Mendelian Genetics – inheritance of traits

Why Peas??

Many varieties (character, traits) Easy to control pollination Could choose distinct characters

First Experiments:

True-breeding parent generation

Why aren’t all the floweres light purple?

Mendel’s Law of SegregationWhite “heritable factor” did not disappear in F1 generation but only purple “heritable factor” was acting.

Alternative version of genes (heritable factors) account for variations in inherited characters.

For each character, an organism inherits two alleles (versions of the gene) one from each parent.

If the two alleles differ, one is fully expressed – DOMINANT; the other does not affect the organism’s appearance - RECESSIVE trait.

The two alleles segregate during gamete production

Mendelian Genetics Terms

• Homozygous• Heterozygous• Phenotype • Genotype• Punnett square• Monohybrid cross• Testcross

Testcross

What happens if looking at two traits simultaneously?

• Dihybrid cross

xTrue-breeding Parent (P)

F1

F2: 9:3:3:1

Mendel’s Law of Independent Assortment

• Each character is independently inherited – segregate independently during gamete formation

• Practice dihybrid crosses• Use rules of probability to predict outcomes of

genetic crosses

http://www.biology.arizona.edu/mendelian_genetics/problem_sets/monohybrid_cross/monohybrid_cross.html

More Complicated Situations…

1. Incomplete dominance

Multiple Alleles

IAIA = A; IAi = AIBIB=B; IBi = B

IAIB= AB ii = O A, B – codominant

i - recessive

Other situations:• Pleiotropy – one gene affects several

phenotypes• Epistasis – one gene affects the phenotypic

expression of another gene• Polygenetic inheritance – additive effect of

two or more genes on a single phenotypic character

Human Disorders that follow Mendelian Patterns of Inhertitance

• Recessively inherited disorders: albinism, cystic fibrosis, Tay-Sachs disease, sickle-cell anemia

• Dominantly inherited disorders: Achondroplasia, Huntington’s disease

Cystic Fibrosis

Tay-Sachs Disease

Achondroplasia

Huntington DiseaseLate Acting – middle ageWestern Europeans

Screening for inherited disorders

Pedigrees

Pedigree – Huntington disease

Pedigree – Sickle cell anemia

Chromosomal Theory of InheritanceThomas Morgan

Fruitflies

4 pairs of chromosomes

Wild type phenotype

Mutant phenotype

White-eyed males only

Sex-linked traitsLocated on Sex chromosomes: X or Y

Females XX; Males XY

X-linked recessive

Hemophilia, color blindness