Mendelian Inheritance Laws/Patterns: Law of Segregation (First Law): Every individual has two
alleles for each trait, which separate (segregate) during gamete formation (meiosis). Each gamete
(sperm or egg) receives only one allele, meaning offspring receive one allele from each parent. Law of
Independent Assortment (Second Law): Genes for different traits are sorted independently of one
another. The inheritance of one trait does not affect the inheritance of another (e.g., seed color does not
influence seed shape). This applies to genes located on different chromosomes. Law of Dominance
(Third Law): In a heterozygote (an individual with two different alleles), one allele acts as dominant,
masking the presence of the recessive allele and determining the phenotype. Violations: Linked
Genes (Violates Independent Assortment): Genes located physically close together on the same chromosome tend to be inherited
together, rather than separating independently. Incomplete Dominance: Heterozygotes show an intermediate phenotype (e.g., pink flowers
from red and white parents) instead of one parent fully dominating. Codominance: Both alleles are expressed equally and simultaneously
in the phenotype, such as in AB blood type. Multiple Alleles: Instead of only two alleles (dominant/recessive), many genes have multiple
variations in a population. Polygenic Traits: Traits are controlled by more than one gene, causing a continuous range of phenotypes (e.g.,
skin color or height) rather than simple distinct groups. Epistasis: The interaction between genes where one gene masks or modifies the effect of another gene at a
different locus. Sex Linkage: Genes carried on sex chromosomes (X or Y) show distinct inheritance patterns compared to autosomal genes. Lethal Alleles: Certain
genotypes are fatal, altering the expected Mendelian ratios in offspring. Epigenetic Effects: Inherited modifications to DNA that affect gene expression without changing
the DNA sequence, which can break the rules of classical inheritance. Cytoplasmic Inheritance: Traits inherited through mitochondrial or chloroplast DNA, which are
inherited only from the mother. Constructing Punnett Squares: mono-use individual alleles, di-use pairs of alleles, tri-use trios of alleles. Epistasis-when an
unrelated gene modifies the depression of another gene. The suppressive gene is epistatic, while the suppressed gene is hypostatic. Recessive Epistasis (9:3:4 ratio):
The presence of two recessive alleles at one locus (homozygous recessive) hides the phenotype at a second locus (e.g., coat color in Labs). Dominant Epistasis (12:3:1
ratio): A dominant allele at one locus masks the phenotype of the second locus. Double Recessive Epistasis (9:7 ratio): Homozygous recessive alleles at either locus
produce the same phenotype. Double Dominant Epistasis (15:1 ratio): A dominant allele at either of the two loci produces the same phenotype. Antagonistic epistasis:
The combination of mutations has a fitness closer to the ancestral state than expected (diminishing returns). Synergistic epistasis: The combination of mutations has a
more severe impact on fitness than expected (exacerbating effects). Linkage: Genetic linkage is the tendency of genes located close together on the same
chromosome to be inherited together during meiosis. Complete Linkage: Occurs when two genes are so close that they are always inherited together, with no crossing
over observed. Incomplete Linkage: Occurs when genes are on the same chromosome but are far enough apart that crossing over occurs, allowing them to separate
sometimes. Sex Linkage: Refers to genes located on sex chromosomes (usually the X chromosome), causing traits to be linked to the sex of the organism. The strength
of linkage is measured by the recombination fraction, ranging from (complete linkage) up to (no linkage/independent assortment). A "LOD score" (logarithm of the odds)
of or higher is traditionally used as evidence that a linkage is not due to chance.
Map Genes Using Recombination Frequency- 1. Perform a Test Cross: Mate a heterozygous
individual with a homozygous recessive individual to observe traits. 2.Identify Offspring Phenotypes:
Identify parental types (look like parents) and recombinant types (new combinations of traits)3.Calculate
Recombination Frequency (RF): RF = ( 4.Convert
# 𝑜𝑓 𝑟𝑒𝑐𝑜𝑚𝑏𝑖𝑛𝑎𝑛𝑡 𝑜𝑓𝑓𝑠𝑝𝑟𝑖𝑛𝑔/𝑡𝑜𝑡𝑎𝑙 # 𝑜𝑓 𝑜𝑓𝑓𝑠𝑝𝑟𝑖𝑛𝑔) 𝑥100
RF to Map Distance: The resulting percentage equals the map units. Example: If the RF between genes A
and B is 15, they are 15 cM apart. 5.Determine Gene Order: Use distances from multiple pairs of genes to
determine their linear order on the chromosome, starting with
the highest RF (furthest apart). If A-C is 20 and B-C is 30, the
order is B-A-C because B and C are farthest apart.
Nondisjunction is the failure of homologous chromosomes or
sister chromatids to separate properly during cell division
(meiosis I, II, or mitosis), resulting in daughter cells with an
abnormal number of chromosomes (aneuploidy). It causes
genetic disorders like Down syndrome/Trisomy 21, Klinefelter
syndrome, and Turner syndrome/Monosomy X, often producing
gametes with an extra or missing chromosome.
Trisomy-embryo inherits extra chromosome.
Monosomy-embryo lacks chromosome. Many nondisjunction events are fatal to the embryo. Somatic recombination, or
V(D)J recombination, is a genetic mechanism in developing B and T lymphocytes where DNA segments (Variable, Diversity,
Joining) are cut and rejoined to create unique receptors. This process generates immense diversity in antibodies and T-cell receptors, allowing the adaptive immune
system to recognize countless pathogens. Occurs in the bone marrow (B cells) and thymus (T cells) to create unique antigen receptors. Specialized enzymes, RAG1 and
RAG2 (Recombination Activating Genes), cut DNA at specific sites (RSS motifs), followed by joining the segments.Diversity Generation: By randomly choosing and
joining V, D, and J segments—and adding random nucleotides at junctions—the immune system can generate an almost infinite variety of receptors. Unlike RNA splicing,
this is a permanent rearrangement of DNA within the lymphocyte's genome. The Hardy-Weinberg equilibrium is a principle stating that allele and genotype
frequencies in a population remain constant from generation to generation in the absence of evolutionary influences. It acts as a baseline model for non-evolving
populations, requiring no mutations, random mating, no gene flow, large population size, and no selection. Genetic drift is a
mechanism of evolution causing random, non-adaptive fluctuations in allele frequencies within a population over generations, having
the strongest impact on small populations. It reduces genetic variation by causing alleles to become fixed or disappear entirely by
chance, rather than through natural selection. Bottleneck Effect: A sharp reduction in
population size due to environmental events (e.g., natural disasters) or human activities,
leaving a small, random sample of survivors with limited diversity. Founder Effect: Occurs
when a small group splits off from a main population to establish a new colony, causing the
new population to have different allele frequencies than the original. Migration(gene flow)is
the transfer of genetic material (alleles) from one population to another through the movement
of individuals or gametes (e.g., pollen). It increases genetic variation within a population but
reduces genetic differences between populations, acting as a homogenizing force in evolution.
Relative fitness is the reproductive success of a specific genotype or phenotype compared to
the most successful variant in a population, usually denoted by 𝑤 on a scale from 0 to 1. It is
calculated as the absolute fitness of a genotype divided by the absolute fitness of the fittest
genotype. . Fittest genotype is w=1, while
𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝐹𝑖𝑡𝑛𝑒𝑠𝑠(ω)= 𝑎𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝑓𝑖𝑡𝑛𝑒𝑠𝑠 𝑜𝑓 𝑔𝑒𝑛𝑜𝑡𝑦𝑝𝑒
𝑎𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝑓𝑖𝑡𝑛𝑒𝑠𝑠 𝑜𝑓 𝑓𝑖𝑡𝑡𝑒𝑠𝑡 𝑔𝑒𝑛𝑜𝑡𝑦𝑝𝑒
other are 1-s(s being selection coefficient. Homolog(Homology): A general term for any gene
or protein that shares a common ancestor. Ortholog (Orthology): Genes in different species that evolved from a
common ancestor through speciation. They typically maintain the same function. Example: Human hemoglobin A and
mouse hemoglobin A. Paralog (Paralogy): Genes in the same or different species that evolved through duplication.
They often have different, though often related, functions.Example: Human hemoglobin A and human hemoglobin B.
Broad-sense heritability( ) is the proportion of total phenotypic variance( ) in a population caused by all genetic
𝐻2𝑉𝑝
differences, including additive, dominance, and interaction (epistatic) variances( ). Expressed as ,it
𝑉𝐺 𝐻2=𝑉𝐺/𝑉𝑃
measures the total genetic contribution to trait variation, ranging from 0 to 1, with higher values indicating stronger
genetic influence. Narrow-sense heritability ( is the proportion of total phenotypic variance attributable specifically
ℎ2)
