Mendelian Genetics How Are Traits Inherited?

Gregor Mendel

• Raised on farm and understood the value of plant breeding.

• At 21, entered priesthood and studied plant breeding in a monastery in the Czech Republic.

• Loved to read especially about natural sciences and was aware of Darwin’s findings.

• Studied the inheritance of traits in pea plants.

• Studied traits that occur in distinct forms.

• Developed true-breeding varieties

• When bred amongst themselves these plants produced offspring identical to the parent for that trait.

• Used mathematical analysis in his studies.

Definitions

• Gametes: reproductive cells produced by sexually reproducing organisms.

•     Two types:

•        male gametes = sperm

•             – In plants: contained in pollen

•        Female gametes = eggs

                    –In plants: contained in ovules

                    –Ovules contained in carpels

Fertilization

• Fertilization: fusion of egg and sperm

         -Self-fertilized:  fusion of sperm and egg from same plant

         -Cross fertilized:  fusion of egg and sperm from two different plants

• Produced hybrids

• F1: first generation

• F2: second generation

Mendel’s Interpretations

• Modified to incorporate today’s vocabulary

–Genes:  the hereditary information that determines a single trait

–Alleles: alternate forms of a gene

• When an organism inherits two identical alleles for a trait, organism is said to be homozygous for the trait.

• When an organism inherits to different alleles for one trait, the organism is called heterozygous for the trait.

Why were all the seeds in the F1 generation round?

• Dominant vs. Recessive Allele

         -Dominant:  an allele that is expressed whenever it is present

         -Recessive:  an allele that is masked whenever the dominant allele is present.

Dominant and recessive alleles influence an organism’s phenotype

Genotype and Phenotype

• Genotype: Genetic makeup of an individual. It is determined by the alleles present for each trait.

• Phenotype: Physical appearance of a trait. It is the expression of the genotype.

Can you look at someone and determine their genotype?

• Testcross is used to determine the genotype of an individual

–Testcross:  crossing an organism with unknown genotype with one that is homozygous recessive for the trait.

–Example:

• Plant that has round (R) seeds crossed with one that is homozygous recessive (rr) for wrinkled seeds

Using Punnett Squares to Predict the Inheritance of Sickle Cell Anemia

Sickle Cell Anemia

• Due to presence of recessive allele denoted by HbS.

• Causes distortion of red blood cells into long, thin sickles.

• Causes pain, tissue damage, and cells to rupture prematurely.

• Carriers: have heterozygous genotypes and do not manifest symptoms.

Punnett Squares

• Used to predict possible offspring genotypes

• Place alleles for each parent on each side

Mother’s genotype         Father’s genotype

   HbA/ HbA    x    HbS/ HbS

Predict inheritance

• Given parents’ genotypes, you can predict offspring’s genotypes and phenotypes

• HbS/ HbS = homozygous recessive results in sickle cell anemia

• Both HbA/ HbS (heterozygous) and HbA/ HbA (homozygous dominant) are normal.

Mendel’s Laws

• Based on Mendel’s data, he formulated two laws:

–Law of Segregation:  A parent contributes only one of its alleles for a trait to each offspring.

–If parent is heterozygous for a trait, the particular allele donated to the offspring is random.

• Law of Independent Assortment: Alleles of one gene are passed to offspring independently of the alleles of other genes.

–Applies to the inheritance of two or more genes simultaneously.

• Can use the inheritance of traits in fruit flies to illustrate the law of independent assortment.

• Dihybrid cross

–A fertilization in which the parents differ in two distinct traits or characteristics.

Probability

• Can be used to better understand segregation and independent assortment.

• Look at coins first…

Chances of tossing a head?

• If you toss a coin, what is the chance of head side landing right-side up?

• What is the chance of tail side landing right-side up?

Probability Rule

• Sum rule: the combined probability of two or more mutually exclusive events occurring is equal to the sum of their individual probabilities

• Example: Parent has Gg genotype. What is the probability that the offspring will receive the g or G allele from the parent?

Probability of g or G = 0.5 + 0.5

Probability of g or G = 1

• Product rule: The joint probability that both of two independent events occur is the product of the individual probabilities of each event.

Example:  Mother has Gg genotype and father has Gg genotype.  What is the probability that the offspring will have gg genotype?

Probability of g from Mom and g from Dad G = 0.5 x 0.5

Probability of gg genotype in offspring = 0.25

Why Aren’t Members of the Same Species Identical?

• Sources of Genetic Variation

–Law of Independent Assortment

–Mutation

Law of Independent Assortment

• Allows for new gene combinations or genetic recombination

• Can mathematically predict the possible combinations

         Number of possible genotypes = 2n

where n = the number of genes or traits considered

          Example:  considering 100 traits:

• 2100 = 1.26765 x 1030

Mutations

• Definition = change in DNA sequence resulting the appearance of a new allele

• Can occur at any time, but not always beneficial

• Some can be harmful and result in death

• Some can result in the organism having traits that make them better suited to the environment

Do Mendel’s Laws Always Apply?

Incomplete Dominance

•A condition where all three genotypes are expressed.
•Phenotypic level: contradicts Mendel’s conclusions.
•Genotypic level:  consistent with Mendel’s laws.

Lethality

•A condition in which the inheritance of a lethal combination of alleles results in death of the organism.
•Lucien Cuenot studied inheritance of coat color in mice.
•Huntington’s disease.

Pleiotropy

• A single gene affects two or more traits.

• Examples:

–One gene affects whether seed coat is round/wrinkled

–Cat fur

Polygenic vs. Monogenic inheritance

• Polygenic: A trait affected by many genes.

–Examples:  Height, weight, skin color

• Monogenic: Traits determined by single gene with two alleles.

–Examples:  Flower color in four-o’clock plants