How are traits inherited? Blending Hypothesis Traits of parents blend together like paint. Randomly mating population should lead to uniform traits. Particulate Hypothesis Traits of parents are in discrete units (genes) which do not mix. Individual units (genes) retain identity

Mendel’s Approach Carefully planned experiments to test blending hypothesis of heredity Used distinctive characteristics of pea plant Studied offspring of 1st and 2nd generations Counted offspring and used quantitative analysis

A genetic cross (Fig.14.1)

Mendel tracked traits for three generations (Fig. 14.2)

Alleles, alternate forms of a gene (Fig. 14.3)

Mendel’s law of segregation (Fig. 14.4)

Mendel’s Principles (Determine from monohybrid crosses) Alternate versions of genes (different alles) account for variations in inherited characteristics. For each character or factor, an organism inherits two alleles, one from each parent. If two alles differ, then one (dominant allele) is expressed. The other (recessive allele) has no noticeable effect. The two alleles separate during gamete formation- Mendel’s Law of Segregation.

Genotype versus phenotype (Fig. 14.5)

A testcross (Fig. 14.6)

Testing two hypotheses for segregation in a dihybrid cross (Fig. 14.7)

Dihybrid Cross--Conclusions Alleles controlling different traits assort independently of one another during the formation of gametes. Mendel’s Principle of Independent Assortment

Mendel’s Principles Rap A pair of genes control each trait. When gametes form, pairs separate. Dominant genes hide recessive ones. Each pair of genes independently runs.

Chance and probability Chance = any situation in which the factors affecting the outcome are so numerous and (taken individually) so weak that we can never hope to determine a cause. Probability = the application of mathematics to the prediction of events happening by chance.

Science and probability Science deals primarily with probabilities and not with certainties: Decay of radioactive atoms Collisions of molecules in a gas Effects of smoking on health Distribution of genes during meiosis

Basic question of probability How often should we expect a particular event to occur in a given number of events? Probability = Probability of flipping heads = on a coin number of desired events number of possible events 1 (heads up desired) 2 (heads or tails up is possible)

Multiplication rule of probability The probability that two or more independent events will occur together in some specific combination is equal to the product of the probabilities of each event occuring separately. The probability that a coin will turn up heads two times in a row is ½ times ½ or ¼. The probability that two dice will turn up one is 1/6 times 1/6 or 1/36.

Addition rule of probability The probability that any one of two or more mutually exclusive events will occur is calculated by adding together their individual probabilities. The probability of turning up an ace in a deck of 52 cards is 1/52 (ace of spades) plus 1/52 (ace of diamonds) plus 1/52 (ace of clubs) plus 1/52 (ace of hearts) or 1/13.

Segregation of alleles and fertilization as chance events (Fig. 14.8)

Spectrum of dominance Complete Dominance Incomplete Codominance Trait of dominant allele hides trait of recessive allele Traits of both alleles blend together Traits of both alleles express themselves Flower color in peas, tongue rolling in humans Flower color in snap-dragons, eye shape in fruit flies Coat color in cows, blood type in humans

Incomplete dominance in snapdragon color (Fig. 14.9)

Multiple alleles for the ABO blood groups (Fig. 14.10)

Pleiotropy One gene often has multiple effects on phenotype. Pleiotropic effects of sickle-cell allele (see Fig. 14.15): Breakdown of red blood cellsphysical weakness, anemia, heart failure. Clumping of cells and clogging of small blood vesselsheart failure, pain, fever, brain damage. Accumulation of sickled cells in spleenspleen damage.

Epistasis: an example of gene interactions (Fig. 14.12)

A simplified model for polygenic inheritance of skin color (Fig. 14

Environmental effects on gene expression: fur color in Siamese cats (warmer areas have lighter coat; cooler areas have darker coat)

Environmental effects on gene expression: flower color in Hydrangea soil pH < 5.5 soil pH > 6.5

Variation in gene expression In multifactorial traits –many factors, both genetic and environmental, collectively influence phenotype. This range of phenotype possibilities is called the norm of reaction). Multifactorial traits Skin pigmentation (exposure to sun vs. genes) Red and white blood cell counts (altitude, activity level vs. genes) Size/shape/greeness of leaves

Pedigree Diagrams Basic Symbols Pedigree Diagrams: I Basic Symbols There are several conventions that are followed in the representation of pedigrees. Traditionally, a circle is the basic symbol for a female. The symbol for a male is a square. A horizontal line connecting a male and female depicts a mating. Reference Campbell, N. E. & Reece, J. B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings.

Hemophilia: An Example In this pedigree, only males are affected, and sons do not share the phenotypes of their fathers. Thus, hemophilia is linked to a sex chromosome–the X. Expression of hemophilia skips generations. Thus, it is recessive. Extensive bruising of the left forearm and hand in a patient with hemophilia. Hemophilia Hemophilia is a condition of excessive bleeding caused by missing clotting factors in the blood. Hemophiliacs are prone to bruising, as illustrated in the photo here, and to other, potentially fatal, risk factors. In this pedigree, there is a trend for only males to express the trait strongly suggesting the role of sex chromosomes. However, the sons do not share the phenotypes of their fathers, so the Y chromosome is not a likely candidate. Thus, we can conclude that the gene for hemophilia is on the X chromosome. Since the trait skips generations, we can assume that an allele for hemophilia is recessive to an allele for normal blood clotting factors. Although rare, a female can be afflicted if she inherits an allele for hemophilia on both X chromosomes. Reference Campbell, N. E. & Reece, J. B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings. Image Reference Young, M. (2005). Pedigree chart. Houston, TX: Baylor College of Medicine, Center For Educational Outreach. Hemophilia A. Retrieved 08-10-2006 from http://medgen.genetics.utah.edu/index.html

Albinism: Parent-Offspring Relationships #1 must transmit “a” to each offspring. The “A” in the offspring must come from the father. Normal father could be either heterozygous or homozygous for an “A.” ** Parent – Offspring Relationships Now, we will consider more of the parent – offspring relationships, starting with individual #1’s family. We already know that individual #1 is a homozygote for “a” because she expresses albinism. We also already have determined that her offspring must have at least one “A” because they all are normal. Thus, we can conclude that the offspring must have received their normal allele from their father, because the mother can contribute only an “a.” All offspring in this family are heterozygotes. We do not have enough information to determine, for sure, whether the father (individual #2) is a heterozygote or is homozygous for the normal allele. If we consider that albinism is a rare genetic trait, however, we could fairly safely guess that the father is homozygous for the normal allele because we expect heterozygotes to be rare in the population. Still, we cannot rule out that the father is carrying the “a” allele. Reference Campbell, N. E. & Reece, J. B. (2002). Biology (6th ed.). San Francisco: Benjamin Cummings. Image Reference Young, M. (2005). Pedigree chart. Houston, TX: Baylor College of Medicine, Center For Educational Outreach.

Fetal diagnosis (Fig. 14.17)