PREDICTING THE INHERITANCE OF GENETIC TRAITS

Recall Mendel’s Experiments True-breeding: “pure-bred” Hybrid: the offspring of two different varieties. P: Parental generation F1: First filial generation (first set of offspring) F2: Second filial generation (second set of offspring) P Generation True-Breeding F1 Generation Hybrid F2 Generation

Mendel’s Inferences Each plant had two “factors” that act as a set of instructions for each characteristics. 2. Each parent donates one of these factors to the offspring. 3. One factor or trait may dominate over the other if it is present.

Sexual Reproduction means Characteristics are Inherited in Pairs Homologous Chromosomes: carry genes associated with the same inherited traits – but may have different versions. Alleles: One of two or more different forms of a gene. These are the Mendelian “factors.”

Genetic Terminology Phenotype: the physical description of an organism’s trait. Genotype: The genetic description of an organism’s trait – the combination of alleles. Homozygous: An organism with two of the same alleles for a particular trait. Heterozygous: An organism with two different alleles for a particular trait.

Mendelian Laws of Genetics Law of Segregation Alleles for each inherited trait separate from each other during gamete formation Each gamete carries one allele for each characteristic. Each gamete contributes its allele for a characteristic during fertilization. Law of Independent Assortment Two alleles for one gene segregate independently of the alleles of the other genes. If A and a are the alleles for one trait, and R and r are the alleles for another trait… Note how each combination of alleles, AB, Ab, ab, and aB are equally likely to occur in each gamete.

The exception… Gene Linkage Genes close to each other on the chromosome are usually inherited together – Linked Genes. They are less likely to be split up by crossing over.

Patterns of Inheritance There are many different types of inheritance, but we will look at 4 major types: Mendelian Dominant & Recessive Inheritance Incomplete Dominance Codominance Sex-linked Inheritance

1) Mendelian Dominant & Recessive Inheritance One allele is designated dominant while the other is recessive. It only takes the presence or absence of the dominant allele to determine the phenotype. There are only two possible phenotypes. Example: Green is dominant and yellow is recessive. What symbols should represent each allele? DOMINANT: _____ RECESSIVE: _____

Homozygous Dominant Homozygous Recessive G G g g GG gg Genotype: __________________ Phenotype: _________________ Genotype: __________________ Phenotype: _________________ Homozygous Recessive G g Genotype: __________________ Phenotype: _________________ Gg

GENETIC CROSSES Genetic Cross: Any type of deliverate breeding between a male and female to produce offspring to carry the genetic material of each parent. Monohybrid Cross: A genetic cross in which one trait is being studied. Punnet Square: A simple grid that shows the different possible genotypes of offspring based on the genotypes of the parents.

USING PUNNETT SQUARES TO PREDICT INHERITANCE Example Question: In a pea plant, purple flowers are dominant over white flowers. If a heterozygous plant is crossed with a white plant, determine the phenotypes of the offspring. Follow these steps! State the symbols for the alleles. Determine the genotypes of the parents – use appropriate symbols. Place the parental genotypes on the top and side of a Punnett square. Cross! The inside of the Punnett square shows the offspring’s genotypes. Summarize the genotypes and phenotypes of the offspring using ratios, fractions, or percentages.

Practice Questions: The allele for black hair is dominant to the allele for red hair colour in a horse. If two heterozygous individuals breed, what is the chance they will have a red-haired offspring?

2) Incomplete Dominance Neither allele is considered recessive. Heterozygous conditions produce a blend of the two traits, creating a third phenotype. The alleles are represented by bare capital letters and the same capital letter with a “prime,” R’ R RR’

Practice Example: A cross between a blue blahblah bird and a white blahblah bird produces offspring that are silver. What would be the phenotypic ratios of offspring produced by one silver and one blue blahblah bird?

3) Codominance Neither allele is considered recessive. Heterozygous conditions produces a phenotype where both alleles for the trait are displayed – think “checkered.” Just like Incomplete Dominance, the alleles are represented by capital letters. W R RW

Practice Example: When white chickens are crossed with black chickens, the offspring have both black and white feathers. If two “spotted” chickens are crossed, what will they the phenotypes of the offspring?

The Inheritance of Sex-Linked Traits next lesson!