Inheritance patterns can be intricate and are often more complicated than those predicted by simple Mendelian genetics.
The relationship between genotype (genetic makeup) and phenotype (physical expression) is rarely straightforward.
The Spectrum of Dominance
Complete Dominance
Definition: Complete dominance occurs when the phenotypes of the heterozygote (individual with two different alleles) and the dominant homozygote (individual with two identical dominant alleles) are indistinguishable.
Example: In pea plants, purple flowers (homozygous dominant) and heterozygous individuals appear the same, both exhibiting purple flowers.
Multiple Alleles of a Single Gene in a Population
Concept: Variation in spelling of a word, such as different forms of the word "color" in various languages (e.g. color [US], colour [UK], couleur [France]).
Sources: Result from mutations.
Examples:
Blood Groups in Humans: Three alleles A, B, O.
Rabbit Hair Color: Four different alleles.
Confounding Features of Inheritance (CFI)
Co-dominance
Definition: In co-dominance, two dominant alleles contribute to the phenotype independently and distinctly.
Example: Human blood groups where IA and IB alleles are expressed simultaneously, resulting in AB blood type with six possible genotypes.
Co-dominance and Blood Groups
Blood Group Determination: The ABO blood type is determined by multiple alleles (IA, IB, and i).
Carbohydrates on blood cell surfaces depend on the allele:
IA and IB code for enzymes that add carbohydrates.
i is recessive to both IA and IB.
Universal Donor and Recipient
Universal Recipient: Type AB+ blood can receive from all blood types.
Universal Donor: Type O- blood can donate to all blood types.
Rh Factor Genes: Rh+ is dominant (presence of Rh factor) and Rh- is recessive (absence of Rh factor).
Order of Dominance in Rabbit Coat Color
Four alleles exist for rabbit coat color gene C:
C (fully dominant) > cch > ch > c (fully recessive).
Incomplete Dominance
Definition: Incomplete dominance occurs when the heterozygote exhibits a phenotype that is intermediate between those of the two homozygotes.
Example: Flower colors where red (CRCR) and white (CWCW) produce pink (CRCW) offspring.
Homozygous dominant (HH): Produces maximum wavy hair protein, resulting in curly hair.
Homozygous recessive (hh): Produces no wavy hair protein, resulting in straight hair.
Heterozygotes (Hh): Produce some wavy hair protein, leading to wavy hair phenotype.
Pleiotropy
Definition: Pleiotropy occurs when a single allele influences multiple phenotypic traits.
Example: Sickle cell disease leading to:
Deformed red blood cells
Physical weakness
Pain and organ damage
Extending Mendelian Genetics for Two or More Genes
Traits Influenced by Multiple Genes
Some traits are influenced by multiple genes leading to two different scenarios:
Epistasis: One gene at one locus can interfere with the expression of a gene at another locus.
Polygenic Inheritance: The additive effect of two or more genes influences a single trait.
Epistasis Example
Gene 1 (B/b) controls color (B = black, b = brown).
Gene 2 (E/e) controls expression:
E = ability to deposit pigment (black or brown).
e = inability (golden retriever).
Golden retrievers can be produced irrespective of the B gene if they possess homozygous recessive for E.
Polygenic Inheritance Example
Traits like skin color and height in humans display a range of phenotypes along a continuum, termed quantitative characters.
Genetic Crosses: Demonstrates that traits show variation dependent on multiple alleles:
Examples from genetic crosses with progeny outcomes.
Human Quantitative Traits
Findings from genomic studies illustrate the complexity of traits:
Over 253,000 individuals studied.
Identified 697 genetic variants explaining significant heritability for adult height.
Genes and pathways identified relating to growth and health implications.
Nature and Nurture: The Environmental Impact on Phenotype
Phenotypes result not solely from genetic factors but are also influenced by the environment, categorized as multifactorial characters.
Despite complexities, Mendel's principles of segregation and independent assortment remain applicable.
The Norm of Reaction
Definition: The norm of reaction describes the range of phenotypes possible from a single genotype influenced by environmental factors.
Example: The color of hydrangeas is affected by soil pH leading to variations in flower color based on acidic or alkaline conditions.
Mendelian Inheritance and Chromosomal Theory
The chromosomal basis supporting Mendelian inheritance describes that genes are located on chromosomes which undergo segregation and independent assortment during meiosis.
Morgan’s Work with Drosophila: Provided foundational evidence correlating loci of genes to specific chromosomes by studying eye color in fruit flies.
Chromosomal Basis of Sex
An organism’s sex is primarily determined by the presence or absence of specific chromosomes (XX for females, XY for males).
The impact of the SRY gene on the Y chromosome is crucial for male sexual development.
Sex-Linked Genes
Definition: Genes located on sex chromosomes, with most sex-linked traits found on the X chromosome.
Males exhibit all traits linked to their single X chromosome, leading to hemizygosity.
Examples of Sex-Linked Traits
Traits in Drosophila, like eye color, where the red trait is dominant over white.
Inheritance dynamics differ for males and females due to chromosome composition and combinations.
Genetic Disorders
Dominantly Inherited Disorders
Dominant alleles result in traits expressed even if one copy is present.
Examples:
Huntington's Disease: A progressive neurological disorder caused by a dominant allele.
Polydactyly: Presence of more than five digits due to dominant inheritance.
Recessively Inherited Disorders
Disorders typically manifest in individuals with homozygous recessive genotypes.
Carriers, individuals heterozygous for recessive alleles, do not display symptoms.
Examples include Sickle Cell Disease and Albinism.
Genetic Testing and Counseling
Genetic counselors help assess the risk of genetic disorders using family histories.
Various tests are available to identify carrier status for over 4,500 diseases, including:
Cystic Fibrosis, Huntington's Disease, and Sickle Cell Disease.
Prenatal Testing Methods
Amniocentesis: Testing amniotic fluid for genetic abnormalities.
Chorionic Villus Sampling (CVS): Testing placental tissue for genetic disorders.
Social Concerns about DNA Screening
Genetic information is sensitive and can carry implications for discrimination or privacy concerns.
The Genetic Information Nondiscrimination Act (GINA) protects individuals from discrimination based on genetic information in health insurance and employment situations.