Chapter 12: Mendel’s Experiments and Heredity Flashcards
Who was Gregor Mendel, and why is he significant?
Father of Genetics: Conducted pioneering experiments with pea plants (1856–1863).
Discovered fundamental laws of inheritance: Law of Segregation and Law of Independent Assortment.
Used quantitative analysis to propose that traits are inherited via discrete units (genes).
Describe Mendel’s experimental methods.
Studied 7 traits in pea plants (e.g., seed shape, flower color).
Used true-breeding plants (produce identical offspring when self-pollinated).
Performed cross-pollination to create hybrids.
Analyzed F1 (first filial) and F2 generations to deduce inheritance patterns.
Define the following terms: Allele, Genotype, Phenotype, Homozygous, Heterozygous.
Allele: Variant form of a gene (e.g., dominant A vs. recessive a).
Genotype: Genetic makeup (e.g., AA, Aa, aa).
Phenotype: Observable trait (e.g., round seeds, wrinkled seeds).
Homozygous: Two identical alleles (AA or aa).
Heterozygous: Two different alleles (Aa).
What is the Law of Segregation?
During gamete formation, alleles separate so each gamete carries only one allele for each gene.
Explains why offspring inherit one allele from each parent.
Demonstrated in monohybrid crosses (e.g., Aa × Aa → 3:1 phenotypic ratio in F2).
What is a monohybrid cross? Provide an example.
A cross between parents differing in one trait (e.g., seed shape: round RR vs. wrinkled rr).
Example: RR × rr → F1 = Rr (all round).
F1 × F1 → F2 = 3:1 (round:wrinkled).
Explain the Law of Independent Assortment.
Alleles for different genes segregate independently during gamete formation.
Applies only to genes on different chromosomes (or far apart on the same chromosome).
Demonstrated in dihybrid crosses (e.g., 9:3:3:1 phenotypic ratio).
What is a dihybrid cross? Provide an example.
A cross between parents differing in two traits (e.g., seed shape and color).
Example: RRYY × rryy → F1 = RrYy (all round/yellow).
F1 × F1 → F2 = 9:3:3:1 phenotypic ratio.
What is a Punnett square, and how is it used?
A grid to predict offspring genotypes/phenotypes from parental gametes.
Steps:
List parental gametes (e.g., RR and rr for Rr × Rr).
Combine gametes in the grid.
Calculate genotype/phenotype ratios.
What is a test cross, and why is it useful?
Crossing an organism with an unknown genotype (e.g., A_ with a homozygous recessive aa) individual.
Purpose: Determine if the unknown genotype is homozygous (AA) or heterozygous (Aa).
Example: If A_ × aa produces all dominant phenotypes, the unknown is AA.
What is incomplete dominance? Provide an example.
Neither allele is dominant; heterozygotes display a blended phenotype.
Example: Snapdragon flowers:
RR (red) × rr (white) → F1 = Rr (pink).
F1 × F1 → 1:2:1 (red:pink:white).
What is codominance? Provide an example.
Both alleles are expressed equally in heterozygotes.
Example: Blood type AB (alleles I_A and I_B produce both A and B antigens).
Define multiple alleles. Provide an example.
A gene with more than two allele variants in a population.
Example: Human blood type (I_A, I_B, i).
What is polygenic inheritance?
A single trait influenced by multiple genes (e.g., skin color, height).
Results in a continuous range of phenotypes (bell curve distribution).
What is pleiotropy? Provide an example.
A single gene affects multiple traits.
Example: Marfan syndrome (gene mutation affects connective tissue, leading to heart, eye, and skeletal issues).
How do sex-linked traits differ from autosomal traits?
Genes located on sex chromosomes (usually X in humans).
Example: Color blindness (X_cY males express it; X_cX_c females need two recessive alleles).
Males are hemizygous (only one X chromosome).
What is a pedigree, and how is it used?
A family tree diagram showing inheritance patterns of a trait.
Uses:
Determine if a trait is dominant/recessive.
Identify carriers (heterozygotes).
Predict risk of genetic disorders.
Compare autosomal and sex-linked disorders.
Autosomal: Affects both sexes equally (e.g., cystic fibrosis, sickle cell anemia).
Sex-linked: More common in males (e.g., hemophilia, Duchenne muscular dystrophy).
List Mendel’s four postulates.
Unit Factors: Traits are controlled by pairs of alleles.
Dominance/Recessiveness: One allele may mask another.
Segregation: Alleles separate during gamete formation.
Independent Assortment: Alleles for different traits segregate independently.
Name exceptions to Mendel’s laws.
Linked genes (genes on the same chromosome violate independent assortment).
Epistasis (one gene affects the expression of another).
Environmental influence (e.g., temperature affecting fur color in Himalayan rabbits).
Why is Mendel’s work foundational to modern genetics?
Provided the framework for understanding DNA, chromosomes, and gene expression.
Basis for genetic engineering, CRISPR, and medical genetics (e.g., carrier screening).
