Genetics Lecture 4: Extensions of Mendelian Genetics and Chromosome Mapping

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Extensions of Mendelian Genetics

Dominant and Recessive Traits

Inheritance patterns can be classified as dominant or recessive, depending on how alleles influence the phenotype. Understanding these patterns is fundamental to predicting genetic outcomes in offspring.

Dominant Trait: Expressed when at least one dominant allele is present (e.g., Aa or AA).
Recessive Trait: Expressed only when two recessive alleles are present (e.g., aa).
Punnett Square: Used to predict the probability of offspring inheriting particular genotypes and phenotypes.
Example: Polydactyly (extra fingers/toes) is a dominant trait. If a heterozygous individual (Aa) mates with a homozygous recessive (aa), half of the offspring are expected to show polydactyly.

Forms of Dominance

Dominance describes how alleles interact to produce a phenotype. There are several forms:

Type	Description
Complete Dominance	Phenotype of the heterozygote is the same as one of the homozygotes.
Incomplete Dominance	Phenotype of the heterozygote is intermediate between the two homozygotes.
Codominance	Phenotype of the heterozygote includes both homozygote phenotypes.
Multiple Alleles	More than two alleles exist for a given trait.

Incomplete Dominance

In incomplete dominance, the heterozygote displays a phenotype that is intermediate between the two homozygotes. This results in a blending of traits.

Example: Crossing red-flowered (R1R1) and white-flowered (R2R2) snapdragons produces pink-flowered (R1R2) offspring.
Genotypic and Phenotypic Ratios: Both are 1:2:1 in the F2 generation.

Codominance

In codominance, both alleles in a heterozygote are fully expressed, resulting in a phenotype that shows both traits distinctly.

Example: Human MN blood group system. Individuals with genotype LMLN express both M and N antigens on red blood cells.
Genotypes and Phenotypes:

Genotype	Phenotype
LMLM	M
LMLN	MN
LNLN	N

Lethal Alleles

Lethal alleles cause death when present in certain genotypes, often altering expected Mendelian ratios.

Example: In mice, the yellow coat color allele (Y) is dominant for color but lethal when homozygous (YY).
Observed Ratio: Cross Yy x Yy yields a 2:1 ratio of yellow to non-yellow offspring, as YY individuals do not survive.
Pleiotropy: A single gene mutation affects multiple traits.

Dominant Lethal Alleles

Dominant lethal alleles cause death even when only one copy is present. These are rare because affected individuals often do not survive to reproduce.

Example: Achondroplasia (a form of dwarfism) is caused by a dominant allele in the FGFR3 gene. Homozygous individuals (AA) do not survive.

Epistasis

Epistasis occurs when the expression of one gene is affected by another gene. The gene doing the masking is epistatic; the gene being masked is hypostatic.

Recessive Epistasis: A recessive allele at one locus masks the expression of alleles at another locus.
Example: Labrador retriever coat color. The e/e genotype at one locus masks the black/brown gene, resulting in yellow coat.

Genotype	Phenotype
B/_; E/_	Black
b/b; E/_	Brown
_/_; e/e	Yellow

Bombay Phenotype: Individuals with hh genotype lack the H antigen, so ABO antigens cannot be expressed, resulting in blood type O regardless of genotype.

Dominant Epistasis

Dominant epistasis occurs when a dominant allele at one locus masks the expression of alleles at another locus.

Example: Squash fruit color. The dominant allele W masks the yellow/green gene, resulting in white fruit.

Genotype	Phenotype
W/_; Y/_	White
W/_; yy	White
ww; Y/_	Yellow
ww; yy	Green

Complementary Gene Interaction

Two genes work together to produce a phenotype. Both must have at least one dominant allele for the trait to be expressed.

Example: Flower color in sweet peas. Both C and P genes must be present for purple color; otherwise, flowers are white.

Novel Phenotypes

Interactions between genes can produce new phenotypes not seen in the parents.

Example: Fruit shape in summer squash. Crosses between disc-shaped and long-shaped squash can produce sphere-shaped offspring.

Penetrance and Expressivity

Penetrance and expressivity describe the variability in phenotype expression among individuals with the same genotype.

Penetrance: Percentage of individuals with a genotype who express the expected phenotype.
Expressivity: Degree to which a trait is expressed in an individual.
Example: Polydactyly may show full extra digits in some individuals, while others may have only skin tags.

Linkage and Crossing Over

Gene Linkage

Linked genes are located close together on the same chromosome and tend to be inherited together. Linkage violates the principle of independent assortment.

Complete Linkage: No crossing over occurs; genes are always inherited together (0% recombination).
Independent Assortment: Genes on different chromosomes or far apart on the same chromosome assort independently (50% recombination).

Crossing Over and Recombination

Crossing over is the physical exchange of chromosome segments between non-sister chromatids during meiosis, resulting in recombinant gametes.

Recombination Frequency: Proportion of recombinant offspring; used to estimate the distance between genes.
Formula:

Example: If 200 recombinants are observed out of 1000 total offspring, recombination frequency is 20%.

Chromosome Mapping

Chromosome mapping uses recombination frequencies to determine the relative positions of genes on a chromosome.

Map Units (centiMorgans): 1% recombination frequency equals 1 map unit.
Application: Mapping genes in Drosophila using crosses between heterozygous and homozygous recessive individuals.

Additional info:

Epistasis and gene interactions are crucial for understanding complex traits and genetic diseases.
Penetrance and expressivity explain why individuals with the same genotype may show different phenotypes.
Chromosome mapping is foundational for genetic research and breeding programs.