Transcript genetics
Mendelian Genetics Introduction to Genetics Gregor Mendel Father of Genetics GENETICS: study of heredity HEREDITY: passing of traits from parent to offspring TRAITS: characteristics Gregor learned about these patterns by studying PEAS Mendel’s Pea Plants Pea plants are normally selfpollinating (fertilizes itself; producing offspring from a single parent) Self-pollinating plants produce offspring identical to the parent. Mendel eliminated the selfpollinating and allowed them to cross-breed so he could observe the offspring of 2 different parents. Medel’s Pea Plants Mendel Studied 7 different traits and looked at 3 generations. P1= the original parent generation F1= first generation of offspring F2= second generation of offspring Mendel’s Conclusions Inheritance is determined by genes that determine the traits AND there are 2 different forms of a gene, called alleles. Some alleles are DOMINANT and some are RECESSIVE Review: How do traits get passed from parent to offspring? Through GAMETES: sex cells Male gamete is the SPERM Female gamete is the EGG FERTILIZATION: union of egg & sperm ZYGOTE: fertilized egg GENES VS. ALLELES Genes Segment of DNA that codes for one protein/trait Most traits are determined by TWO genes. Remember chromosomes have a homologous match…two chromosomes with similar genetic information EXAMPLE: Hair color, eye color, height in pea plants Alleles Each form of a gene is called an ALLELE EXAMPLE: brown or blond blue, hazel, brown tall, short Law of Dominance DOMINANT (allele) Always expressed if present Only need ONE dominant allele to be expressed EXAMPLES: dark hair, brown eyes, tall peas RECESSIVE (allele) May be present but will not be expressed if paired w/ dominant allele Requires TWO recessive alleles for recessive trait to be expressed EXAMPLES: light hair, blue eyes, short peas Law of Segregation Gene pairs separate during formation of gametes (meiosis) Each gamete gets ONE allele from a pair Each parent passes down one allele to their offspring Offspring get 2 alleles (one from each parent) Preparing for Genetic Crosses Letters will represent alleles Use the first letter of the dominant trait Capital letter = dominant trait Lower case letter = recessive trait EXAMPLE: height in pea plants TALL is dominant over short, so… T = tall t = short Genotype vs. Phenotype All possible combinations using T, t: TT, tt, Tt, tT GENOTYPE: actual genes in the pair; can’t see…they’re on chromosomes…but they determine the individual’s traits Use terms: homozygous: both genes the same (TT, tt) heterozygous: genes different; 1 dominant, 1 recessive (Tt, tT) Genotype vs. Penotype PHENOTYPE: the trait that is expressed; the ‘physical’ appearance Phenotype Genotype TT…tall (homozygous dom) tt….short (homozygous rec) Tt….tall (heterozygous) tT….tall (heterozygous) Mendelian Traits Traits that have 2 alleles EXAMPLE: cleft chin If a person inherits two dominant alleles (CC) or one dominant & one recessive (Cc), they will have a cleft chin (top) If a person inherits 2 recessive alleles (cc), they will not have a cleft chin (bottom) Solving Genetic Crosses Use PUNNETT SQUARES Diagram used to predict the offspring given genotypes of parents. The parents genotypes go on outside of square (one parents’ across top; other parents’ down left side Monohybrid Cross These involve just ONE trait; therefore, you will see just ONE letter (but it can vary in size). Ex. Aa x Aa or AA x aa Test Cross Just because you know the phenotype, does not mean you know the genotype for a particular trait! To identify the genotype of an organism with a dominant trait, you must perform a test cross. This means you mate the organism of the unknown genotype with a homozygous recessive organism. Ex. You need to find the genotype of a brown rat. Brown is dominant to white. The genotype of the brown rat could be _____ or _______.