Chapters 9 & 12: Genetics Heredity – The passing of traits from parents to offspring Genetics – The study of heredity Gregor Mendel • Austrian monk • Bred pea plants • 1860 - developed laws of heredity mms://204.13.204.36/Video9/mendelslaw.asf •He cross-pollinated plants •He bred plants to be pure for certain traits •Ex: Tall parent tall offspring Short parent short offspring •Then he cross-bred plants with opposite traits •Tall x short •Round x wrinkled •Yellow x green •Parents - P generation – tall x short •Offspring – F1 generation (1st filial generation) •All offspring were tall (short trait disappeared) •Allowed F1 generation to self-pollinate •F2 generation – 75% tall to 25% short (short trait reappeared) •Repeated many times – always same ratios for each generation (see results slide #2) Mendel’s Conclusions: •There are 2 factors for every trait (today we know these factors to be genes – 1 from mother, 1 from father) •One of these factors can be dominant over the other (the recessive trait) •This is known as the Law of Dominance •The factors separate when the gametes (eggs & sperm) are formed –The Law of Segregation •Each gamete only has 1 factor from each pair (haploid) •Fertilization gives each new individual 2 factors again (diploid) •Mendel then crossed pure plants that differed in 2 traits •Ex: yellow, round peas crossed with green, wrinkled peas •F1 generation always showed dominant traits •F2 generation had the following results: (see next slide) F2: 9 yellow, round 3 yellow, wrinkled 3 green, round 1 green, wrinkled •Based on these results, Mendel concluded that pairs of factors separate independently during meiosis – The Law of Independent Assortment •Ex: Below, hairline and finger length are not dependent on each other • Alleles - various forms of a trait • Ex: tall and short height curly and straight hair brown and blue eyes Widow’s peak The Epicanthal Fold (eye fold) Genotype and Phenotype • Genotype refers to the genes of an individual; can be represented by two letters • Homozygous - both alleles are the same • Homozygous dominant - WW • Homozygous recessive - ww • Heterozygous – alleles are different - Ww • Phenotype refers to the appearance of the individual. • Both WW and Ww result in widow’s peak, the dominant trait • ww will result in no widow’s peak, the recessive trait Monohybrid Crosses • Considers only one trait. • Punnett square – chart used to determine probability •Ratio shows # of offspring with dominant vs. recessive trait Probability • Determine the odds of an event occurring. • Expressed as fraction or percentage • Ex: (1/4) or 25% • The probability that two or more independent events will occur together is the product of their chances occurring separately • Ex: odds of having a boy = ½ Odds of having 2 boys = (1/2) x (1/2) = (1/4) • The chance of widow’s peak: • WW or Ww = 75% or ¾ • Chance of a continuous hairline: • ww = 25% or 1/4 •Odds of having 3 children with a continuous hairline: •(1/4) x (1/4) x (1/4) = (1/64) Dihybrid Cross • Two traits are considered • Genotypes of the parents require four letters (two for each trait). • Codominance - both alleles are equally expressed in a heterozygote • Ex: Blood type – AB blood • Incomplete dominance – heterozygous genotype shows an intermediate phenotype, representing a blending of traits. • Ex: Curly, wavy, or straight hair in Caucasians ABO Blood Types • How your book shows blood type: Blood type (phenotype) A Genotype IAIA or IA i B IBIB or IBi AB IA IB O ii •How your teacher shows blood type: Blood type (phenotype) A Genotype AA or AO B BB or BO AB AB O OO Inheritance of blood type Incomplete dominance • Other examples of incomplete dominance: • Plants called four o’clocks RR – red RR’ – pink R’R’ – white R R RR • So a cross RR’ R’ between two pink plants produces 1 red, 2 pink, and 1 white plant R’ RR’ R’R’ • Another example includes Sickle cell disease in humans • HbA represents normal hemoglobin; and HbS represents the sickled condition – HbAHbA – normal – HbSHbS – sickle-cell disease – HbAHbS - have the intermediate condition called sickle-cell trait. • Heterozygotes have an advantage in malariainfested Africa because the pathogen for malaria cannot exist in their blood cells. Sex determination: •Female – XX •Male – XY •Always 50% X chance of having a boy or a girl Y •Male determines gender of baby X X XX XX XY XY Sex-Linked Traits • Traits controlled by genes on the X or Y chromosomes • X-linked or Y-linked • Most X-linked traits are recessive, so a female would have to have two recessive genes to express the trait; a male would only need one. • Y-linked traits are only passed from father to son • Examples of X-linked traits include Color blindness, Hemophilia, Muscular Dystrophy, Fragile X Syndrome Phenotype Normal female Carrier female Affected female Normal male Affected male Gentoype XBXB XBXb XbXb XBY XbY Cross involving an X-linked allele Pedigree Charts • Constructed to show the pattern of inheritance of a characteristic within a family. • The particular pattern indicates the manner in which a characteristic is inherited (suggests X-linked, dominant, etc.) Symbols used: Normal female Carrier female Affected female Normal male Affected male Male can be a carrier for an autosomal trait. Autosomal recessive pedigree chart Autosomal dominant pedigree chart Amniocentesis • Uses a needle to extract amniotic fluid from the uterus of a pregnant woman from the 14th to 17th week of pregnancy. • Up to 400 chromosome and biochemical problems can be detected by culturing fetal cells that are in the amniotic fluid. • 0.3% chance of miscarriage with this procedure – do this test only if certain risk factors are present. Amniocentesis Chorionic Villi Sampling (CVS) • Uses a thin suction tube to sample chorionic cells from the placenta as early as the fifth week of pregnancy. • Chorionic cells are found in the placenta • The cells do not have to be cultured, and karyotyping can be done immediately. • 0.8% risk of miscarriage but can be performed earlier than amniocentesis. Chorionic villi sampling