Extensions to Mendelian Inheritance – Lecture Review

What are the three general rules of Mendelian inheritance?

(1) Genes are passed from cell to cell and generation to generation unaltered; (2) Genes obey the Law of Segregation; (3) Genes obey the Law of Independent Assortment.

Which genetic phenomenon violates Mendel’s Law of Independent Assortment?

Linkage—genes located on the same chromosome and inherited together.

How does complete dominance differ from incomplete dominance?

In complete dominance, heterozygotes display the same phenotype as dominant homozygotes, whereas in incomplete dominance the heterozygote phenotype is intermediate between the two homozygotes.

Give the definition of codominance.

A form of inheritance in which two different dominant alleles are both fully and independently expressed in the phenotype of the heterozygote.

What human blood-group system illustrates multiple alleles and codominance?

The ABO blood group, controlled by three alleles (IA, IB, i) where IA and IB are codominant and i is recessive.

Which allele combinations produce blood type AB?

Genotype IAIB produces the AB phenotype because both IA and IB alleles are expressed.

Define pleiotropy and provide an example.

Pleiotropy occurs when one gene affects multiple phenotypic traits; sickle-cell disease is an example, leading to abnormal hemoglobin, pain, organ damage, and malaria resistance in heterozygotes.

What is polygenic inheritance?

A pattern in which two or more genes jointly influence a single phenotypic trait, such as human skin color.

In a trihybrid AaBbCc cross for skin color, how many different phenotypic classes are expected?

Seven classes, based on the number of ‘dark-skin’ alleles (0–6).

Explain epistasis using Labrador coat color.

In Labradors, one locus (B/b) codes for pigment color (black or brown) and a second locus (C/c) controls pigment deposition; cc individuals are yellow regardless of B/b genotype, producing a 9:3:4 phenotypic ratio in dihybrid crosses.

What ratio is typical of recessive epistasis in a dihybrid cross?

9 (AB) : 3 (aaB) : 4 (Abb + aabb).

Define epigenetics.

Heritable, reversible changes in gene expression that do not involve alterations in the DNA sequence, such as DNA methylation and histone modification.

What is genomic imprinting?

An epigenetic phenomenon where certain genes are expressed in a parent-of-origin–specific manner due to differential DNA methylation established during gametogenesis.

Which enzyme activity is most associated with DNA methylation maintenance?

DNA methyltransferase that copies existing methylation patterns during DNA replication.

What structure forms when one X chromosome is inactivated in female mammals?

A Barr body, the condensed, transcriptionally inactive X chromosome.

Why do tortoiseshell female cats display patchy coat colors?

Random X-chromosome inactivation leads to cellular mosaics, expressing either the orange or black allele in different patches.

Do male mammals form Barr bodies? Explain.

No; males possess only one X chromosome, which remains active, so no inactivation or Barr body formation occurs.

Describe extranuclear (cytoplasmic) inheritance.

Transmission of genes located in organelle genomes (mitochondria, chloroplasts/proplastids), usually inherited maternally in most animals and plants.

What classic experiment demonstrated chloroplast inheritance in four-o’clock plants?

Carl Correns’ reciprocal crosses showed that leaf color (green, white, variegated) follows the phenotype of the maternal parent because chloroplasts come from the egg, not pollen.

Why can mitochondrial diseases affect many tissues simultaneously?

Mitochondria are involved in energy metabolism; defects impair ATP production in energy-demanding organs such as brain, muscle, heart, and kidneys.

Name a human disorder caused by mutant mitochondrial DNA.

MELAS syndrome (Mitochondrial Encephalopathy, Lactic Acidosis, and Stroke-like episodes).

What principle explains why reciprocal crosses with extranuclear genes give different results?

Maternal inheritance—offspring phenotype depends on the genotype of the egg’s cytoplasm rather than the sperm.

How can environment influence phenotype despite identical genotypes?

External factors (nutrition, toxins, lifestyle) interact with genes, producing multifactorial traits such as heart disease, diabetes, or certain cancers.

Give an example of diet-induced epigenetic change in animals.

Royal jelly fed to honey-bee larvae triggers epigenetic modifications that develop a queen phenotype instead of a worker.

What is linkage and how is it detected?

Physical proximity of genes on the same chromosome causing them to be inherited together; detected by recombination frequencies lower than 50 % in genetic crosses.

Which Mendelian law does cytoplasmic genome inheritance challenge?

The Law of Segregation, as organelle genes are not equally segregated from both parents.

Can epigenetic marks be transmitted across generations? Provide evidence.

Yes; certain DNA methylation patterns (e.g., at imprinted loci) and histone modifications can persist through gametes and influence offspring phenotypes.

What is the difference between de novo and maintenance methylation?

De novo methylation establishes new methyl groups on previously unmethylated DNA during gametogenesis or early development, whereas maintenance methylation copies existing patterns during DNA replication.

Why do environmental and genetic factors together cause ‘multifactorial’ diseases?

Because disease susceptibility arises from interactions between multiple genes and diverse environmental influences, producing complex inheritance patterns that deviate from simple Mendelian ratios.