APBIO- Chpt 14A “Overview of Gregor Medel’s Discoveries”

Warm-Up: Compare and contrast the blending hypothesis and particulate hypothesis for heredity.

Answer to Warm-Up

• SIMILARITIES:• Both hypotheses are related to the study and ideas of genetics and heredity. Both

were accepted, at some point in time, as the means for understanding genes and traits.

• DIFFERENCES:• “Blending” hypothesis:• This hypothesis proposes that the genetic material contributed by each parent mixes

in a manner analogous to the way blue and yellow paints blend to make green.• Over many generations, a freely mating population should give rise to a uniform

population of individuals.• However, the “blending” hypothesis appears incorrect as everyday observations and

the results of breeding experiments contradict its predictions.• An alternative model, “particulate” inheritance: • Proposes that parents pass on discrete heritable units - genes - that retain their

separate identities in offspring.• Genes can be sorted and passed on, generation after generation, in undiluted form.

Objectives

• (1) Students will be able to describe the patterns of inheritance that Mendel’s data revealed.

• (2) Students will be able to summarize Mendel’s law of segregation.

• (3) Students will be able to explain how there can be many versions of one gene.

• (4) Students will be able to describe how genes influence the developments of traits.

• (5) Students will be able to describe monohybrid and dihybrid crosses.

• (6) Students will be able to explain how heredity can be illustrated mathematically.

Mendel & Heredity

• KEY CONCEPT: Mendel’s research showed that traits are inherited as discrete units.

Mendel & Heredity

• Mendel laid the groundwork for genetics.• Traits are distinguishing

characteristics that are inherited.• Genetics is the study of biological

inheritance patterns and variation.• Gregor Mendel showed that traits

are inherited as discrete units.• Many in Mendel’s day thought

traits were blended.

Mendel & Heredity

• Mendel’s data revealed patterns of inheritance.• He made three key decisions in his experiments.

1. Use of purebred plants2. Control over breeding3. Observation of seven

“either-or” traits

Mendel’s Experiment

• Mendel used pollen to fertilize selected pea plants.• P generation crossed to produce F1 generation• Interrupted the self-pollination process by removing

male flower parts

• Mendel allowed the resulting plants to self-pollinate.• Among the F1 generation, all plants had purple flowers• F1 plants are all heterozygous• Among the F2 generation, some plants had purple

flowers and some had white

• Mendel observed patterns in the first and second generations of his crosses.

Mendel & Heredity

• Mendel drew some important conclusions:• Traits are inherited as discrete units.• Organisms inherit two alleles (copies of each

gene), one from each parent.• The two copies segregate during gamete

formation.• The last two conclusions are called the law

of segregation.

You Tube Video Clip- Gregor Mendel

• http://www.youtube.com/watch?v=aDpLDBaEBjk&feature=related

Reflection

• (1) Describe the patterns of inheritance that Mendel’s data revealed.• (2) Summarize Mendel’s law of segregation.

Traits, Genes, & Alleles

• KEY CONCEPT :Genes encode proteins that produce a diverse range of traits.

Traits, Genes, & Alleles

• The same gene can have many versions. • Gene- piece of DNA that

directs a cell to make a certain protein.

• Each gene has a locus, aspecific position/location.

Traits, Genes, & Alleles

• Allele- any alternative form of a gene occurring at a specific locus on a chromosome. • Each parent donates one allele

for every gene.• Homozygous describes two

alleles that are the same at a specific locus.

• Heterozygous describes two alleles that are different at a specific locus.

Traits, Genes, & Alleles

• Genotype- refers to the makeup of a specific set of genes.• i.e., Bb, BB, bb

• Phenotype- is the physical expression of a trait.• i.e., blue eyes, blonde hair, tall

• Two organisms can have the same phenotype but have different genotypes if one is homozygous dominant and the other is heterozygous.

Traits, Genes, & Alleles

• Alleles- represented using letters.• A dominant allele is expressed

as a phenotype when at least one allele is dominant.

• A recessive allele is expressed as a phenotype only when two copies are present.

• Dominant alleles are represented by uppercase letters; recessive alleles by lowercase letters.

Traits, Genes, & Alleles

• Both homozygous dominant (i.e., BB) and heterozygous (i.e., Bb) genotypes yield a dominant phenotype.• Most traits occur in a range and do not

follow simple dominant-recessive patterns.

Reflection

• (3) Explain how there can be many versions of one gene.• (4) Describe how genes influence the

development of traits.

Traits & Probability

• KEY CONCEPT:The inheritance of traits follows the rules of probability.

Traits & Probability

• Punnett squares illustrate genetic crosses. • The Punnett square is a grid

system for predicting all possible genotypes resulting from a cross.• Axes- possible gametes

of each parent.• Boxes- possible genotypes

of the offspring.• The Punnett square yields the

ratio of possible genotypes and phenotypes.

Traits & Probability

• Monohybrid crosses examine the inheritance of only one specific trait.

• homozygous dominant-homozygous recessive: all heterozygous, all dominant

Traits & Probability

• heterozygous-heterozygous—1:2:1 homozygous dominant: heterozygous:homozygous recessive; 3:1 dominant:recessive

Traits & Probability

• heterozygous-homozygous recessive—1:1 heterozygous:homozygous recessive; 1:1 dominant:recessive

• A testcross is a cross between an organism with an unknown genotype and an organism with the recessive phenotype.

Traits & Probability

• Dihybrid cross involves two traits. • Mendel’s dihybrid crosses with

heterozygous plants yielded a 9:3:3:1 phenotypic ratio.

• Mendel’s dihybrid crosses led to his second law:• The law of independent

assortment states that allele pairs separate independently of each other during meiosis.

Traits & Probability

• Heredity patterns can be calculated with probability. • Probability is the likelihood that something will happen.• Probability predicts an average number of occurrences, not an exact number

of occurrences.• Probability = number of ways a specific event can occur number of total possible outcomes

Probability applies to random events such as meiosis and fertilization.

Traits & Probability

• Mendel’s laws of segregation and independent assortment reflect the same laws of probability that apply to tossing coins or rolling dice.

• The probability scale ranged from zero (an event with no chance of occurring) to one (an event that is certain to occur).• (1) We can use the rule of multiplication to determine the chance that

two or more independent events will occur together in some specific combination.

• (2) The rule of addition also applies to genetic problems. Under the rule of addition, the probability of an event that can occur two or more different ways is the sum of the separate probabilities of those ways.

• We can combine the rules of multiplication and addition to solve complex problems in Mendelian genetics.

Summary of Mendel

• Mendel’s laws of independent assortment and segregation explain heritable variation in terms of alternative forms of genes that are passed along according to simple rule of probability.

• These laws apply not just to garden peas, but to all other diploid organisms that reproduce by sexual reproduction.

• Mendel’s studies of pea inheritance endures not only in genetics, but as a case study of the power of scientific reasoning using the hypothetico-deductive approach.

Reflection

• (5) Describe monohybrid and dihybrid crosses.• (6) Explain how heredity can be illustrated

mathematically.