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Biology
Grade 9-12
Genetic Disorders Reference Cheat Sheet
A printable reference covering Mendelian inheritance, pedigree patterns, genetic disorder types, Punnett squares, carrier probability, and screening concepts for grades 9-12.
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The core ideas include dominant, recessive, X-linked, codominant, and chromosomal inheritance patterns. Punnett squares and probability rules help estimate the chance that a child will inherit a genotype or show a disorder. Pedigrees show family relationships across generations and can reveal whether a trait is likely autosomal or sex-linked. Genetic testing, carrier screening, and counseling can help families understand risk, but they do not always predict symptoms perfectly.
Key Facts
- In an autosomal dominant disorder, one affected allele is enough to show the trait, so Aa or AA individuals are affected.
- In an autosomal recessive disorder, two affected alleles are needed to show the trait, so aa individuals are affected and Aa individuals are carriers.
- If two carriers of an autosomal recessive disorder have a child, the expected probabilities are 25% affected, 50% carrier, and 25% unaffected non-carrier.
- In X-linked recessive inheritance, males are more often affected because they have one X chromosome, so one recessive allele on the X can cause the disorder.
- A Punnett square shows possible allele combinations, but it gives probabilities for each child, not a guaranteed pattern for a family.
- Codominance means both alleles are expressed, as in blood type AB, where the A and B alleles both appear in the phenotype.
- Nondisjunction is the failure of chromosomes to separate correctly during meiosis, which can cause disorders such as trisomy 21.
- Genotype refers to allele combination, while phenotype refers to the observable trait or condition.
Vocabulary
- Allele
- An allele is a version of a gene that can influence a specific trait.
- Carrier
- A carrier has one recessive disorder allele and one normal allele, so they usually do not show the disorder but can pass the allele to offspring.
- Pedigree
- A pedigree is a family tree diagram used to track how a trait or disorder is inherited across generations.
- Autosomal
- Autosomal describes a gene located on one of the non-sex chromosomes.
- X-linked
- X-linked describes a gene located on the X chromosome, often producing different inheritance patterns in males and females.
- Nondisjunction
- Nondisjunction is an error in cell division where chromosomes fail to separate properly, leading to cells with too many or too few chromosomes.
Common Mistakes to Avoid
- Confusing dominant with common is wrong because a dominant disorder can be rare if the affected allele is uncommon in the population.
- Calling every unaffected person a carrier is wrong because carriers apply mainly to recessive conditions and must have one recessive disorder allele.
- Assuming Punnett square ratios predict exact family outcomes is wrong because each child is an independent event with the same probabilities.
- Mixing up autosomal and X-linked patterns is wrong because autosomal traits affect males and females similarly, while X-linked traits often show sex-based differences.
- Forgetting that environment can affect phenotype is wrong because some genetic disorders vary in severity due to diet, treatment, lifestyle, or other genes.
Practice Questions
- 1 Two parents are carriers for an autosomal recessive disorder. What is the probability that their child will be affected?
- 2 A father has an autosomal dominant disorder and is heterozygous, Aa. The mother is unaffected, aa. What is the probability that a child will inherit the disorder?
- 3 A carrier mother for an X-linked recessive disorder has children with an unaffected father. What is the probability that a son will be affected?
- 4 A pedigree shows that a disorder appears in every generation and affects both males and females. Explain why this pattern may suggest autosomal dominant inheritance.