Transmission Genetics 2

Transmission Genetics Overview

Lecture and Lab Assignments

Review of Genetic Principles

Probability Calculations

  • Students must be able to:

    • Calculate probabilities

    • Understand and apply product and sum rules

    • Interpret genotypes and phenotypes

Monohybrid Cross Review

  • Example: In pea plants, round seeds (R) are dominant to wrinkled seeds (r). Crossing two heterozygous plants (Rr × Rr).

  • Questions:

    • Probability of producing a wrinkled seed (rr)?

    • Probability of producing a round but heterozygous plant?

  • Key Principle: Law of Segregation must be applied.

Limitations of Punnett Squares

  • Giant Punnett Squares Not Used: Instead, utilize the product and sum rules.

    • Product Rule: Represents “AND” scenarios in genetic crosses.

    • Sum Rule: Represents “OR” scenarios.

    • Law of Independence: Important for understanding genetic outcomes.

Dihybrid Cross Review

  • Traits in Peas: Round (R) is dominant to wrinkled (r); yellow (Y) is dominant to green (y).

  • Cross: Two heterozygous plants (RrYy × RrYy).

  • Questions:

    • Probability of producing a round, yellow seed?

    • Probability of producing a wrinkled, green seed?

    • Probability of producing a round seed regardless of color?

Understanding Sex-Linked Traits

Basic Concepts

  • Historical Context: Mendel's traits chosen were autosomal, unaffected by sex chromosomes.

  • Sex Chromosomes: Some organisms exhibit traits dependent on sex chromosomes, leading to deviations from expected 3:1 ratios (e.g., red/white eye color in Drosophila).

Differences in Trait Expression

  • Sex Determination:

    • Organisms may have sex determined by specific homologous pairs.

    • Female: X/X (homogametic)

    • Male: X/Y (heterogametic)

  • Note on Complexity: The understanding of sex determination is nuanced, with potential for chromosomal disorders, leading to combinations like X/0, X/X/X, X/X/Y.

X and Y Linked Traits

  • X-Linked Traits: Genes that exist on the X chromosome but not on the Y are classified as X-linked.

  • Y-Linked Traits: Genes that exist on the Y chromosome but not on the X are classified as Y-linked.

Example of X-Linked Trait in Animals

  • Case Study: "unicorn" gene that produces horns on horses with the allele for horns being recessive (h) to normal (H).

  • Questions:

    • Can a male exhibit the horn trait if X-linked? How many copies would it require? What about females?

    • Can a male exhibit the horn trait if Y-linked? What about females?

Recessive and Dominant Systems in Sex-Linked Traits

  • Z-Linked Traits in Males:

    • Single allele will dictate phenotype.

  • X-Linked Traits in Females:

    • Must consider dominant and recessive alleles.

  • Y-Linked Traits:

    • Cannot manifest in females; one allele dictates phenotype in males.

Notation and Examples for Sex-Linked Traits

  • Example with Beetles: Body color notation

    • Yellow body color is X-linked and recessive:

    • Heterozygous female: XG Xg

    • Female with yellow body: Xg Xg

    • Male with yellow body: Xg Y

    • Gametes: Xg (yellow allele), XG (wild-type), Y (Y-gamete).

Punnett Squares for Sex Chromosomes

  • Example Square for X-linked recessive trait (Bedhead Hair):

    • Normal: B

    • Bedhead: b

    • Punnett layout:

    • Cross: XBXb × XBY → Outcomes: XBXb (carrier female), XBY (normal male), XbY (affected male).

  • Reinforce rules of thumb regarding X-linked dominant and recessive traits:

    • Affected fathers pass to all daughters, no sons affected.

    • Affected mothers can pass to all sons.

Common X-Linked Traits in Humans

  • Notably recessive conditions include:

    • Red-green color blindness

    • Hemophilia A (Factor VIII deficiency)

    • Hemophilia B (Factor IX deficiency)

    • Duchenne muscular dystrophy

    • Becker muscular dystrophy

    • G6PD deficiency

Probability Example: Hemophilia Transmission

  • Genotype Representation:

    • XH = normal allele

    • Xh = hemophilia allele (recessive)

  • Parental Genotypes:

    • Mother: carrier → XH Xh

    • Father: affected → Xh Y

  • Offspring Genotype Outcomes:

    • Punnett Square Analysis yields: XHXh (carrier daughter), XhXh (affected daughter), XHY (normal son), XhY (affected son).

Application of Knowledge in Genetic Analysis

  • Mendel’s experimental crossings established the foundation for understanding inheritance patterns.

  • For humans, crosses are not feasible; thus, inheritance is inferred through family pedigrees (recorded ancestry of a person or family).

Pedigree Analysis

  • Visual Examples: Family trees showing inheritance patterns with symbols to denote traits.

    • Female: ○ , Male: ■

    • Expressed traits vs non-expressed traits are shown through filled or empty symbols.

    • Connections for deceased, parents, siblings, and twins, and generations using appropriate notation.

Pedigree Algorithms for Inheritance Modes

  • Inheritance modes to discern:

    • X-Linked Dominant

    • X-Linked Recessive

    • Y-Linked

    • Autosomal Dominant

    • Autosomal Recessive

  • Process Steps:

    1. Determine if trait can be Y-linked.

    2. Ascertain if dominant or recessive.

    3. If dominant, determine if X-linked or autosomal.

    4. If recessive, decide if X-linked or autosomal

Detailed Pedigree Analysis Steps

  • Dominance Assessment:

    • If two affected parents have unaffected offspring, the trait is dominant.

    • If unaffected parents have affected offspring, the trait is recessive.

  • Stepwise Analysis in Pedigree:

    • Examine X-linked traits for father to son transmission and offspring patterns.

Practice Questions

Suggested Optional Exercises

  • Chapter 2: 2, 8, 10, 12, 14, 16, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 44

  • Chapter 3: 2, 4, 8, 10, 12, 14, 16, 18, 22, 24, 26, 28, 30