Mendelian Inheritance

• The concept of inheritance was first described by the Monk Gregor Mendel as he documented the inheritance patterns viewed in flowering pea plants. He later assigned the gene as the basic unit of inheritance.

Basics: The Vocabulary of Genetics• character - a heritable feature (hair color, height, ...) • trait - variations of characters (brown vs blond) • true breeding plant - self pollination leads to a offspring of the same

variety (all white or all purple flowers) • hybridization - the cross pollination of two true breeding varieties of

plants • P - Generation = true breeding parents • F1Generation = hybrid offspring of the P cross

• F2Generation = self-pollinated offspring of the F1 • allele - alternate versions of a gene • dominant allele - codes for a trait that is always expressed (symbolized

by a capital letter) • recessive allele - has no noticeable contribution to an organism's

appearance if a dominant allele is also present(symbolized by a lower cased letter of the dominant trait)

• wildtype - the dominant trait expressed in the highest ratio in nature • genotype - organisms genetic makeup • phenotype - organisms outward appearance • homologous - having the same structure (two copies of the same

chromosome are homologous) • homozygous - having one allelic copy of agene, true breeding (PP or pp) • heterozygous - having 2 different alleles for a gene

Mendel's Model• Alternate versions of genes account for

variations in inherited characteristics. • For each characteristic, an organism

inherits two alleles. One from each parent.

• If 2 alleles inherited differ, the dominant one is expressed

• Law of Segregation - Two alleles for a heritable characteristic are separated during gametogenesis and end up indifferent gametes

• Law of independent assortment - each pair of alleles segregates independently of other pairs of alleles during gamete formation

Mendel's Model• 7 basic pea plant traits

– Was fortunate that traits were found on different chromosomes and that traits were only the result of 2 different alleles

Punnett Square • A punnett square can be used to predict the allelic frequencies

when true breeding parents are crossed.– draw a square having twice the number of side boxes as number of

alleles. – segregate the alleles on the side of the boxes (if more than one trait

you must segregate as groups) – recombine the alleles in the boxes – count the ratios

• Notice that the genotype ratio for the F1 cross is 1:2:2:1 & the phenotype ratio is 3:1 with the dominant trait (purple) expressed in the hybrids.

dihybrid crosses

• For dihybrid crosses the results are still predictable and are as follows: R - round, r - wrinkled, Y - yellow, y - green

Multiplication- addition rule:• Multiplication- addition rule:• You can use the allelic frequencies to predict the outcome of a cross.• Given the Genotypes RrYy & rrYy, what is the frequency of offspring

exhibiting both recessive traits?• rr = 1/2• yy = 1/4• 1/2 x 1/4 = 1/8• Chance for both dominant?• Rr = 1/2• YY = 1/4• Yy = 1/2• 1/2 x (1/4 + 2/4) = 3/8