Standard VII- Applying Mendel’s Laws Biology AHSGE Standard VII- Applying Mendel’s Laws

Heredity The passing of characters from parents to offspring Genetics- Branch of Biology dealing with heredity Gregor Mendel- Austrian monk who developed rules of heredity Experimented with pea plants

Mendel’s Experiments Monohybrid cross- Cross of parents involving one pair of contrasting traits True breeding- All offspring display one characteristic P generation- First two individuals crossed F1 generation (filial)- First offspring from P generation F2 generation- Second offspring resulting from F1 generation

Mendel’s Hypotheses For each inherited trait, an individual has two copies of the gene- one from each parent. There are alternative versions of genes- alleles When two different alleles are present, the individual may completely express one, while the other allele has no observable affect Dominant- Expressed allele; only one copy needed for expression; capital letter Recessive- Allele not expressed when dominant allele is present; need both copies for expression; lowercase letter When gametes are formed, each gene’s alleles separate independently, therefore gametes have one allele for each gene

Modern Terms Homozygous- Having two of the same alleles for a gene Ex: AA, BB, aa or bb Heterozygous- Having two different alleles for a particular gene Ex: Aa or Bb Genotype- The set of alleles for a characteristic or trait (the letters) Phenotype- The expressed trait or physical appearance of a genotype

Laws of Heredity The Law of Segregation- When gametes are formed, each gene’s alleles separate (segregate) independently, therefore gametes have one allele for each gene The Law of Independent Assortment- Alleles of different genes separate independently of one another during meiosis

Punnett Square Diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in the cross Test cross- An individual whose phenotype is dominant, but whose genotype is not known, is crossed with a homozygous recessive individual Monohybrid cross- Cross of parents involving one pair of contrasting traits Dihybrid Cross- Cross of parents involving two pairs of contrasting traits

Monohybrid Cross Cross a heterozygous, brown-haired woman with a heterozygous, brown-haired man; Brown hair is dominant, while blonde is recessive

Probability The likelihood that a specific event will occur Expressed as percentages or ratios Probability = number of one kind of possible outcome total number of all possible outcomes Genotypic ratio- Ratio of genotypes from a punnett square Phenotypic ratio- Ratio of phenotypes from a punnett square

Punnett Square and Probability - Probability of offspring with brown hair; blonde Genotypic Ratio Phenotypic Ratio

Dihybrid Cross Cross two heterozygous pea plants for seed shape (R= round; r= wrinkled) and color (Y= yellow; y= green); RrYy 1st- Complete a monohybrid cross of the two traits for each parent to determine possible gametes

Dihybrid Cross

Dihybrid Cross Calculate the genotypic and phenotypic ratios for the previous cross Genotypic Ratio 1 RRYY: 3 RRYy: 2 RrYY: 3 RrYy: 1 RRyy: 2 Rryy: 1 rrYY: 2 rrYy: 1rryy Phenotypic Ratio 9 Round; yellow: 3 Round; green: 3 Wrinkled; yellow: 1 Wrinkled; green

Inheritance of Traits Pedigree- Family history that shows how a trait is inherited over several generations Circles- female Squares- male Horizontal lines show mating Vertical lines indicate offspring Completely shaded are affected Partial shading represents a carrier

Pedigree-Albinism

Albino

Sex-linked Genes Allele is found on the X or Y chromosome XX- female; XY- male Mostly carried on X and recessive Males receiving this on the X would exhibit the condition due to having only one X (Ex: Hemophilia)

Hyperchol-esterolemia Genetic Disorder Dominant or Recessive Symptom Defect Frequency Sickle Cell Anemia Recessive Poor blood circulation Abnormal hemoglobin 1/500 (African american) Hyperchol-esterolemia Dominant Excessive cholesterol in blood; heart disease Abnormal cell surface receptor for cholesterol 1/500 Tay-Sachs Disease Deterioration of nervous system; death Defective form of brain enzyme 1/3,500 (Ashkenazi Jews) Cystic Fibrosis Mucus clogging or organs; do not survive to old age Defective chloride-ion transport protein 1/2,500 (Caucasians) Hemophilia A Sex-linked recessive Failure of blood to clot Defective form of blood-clotting factor 1/10,000 (males) Huntington’s Disease Gradual deterioration of brain tissue in middle age Inhibitor of brain cell metabolism 1/10,000

Down’s Syndrome Chromosomal mutation resulting in mental retardation and physical abnormalities 1 in every 1,000 people born Having an extra chromosome on the 21st pair of human chromosomes Trisomy-21 Caused by nondisjunction of the 21st pair during Anaphase I of meiosis