In a monohybrid cross between two heterozygous parents (Bb × Bb), what is the expected phenotype ratio in the offspring?

3 dominant : 1 recessive — 75% of offspring show the dominant phenotype (BB or Bb), and 25% show the recessive phenotype (bb) 1:1 — half show the dominant phenotype and half show the recessive phenotype All offspring show the dominant phenotype — heterozygous parents can only produce dominant offspring 2:1:1 — two dominant, one intermediate, one recessive

What is a test cross and why is it useful?

A test cross mates an organism with an unknown genotype with a homozygous recessive organism (aa) — if any offspring show the recessive trait, the unknown parent must be heterozygous (Aa) A test cross is used to determine if a genetic disorder is dominant or recessive A test cross involves crossing siblings to see if they share the same genotype A test cross is a genetic test performed in a laboratory using DNA sequencing

What is a pedigree chart and what information does it show?

A pedigree chart is a diagram showing inheritance of a trait across multiple generations of a family — circles represent females, squares represent males, and filled symbols represent affected individuals A pedigree chart is a list of all genes in an organism's genome arranged by chromosome A pedigree chart is a type of Punnett square used for tracking multiple traits at once A pedigree chart shows the evolutionary history of a species

What does it mean if a trait 'runs in families'?

It means the trait has a genetic component — the underlying DNA variants that cause or predispose to the trait are passed from parent to child more often than they would appear by chance It means the trait is contagious and is spread through close family contact It means all family members will definitely develop the condition at some point It means the trait is caused entirely by the environment and not by genes

What is cystic fibrosis and how is it inherited?

Cystic fibrosis is an autosomal recessive disease caused by mutations in the CFTR gene — it causes thick mucus buildup in the lungs and digestive system, and a child must inherit two defective copies to be affected Cystic fibrosis is an autosomal dominant disease — one copy of the mutated gene is enough to cause the condition Cystic fibrosis is caused by a viral infection in infancy and is not inherited Cystic fibrosis only affects females because the gene is located on the X chromosome

What is Huntington's disease and what makes its inheritance pattern unusual?

Huntington's disease is an autosomal dominant neurodegenerative disorder — a single copy of the mutated HTT gene is enough to cause it, meaning every affected person has a 50% chance of passing it to each child Huntington's disease is recessive — both parents must carry the mutation for a child to be affected Huntington's disease is caused by an infection that attacks the brain in old age Huntington's disease skips generations and is therefore considered a recessive trait

What is sickle cell disease and why might the sickle cell allele be common in some populations?

Sickle cell disease is an autosomal recessive condition caused by a single base change in the hemoglobin gene — the sickle allele is common in malaria-endemic regions because carriers (with one copy) have improved resistance to malaria Sickle cell disease is dominant and affects anyone who inherits even one copy of the sickle allele The sickle cell allele is common because it increases athletic performance by improving oxygen delivery Sickle cell disease is caused by a chromosomal deletion, not a single gene mutation

What is Down syndrome and how does it occur?

Down syndrome (Trisomy 21) occurs when a person has three copies of chromosome 21 instead of two — usually due to a non-disjunction error during meiosis in which chromosomes fail to separate properly Down syndrome is caused by a dominant mutation on chromosome 21 inherited from one parent Down syndrome occurs when chromosome 21 is completely missing from the genome Down syndrome is caused by an infection during pregnancy that damages chromosome 21

What are multiple alleles and how do they affect inheritance?

Some genes have more than two possible alleles in the population (though each individual still carries only two) — human ABO blood type is controlled by three alleles (I^A, I^B, i), producing four possible blood types Multiple alleles means an individual can carry three or more copies of the same gene Genes with multiple alleles always produce more than two possible phenotypes Multiple alleles only occur on sex chromosomes

What does 'probability' mean in the context of genetics?

Probability in genetics gives the chance that a particular outcome will occur — a 25% chance means that outcome is expected in 1 out of 4 offspring on average, but does not guarantee it in any individual case A 25% probability means exactly 1 out of every 4 offspring will always be affected Genetic probabilities only apply to plants — human inheritance does not follow probability rules If the first child is unaffected, the next child is more likely to be affected to 'balance out'