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)

1. Alternate versions of genes (different alles) account for variations in inherited characteristics.

2. For each character or factor, an organism inherits two alleles, one from each parent.

3. If two alles differ, then one (dominant allele) is expressed. The other (recessive allele) has no noticeable effect.

4. 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 eventsnumber 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

DominanceCodominance

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 cellsphysical

weakness, anemia, heart failure.– Clumping of cells and clogging of small blood

vesselsheart failure, pain, fever, brain damage.– Accumulation of sickled cells in spleenspleen

damage.

Epistasis: an example

of gene interactions (Fig. 14.12)

A simplified model for polygenic inheritance

of skin color (Fig. 14.12)

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

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.

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.”

**

Fetal diagnosis (Fig. 14.17)