Ch.5 Gene Interaction and Mapping Study Guide

Biology 22: Fall 2024 - Gene Interaction and Mapping Study Notes

Course Overview

  • DNA LAB

  • DARK SIDE of molecular diagnostics

  • Established in 1977

Goals

  • Review non-Mendelian genetics:

    • Single gene genetics

    • Multiple gene genetics

    • Sex and genes

    • Genes on chromosomes

    • Environmental effects

Preliminary Notes

  • Math will be utilized throughout the lesson; students should have scratch paper available.

Non-Mendelian Genetics

Single Gene Non-Mendelian Genetics

Incomplete Dominance

  • Definition: A form of inheritance where heterozygous alleles lead to a blended phenotype.

    • Example: Crossing red (RR) and white (WW) tulips yields pink tulips (RW).

    • Important Concept: This occurs because there's a mix of alleles rather than one being completely dominant or recessive.

Codominance

  • Definition: A genetic scenario wherein both alleles are fully expressed in the phenotype.

  • Example: Crossing pink (RW) and white (WW) camellias produces flowers exhibiting both pink and white patches (RW).

  • Key understanding: Not all alleles fit into the dominant-recessive framework; codominance shows simultaneous expression.

Blood Type Genetics

  • Blood types are categorized by the presence of A and/or B antigens on red blood cells (RBCs).

    • Type O: No antigens present.

  • Important genotypes:

    • A: $I^A I^A$ or $I^A i$

    • B: $I^B I^B$ or $I^B i$

    • AB: $I^A I^B$ (co-dominant type)

    • O: $ii$

Blood Type Allocation

  • A and B antigens represent co-dominance; they can express together if present in genotype.

  • If neither antigen is present, the blood type is O.

Example Problem: Cross Analysis

  • Example Cross: $AO imes BO$ yields

    • 1/4 Type AB

    • 1/4 Type B

    • 1/4 Type A

    • 1/4 Type O

  • Caller engagement: Asking about parents' blood types generates awareness of the genetic implications in blood inheritance patterns.

Rhesus Factor

  • Definition: The Rh blood group system involves proteins on RBCs; Rh(D) is a primary focus as it is completely dominant over recessive (Rh-).

Rh Incompatibility Risks in Pregnancy

  • A mother's Rh-negative blood type with an Rh-positive fetus leads to potential complications like

    • Jaundice

    • Liver failure

    • Heart failure

    • Neurological damage or fetal death

  • Specific at-risk scenarios occur during the second pregnancy with Rh-positive offspring leading to a risk level of approximately 15%.

RhIg Treatment

  • RhIg (Rho(D) immune globulin) is given to Rh-negative mothers during their first pregnancy; it prevents antibody formation against Rh-positive cells by essentially 'tricking' the immune system.

  • RhIg must be administered before the mother makes Rh(D) antibodies to be effective.

James Harrison Case Study

  • James Harrison's blood displays high concentrations of Rho(D) antibodies, leading to a lifelong commitment to blood donation until age limitations were enacted.

Lethal Alleles

  • Definition: Genotypes that lead to early embryonic death or abortive offspring.

  • Important concept: Identification of lethal alleles is made when a genotype does not appear in offspring sets.

Dominance Hierarchy

  • A scenario where multiple alleles of a gene exist with specific dominance relationships.

  • Example: In ducks, allele hierarchy shows MR (dominant) > M (moderate) > md (least dominant).

  • Practice Question: If a restricted duck genotype (MRM) mates with another (MRmd), assess potential offspring phenotypes.

Pleiotropy

  • Definition: The effect of a single gene on multiple phenotypes.

  • Example: A mutation in the beta-globin gene causing sickle cell anemia can influence

    • Blindness

    • Liver dysfunction

    • Heart attack susceptibility.

Molecular vs Physiological Dominance

  • Analysis between molecular functions (presence of proteins) and physiological traits (disease manifestation).

  • Example: Cystic fibrosis features a mutated CFTR protein affecting chloride regulation, having different outcomes at molecular versus physiological levels.

    • Heterozygous carriers expressing both normal and mutated CFTR exhibit co-dominance molecularly but are recessive physiologically since they do not manifest the disease.

Multiple Gene Non-Mendelian Genetics

Polygenic Traits

  • Definition: Traits influenced by two or more genes; often displayed in continuous distributions.

  • Examples include height, skin color, and eye color.

Epistasis

  • Occurs when one gene affects the expression of another.

  • Analogy: A light switch controlling a bulb, where the switch represents the epistatic gene and the bulb the hypostatic gene.

Recessive Epistasis Example

  • In Labrador retrievers, coat color depends on two genes:

    • Gene 1 influences color directly (B for black, b for brown)

    • Gene 2 controls color deposition (C for color, c for no deposition).

  • Genotypes yielding variations in coat color outcomes.

    • Example practice problem: Mom = BbCc, Dad = bbcc; determine colors of puppies from this genetic cross.

Relation to ABO Blood Type

  • The ABO blood group system involves a recessive epistatic gene influencing the A and B antigens dependent on the presence of H, sullying blood type O as a result of the absence of sugar addition.

Duplicate Recessive Epistasis

  • Description: This involves two levels of epistasis, where both enzyme functionality is required for trait production.

  • Example in snails whose color depends on both enzyme production pathways.

  • Practice problem: Crossing a snail with genotype aaBb and Aabb; analyze the color percentages of offspring.

Dominant Epistasis

  • The epistatic gene works dominantly rather than recessively.

  • Practice Problems: Determine the probability of yellow offspring when crossing a yellow (WwYy) squash with a green one.

Sex and Genes

Sex-Influenced Traits

  • Traits expressed differently based on sex, such as Bb being dominant in males and recessive in females.

Sex-Limited Interaction

  • Definitions of Key Concepts:

    • Penetrance: % of individuals whose expressed phenotype matches their genotype.

    • Expressivity: The degree of manifesting a phenotype.

Cytoplasmic Inheritance

  • The egg predominantly contributes to the cytoplasm of the zygote, including organelles like mitochondria.

  • Example study on the random segregation of mutated versus wild-type mitochondria and the resulting phenotypic variations in cell divisions.

Genetic Maternal Effect

  • Maternal genotype dictates the embryo's phenotype, highlighted through breeding studies.

Genomic Imprinting

  • Mechanism through which certain genes exhibit different expressions based on whether they are inherited from the mother or father, primarily through methylation processes.

Sex-Linked Genes

  • Definition: Genes located on sex chromosomes; X-linked genes are notably more numerous than Y-linked genes.

  • Discussion points: Female (XX) allelic combinations and male (XY) hemizygosity in expressing sex-linked traits.

Punnett Squares Example

  • Representation of X-linked inheritance patterns through visual methods in Punnett squares.

Practice on Sex-Linked Pedigrees

  • Steps for analyzing sex-linked inheritance within familial structures to identify definitive patterns.

    • Evaluate male prevalence, maternal influence, and assess patterns derived from parental genotypes.

Examination of the Y-Chromosome

  • Analysis of the limited gene pool present on the Y-chromosome compared to the gene-rich X-chromosome, including evolutionary trends.

X-Chromosome Inactivation

  • The random inactivation of one X chromosome in females resulting in phenotypic mosaics, illustrated through case examples in cats demonstrating black and orange fur variations due to inactivation.

Environmental Effects on Genetics

Anticipation

  • The concept where traits become increasingly pronounced or expressed earlier with generational passage due to genetic accumulation.

External Influences on Phenotype

  • Environmental factors can greatly affect genetic expression, exemplified by hydrangea color variation due to soil acidity.

Nature vs. Nurture Expression

  • Temperature dependencies on certain phenotypic expressions, for example in Drosophila's vestigial mutation variation.

Conclusion

  • Comprehensive study on gene interactions, mapping, and effects influenced by multiple variables in genetic expression across different organisms is vital for understanding complex traits. This encapsulates Mendelian and non-Mendelian genetic principles which play significant roles in current genetics and biological research.