Notes on Genes and Inheritance Module
Module 10: Genes and Inheritance - BIOL 1345
Learning Objectives (Study Guide)
By the end of this lesson, you will be able to:
Explain the difference between phenotype & genotype.
Explain the effects of chromosome segregation & independent assortment on gamete formation and the inheritance of alleles.
Explain the difference between dominant & recessive alleles.
Explain the difference between homozygous & heterozygous genotypes.
Use the genotype of an organism to predict its phenotype.
Identify the sex of individuals in a pedigree.
Use a pedigree to predict the genotype of individuals.
Use a pedigree to identify the inheritance pattern of a trait.
Explain how autosomal traits & X-linked traits are inherited differently.
Explain how incomplete dominance affects gene expression.
Explain how a quantitative trait is expressed.
Explain how the inheritance of linked genes is different from the inheritance of unlinked genes.
Draw Punnett squares to estimate phenotype and genotype probabilities.
How Does Someone Get Type I Diabetes?
Type I Diabetes is characterized as an inherited autoimmune disorder where the immune system attacks the beta cells of the pancreas.
Consequences:
The beta cells cease insulin production.
Resulting high blood sugar levels since glucose cannot be taken up by cells.
Cells become deprived of glucose, prompting the liver to release additional glucose into the bloodstream.
Genetics
Genetics: The field of science that studies genes, focusing on:
How genes are inherited
How genes are expressed
The modern field has roots in the experiments conducted with pea plants by Gregor Mendel, a German-Czech friar who selectively bred thousands of pea plants.
Mendel made observations on their physical characteristics (phenotype) to predict their genetic makeup (genotype).
Mendel's conclusions accurately reflect current molecular observations, even though he did not know about chromosomes or DNA.
Meiosis: A Review
Meiosis is the process that produces haploid cells known as gametes.
Each gamete contains one of each chromosome.
Upon fertilization, two haploid gametes combine to form a diploid cell, which undergoes mitosis to develop into a functioning multicellular organism, inheriting genetic information from both parents.
Genetics Terminology
Phenotype: The physical appearance or measurable attribute resulting from gene expression (e.g., purple vs. white flowers).
Genotype: The specific allele combination for a particular gene (e.g., PP, Pp, or pp).
Important Note: Do not mix letters when representing one trait.
Genotypes
Types of Genotypes:
Homozygous Genotype: Contains two of the same allele.
Homozygous Dominant: Two copies of the dominant allele (e.g., PP).
Homozygous Recessive: Two copies of the recessive allele (e.g., pp).
Heterozygous Genotype: Contains two different alleles (e.g., Pp).
Allele Representation:
Uppercase letters denote dominant alleles.
Lowercase letters denote recessive alleles.
Overview of Mendel’s Experiments
Mendel’s Model System: Studied inheritance of traits using pea plants. Pea plants self-fertilized, yielding true-breeding (homozygous) lines in the parental (P) generation.
He performed manual crosses to create hybrids (F1 generation) which were then allowed to self-fertilize producing the F2 generation.
Pea Traits
Trait: A variation in the physical appearance of a heritable characteristic, selected traits were discrete with two clear phenotypes.
Mendel’s Conclusions
Conclusion #1: Law of Dominance:
Some alleles are dominant, meaning they are expressed visibly.
Some alleles are recessive, which do not manifest if a dominant allele is present.
To express recessive alleles, an individual must have two copies of that allele.
Important Note: "True-breeding" refers to homozygous lines.
Let’s Practice!
Determining Dominant Allele: Based on phenotypes and genotypes (e.g., identifying between purple and white alleles).
Identify whether they are homozygous or heterozygous for the petal color gene.
Conclusion #2: The Law of Independent Assortment:
Alleles for each gene are inherited independently from one another.
Inheriting a specific allele for one gene does not determine the allele inherited for another gene on a different chromosome.
Conclusion #3: The Law of Segregation:
Despite individuals possessing two copies of each gene, these alleles are separated during gamete formation.
Offspring inherit only one allele from each parent for each gene.
Let’s Practice: Punnett Squares
Draw Punnett squares for the P generation and F1 generation cross, listing chances for both genotype and phenotype outcomes.
Reminder: A heterozygous genotype leads to the dominant phenotype due to the dominance of one allele over the other.
Summary of Mendel’s Work: The Laws of Heredity
Law of Dominance: Alleles can be dominant or recessive.
Law of Segregation: Individuals have two alleles and pass on one randomly to offspring.
Law of Independent Assortment: Traits are inherited independently of one another, without influencing each other's inheritance.
Mendel’s work spanned eight years and included 28,000 plants, leading to greater recognition posthumously approximately 20 years after his death.
Autosomes vs. Sex Chromosomes
Autosomes: Contain genes regulating general functions, metaphorically numbering 1-22 in humans.
Sex Chromosomes: Involve genes critical for sex determination (XX in females and XY in males), but also encompass non-sex-specific functions like color vision and blood clotting.
Inheritance Patterns: Pedigrees
Geneticists employ pedigrees to chart allele inheritance patterns in families, where:
Males are represented as squares (□) and females as circles (Ο).
Horizontal lines denote mating; filled shapes signify individuals expressing a trait (often disorders).
Autosomal Dominant Disorders
Illustrated Family Tree: In autosomal dominant disorders, both males and females are equally impacted, typically appearing in all generations without skipping.
Autosomal Recessive Disorders
Illustrated Family Tree: In autosomal recessive disorders, both genders are equally affected, and the trait may skip generations.
Let’s Practice!
For numbered individuals (i.e., #1, #2, #3), determine their sex and genotypes.
Sex-linked Traits
Definition: Traits correspond to genes situated on the X or Y chromosome and are termed sex-linked.
Females possess two X chromosomes, compared to males with one X chromosome.
Inheritance of Sex Chromosomes
Males transmit their Y chromosome solely to sons, meaning they cannot pass on X-linked disorder alleles to them.
Males pass their X chromosome to daughters, which means that affected males will always pass on X-linked disorder alleles to daughters.
X-linked Recessive Disorders
An affected male passes the X-linked recessive disorder to all daughters (carriers), but none of the sons inherit the allele.
For a carrier female, there is a 50% probability that each son will be affected and each daughter will be a carrier.
Example: Color Blindness
Color blindness is classified as an X-linked recessive trait, where about half of the sons from a carrier mother are anticipated to be color-blind, while half of the daughters are expected to be carriers.
X-linked recessive disorders are predominantly more common in males and may “skip” generations through carrier females.
Beyond Simple Dominance and Recessiveness
While many genes exhibit predicted dominance/recessiveness, there are occasions where:
Alleles do not adhere strictly to the dominant or recessive framework.
An individual's genotype does not wholly elucidate their phenotype.
Incomplete Dominance
Expressing traits under incomplete dominance indicates neither allele is fully dominant, thus heterozygotes reveal an intermediate phenotype.
Example: Human hair texture is a trait where:
Straight hair (c) and curly hair (C) are alleles.
Homozygous individuals possess either straight or curly hair; heterozygotes showcase a distinct wavy hair phenotype, positioned between straight and curly.
Quantitative Traits
Quantitative Traits: Characterized by a broad range of phenotypic possibilities existing along a continuum, without distinct types.
Example: Human skin color is modulated by three genes, each with two alleles, creating 64 potential allele combinations from parental gametes.
Linked Genes
Mendel’s Law of Independent Assortment applies if genes are situated on different chromosomes.
However, genes located on the same chromosome are termed linked genes, typically inherited together; crossing over can separate linked gene alleles.
Example: Hair & Eye Color
Genes influencing hair and eye color, such as OCA2 and HERC2, reside on Chromosome #15.
Because of genetic linkage, the combinations of alleles from parents are often inherited intact, explaining commonalities in hair and eye color pairings.
To Prepare for the Exam
Review class notes, utilizing the eTextbook and other resources to augment lecture materials.
Complete homework, revisiting difficult concepts proactively.
Reflect on learning progress throughout the semester as the course approaches completion.