Chapter 12: Genes, Chromosomes, & Heredity - In-Depth Notes

I) Vocabulary Needed

Homologous Chromosomes (Homologs)

• Homologous pairs consist of 23 pairs of chromosomes, with each pair appearing similar in structure but containing different genetic information that contributes to genetic diversity.

• The first 22 pairs are referred to as somatic chromosomes or autosomes, while the 23rd pair represents the sex chromosomes (X or Y).

• Locus: A specific position on a chromosome where a gene resides, which serves as a marker for genetic mapping and studying gene functions.

• Allele: Various forms or mutations of a gene that govern the same trait, illustrating how genetic variation can lead to differences among individuals.

• Homologous chromosomes share the same gene at corresponding loci but can carry different alleles, thus influencing an organism's phenotype.

Homozygous vs Heterozygous

• Homozygous: When both alleles of a gene are identical, either dominant or recessive (e.g., (ww) or (WW)).

• Heterozygous: When the alleles of a gene are different, one dominant allele and one recessive allele (e.g., (Ww)), leading to the expression of the dominant trait.

Dominant vs Recessive Alleles

• Dominant allele: The allele that is expressed in the phenotype even when only one copy is present in a heterozygous formation (represented in uppercase letters, e.g., (N)).

• Recessive allele: An allele that requires two copies (homozygous formation) to be expressed in the phenotype; it does not manifest in the presence of a dominant allele (represented in lowercase letters, e.g., (n)).

Genotype vs Phenotype

• Genotype: The entire genetic makeup of an organism, encompassing all alleles present (e.g., (Ww, ww)).

• Phenotype: The visible or expressed traits of an organism, which may include physical characteristics and physiological properties like color or behavior (e.g., widow's peak).

II) Mendelian Genetics

Shorthand Representation of Genotypes: Genotype representation largely simplifies understanding genetics and inheritance patterns using various allele combinations.

Monohybrid Cross (1 trait):

• Example: Parental Genotypes = (Ww), which implies traits are derived from two different alleles received from each parent.

• Gamete genotypes lead to different potential offspring genotypes through a systematic approach known as a Punnett Square.

Punnett Square Example for Monohybrid Cross

• Parental Gametes:

  • (W) x (w)

• Offspring ratios can be evaluated from this cross demonstrating inheritance patterns, such as:

  • E.g., (1/4 WW, 1/2 Ww, 1/4 ww) indicates that 25% of the offspring are homozygous dominant, 50% are heterozygous, and 25% are homozygous recessive.

Dihybrid Cross (with unlinked loci):

• This approach examines two traits simultaneously, each trait being determined by two alleles.

• For example, the genotype (WwNn) considers traits such as Widow’s peak (hairline shape) and cystic fibrosis (a genetic disorder).

Punnett Square Example for Dihybrid Cross

• Parental Gametes:

  • (WN), (Wn), (wN), (wn)

• Offspring ratios are calculated in a similar manner to monohybrid crosses, examining combinations of both traits to yield different phenotype ratios in offspring.

Linked vs Unlinked Genes:

• Linked Genes: Genes that reside closely together on the same chromosome tend to be inherited together, affected by recombination rates during meiosis.

• Unlinked Genes: These genes are situated on different chromosomes and assort independently during gamete formation, following Mendel's laws of inheritance.

III) Gene Interactions

Incomplete Dominance:

• This genetic phenomenon occurs when the dominant allele is not completely expressed, leading to a blend of traits in the phenotype.

• Example: Hair color, where the allele for brown (B) combines with an absence of brown (b), resulting in:

  • Incomplete Dominance Example:

  • Genotypes yield phenotypes:

  • (BB): brown, (Bb): tan, (bb): beige.

Codominance:

• In this interaction, both alleles in a heterozygous genotype are fully expressed, contributing distinctly to the phenotype.

• Example: ABO blood types demonstrate distinct contributions of different alleles such as:

  • Codominance Example:

  • Alleles: (IA, IB, i), where (IAIB) genotypes result in Type AB blood, showcasing the expression of both A and B antigens.

Pedigree Analysis:

• This method tracks inheritance patterns of genetic traits throughout generations, providing insight into the likelihood of genetic conditions appearing in descendants.

• For instance, analyzing the genotypes of parents where one has Type AB and the other Type O can help identify possible genotypes of offspring.

Polygenic Inheritance:

• Traits influenced by multiple genes exhibit a spectrum of phenotypes rather than clear-cut distinctions.

• Examples can be seen in human characteristics such as skin color and height, where various loci contribute quantitatively to the phenotypic expression.

Polygenic Inheritance Example:

• Many loci contribute to complex traits; the resultant frequency distribution across a population indicates widespread variability.

Pleiotropy:

• This occurs when a single gene impacts multiple phenotypic traits, reflecting the multifaceted effects that certain genes can have on the organism.

• For example, a gene may affect pigmentation in skin, hair, and eyes simultaneously.

Epistasis:

• One gene’s expression can modify or mask the expression of another gene, illustrating the complex interactions between different genes.

• An example in dogs: in labradors, one gene determines fur color while another gene influences pigment distribution leading to different phenotypic outcomes.

Penetrance:

• This denotes the percentage of individuals with a particular genotype that express the associated phenotype, illustrating variability in expression.

• Example: Huntington's Disease showcases a high penetrance of 95%, meaning most individuals carrying the allele exhibit symptoms of the disease.

Expressivity:

• This refers to the extent to which a genotype is expressed in phenotypes, revealing the degree of variation in trait manifestations among individuals with the same genotype.

• Example: Polydactylism highlights varying expressivity, where individuals may have additional digits with differing degrees of severity.

Influences on Phenotype

• Environmental factors such as temperature, nutrition, and light interact with genetic predispositions, significantly influencing the expression and development of phenotypic traits, emphasizing the complexity of phenotype development beyond just genetic inheritance.