“Pure” lines Reliably produce progeny like parents. Now often called inbred or isogenic lines. Mendel worked with pure lines of garden peas.

Garden peas normally reproduce by self-pollination.

Mendel could control parentage of crosses.

P1 = parents Pure lines in this case. X F1 = 1st generation In this example, one trait dominates over the other. F2 = 2nd generation Produced by selfing. The hidden trait re-appears!

Dominance means that the phenotype will reflect the presence of that allele equally, whether or not the other allele is present. Short hair is dominant to long hair in trolls. Dominance does NOT mean: Better (there are dominant lethal alleles) More common (dominant alleles can be uncommon) Also -- for many traits the heterozygote is intermediate between the two homozygotes, reflecting incomplete or co-dominance. More on this later…..

Segregation= separation of two alleles of parent into gametes. “particulate Inheritance”

Do alleles for different genes segregate together? For example, must green peas be wrinkly?

Independent Assortment. Transmission for alleles of one trait is independent of transmission for (most) other traits.

“Dihybrid cross” between parents heterozygous for two traits. Get famous 9:3:3:1 ratio of phenotypes.

Testcross confirms predictions of independent assortment. Recessive parent used to reveal genotype of hypothesized RrYy (dominant phenotype is difficult to genotype!).

Practice problems for Mendelian principles. Next, extension of Mendel’s rules: 1)sex-linked genes.

Drosophila has same system.

red eyes dominant Females have 2 copies of allele; Males have only one, Inherited from mother.

This slide shows meiosis to produce sperm. Meiosis to produce eggs would begin with two X-chroms, which can cross over (recombine).

Do sex-linkage problems from

About 10% of human males cannot distinguish long & medium wavelengths (red & green). The opsin gene for short wavelengths is on an autosome. Which chromosome carries the genes for long & medium wavelengths?

Pedigree analysis can reveal sex-linkage in humans. These two had 9 kids. Work backwards to identify carriers.

Extension of Mendel’s rules: 1) Sex linkage 2) Incomplete/co-dominance

Enzymes generally show co-dominance, with the heterozygote producing both forms of the enzyme This slide actually shows a pcr, so reveals genotype rather than phenotype. But, gels of enzymes look very much like this, with the heterozygote producing two types of enzymes that migrate differently, and may have different activities in particular environments. ADH is an example.

Extension of Mendel’s rules: 1) Sex linkage 2) Incomplete/co-dominance 3) Multiple alleles at a locus

Distribution of ADH* alleles in human populations. There are generally >2 common alleles in each population. *Alcohol dehydrogenase-4 Class 1

Red blood cell membranes have stuff sticking out….proteins, carbohydrate, etc. The ABO molecules are carbohydrates. From cell membrane

There are three common alleles at the ABO locus. Blood typeMembrane carbohydrates Genotype Ononeii AAI A I A or I A i BBI B I B or I B i AB IAIBIAIB What are the dominance relationships among the alleles? (In some cases, complete and in some co-dominance.)

Extension of Mendel’s rules: 1) Sex linkage 2) Incomplete/co-dominance 3) Multiple alleles at a locus 4) Epistasis -- interactions among alleles of DIFFERENT loci affect phenotype.

CC or Cc ColorBB or BbBlack Agouti cc bb Albino Brown Agouti Genes C and B both affect hair color in mice

What will be the F2 ratios of a cross between pure lines of black and albino mice?

LINKAGE Image from

Alleles on the same chromosome don’t assort independently.

Quantitative traits

Complex diseases influenced by multiple genes in interaction with the environment: Diabetes Cardiovascular disease Cancer Alzheimer’s disease Schizophrenia Epilepsy…. Go to Type in Alzheimer; you will see the locations 29 genes currently associated with the disease. You can then click on a chromosome to see information about each gene.