Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 14.4 Many Human Traits Follow Mendelian Patterns of Inheritance
Human genetics can be understood using Mendelian principles, even though ethical and practical constraints prevent direct experiments. Tools like pedigree analysis and genetic testing help track and predict inheritance patterns of both harmless and serious traits.
Pedigree Analysis
- A pedigree is a family tree that traces traits across generations
- Helps geneticists analyze inheritance patterns when direct experimentation isn't possible
- Traits can be identified as dominant or recessive based on observed patterns
- Used to calculate probabilities of offspring inheriting certain traits
- Example traits:
- Widow’s peak: modeled as a dominant trait
- Inability to taste PTC: modeled as a recessive trait
- Pedigrees can also be used to evaluate risk for genetic disorders before conception
Recessively Inherited Disorders
- Thousands of disorders are caused by recessive alleles
- Heterozygotes (carriers) have normal phenotypes due to one functioning allele
- Disorder only appears in individuals homozygous for the recessive allele
- Examples:
- Albinism: lack of pigmentation
- Cystic fibrosis: thick mucus buildup affecting lungs and digestion
- Sickle-cell disease: abnormal hemoglobin shapes red blood cells
- Carriers are typically healthy but can pass the allele to offspring
- Genetic risk increases with consanguineous (related) mating
Sickle-Cell Disease: A Case of Evolution
- Common in people of African descent
- Caused by a single amino acid change in hemoglobin
- Homozygotes (ss) suffer from anemia, pain, and organ damage
- Heterozygotes (Ss) have sickle-cell trait—some symptoms but usually healthy
- Heterozygote advantage: carriers are more resistant to malaria
- Explains high frequency of the allele in malaria-endemic regions
Dominantly Inherited Disorders
- Dominant disorders are less common but can be serious
- Achondroplasia: a form of dwarfism, heterozygous individuals are affected
- Huntington’s disease: late-onset nervous system degeneration caused by a dominant allele
- Dominant alleles can persist if symptoms appear after reproductive age
- Genetic testing can now identify Huntington’s carriers before symptoms show
Multifactorial Disorders
- Caused by genetic and environmental factors
- Often polygenic (affected by many genes)
- Examples: heart disease, diabetes, cancer, schizophrenia
- Lifestyle choices influence expression of genetic risk
Genetic Testing and Counseling
- Carrier testing can identify individuals who carry recessive alleles
- Fetal testing:
- Amniocentesis (15–16 weeks): analyzes amniotic fluid
- Chorionic Villus Sampling (CVS) (10–11 weeks): samples placental tissue
- Both can detect chromosomal and molecular genetic disorders
- Noninvasive tests: blood-based screening and ultrasound
- Newborn screening (e.g., PKU) enables early treatment of certain disorders
- Ethical concerns: privacy, discrimination, and difficult family decisions
In a Nutshell
Many human traits follow Mendelian inheritance, from harmless features to severe genetic disorders. Pedigree analysis, genetic testing, and counseling help assess risk, while multifactorial traits show how genes and environment work together in shaping health and disease.