Non-Mendelian Genetics Overview

Non-Mendelian Inheritance Patterns

• Some traits do not follow Mendel's predicted inheritance patterns.
• Sex chromosomes transmit traits differently in males and females.
• Genes close together on the same chromosome do not always assort independently.
• Mitochondrial and chloroplast DNA have unique inheritance patterns.

Human Sex Chromosomes

• The small region of homology in sex chromosomes allows them to pair up and segregate during meiosis.
• The X and Y chromosomes share a small region of homology.
• Very few genes on the X chromosome have counterparts on the Y chromosome.

Segregation of Sex Chromosomes

• Random fertilization of gametes (sperm and egg) typically results in a 1:1 ratio of XX and XY offspring.
• At birth, there's a slight excess of males.

X-Linked Genes

• T.H. Morgan discovered white-eyed fruit flies.
• Crossing a white-eyed male with a red-eyed female produced all red-eyed F1 offspring.
• The F1 generation showed a typical recessive inheritance pattern.
• Crossing F1 offspring revealed different inheritance patterns than Mendel predicted.

Inheritance Patterns of X-Linked Genes

• White eyes reappeared in the F2 generation, but only in males.
• All F2 females had red eyes.
• In the F2 offspring, the male ratio of red-eyed to white-eyed flies was 1:1.

X-Linkage

• Most genes on the X chromosome do not have counterparts on the Y chromosome.
• A recessive mutation on an X-linked gene is expressed in males.
• Males with X-linked recessive traits will have carrier daughters and unaffected sons.

Heterozygous XX Cross

• A recessive allele on the X chromosome can alternate between generations of males and females.
• In heterozygous female offspring, half are heterozygous carriers and half of the male offspring are expected to be affected.

Morgan's White-Eye Fruit Fly Crosses

• Morgan's work supported the hypothesis of X-linkage.
• His experimental data and predictions for future crosses were consistent with X-linkage.

Nondisjunction

• Nondisjunction is a rare meiotic event where chromosomes fail to separate properly during meiosis.
• In nondisjunction, some eggs have more than one copy of the X chromosome, or no X chromosomes.
• The genotypes XXX and OY were not observed; embryos with these constitutions do not survive.

Nondisjunction in Fruit Flies

• Nondisjunction in fruit flies has produced exceptional flies.
• The white-eye gene is located on the X chromosome.
• Bridges' work provided the first experimental evidence of nondisjunction.

X-Linked Recessive Mutation in Humans: Color Blindness

• Red-green color blindness is an illustration of X-linked recessive mutations.
• Affected males often have unaffected offspring, as heterozygous daughters are typically carriers.
• Affected fathers can have their sons affected through their heterozygous daughters.

Crossing Over Without Recombination

• Crossing over does not always occur between genes of interest.
• Nonrecombinant chromosomes have the same configuration of alleles as one parental chromosome.

Crossing Over With Recombination

• Crossing over occurs in prophase I of meiosis.
• Crossing over produces recombinant and nonrecombinant chromosomes.
• Recombinant chromosomes show a nonparental combination of alleles.

Frequency of Recombination

• The physical distance between two genes on a chromosome affects their recombination frequency.
• Genes closer together have a lower recombination frequency.
• The frequency of recombination between genes ranges from 0% to 50%.

Y-Linked Genes

• Y-linked genes are located in a unique area of the Y chromosome.
• They do not undergo crossing over with the X chromosome.
• Y-linked genes are passed from fathers to sons only.

Y Chromosome Haplotypes and Ancestry

• Each unique combination of nucleotides on the Y chromosome is a haplotype..
• The accumulation of mutations over time helps trace lineages of Y chromosomes.

Worldwide Y Chromosome Lineages

• Worldwide, Y-chromosome haplotypes tend to cluster geographically.

Inheritance of Mitochondrial and Chloroplast DNA

• Mitochondria and chloroplasts were originally prokaryotic organisms engulfed by eukaryotic cells.
• They have their own genomes containing genes for many enzymes.
• Organelle inheritance patterns vary by organism (maternal or paternal or biparental).

Patterns of Inheritance in Cytoplasmic Organelles

• Three major inheritance patterns exist for cytoplasmic organelles:
• Maternal inheritance
• Paternal inheritance
• Biparental inheritance

Inheritance of Mitochondrial DNA

• Mitochondrial DNA mutations can result in red patches in muscle fibers.
• Mitochondrial DNA is maternally inherited, meaning males do not transmit this trait.

Inheritance of Mitochondrial DNA and Ancestry

• Mitochondrial DNA (mtDNA) mutations accumulate over time.
• mtDNA can be used to trace ancestry because it does not undergo recombination and is maternally inherited.