This comprehensive study guide provides answers and explanations to common heredity questions, covering key concepts like Mendelian genetics, gene expression, and genetic variations. Whether you're preparing for an exam or simply looking to solidify your understanding of heredity, this guide will help you master the fundamentals.
I. Mendelian Genetics: The Basics
A. Key Terms & Definitions:
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Allele: A variant form of a gene. For example, the gene for flower color in pea plants might have an allele for purple and an allele for white.
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Gene: A segment of DNA that codes for a specific trait.
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Genotype: The genetic makeup of an organism, represented by the combination of alleles (e.g., homozygous dominant (AA), heterozygous (Aa), homozygous recessive (aa)).
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Phenotype: The observable characteristics or traits of an organism (e.g., purple flowers, white flowers).
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Homozygous: Having two identical alleles for a particular gene (AA or aa).
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Heterozygous: Having two different alleles for a particular gene (Aa).
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Dominant Allele: An allele that masks the expression of another allele (represented by a capital letter).
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Recessive Allele: An allele whose expression is masked by a dominant allele (represented by a lowercase letter).
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Punnett Square: A diagram used to predict the genotypes and phenotypes of offspring from a genetic cross.
B. Practice Problems (with answers):
Problem 1: In pea plants, tallness (T) is dominant to shortness (t). What are the possible genotypes and phenotypes of offspring from a cross between a homozygous tall plant (TT) and a homozygous short plant (tt)?
Answer: All offspring will have the genotype Tt (heterozygous) and the phenotype tall. The Punnett Square would show:
| T | T | |
|---|---|---|
| t | Tt | Tt |
| t | Tt | Tt |
Problem 2: Brown eyes (B) are dominant to blue eyes (b). A heterozygous brown-eyed individual (Bb) marries a blue-eyed individual (bb). What are the possible genotypes and phenotypes of their children?
Answer: The Punnett Square shows:
| B | b | |
|---|---|---|
| b | Bb | bb |
| b | Bb | bb |
There's a 50% chance of a brown-eyed child (Bb) and a 50% chance of a blue-eyed child (bb).
II. Beyond Mendelian Genetics: Exploring Complex Inheritance
A. Incomplete Dominance: Neither allele is completely dominant; the heterozygote shows a blended phenotype. (e.g., red flower (RR) x white flower (rr) = pink flower (Rr)).
B. Codominance: Both alleles are fully expressed in the heterozygote. (e.g., AB blood type).
C. Multiple Alleles: More than two alleles exist for a gene (e.g., ABO blood types).
D. Polygenic Inheritance: Traits controlled by multiple genes (e.g., human height, skin color).
E. Sex-Linked Traits: Traits located on sex chromosomes (X or Y). (e.g., color blindness, hemophilia).
III. Gene Expression and Genetic Variations
A. Gene Expression: The process by which information encoded in a gene is used to synthesize a functional gene product (protein).
B. Mutations: Changes in the DNA sequence that can lead to variations in traits. These can be caused by various factors, including errors during DNA replication or exposure to mutagens.
C. Genetic Variations: Differences in DNA sequences among individuals, leading to diversity in traits. These variations are the raw material for evolution.
This study guide provides a solid foundation for understanding heredity. Remember to consult your textbook and class notes for further details and examples. Practice solving more problems to reinforce your understanding!
