Patterns of Inheritance: Key Concepts and Applications

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Patterns of Inheritance

Key Terms and Definitions

This section introduces essential vocabulary for understanding genetic inheritance. Mastery of these terms is foundational for analyzing genetic patterns and disorders.

Multiple alleles: More than two alternative forms of a gene (alleles) exist in a population, though an individual can only carry two alleles for each gene. Example: ABO blood group system.
Pleiotropy: A single gene influences multiple, seemingly unrelated phenotypic traits. Example: The gene responsible for sickle cell anemia also affects resistance to malaria.
Continuous variation: Traits that show a range of phenotypes due to the additive effects of multiple genes (polygenic inheritance), such as height or skin color.
Epistasis: Interaction between genes where one gene masks or modifies the expression of another gene at a different locus.
Incomplete dominance: The heterozygote phenotype is intermediate between the two homozygotes. Example: In snapdragons, red and white flowers produce pink offspring.
Codominance: Both alleles in a heterozygote are fully expressed. Example: AB blood type in humans.
Polygeny (Polygenic inheritance): Multiple genes contribute to a single trait, resulting in continuous variation.
Multifactorial trait: Traits influenced by both genetic and environmental factors. Example: Heart disease.
Sex chromosome: Chromosomes involved in determining the sex of an organism (X and Y in humans).
X chromosome: One of the two sex chromosomes; females have two, males have one.
Y chromosome: The sex chromosome present only in males, carrying the SRY gene that determines maleness.
Autosome: Any chromosome that is not a sex chromosome.
Inborn error of metabolism: Genetic disorders resulting from defects in metabolic pathways. Example: Phenylketonuria (PKU).
Pedigree: A diagram showing the inheritance of a trait across generations in a family.
Autosomal dominant: A trait that is expressed when at least one dominant allele is present on an autosome.
Autosomal recessive: A trait that is expressed only when two recessive alleles are present on an autosome.
X-linked dominant: A dominant trait carried on the X chromosome.
X-linked recessive: A recessive trait carried on the X chromosome, more commonly expressed in males.
Karyotype: An organized profile of an individual's chromosomes, used to detect chromosomal abnormalities.
Non-disjunction: Failure of chromosomes to separate properly during meiosis, leading to abnormal chromosome numbers.
Aneuploidy: The presence of an abnormal number of chromosomes in a cell (e.g., trisomy 21 in Down syndrome).
Barr body: An inactivated X chromosome in female cells, visible as a dense spot in the nucleus.
Dosage compensation: Mechanism that balances the expression of X-linked genes between males and females.
Klinefelter syndrome: A chromosomal disorder in males with an extra X chromosome (XXY).
Down syndrome: A genetic disorder caused by trisomy 21 (an extra copy of chromosome 21).
Turner syndrome: A chromosomal disorder in females with only one X chromosome (XO).
Translocation: A chromosomal abnormality where a segment of one chromosome is transferred to another chromosome.

ABO Blood Group Inheritance

The ABO blood group is determined by three alleles (IA, IB, i) and exhibits both multiple allelism and codominance.

Alleles: IA (A antigen), IB (B antigen), i (no antigen).
Inheritance pattern: IA and IB are codominant; i is recessive.
Possible genotypes and phenotypes:

Genotype	Phenotype (Blood Type)
IAIA or IAi	A
IBIB or IBi	B
IAIB	AB
ii	O

Pleiotropy Example

Sickle cell anemia: The sickle cell gene affects hemoglobin structure, red blood cell shape, and resistance to malaria.

Incomplete Dominance vs. Codominance

Incomplete dominance: Heterozygotes have an intermediate phenotype. Example: Familial hypercholesterolemia.
Codominance: Both alleles are fully expressed in heterozygotes. Example: AB blood type.

Epistasis and Phenotype

Epistasis: One gene affects the expression of another gene. Example: Coat color in Labrador retrievers.

Environmental Effects on Phenotype

Role: Environmental factors such as nutrition, temperature, and exposure to chemicals can influence gene expression and phenotype.
Example: Height is influenced by both genes and nutrition.

Polygenic Inheritance

Definition: Traits controlled by two or more genes, often resulting in continuous variation.
Example: Human skin color.

Inborn Errors of Metabolism

Definition: Genetic disorders caused by defects in specific enzymes of metabolic pathways.
Example: Phenylketonuria (PKU), where the body cannot metabolize phenylalanine.

Pedigree Analysis

Use: Pedigrees help geneticists trace inheritance patterns and predict genotypes.
Symbols: Squares represent males, circles represent females, shaded shapes indicate affected individuals.

Sex Chromosomes vs. Autosomes

Sex chromosomes: X and Y chromosomes determine biological sex.
Autosomes: All other chromosomes (22 pairs in humans).

Karyotypes and Chromosomal Abnormalities

Karyotype: A visual display of chromosomes used to detect abnormalities such as aneuploidy or translocations.
Abnormalities: Extra or missing chromosomes (e.g., trisomy 21), structural changes (e.g., translocations).

Sex Determination and the SRY Gene

Sex determination: Presence of the Y chromosome (and SRY gene) leads to male development.
SRY gene: Triggers testis development.

Sex-Linked Genes and Inheritance

Sex-linked genes: Genes located on sex chromosomes, especially the X chromosome.
Types: X-linked and Y-linked inheritance.

X-Linked Diseases in Males

Reason: Males have only one X chromosome, so recessive X-linked traits are more likely to be expressed.

Gene Dosage and X-Chromosome Inactivation

Gene dosage: The number of copies of a gene affects the amount of gene product.
X-inactivation: In females, one X chromosome is randomly inactivated (Barr body) to balance gene expression.

Inheritance of X-Linked Disorders

Pattern: X-linked recessive disorders are more common in males. Example: Hemophilia.

Aneuploidy and Nondisjunction

Aneuploidy: Abnormal number of chromosomes due to nondisjunction during meiosis.
Consequences: Disorders such as Down syndrome (trisomy 21), Turner syndrome (XO), and Klinefelter syndrome (XXY).

Common Chromosomal Disorders

Down syndrome: Trisomy 21, caused by nondisjunction.
Turner syndrome: Monosomy X (XO), affects females.
Klinefelter syndrome: XXY, affects males.
Translocation: Rearrangement of chromosome segments, can lead to genetic disorders.

Additional info: For each disorder, chromosomal abnormalities typically arise during gamete formation due to errors in meiosis (nondisjunction or improper crossing over).