#AP Biology: Genetics - The Night Before 🧬

Hey! Let's get you feeling confident about genetics for your AP Bio exam tomorrow. We're going to break down the key concepts, hit the high-yield stuff, and make sure you're ready to ace it! Let's dive in!

#Mendelian Genetics: The Basics

• Gregor Mendel, the father of modern genetics, laid the foundation for our understanding of inheritance. His laws are crucial!
• He figured out how traits are passed down through generations.

• DNA and RNA are the molecules that carry genetic information. 🧬
• Ribosomes (protein factories) are essential in all life forms because they use RNA to create proteins.

• Law of Segregation: During gamete (sperm/egg) formation, allele pairs separate, so each gamete gets only one allele per trait. Think of it like a fair split. ⚖️
• Law of Independent Assortment: Alleles for different traits separate independently of each other during gamete formation. This means that the inheritance of one trait doesn't affect the inheritance of another. 💡

#Essential Vocabulary: Know Your Terms

• Phenotype: The physical appearance of a trait. (e.g., red flowers, tall plants). Remember: Phenotype = Physical. 🌸
• Genotype: The genetic makeup of a trait (the specific alleles). (e.g., AA, Aa, aa). Remember: Genotype = Genes. 🧬
• Allele: A version of a gene (dominant or recessive). Most genes have two alleles.
• Homozygous Recessive: Two recessive alleles (e.g., aa). Only this genotype shows the recessive phenotype. 🔑
• Dominant: An allele that masks the expression of a recessive allele (e.g., A). 💪
• Recessive: An allele that is masked by a dominant allele (e.g., a). 🙈
• Homozygous Dominant: Two dominant alleles (e.g., AA). Shows the dominant phenotype.
• Heterozygous: One dominant and one recessive allele (e.g., Aa). Shows the dominant phenotype. 🎭

#Punnett Squares: Predicting Inheritance

• Punnett squares are tools to predict the probability of offspring genotypes and phenotypes.
• They work because of Mendel's laws.

#Example: Monohybrid Cross

• Let's say we cross a heterozygous (Yy) and homozygous recessive (yy) individual:

  
  • Caption: A Punnett square showing a cross between a heterozygous (Yy) and homozygous recessive (yy) individual. The offspring have a 50% chance of showing the dominant phenotype and a 50% chance of showing the recessive phenotype.

• The offspring will have a 50% chance of showing the dominant phenotype (yellow) and a 50% chance of showing the recessive phenotype (green).

#Non-Mendelian Inheritance: When Things Get Tricky

• Not all traits follow simple dominant/recessive patterns. These are called non-Mendelian traits.
• Punnett squares can still be used, but the ratios might be different.

#Dihybrid Crosses: Two Traits at Once

• Dihybrid crosses examine the inheritance of two genes at the same time.

• The Punnett square will have 16 boxes.

  
  • Caption: A Punnett square showing a dihybrid cross. Note the 9:3:3:1 phenotypic ratio.

• Phenotypic Ratio: A dihybrid cross between two heterozygotes usually results in a 9:3:3:1 phenotypic ratio. (Remember, this is for phenotypes, not genotypes!) 💡

#Sex-Linked Traits: Genes on the X Chromosome

• Sex-linked genes are located on the X or Y chromosomes. Most are on the X chromosome.

• Examples: color blindness, hemophilia.

• Males (XY) are more likely to be affected by X-linked recessive traits because they only have one X chromosome. ♂️

• Females (XX) need two copies of the recessive allele to express the trait.

• A female with one copy of a recessive X-linked allele is a carrier. She doesn't express the trait but can pass it on to her children. 👩‍👧‍👦

  
  • Caption: A Punnett square showing a cross between a carrier female and a normal male. Note the different probabilities for male and female offspring.

#Final Exam Focus

• High-Priority Topics:
  • Mendel's Laws (segregation, independent assortment)
  • Punnett squares (monohybrid, dihybrid, sex-linked)
  • Key vocabulary (phenotype, genotype, allele, homozygous, heterozygous, dominant, recessive)
  • Non-Mendelian inheritance patterns
• Common Question Types:
  • Predicting offspring genotypes and phenotypes from Punnett squares.
  • Analyzing inheritance patterns from pedigrees.
  • Applying probability rules to genetic crosses.
  • Identifying sex-linked traits and their inheritance patterns.
• Last-Minute Tips:
  • Read each question carefully! Don't rush.
  • Draw out Punnett squares to visualize the crosses.
  • Double-check your work, especially your calculations.
  • Stay calm and confident. You've got this! 💪

#Practice Questions

Practice Question

Multiple Choice Questions:

1. In a certain species of plant, the allele for tall stems (T) is dominant over the allele for short stems (t). If a heterozygous plant is crossed with a homozygous recessive plant, what is the probability of the offspring having short stems? a) 25% b) 50% c) 75% d) 100%

2. A woman who is a carrier for a sex-linked recessive trait marries a man who does not have the trait. What is the probability that their son will have the trait? a) 0% b) 25% c) 50% d) 100%

Free Response Question:

In fruit flies, gray body color (G) is dominant to black body color (g), and normal wings (W) are dominant to vestigial wings (w). A cross is made between a heterozygous gray-bodied, normal-winged fly and a black-bodied, vestigial-winged fly.

(a) What are the genotypes of the parents? (b) Draw a Punnett square to show all possible genotypes of the offspring. (c) What is the phenotypic ratio of the offspring? (d) If a cross between two of the offspring with the genotype GgWw is made, what is the probability of getting a fly with a black body and vestigial wings?

Scoring Breakdown:

(a) (2 points) - Parent 1 genotype: GgWw (1 point) - Parent 2 genotype: ggww (1 point)

(b) (4 points) - Correct Punnett square with 16 boxes (1 point) - Correct genotypes of the offspring (3 points)

(c) (2 points) - Phenotypic ratio: 1:1:1:1 (2 points)

(d) (3 points) - Correctly calculate the probability of getting ggww offspring from GgWw x GgWw cross (3 points)

Combined Unit Question:

3. Explain how the concepts of meiosis and Mendel's Law of Segregation are related. How does this process contribute to genetic variation in offspring?

Alright, you've got this! Go get that 5! 🚀