Chapter 14 – Mendel and the Gene Idea

Mendelian Genetics Character – heritable feature Trait – variant for a character True-bred – all offspring of same variety Hybridization – crossing of 2 different true-breds P generation – parents F1 (first filial) generation F2 (second filial) generation

Mendel’s Law of Segregation Alternative versions of genes (alleles) account for variations in inherited characteristics For each character, an organism inherits 2 alleles, one from each parent If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance The alleles for each character segregate (separate) during gamete production (meiosis).

Genetic Vocabulary Punnett square: predicts the results of a genetic cross between individuals of known genotype Homozygous: pair of identical alleles for a character Heterozygous: two different alleles for a gene Phenotype: an organism’s traits, physical appearance Genotype: an organism’s genetic makeup Testcross: breeding of a recessive homozygote with a dominate phenotype (but unknown genotype)

Law of Independent Assortment When tracking the inheritance of 2 or more traits at the same time. Each pair of alleles segregates independently of other pairs of alleles during gamete formation. Genes must be located on different chromosomes.

Exceptions to Mendel’s Rules Challenge: a rooster with grey feathers is mated with a hen of the same phenotype. Among their offspring, 15 chicks are grey, 6 are black, and 8 are white. What is the simplest explanation for this pattern? What phenotypes would you expect in the offspring resulting from a cross between a grey rooster and a black hen? Incomplete dominance: heterozygous genotype expresses a phenotype that is between the phenotypes of the 2 parents, Ex: snapdragons Codominance: heterozygous genotype expresses a phenotype that is a combination of both phenotypes, Ex: cattle/horse coat color (roan) and Tay-Sachs disease in humans

Exceptions cont. Multiple alleles: 3 or more alleles for a given trait, Ex: ABO blood types – has 3 alleles, IA, IB, and i. Challenge: A man with type A blood marries a woman with type B blood. Their child has type O blood. What are the genotypes of these individuals? What other genotypes, and in what frequencies, would you expect in offspring from this marriage?

Exceptions cont. Pleiotropy: genes with multiple phenotypic effects, Ex: sickle-cell

Exceptions cont. Epistasis: a gene at one locus affects the phenotypic expression of a gene at a second locus, Ex: mice coat color, Labrador retriever coat color Challenge: A pale yellow Labrador retriever is mated with a black retriever. In their litter of puppies all 4 Labrador colors are observed. What are the genotypes of the parent dogs? Determine the probability of each color of puppy

Exceptions cont. Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character, Ex: human skin pigmentation and height

Human traits Pedigree – shows family history for a trait Two examples: widow’s peak and earlobes

Human traits cont. Recessively inherited disorders: Cystic fibrosis – most common lethal genetic disease in US (mainly European descent), most die before age of 5 Sickle-cell disease – hemoglobin sickle cells aggregate into large clumps causing clots (most common inherited disease in African-Americans) Carriers – heterozygous for a recessive trait, may suffer from some aspects of the disorder and can transmit the disorder to their offspring Dominantly inherited disorders: Acondroplasia – dwarfism Huntington’s disease – degenerative disease of the nervous system, phenotypic affects visible around 40 years of age

Genetic Testing – fetal testing