Genetics and Inheritance

The Science of Genetics

• Early explanations of heredity have been disproven by modern science.
• Hippocrates' theory of Pangenesis:
  • Particles from each part of the body travel to eggs or sperm.
  • These particles are passed on to offspring.
• Early 19th-century biologists proposed the 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

• Purebreeding 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 and the Punnett Square

• Law of Segregation: Occurs during MEIOSIS.

Mendel’s Hypotheses About Inheritance

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.

Diploid Genes and Law of Dominance

3. If the two alleles of an inherited pair differ:
   • 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.

• Law of Dominance

Autosomal Dominant Disorders

• Achondroplasia.
• Huntington’s disease.
• Hypercholesterolemia.

Autosomal Recessive Disorders

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

Law of Segregation

4. 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

5. 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

• Mendel's observation support maternal and paternal alleles line up at metaphase I independently if genes for seed color and shape not linked

Rules of Probability

• Rule of multiplication:
  • Bb male x Bb female.
  • What is the chance the offspring will be bb? For both parents
  • (\frac{1}{2}) gametes have B.
  • (\frac{1}{2}) gametes have b.
  • (\frac{1}{2}) sperm b x (\frac{1}{2}) egg b = (\frac{1}{4})

Rules of Probability: Rule of Addition

• Bb male x Bb female
• What is the chance for offspring with genotype Bb?
• Option 1:
  • B could come from sperm (\frac{1}{2}) and b from egg (\frac{1}{2}): (\frac{1}{2}) x (\frac{1}{2}) = (\frac{1}{4}) chance.
• Option 2:
  • b could come from sperm (\frac{1}{2}) and B from egg (\frac{1}{2}): (\frac{1}{2}) x (\frac{1}{2}) = (\frac{1}{4}) chance.
• Add separate probabilities:
  • \frac{1}{4} + \frac{1}{4} = \frac{1}{2} chance of 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 because 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.

Linkage: Sex-Linked Disorders

• X-linked recessive trait:
  • Hemophilia.
  • X^HX^H, X^HX^h, X^hX^h, X^HY, X^hY
  • Phenotypes?
• Gene on sex chromosome.
• X-linked recessive trait:
  • Red-green colorblindness.
  • Genotypes? ……phenotypes…?
• Y-linked trait: hairy auricle
  • XY^h
  • Not in the text.