Chapter 20 Flashcards
(30 cards)
What is a gene?
A DNA segment that carries the instructions (code) for making one functional product—usually a protein.
Define allele.
Alternative versions of the same gene (e.g., F vs. f for freckles).
Genotype vs. phenotype?
Genotype = actual allele pair (AA, Aa, aa). Phenotype = observable trait produced (freckled / non‑freckled).
Homozygous vs. heterozygous.
Homozygous = two identical alleles (AA or aa). Heterozygous = two different alleles (Aa).
What is a dominant allele?
Masks a recessive allele in the heterozygote; its trait appears with only one copy.
What is a recessive allele?
Expressed only when both alleles are recessive (aa).
What is the Law of Segregation (Mendel)?
Each parent’s two alleles separate into different gametes—each sperm/egg gets just one allele.
What does the Law of Independent Assortment state?
Allele pairs on different chromosomes segregate into gametes independently of one another.
What is the purpose of a Punnett square?
Predicts probabilities of offspring genotypes/phenotypes from known parent genotypes.
When does a monohybrid 3:1 phenotypic ratio occur?
Two heterozygotes (Aa × Aa) cross for a trait showing complete dominance.
What does a dihybrid 9:3:3:1 ratio indicate?
A double‑heterozygote cross (AaBb × AaBb) with genes on different chromosomes.
Give an example of complete dominance.
Freckles (F) over no‑freckles (f).
Give an example of codominance.
ABO blood type AB—both IA and IB expressed fully.
Give an example of incomplete dominance.
Straight (SS) × curly (ss) hair gives wavy (Ss) hair.
What is a multiple‑allele system in humans?
ABO blood group (IA, IB, i).
Define polygenic trait and give an example.
Trait controlled by many genes → continuous range (height, skin colour).
What is pleiotropy?
One gene → many effects; sickle‑cell allele causes RBC sickling and organ damage.
What are linked genes?
Genes on the same chromosome that tend to be inherited together; crossing‑over can unlink them.
What are key points of X‑linked recessive inheritance?
- More common in males (one X copy). * Sons inherit mutant allele only from mother. * Daughters can be carriers.
What are common X‑linked disorders?
- Red‑green colour blindness * Hemophilia A/B * Duchenne muscular dystrophy
Give an example of a sex‑influenced trait.
Male‑pattern baldness—dominant in presence of high testosterone (males), recessive in females.
What is chromosome deletion and an example disorder?
Loss of tip of chromosome 5 → Cri‑du‑chat syndrome (cat‑like cry, delays).
What is chromosome duplication and an example disorder?
Repeated sequence on X → Fragile X syndrome (most common inherited intellectual disability).
What is amniocentesis, and when is it performed?
Weeks 15-20; needle removes amniotic fluid → fetal cells tested for chromosomal & gene defects.
