

An allele whose phenotypic effect is completely masked or concealed in the presence of a dominant allele.

Principle of segregation

Mendel's first law stating that the two alleles for a gene separate during gamete formation so that each gamete carries only one allele for each gene.

Pedigree analysis

The systematic study of a family tree diagram to trace the inheritance pattern and transmission of specific traits across multiple generations.

Square symbol in a pedigree

Represents a male individual.

Circle symbol in a pedigree

Represents a female individual.

Shaded symbol in a pedigree

Represents an individual who physically expresses the trait or condition being analyzed.

Dominant inheritance pattern in a pedigree

A pattern where the trait typically appears in every generation without skipping, and affected offspring almost always have at least one affected parent.

Recessive inheritance pattern in a pedigree

A pattern where the trait frequently skips generations, allowing two unaffected heterozygous carrier parents to produce an affected child.

Dihybrid cross

A mating experiment between two organisms that simultaneously evaluates the inheritance patterns of two distinct genes or traits.

Principle of independent assortment

Mendel's second law stating that alleles of distinct unlinked genes segregate independently into gametes during meiosis.

Expected phenotypic ratio of a standard dihybrid cross ($AaBb \times AaBb$)

$9:3:3:1$, assuming independent assortment and complete dominance.

Gametes produced by genotype $AaBb$

Four equal combinations: $AB$, $Ab$, $aB$, and $ab$.

Rule of addition in genetics

A probability rule stating that the likelihood of either of two mutually exclusive events occurring is determined by adding their individual probabilities.

Rule of multiplication in genetics

A probability rule stating that the joint likelihood of two independent events occurring together is calculated by multiplying their individual probabilities.

Testcross

A breeding experiment crossing an organism displaying a dominant phenotype of unknown genotype with a homozygous recessive ($aa$) individual.

Primary purpose of a testcross

To establish whether an individual with a dominant phenotype is homozygous dominant or heterozygous based on offspring phenotypes.

Incomplete dominance

An inheritance pattern in which heterozygous individuals display a blended, intermediate phenotype between the two homozygous phenotypes.

Codominance

An inheritance pattern in which both distinct alleles are simultaneously and fully expressed in the phenotype of a heterozygote.

Difference between incomplete dominance and codominance

Incomplete dominance yields an intermediate, blended phenotypic state, whereas codominance results in both parental phenotypes being distinctly and simultaneously observable.

Polygenic inheritance

An inheritance pattern in which a single continuous physical trait is collectively controlled by the additive effects of multiple genes.

Pleiotropy

A genetic condition where a single individual gene influences multiple, distinct, and seemingly unrelated phenotypic traits.

Multiple-allele inheritance

A genetic scenario where three or more alternate alleles for a single gene exist within a given population.

Epistasis

A non-allelic gene interaction where the phenotypic expression of one gene locus masks, inhibits, or alters the expression of a gene at another locus.

ABO blood group alleles

A multiple-allele system consisting of $I^A$, $I^B$, and $i$, wherein $I^A$ and $I^B$ are codominant with each other and dominant over recessive $i$.

ABO blood group genotypes

Type A ($I^AI^A$ or $I^Ai$), Type B ($I^BI^B$ or $I^Bi$), Type AB ($I^AI^B$), and Type O ($ii$).

Environmental influence on phenotype

The modification of an organism's phenotypic expression due to external parameters such as diet, temperature, and sun exposure.

Offspring phenotype from $YY \times yy$ (yellow dominant to green)

$100\%$ yellow, because all offspring receive the heterozygous genotype $Yy$.

Offspring blood types from a cross between AB and O parents

Type A or Type B only (genotypes $I^Ai$ and $I^Bi$), with no chance of type AB or O.

Phenotypic possibilities when red ($RR$) and white ($WW$) flowers show incomplete dominance

Red ($RR$), pink ($RW$), or white ($WW$).

Expected phenotypic ratio from a cross between two heterozygotes ($Aa \times Aa$)

$3:1$ (three showing the dominant phenotype to one showing the recessive phenotype).

Percentage of offspring inheriting Marfan syndrome from an affected heterozygote ($Mm$) and an unaffected parent ($mm$)

$50\%$, because each offspring has a $1/2$ probability of inheriting the dominant allele $M$.

Possible offspring blood types from two parents with blood type AB

Type A, Type B, or Type AB in a $1:2:1$ ratio ($I^AI^A$, $I^AI^B$, and $I^BI^B$).

Types of gametes produced by genotype $PpTt$

Four unique gamete combinations: $PT$, $Pt$, $pT$, and $pt$.

Expected fraction of white and tall offspring in a $PpTt \times PpTt$ cross

$3$ out