Chapter 14: Mendel and the Gene Idea

Mendel’s Ideas based on Observations Alternative alleles account for variations in inherited characteristics For each character, an organism inherits two alleles, one from each parent If two alleles differ, the dominant allele is expressed, the recessive allele has no effect The two alleles segregate during gamete formation.

Other common terms Homozygous vs. heterozygous – Homozygous dominant – Homozygous recessive – Heterozygous Genotype vs. phenotype – Phenotype is physical – Genotype describes collection of genes

Solving problems (basics) Read the problem Write down the parents’ genotypes (if possible) Punnet square Answer the question

Example: Having attached earlobes is recessive to free hanging. If a parent that is heterozygous has a child with a parent that has attached earlobes, what is the chance that their first child will have free-hanging earlobes?

Test Cross Done to determine the genotype of a dominant phenotype individual Cross the individual with a homozygous recessive individual Look at the ratio of the offspring IQ 14.2

Dihybrid Cross Do IQ 14.3

Using Probabilities IQ 14.4, 14.5

Extensions of Basic Mendian Genetics Incomplete dominance – Heterozygotes have their own phenotype – Ex. Red X white snapdragons  PINK Blended due to amount of pigment production being intermediate

Codominance – Heterozygotes have both traits equally expressed (separate, distinguishable expression – not intermediate)

Multiple Alleles – Most genes exist in more than two allelic forms Ex. ABO blood groups, A and B show codominance, O is recessive – IQ 14.6

Pleiotropy: – One gene affects an organism in many ways Ex. Cystic fibrosis – Defective Cl- channel causes mucus to coat cells  poor absorption of nutrients, bronchitis, bacterial infections

Epistasis – A gene at one locus affects the expression of a gene at another locus Ex. Mouse color – Black is dominant to brown at one locus – At another gene, the dominant trait is for display of color. Any mouse homozygous recessive at this locus will be albino – IQ 14.7

Polygenic inheritance – An additive affect of 2 or more genes on a single phenotypic character Ex. Human skin color, affected by 3+ genes Each dominant allele gives a “dose” of dark color – IQ 14.8

Pedigree charts Used to track inheritance patterns in humans Males are squares, circles are females Offspring listed in order of their birth Shaded symbols have the trait being traced