Chapter 9 Patterns of Inheritance Mendelian Genetics Chapter 9 Patterns of Inheritance

Gregor Mendel 1850s, Mendel studied inheritance in pea plants by breeding them Peas are good because they have distinctive traits, male and female parts, and are easy to manipulate He crossed true breeding with other true breeding as well as with hybrids

Gregor Mendel P = parental generation F1 = First filial generation (P X P) F2 = Second filial (F1 X F1) True breeding (homozygous) = plants that only breed to produce one phenotype Hybrid (heterozygous) = results of crosses between two plants that breed true for different phenotypes for the same trait

Gregor Mendel Genotype = genetic makeup of an organism Sequence of the gene Phenotype = expressed genotype Visible or measurable traits – based upon genotype

Gregor Mendel He crossed a true breeding purple-flowered plant with a true breeding white-flowered plant All of the F1 generation was purple

Gregor Mendel He then crossed two F1’s with each other The results (F2’s) were roughly 3 purples to 1 whites White trait was absent in F1’s but reappeared in the F2’s

Gregor Mendel The reappearance of the white trait meant that it was not altered in the F1, just “hidden” Mendel did the same crosses, and got similar results, with other traits of the pea plant

Gregor Mendel

Gregor Mendel Four ideas spawn from Mendel’s work: Alleles account for different traits Organisms inherit two alleles, one from each parent (may be same or different) If alleles are different, then one is dominant over the other The two alleles segregate (law of segregation) during gamete formation

Punnett Square Predicts the statistical results of a cross

Punnett Square A test cross, breeding a homozygous recessive with a dominant phenotype, but unknown genotype, can determine the identity of the unknown allele

Punnett Square One Factor Crosses = crosses with only one trait Monohybrid crosses = crossing one heterozygote with another Two Factor Crosses = crosses with two traits Dihybrid crosses = crossing one organism that is heterozygous for both traits with another that is heterozygous for both traits

Punnett Square In monohybrid crosses, he noticed that genotypic and phenotypic ratios could be predicted Genotypic ratio = 1:2:1 Phenotypic ratio = 3:1

Punnett Square Example of a dihybrid cross: Crossing one plant that is heterozygous for seed shape and yellow seed coat (RrYy) with another plant that is also heterozygous for both traits (RrYy) RrYy X RrYy Dihybrid results point to the law of independent assortment

Independent Assortment In our example (RrYy X RrYy), both parents can produce four possible gametes RY Ry rY Since both parents have four possible gametes, there are 16 possible combinations for fertilization

Independent Assortment

Independent Assortment These possible combinations produce a genotypic ratio of 1:2:1:2:4:2:1:2:1 It also produces a phenotypic ratio of 9:3:3:1 The law of independent assortment simply means that during gamete formation, allelic pairs that code for different traits assort independent of each other

Dominance Mendel was very lucky to have studied peas They follow simple dominance laws Not all cells follow the simple laws of dominance vs. recessiveness

Variations Incomplete dominance = neither allele is shown in its dominant form; an intermediate phenotype is shown Genotypic and phenotypic ratios are the same (1:2:1)

Variations Codominance = heterozygote expresses a phenotype that is distinct from and not intermediate between those of the two homozygotes Ex. Human AB blood type

Variations Codominance

Variations Multiple alleles – gene exists as more than two alleles in the population Rabbit coat color gene has 4 alleles: C, c, cch & ch 5 phenotypes 10 genotypes

Variations Pleiotropy – one gene affects more than one phenotypic characteristic Ex: sickle-cell disease affects much more than just the overall conformation of the hemoglobin protein

Variations Epistasis – one gene at one locus affects the phenotype of another gene at another locus Ex: mice coat color depends on two genes One, the epistatic gene, determines whether or not pigment will be deposited in hair Presence (C) is dominant to absence (c) The second determines whether the pigment to be deposited is black (B) or brown (b) An individual that is cc has a white (albino) coat regardless of the genotype of the second gene

Variations – epistasis A cross between two black mice that are heterozygous (BbCc) will follow the law of independent assortment However, unlike the 9:3:3:1 offspring ratio of an normal Mendelian experiment, the ratio is 9 black, 3 brown, and 4 white

Variations Polygenic – one phenotype determined by the combined effect of more than one gene Ex: human skin color, eye color, height

Variations

Variations Multifactorial Traits – determined by the combined effect of one or more genes plus the environment Ex: heart disease, body weight, intelligence

Pedigrees Autosomal Recessive Traits: located on non-sex chromosomes affects males and females parents must be carriers or affected affected individuals are homozygous recessive Ex: Albinism, Cystic fibrosis, Phenylketonuria, Sickle cell disease

Autosomal Recessive

Pedigrees Autosomal Dominant Traits: located on non-sex chromosomes affects males and females at least one parent is affected does not skip generations affected individuals are homozygous dominant or heterozygous Ex. Achondroplasia, Huntington disease, Lactose intolerance, Polydactyly

Autosomal Dominant