Genetics/mendelian Flashcards
(20 cards)
What are lethal alleles and how are they inherited?
Lethal alleles cause death when present in certain genotypes. They often result from mutations in essential genes and are typically recessive.
Mexican hairless dogs: Heterozygous (Hh) dogs are hairless; homozygous dominant (HH) is lethal.
Achondroplastic dwarfism: Autosomal dominant; homozygous dominant (AA) is lethal in humans.
Differentiate between sex-linked and sex-limited traits.
Sex-linked: Traits determined by genes on sex chromosomes (e.g., X-linked recessive hemophilia).
Sex-limited: Traits expressed in only one sex due to hormonal differences (e.g., beard growth in males, breast development in females).
Compare incomplete dominance and codominance.
Incomplete dominance: Heterozygotes display an intermediate phenotype (e.g., pink flowers from red × white parents).
Codominance: Both alleles are expressed simultaneously (e.g., AB blood type in humans).
Define penetrance and expressivity.
Penetrance: The proportion of individuals with a genotype that show the associated phenotype (e.g., Huntington’s disease is 100% penetrant).
Expressivity: The degree to which a phenotype is expressed (e.g., polydactyly severity varies)
What is pleiotropy? Provide an example.
A single gene affecting multiple traits. Example: Porphyria variegata causes skin sensitivity, abdominal pain, and neurological issues due to a heme synthesis defect.
Genetic Heterogeneity
Different genes producing the same phenotype (e.g., 132 genes linked to hearing loss). Contrast with allelic heterogeneity (multiple alleles at one locus, e.g., 1,600+ CFTR mutations causing cystic fibrosis).
What is a phenocopy? Give an example.
An environmentally induced trait mimicking an inherited one. Example: Thalidomide-induced limb defects resembling genetic phocomelia.
How is the Chi-squared test used in genetics?
It compares observed vs. expected phenotypic ratios to test inheritance hypotheses. Formula:
Degrees of freedom = (number of categories - 1). If χ2χ 2 < critical value, the hypothesis is accepted.
Autosomal Recessive Disorders
Q: Describe the inheritance of cystic fibrosis and sickle cell anemia.
Both are autosomal recessive:
Cystic fibrosis: Caused by CFTR mutations (e.g., ΔF508); symptoms include thick mucus and lung infections.
Sickle cell anemia: Caused by HbS mutation; leads to rigid, sickle-shaped red blood cells.
X-Linked Recessive Inheritance
Q: How does X-linked recessive inheritance work?
Males (XY) are more frequently affected because they inherit one X from the mother. Females (XX) must inherit two recessive alleles. Example: Hemophilia B.
Summarize Mendel’s laws.
Law of Segregation: Alleles separate during gamete formation.
Law of Independent Assortment: Alleles for different traits segregate independently (applies to unlinked genes).
What phenotypic ratio arises from a dihybrid cross?
9:3:3:1 ratio in F2 generation (e.g., Mendel’s pea plants with seed color and shape).
What does the teosinte hypothesis propose?
Modern maize evolved from teosinte via 5 key gene changes, as shown by George Beadle using Mendelian genetics.
What is gene therapy?
Introducing healthy genes to replace defective ones (e.g., leptin gene therapy in mice). Limited success in humans due to complex multigenic diseases.
Name ethical concerns in genetics.
Genetic privacy, GMO controversies, eugenics risks, and prenatal testing dilemmas (e.g., embryo selection).
Chromosome StructureDescribe human chromosome organization.
46 chromosomes (22 autosome pairs + XX/XY). Each chromosome has a centromere, telomeres, and sister chromatids (during replication).
Explain the central dogma of molecular genetics.
DNA → RNA → Protein. Genes are transcribed to mRNA, which is translated into proteins.
How is polydactyly inherited?
Autosomal dominant with incomplete penetrance and variable expressivity (extra fingers/toes may vary in number and severity).
What causes Fragile X syndrome?
X-linked dominant disorder caused by CGG triplet repeats in the FMR1 gene, leading to intellectual disability and behavioral issues.
Why did Mendel use pea plants?
Peas have distinct traits, short generations, and allow controlled crosses. He established pure-breeding lines and quantified results (e.g., 3:1 ratios).
