Genetics and Inheritance: Human Biology Chapter 19 Study Notes

Study Guide - Smart Notes

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

Genetics and Inheritance

Introduction to Genetics

Genetics is the study of genes, heredity, and the variation of inherited characteristics. It explores how traits are passed from one generation to the next and how genetic information is organized and expressed in living organisms.

Gene: A segment of DNA that contains instructions for building proteins. Only about 1.5–2.5% of the human genome consists of protein-coding genes.
Genome: The complete set of DNA in an organism, including all of its genes.
Inheritance: The process by which genetic information is passed from parents to offspring.
Humans inherit two sets of DNA, one from each parent, resulting in genetic variation among siblings (except identical twins).

Chromosomes and Genetic Organization

Chromosomes are structures within the cell nucleus composed of DNA and proteins. They carry genetic information and are visible during cell division.

Chromosome: A single molecule of DNA associated with proteins, forming a compact structure.
Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes (X and Y).
Homologous chromosomes: Chromosome pairs that are similar in size, shape, and banding pattern but may carry different alleles.
Allele: Alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
Homozygous: Having two identical alleles for a particular gene.
Heterozygous: Having two different alleles for a particular gene.

Karyotype

A karyotype is a composite display of an individual's chromosomes, arranged in pairs from largest to smallest. It is used to identify chromosomal abnormalities and differences between the X and Y chromosomes.

Genotype and Phenotype

The genotype is the genetic makeup of an individual, while the phenotype is the observable physical and functional traits.

Genotype: The complete set of alleles in an individual.
Phenotype: Observable traits such as hair color, eye color, blood type, and disease susceptibility.
Phenotype is influenced by both inherited alleles and environmental factors.

Patterns of Genetic Inheritance

Genetic inheritance follows specific patterns, which can be predicted using tools like Punnett squares.

Punnett Square: A diagram used to predict the outcome of genetic crosses by showing possible combinations of parental alleles.

Mendelian Principles

Gregor Mendel established the basic principles of genetics through experiments with pea plants.

Law of Segregation: The two alleles of each gene separate during gamete formation and end up in different gametes.
Law of Independent Assortment: Genes for different traits are separated independently during meiosis, provided they are on different chromosomes.

Dominant and Recessive Alleles

Alleles can be dominant or recessive, affecting how traits are expressed.

Dominant allele: Masks the expression of its complementary allele and is always expressed if present.
Recessive allele: Only expressed if the individual is homozygous for the recessive allele.
Example: Freckles (dominant F allele) and ability to taste PTC (dominant T allele).

Genetic Disorders

Some genetic disorders are caused by recessive or dominant alleles.

Cystic fibrosis: Caused by two copies of a defective recessive allele (CFTR gene).
Polydactyly: Extra fingers or toes caused by a dominant allele.
Achondroplasia: Dwarfism caused by a dominant allele.

Complex Patterns of Inheritance

Incomplete Dominance: Heterozygotes express a phenotype intermediate between both homozygotes (e.g., wavy hair).
Codominance: Both alleles are expressed equally in heterozygotes (e.g., ABO blood types, sickle-cell trait).
Polygenic Inheritance: Traits influenced by multiple genes, resulting in a continuous range of phenotypes (e.g., height, skin color).

Environmental Influence and Epigenetics

Environmental factors can affect gene expression and phenotype, sometimes through epigenetic modifications.

Epigenetic inheritance: Modification and transmission of traits without changes in DNA sequence (e.g., DNA methylation).
Environmental factors such as nutrition can influence traits like height.

Linked Genes and Sex-Linked Inheritance

Linked alleles: Genes located on the same chromosome may be inherited together unless separated by crossing over.
Sex-linked inheritance: Traits determined by genes on the X or Y chromosomes.
X-linked recessive traits: More common in males; examples include color blindness and hemophilia.
Sex-influenced traits: Traits affected by hormones, such as baldness.

Chromosomal Alterations

Changes in chromosome number or structure can lead to genetic disorders.

Nondisjunction: Failure of chromosomes to separate during meiosis, resulting in abnormal chromosome numbers (e.g., Down syndrome, Klinefelter syndrome).
Deletions and translocations: Structural changes in chromosomes can cause disorders such as Cri-du-chat syndrome.

Summary Table: Common Chromosomal Alterations

Alteration	Chromosome	Phenotype
Down syndrome	Trisomy 21	Developmental disabilities, distinct physical features
Klinefelter syndrome	XXY	Tall, sterile male, mild mental impairment
Turner syndrome	XO	Short, sterile female, normal intelligence
Cri-du-chat syndrome	Deletion on 5	Intellectual disability, distinctive cry
XYY syndrome	XYY	Tall male, otherwise normal

Inherited Disorders

Phenylketonuria (PKU): Recessive disorder; lack of enzyme to metabolize phenylalanine.
Tay-Sachs disease: Recessive disorder; lack of lysosomal enzyme, common in Ashkenazi Jews.
Huntington disease: Dominant-lethal allele; always expressed in heterozygotes, onset in midlife.

Key Equations

Punnett Square Probability:

Law of Segregation:

Law of Independent Assortment:

Additional info:

Epigenetic changes can explain differences in identical twins as they age.
Some genetic traits provide evolutionary advantages, such as the sickle cell allele protecting against malaria.