Patterns of Inheritance Chapter 9

Genetics The science of heredity. A distinct genetic makeup results in a distinct set of physical and behavioral characteristics.

History of Genetics Biologists in the 19th century observed inheritance patterns in plants and concluded that offspring inherit traits from both parents. The favored explanation for inheritance then became the “blending” hypothesis. This is the idea that the hereditary materials contributed by the male and female parents mix in forming the offspring.

History of Genetics – Gregor Mendel Modern genetics began in the 1860’s when a monk named Gregor Mendel experimented with breeding garden peas. With a history in mathematics, his research implemented a great deal of statistics. He stressed that the heritable factors (genes) retain their individuality generation after generation (no blending).

History of Genetics – Gregor Mendel He studied pea plants because they had short generation times, they produced large numbers of offspring, and they came in many varieties. Character: flower color, height, seed shape, pod color, etc. Traits (each variant for a character) purple/white flower, tall/short height, round/wrinkled seed, green/yellow pod color.

Characters Traits Mendel chose to study 7 characters, each of which occurred in two distinct forms.

History of Genetics – Gregor Mendel He could strictly control mating of pea plants. Pea plants usually self-fertilize – sperm carrying pollen grains released from the stamens land on the egg containing carpel of the same flower. He could also cross-fertilize – fertilization of one plant by pollen from a different plant.

Cross fertilization

History of Genetics – Gregor Mendel He worked with plants until he was sure he has a true-breed (one that produced offspring all identical to the parent if self-fertilized). Example – parent plant had purple flowers, and if self fertilized it would only produce purple flowered plants generation after generation.

History of Genetics – Gregor Mendel Once he had a true-breed, he then investigated what would happen if he crossed true-breeding varieties with each other. Example – what offspring would result from cross-fertilization of true-breeds? purple flowers x white flowers This offspring of two different true-breeds is called a hybrid. (the fertilization is called hybridization, or cross.)

History of Genetics – Gregor Mendel P generation – the true-breeding parental plants. F1 generation – the hybrid offspring. (F stands for filial, Latin for “son”) F2 generation – result self-fertilization of the F1 plants.

History of Genetics – Gregor Mendel Mendel tracked and recorded the inheritance of characters, & the results lead him to formulate several ideas about inheritance. Let’s look at his monohybrid-cross (parent plants differ in only 1 character).

History of Genetics – Gregor Mendel He crossed a true-breed purple flower with a true-breed white flower. He observed that the F1 generation were all purple flowers. Self-fertilizing the F1 generation he found that the F2 generation had a ratio of 3:1 (3 purple for every 1 white). He concluded that the white trait did not disappear, and that they MUST carry two factors for the flower color character. He called these alleles – alternative versions of a gene.

Homologous chromosomes Alleles reside at the same locus on homologous chromosomes.

History of Genetics – Gregor Mendel Dominant allele – is always expressed if present. (like a trump card) PP or Pp Recessive allele – is only expressed if the dominant allele is NOT present. pp

History of Genetics – Gregor Mendel A homozygous genotype has identical alleles. PP or pp A heterozygous genotype has two different alleles Pp

Law of Segregation: A sperm or egg carries only one allele for each inherited character. because allele pairs separate from each other during the production of gametes.

Genetic composition & appearance Genotype – organism’s genetic makeup. (Represented by letters) Phenotype – organism’s expressed or physical traits.

Genetic composition & appearance Carriers – organisms that are heterozygous, they carry the recessive allele for a trait but phenotypically only the dominant trait is expressed.

Types of hybrid crosses Monohybrid cross – the parents differ in only one character. Green or Yellow seeds. Dihybrid cross – parents differ in two characters. (Round or Wrinkled) and (Green or Yellow) seeds.

Law of Independent Assortment: Each pair of alleles segregates independently of other pairs of alleles during gamete formation.

Punnett Square Punnett squares are used to show the probability of what genotypes the offspring could have.

Test Cross Used to determine the genotype of a unknown character. Used to verify if organism is in fact a true-breed. B = black. The dominant allele. But, is it BB or Bb? This is unknown.

Test Cross Mate organism with unknown genotype, with an organism that has a homozygous recessive genotype. The appearance of the offspring reveals the unknown genotype.