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