Genetics and Inheritance Notes

The Science of Genetics

• Early attempts to explain heredity were later rejected by science.
• Hippocrates' theory of Pangenesis:
  • Particles from each part of the body travel to eggs or sperm and are passed on.
• Early 19th-century biologists' blending hypothesis:
  • Traits from both parents mix in the offspring.

Chapter 9: Inheritance

• Gregor Mendel:
  • An Austrian Monk.
  • Grew up in a rural area.
  • Father of genetics.
  • Began breeding pea plants in 1857.
  • Published his results in 1866.
  • Concluded that parents pass discrete “heritable factors” responsible for traits in offspring.

Why Peas?

1. Have easily distinguishable traits.
2. Are self-pollinating plants.
3. Can easily be cross-pollinated.
4. Have a short life cycle.
5. Produce large numbers of offspring.

Why Was Mendel Successful?

• He Studied Physics, Mathematics, and Chemistry at the University of Vienna.
• He was thorough in his experiments.
• He selected traits that were easily observable.

Mendel’s Monohybrid Cross

• Pure breeding parents:
  • Purple
  • White
• First generation (F1):
  • Offspring all purple.
• Inbred F1 to produce 2nd generation (F2):
  • Purple and white offspring
  • 3:1 ratio
• Concluded F1 had purple and white inheritance factors, but only the dominant factor was visible.

Genetics Terminology

• Characteristic vs. trait
• Dominant vs. recessive
• Genotype vs. phenotype
• Homozygous vs. heterozygous
  • Homozygous (purebred)
  • Heterozygous (hybrid)

Terminology: Chromosomes, Genes, Alleles

Mendel’s Monohybrid Cross

• Punnett square
• LAW OF SEGREGATION: Which stage of MEIOSIS?

Five Hypotheses About Inheritance Formed by Mendel

1. Gene Variation
   • There are alternative forms of genes that account for variations in inherited characteristics.
2. Diploid Genes
   • For each characteristic, an organism inherits two alleles, one from each parent. These alleles may be the same or different.

Alleles and Genotypes

• TT: Homozygous for the dominant allele
• aa: Homozygous for the recessive allele
• Bb: Heterozygous

Law of Dominance

• If the two alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele; the other has no noticeable effect on the organism’s appearance and is called the recessive allele.

Autosomal Dominant Disorders

• Achondroplasia
• Huntingtons disease
• Hypercholesterolemia

Autosomal Recessive Disorders

• Most human genetic disorders are recessive.
• Cystic fibrosis
• Tay-Sachs
• PKU

Law of Segregation

• A sperm or egg carries only one allele for each inherited trait because allele pairs separate (segregate) from each other during the production of gametes.

Independent Assortment

• Each pair of alleles segregates independently of the other pair of alleles during gamete formation.
• The inheritance of one trait has no effect on the inheritance of another.

Mendel's Dihybrid Cross: Independent Assortment

• Discusses Mendel's observation that maternal & paternal alleles line up at metaphase I independently if genes for seed color and shape are not linked.

Rules of Probability

• Rule of multiplication
  • Example: Bb male x Bb female
  • What is the chance that offspring will be bb?
  • 1/2 gametes have B
  • 1/2 gametes have b
  • 1/2 sperm b x 1/2 egg b = 1/4

Rules of Probability

• Rule of addition
  • Example: Bb male x Bb female
  • What is the chance for offspring with genotype Bb?
  • Option 1:
    • B could come from sperm (1/2) and b from egg (1/2): 1/2 \, x \, 1/2 = 1/4 chance
  • Option 2:
    • b could come from sperm (1/2) and B from egg (1/2): 1/2 \, x \, 1/2 = 1/4 chance
    • Add separate probabilities: 1/4 + 1/4 = 1/2 chance Bb offspring

Summary: Mendel’s Laws and Meiosis

• Independent assortment
• Segregation of alleles

Summary: Mendel’s Laws

• Monohybrid cross (Bb \, x \, Bb)
  • Principle of segregation
  • Each gamete carries 1 allele of a gene
  • When do alleles separate from each other during meiosis? (Anaphase I of Meiosis I)
• Dihybrid cross (RrYy \, x \, RrYy)
  • Principle of independent assortment
  • When genes are on different chromosomes, each allele of that gene is assorted into the gamete independently of the other
  • When do different chromosomes line up during meiosis? (Metaphase I of Meiosis I)

Non-Mendelian Inheritance

• Mendel got lucky?
  • All characteristics he studied were determined by 2 alleles, 1 completely dominant over the other
  • Many characteristics have >2 alleles and/or not always complete dominance of any 1 allele

Extensions of Mendel’s Hypotheses

• Incomplete dominance
  • Snapdragon color
  • Hypercholesterolemia
• Codominance
  • ABO blood groups
  • Roan cattle (mixture of white and colored hair)
• Pleiotropy
  • Sickle-cell trait/disease
• Polygenic inheritance
  • Skin color

Blood Groups

• Blood Group (Phenotype), Genotypes, Fraction of population
  • O: ii, 45%, Antibodies Present in Blood - Anti-A and Anti-B, Universal donor
  • A: AA or i^Ai,40%, Antibodies Present in Blood - Anti-B
  • B: BB or i^Bi, 11%, Antibodies Present in Blood - Anti-A
  • AB: i^Ai^B, 4%, Universal recipient

Sickle-Cell Anemia Example

• Individual homozygous for sickle-cell allele.
• Sickle-cell (abnormal) hemoglobin.
• Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped.
• Breakdown of red blood cells.
• Sickled cells.
• Clumping of cells and clogging of small blood vessels.
• Accumulation of sickled cells in spleen.
• Physical weakness, anemia.
• Heart failure.
• Pain and fever. Damage to Spleen and other organs.
• Impaired mental function.
• Pneumonia and other infections.
• Paralysis and Rheumatism.
• Kidney failure.

Sex Chromosomes

• Male: 44 + XY
• Female: 44 + XX
• Sperm: 22 + X or 22 + Y
• Egg: 22 + X
• Offspring: 44 + XX (Female) or 44 + XY (Male)

Linkage: Sex Linked Disorders

• Gene on sex chromosome
• X-linked recessive trait
  • Red-green colorblindness
  • Genotypes & Phenotypes? Need more info
• Y linked trait: hairy auricle
  • XY^h
  • Not in text