PPT 9

Lecture Overview

• Date: 8/14/2023

• Lecture Title: Little Johnny inherited the lazy gene

• Lecture Focus: Mendelian genetics, inheritance patterns, and experiments

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• Title: "8/14/2023 Lecture 9 Little Johnny inherited the lazy gene"

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Gregor Mendel

• Background:

  • Born in Czech Republic, small farm.

  • Conducted agricultural training in basic school.

  • Attended university, became a monk, and later a teacher.

  • Started research on pea plants in 1865.

• Research Insights:

  • Lacked cellular knowledge; tracked traits and made predictions.

  • Chose pea plants for characteristics that are easy to track.

Mendelian Genetics

• Key Findings:

  • 7 inherited characteristics that do not blend:

    1. Flower color: red or white (never pink)

    2. Position

    3. Seed color

    4. Shape

    5. Pod shape

    6. Color

    7. Stem length

• Definitions:

  • Character: Heritable feature varying among individuals.

  • Traits: Variations of characters (e.g., purple flower).

  • Gene: Code for a characteristic (e.g., eye color).

  • Alleles: Alternative versions of a gene.

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Research Methods

• Crossing Pea Plants:

  • Scientists study inheritance by mating true-breeding varieties.

  • Example:

    • Pollen from white flower -> purple flowers in F1 generation.

    • Results confirmed with reciprocal crosses (purple to white).

Results of Mendel's Crosses (Table 14.1)

• Character Traits and Ratios (F2 Generation):

  • Flower color (purple:white) = 705:224 (3.15:1)

  • Flower position (axial:terminal) = 651:207 (3.14:1)

  • Seed color (yellow:green) = 6022:2001 (3.01:1)

  • Seed shape (round:wrinkled) = 5474:1850 (2.96:1)

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Gene Locus

• Each gene has a specific location (locus) on a chromosome.

• Loci: Plural of locus, remains constant across individuals.

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Homologous Chromosomes

• Each diploid organism has two loci for each gene.

• Homologous chromosomes carry same genes, may hold different alleles (e.g., eye color).

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Genetic Terminology

• Alleles: Dominant and recessive traits.

  • Dominant Allele: Expressed in appearance, written in uppercase (e.g., A).

  • Recessive Allele: Not visible in appearance, written in lowercase (e.g., a).

• Homozygous: Two identical alleles (e.g., AA or aa).

• Heterozygous: Two different alleles (e.g., Aa).

• Phenotype: Outward expression (e.g., purple flowers).

• Genotype: Genetic makeup/allele pattern.

Alleles Summary

• Combinations:

  • Purple = Dominant (P), White = Recessive (p)

  • Homozygous Dominant: PP

  • Homozygous Recessive: pp

  • Heterozygous: Pp

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Mendel’s Laws

1. Law of Segregation:

   • Alternative gene versions (alleles) cause variation.

   • For each character, an organism inherits two alleles (one from each parent).

2. Law of Independent Assortment:

   • Genes assort into gametes in all possible combinations.

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Mendel’s Model: Key Concepts

1. Inherited characteristics controlled by discrete units (genes).

2. Individuals possess two alleles for each gene (diploid).

3. Law of Segregation: Alleles segregate during gamete formation.

4. Alleles combine at fertilization giving a new individual two alleles.

Modern Explanation

• Based on meiosis:

  • Diploid organisms produce haploid (n) gametes, resulting in 4 sex cells.

  • Law of Segregation: Distinct separation of maternal and paternal alleles during anaphase I.

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Inquiry Experiment: Mendel’s Findings

• Investigated F2 generation from self-pollination of F1 hybrids.

• Crossed true-breeding purple and white plants. Result F1 hybrids were purple, then observed F2 generation for flower color.

• Ratios:

  • 705 purple to 224 white.

  • Gametes: Each true-breeding parent has two identical alleles.

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Punnett Squares

• Standard Heterozygous Cross Example:

  • Monohybrid Cross: Phenotypic ratio 3:1 (AA x aa).

• Dihybrid Cross: Focuses on two traits with expected ratio 9:3:3:1.

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Caution on Ratios

• Observed ratios are statistical outcomes of many crossings, not individual crosses.

• Emphasizes understanding of probability in genetics.

Genetic Problem-Solving Procedure

1. Read the problem; clarify questions.

2. Determine gene notation and alleles.

3. Set up genotypes for phenotypes.

4. Define parent genotypes.

5. Use Punnett square for genetic crosses and record results.

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Sample Genetics Problems

1. Albinism probability (Harry and Sally case) setup.

   • Notation: A and a

   • Parents: Harry = aa, Sally = Aa

   • Set up and analyze Punnett square.

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Problem Case Study: Tay Sachs Disease

• Genetic risk assessment for future children with Tay Sachs in Clara and Robert's family.

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Rules of Probability in Genetics

• Independent Events:

  • Probability of two independent events together equals product of separate probabilities.

• Mutually Exclusive Events:

  • Probability of one or another event equals sum of individual probabilities.

Summary Example

• Children probability analysis (e.g., coin toss, girl/boy situation).

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Understanding Probability in Family Planning

• Genetics case analysis for Harry and Sally, both carriers for Cystic Fibrosis.

• Outcome Probability: 1/16 chance of both children having CF based on genetic cross.

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Dihybrid Cross Exploration

• Diagrams in dihybrid crosses indicate that two traits focus independently, establishing Mendel's Law of Independent Assortment.

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Dihybrid Cross Mechanisms

• Characteristics and examples of dihybrid crosses examined.

  • e.g. Black and brown guinea pigs.

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Dihybrid Punnett Squares

• Example of gamete combination calculations from genotype to phenotype ratios.

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Summary of Dihybrid Cross Outcomes

• Explanation of gamete segregation and phenotypic outcomes in F2 generations.

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Gamete Production Calculation

• Example showing how to determine gametes from various genotypes and heterozygote indications.

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Review of Mendel's Laws

• Summary of Mendel's findings on segregation and independent assortment through testing and gamete formation processes.