Genetics Study Guide
Test Crosses
Test crosses demonstrate how traits are passed to the next generation.
Examples include:
Homozygous dominant crossed with homozygous recessive.
Heterozygous crossed with homozygous recessive.
Previous discussions covered the ratio of 3:1 and 1:1.
Basics of Genetic Terms
Heterozygous: Two different alleles for a gene (e.g., big P, little p).
Homozygous: Two of the same alleles for a gene (e.g., homozygous dominant BB, homozygous recessive bb).
Distinction in letter notation:
Big letters represent dominant alleles.
Little letters represent recessive alleles.
Practice: Add flourishes to distinguish similar letters (e.g., cursive lowercase letters).
Example - Cross of Two Heterozygotes
Alleles: Big P (purple flower) and little p (white flower).
Genotypic Ratios:
Homozygous dominant, homozygous recessive, and heterozygous offspring.
Phenotypic Ratios:
3 Purple (75%) : 1 White (25%).
Prediction of offspring if 1000 are produced: Expect close to 750 purple and 250 white.
More Punnett Squares
Different parental combinations lead to varying ratios:
Two heterozygotes result in a 3:1 ratio (3 purple: 1 white).
Homozygous recessive crossed with homozygous dominant results in all offspring being purple.
Homozygous recessive crossed with homozygous recessive results in all offspring being white.
Punnett Squares for Probability
Used to explain the ratio of male to female births (1:1).
Female passes X chromosome; male passes X or Y chromosome.
Probability of male or female offspring is always 50%.
Dihybrid Cross
A cross that considers two traits.
Example alleles:
Round (Big R) vs. Wrinkled (little r).
Yellow (Big Y) vs. Green (little y).
Phenotypic ratio from a dihybrid cross:
Typically results in 9:3:3:1 ratio.
Probability in Genetics
Individual event probability remains the same regardless of previous births (e.g., probability of having a girl remains 50% for every birth).
Multiplication of probabilities: When considering multiple successive events (e.g., births), multiply the probabilities of each individual event.
Example of Calculation for Girls:
50% chance for the first girl, same for second, and so forth.
Probability of having four girls in a row = 0.5^4 = 0.0625 or 6.25%.
Sampling Error
Occurs when observed outcomes diverge from expected probabilities in small datasets.
Large datasets tend to closely align with probability predictions.
Additional Genetic Concepts
Incomplete Dominance: A situation where neither allele is completely dominant; results in a blended phenotype (e.g., red flower and white flower yield pink flowers).
Codominance: A situation where both alleles are fully expressed in the phenotype (e.g., blood types A and B produce AB blood type).
X-linked Traits
Traits associated with genes on the X chromosome.
Males have one X (hemizygous), while females have two.
Common example: red-green color blindness.
Probability for offspring in a Punnett square illustrating male and female inheritance of X-linked traits.
Pleiotropy and Epistasis
Pleiotropy: One gene influences multiple traits (e.g., gene affecting hair color and also affecting skin tone).
Epistasis: One gene affects the expression of another gene (e.g., one gene might block the phenotypic expression of another).
Polygenic Inheritance
Traits controlled by multiple genes (e.g., skin color, height).
Typically exhibits a bell curve distribution.
Multifactorial Traits
Traits influenced by multiple genes and environmental factors (e.g., height can be influenced by nutrition, genetics).
Pedigree Analysis
A diagram that shows the occurrence of a trait across generations in a family tree.
Squares represent males, circles for females, and shading indicates affected individuals.
Diamonds can indicate unknown genders in pedigrees.
Can be used to track inheritance patterns of genetic traits.
Conclusion and Next Steps
Review of Mendelian genetics and inheritance.
Prepare for lab practice on genetic concepts and probabilities associated with specific traits.