Mendelian Genetics

Introduction to Genetics GENETICS – branch of biology that deals with heredity and variation of organisms. Chromosomes carry the hereditary information (genes) Arrangement of nucleotides in DNA DNA - RNA - Proteins

Gregor Mendel Gregor Johann Mendel Austrian Monk, born in what is now Czech Republic in 1822 Son of peasant farmer, studied Theology and was ordained priest O.S.A. Went to the university of Vienna, where he studied botany and learned the Scientific Method

Mendel’s Peas Mendel looked at 7 traits in pea plants

Mendel Mendel was the first biologist to use Mathematics – to explain his results quantitatively. Mendel predicted Described the concept of genes Determined that genes occur in pairs Described that one gene of each pair is present in the gametes (one from the sperm and one from the egg.

Important Terminology Homozygous – having identical genes (one from each parent) for a particular characteristic (TT, tt) . Heterozygous – having two different genes for a particular characteristic (Tt) Dominant – the allele of a gene that masks or suppresses the expression of an alternate allele; the trait appears in the heterozygous condition. (Tt, TT) Recessive – an allele that is masked by a dominant allele; does not appear in the heterozygous condition, only in homozygous. (tt)

Check for Understanding #1

Terminology Cont’d Phenotype – the physical appearance of an organism. Genotype – the genetic makeup of an organisms Gamete – a specialized sex cell that contains HALF of the genetic information necessary for expression of a trait Monohybrid cross: a genetic cross involving a single pair of genes (one trait); parents differ by a single trait. P = Parental F1 = First filial generation; offspring from a genetic cross. F2 = Second filial generation of a genetic cross

Monohybrid Cross Parents differ by a single trait. Crossing two pea plants that differ in stem size, one tall one short T = allele for Tall t = allele for dwarf TT = homozygous tall plant t t = homozygous dwarf plant

Punnett Square A useful tool to do genetic crosses For a monohybrid cross, you need a square divided by four…. We use the Punnett square to predict the genotypes and phenotypes of the offspring.

Using a Punnett Square Steps: 1. determine the genotypes of the parent organisms 2. write down your "cross" (mating) 3. draw a p-square Parent genotypes: TT and t t Cross T T ´ t t

Using a Punnett Square cont’d 4. "split" the letters of the genotype for each parent & put them "outside" the p-square 5. determine the possible genotypes of the offspring by filling in the p-square 6. summarize results (genotypes & phenotypes of offspring) H h H h HH Hh Hh hh

Mendels Principles Principle of Dominance: One allele masked another, one allele was dominant over the other in the F1 generation. Principle of Segregation: When gametes are formed, the pairs of hereditary factors (genes) become separated, so that each sex cell (egg/sperm) receives only one kind of gene.

Real Life Case: Cystic Fibrosis IF two parents carry the recessive gene of Cystic Fibrosis (cf), that is, they are heterozygous (Cf cf), one in four of their children is expected to be homozygous for cf and have the disease: Cf cf Cf Cf = normal Cf cf = carrier, no symptoms cf cf = has cystic fibrosis Cf cf CfCf Cfcf Cfcf cfcf

Dihybrid Cross Dihybrid crosses Matings that involving parents that differ in two genes (two independent traits) For example flower color A = purple (dominant) a = white (recessive) and stem length: B = tall b = dwarf

Dihybrid Cross Ratios 1 AABB 2 AABb 2 AaBB 4 AaBb 1 AAbb 2 Aabb 1 aaBB Tall; Purple Flowers (9) Tall; White Flowers (3) Dwarf; Purple Flowers (3) Dwarf; White Flowers (1)

Principle of Independent Assortment Based on these results, Mendel postulated the Principle of Independent Assortment: “Members of one gene pair segregate independently from other gene pairs during gamete formation” Genes get shuffled – these many combinations are one of the advantages of sexual reproduction

Incomplete Dominance Snapdragon flowers come in many colors. If you cross a red snapdragon (RR) with a white snapdragon (rr) You get PINK flowers (Rr)

Incomplete Dominance When F1 generation (all pink flowers) is self pollinated, the F2 generation is 1:2:1 red, pink, white

Other Examples Hair Texture:  Curly hair is incompletely dominant (HH) to straight hair (SS).  Those who have wavy hair are intermediate (HS). Inter-eye Distance:  Close-set eyes are incompletely dominant (DD) to eyes set far apart (FF).  Medium-set eyes are DF Lip Protrusion:  Protruding lips are incompletely dominant (PP) to nonprotruding lips (NN).  Slightly protruding lips are intermediate (PN).

Codominance Two alleles that are equally dominant One allele that is recessive to both When both dominants are present, both show, not a mid-range. Roan coat color in ungulates (cows!!) -giving both brown and white HAIRS!

Codominance in Human Blood Two alleles are expressed (multiple alleles) in heterozygous individuals. Example: blood 1. type A = IAIA or IAi 2. type B = IBIB or IBi 3. type AB = IAIB 4. type O = ii

Polygenic Traits More than one allele exists within a population Example: human blood groups A, B, AB, O

Summary of Genetics Chromosomes carry hereditary info (genes) Chromosomes (and genes) occur in pairs New combinations of genes occur in sexual reproduction Monohybrid vs. Dihybrid crosses Mendel’s Principles: Dominance: one allele masks another Segregation: genes become separated in gamete formation Independent Assortment: Members of one gene pair segregate independently from other gene pairs during gamete formation