Genetics and Chromosomal Inheritance

Genes Located on Chromosomes

• Mendel's hereditary factors are genes located on chromosomes.
• Early 20th century: Thomas Morgan's experiments provided evidence for the chromosome theory of inheritance.

Morgan's Experimental Organism: Fruit Flies

• Drosophila melanogaster (fruit flies) are good for genetic studies.
  • Produce lots of offspring.
  • Short generation times (bred every two weeks).
  • Only four pairs of chromosomes.
• Morgan looked for wild type (normal) and mutant phenotypes in fruit fly populations.
  • Wild type: Red eyes.
  • Mutant: White eyes.

Experiment

• Crossed a male mutant (white eyes) with a female with red eyes.
• F1 generation: All had red eyes.
• F2 generation: 3:1 ratio of red to white eyes, but only males had white eyes.
• Conclusion: The white eye mutant allele is located on the X chromosome, supporting the chromosome theory of inheritance.

Chromosomal Basis of Sex Determination

• Humans and other animals have sex chromosomes (X and Y).
• Females: Two X chromosomes (XX).
• Males: One X and one Y chromosome (XY).
• The X and Y chromosomes pair together as homologs due to short homologous segments.
• The Y chromosome is significantly smaller than the X chromosome.
• The Y chromosome carries fewer genes compared to the X chromosome.
• SRY gene (sex-determining region on the Y) is responsible for testis development in embryos.

Sex-Linked Genes

• Sex linked gene: Any gene located on a sex chromosome.
  • Y-linked genes: Genes on the Y chromosome (very few).
  • X-linked genes: Genes on the X chromosome (many not related to sex).

Inheritance Patterns

• Recessive X-linked trait expression:
  • Females: Must have two copies of the allele (homozygous).
  • Males: Only need one copy of the allele (hemizygous).
• X-linked recessive disorders are more common in males than females.

X-Linked Recessive Disorders in Humans

• Examples: Color blindness, Duchenne muscular dystrophy, and hemophilia.

Mating Examples and Phenotype Expression

• Females not carrying recessive alleles: No females express the X-linked recessive allele.
• Carrier female crossed with a normal male: Only males express the recessive allele.
• Carrier female mating with a male who has the recessive allele: Both female and male offspring can have the recessive trait.

Linked Genes

• Genes on the same chromosome tend to be inherited together (linked).
• Morgan's experiments with fruit flies examined how linkage affects the inheritance of two different characters.
• Traits studied: Body color and wing size.
  • Wild type: Gray body and normal wings.
  • Mutant: Black body and vestigial (smaller) wings.
• F1 hybrid: Gray body and normal wings.
• Test cross: F1 hybrid female crossed with a recessive male (black body, vestigial wings).

Expected Ratios

• Genes on different chromosomes: 1:1:1:1 ratio in test cross products.
• Genes on the same chromosome: Only wild type or double mutant phenotypes.
• Results: Mostly wild type and black vestigial, but some gray vestigial and black normal offspring.

Genetic Recombination

• Production of offspring with trait combinations differing from either parent.
• Parental types: Phenotypes matching one of the parental phenotypes.
• Recombinant types (recombinants): Offspring with non-parental phenotypes or new combinations of traits.
• 50% frequency of recombination: Two genes on different chromosomes.

Incomplete Linkage

• Morgan's studies showed genes can be linked, but linkage can be incomplete.
• Recombinant phenotypes observed, indicating incomplete linkage.
• Crossing over of homologous chromosomes: Mechanism that physically breaks the connection between genes on the same chromosome.
• Crossing over: Different parts of adjacent homologous chromosomes switch, resulting in recombinant phenotypes.

Meiosis

• Duplication of chromosomes.
• Crossing over between homologous chromosomes in meiosis I.
• Mixing of alleles.
• Variation in the combination of alleles from F1 hybrids.

Recombination Frequency

• The number of recombinants divided by the total number of offspring.
• Indicates the percentage of recombination occurring.
• Low frequency: Small number of recombinants.
• Crossing over generates increased genetic variation with recombinant gametes.

Test Crosses and Linked Genes

• Test crosses involving linked genes do not always give a 1:1:1:1 phenotypic ratio.
• Alleles in linked genes are inherited together.
• The distance between linked genes on a chromosome affects the probability of recombination.
• Closer genes are more likely to be linked.

Genetic Diversity

• Recombinant chromosomes bring alleles together in new combinations.
• Genetic diversity is the raw material upon which natural selection works.

Alterations of Chromosome Numbers or Structures

• Large-scale chromosomal alterations in humans and other mammals are a common cause of spontaneous abortions or developmental disorders.
• Plants are more tolerant of these genetic changes.

Abnormal Chromosome Numbers

• Nondisjunction: Pairs of homologous chromosomes don't separate normally during meiosis.
  • Can occur in meiosis I or meiosis II.
  • Results: Gametes with three chromosomes or gametes with one chromosome.
• Aneuploidy: Offspring with abnormal numbers of particular chromosomes.
  • Monosomic zygotes: Only one copy.
  • Trisomic zygotes: Three copies.
• Nondisjunction: occurs in 10-20 % of human conceptions, main reason for pregnancy loss.

Polyploidy

• Organism has more than two complete sets of chromosomes.
  • Triploidy: Three sets.
  • Tetraploidy: Four sets.
• Common in plants, rare in animals.

Alterations in the Structure of Chromosomes

• Chromosome breaks and reassembles in different ways.
  • Deletion: Segment of the chromosome is removed.
  • Duplication: Region of the chromosome is duplicated.
  • Inversion: Segment is switched around.
  • Translocation: Segment from one chromosome switches with a segment from a non-homologous chromosome.

Aneuploidy Disorders

• Aneuploidy of sex chromosomes:
  • XXX females: healthy with no unusual physical features.
  • Klinefelter syndrome: Males with two X chromosomes and one Y chromosome.
  • Turner syndrome: Females with just one X chromosome; sterile and the only known viable monosomy in humans.