Lecture #24 **change PPT Guide # to 27 Introduction to Mendel Honors Biology Ms. Gaynor

Transmission (passing down) of Traits How? One possible explanation of heredity is a “blending” hypothesis genetic material contributed by two parents mixes

Another Hypothesis An alternative to the blending model is the hypothesis of inheritance (genes) Parents pass on discrete heritable units (factors) called genes

Gregor Johann Mendel 1843 Documented a mechanism of inheritance through his experiments with garden peas Figure 14.1

Gregor Johann Mendel Mendel discovered the basic principles of heredity By breeding garden peas in carefully planned experiments

Mendel’s Experimental Method Why use pea plants? available in many varieties easy to get he could strictly control which plants mated with which Grow quickly

Pea Plant Fertilization Self fertilization : mate with self  produce identical offspring TRUE or PURE breeds Cross fertilization : mate with another  can produce different offspring HYBRIDS

SELF Cross B A

Procedure: Crossing pea plants

Mendel’s Procedures Mendel chose to track Only those characteristics (traits) that varied in an “either-or” manner Mendel also made sure that He started his experiments with varieties that were “true-breeding”

Mendel’s Results Mendel only looked at “either-or” characters Ex: Purple OR white flowers Mendel started his experiments with “true-breeding” Made through self fertilization so plants are “TRUE” for only 1 trait Known as HOMOZYGOUS for trait

What was Mendel’s Procedure? 1. He made 14 “TRUE BREEDS” 1 for EACH trait he looked at These are the original parents Are called the P generation

Mendel’s Generations Original Parents Are called the P generation The hybrid (mixed) offspring of the P generation Are called the F1 generation When F1 individuals self-pollinate The F2 generation is produced

What was Mendel’s Procedure? 2. He used cross fertilization to mate 2 true breeds for same gene Ex: Purple vs white flower color 3. He collected the offspring (progeny) The hybrid (mixed) offspring of the P generation Are called the F1 generation

What was Mendel’s Procedure? 4. He crossed (using cross fertilization) male and female from F1 progeny When F1 individuals are mated together The F2 generation is produced

Generations (in general) P = parental generation of a cross F1 = the first generation after the parental (the results of the 1st cross) F2 = a cross between F1 individuals yields F2 (2nd cross)

What did Mendel Discovered? A 3:1 ratio, purple to white flowers, in the F2 generation P Generation (true-breeding parents) Purple flowers White  F1 Generation (hybrids) All plants had purple flowers F2 Generation Where did the white color go?

What did Mendel’s Conclude? Mendel reasoned that In the F1 plants, only 1 factor (ex: purple flower) was affecting physical outcome color in hybrids This factor was dominant and the hidden factor was recessive

What are Mendel’s factors? Mendel’s “factors” are now called alleles Alternative version or form of a gene Figure 14.4 Allele for purple flowers SAME locus for flower-color gene Homologous pair of chromosomes Allele for white flowers F f

Alleles Alleles = alternative forms of genes (Mendel’s “factors”) Each individual has 2 alleles for the same gene (because there are 2 homologous chromosomes) Each parent passes one allele for each gene to his/her offspring

h or H

Useful Genetic Vocabulary Homozygous A pair of IDENTICAL alleles for that gene Exhibits true-breeding aa = homozygous recessive (or just recessive) HH = homozygous dominant Heterozygous A pair of alleles that are different for that gene Aa or Hh

Recessive is… Recessive an allele that does NOT produce a characteristic effect when present with a dominant allele only expressed when the determining allele is present in the homozygous condition aa or hh

Dominant is… Dominant an allele that produces the same phenotypic effect whether inherited with a homozygous or heterozygous allele Aa or AA, Hh or HH The allele that is expressed Not necessarily better, stronger, etc.

Mendel’s Law of Segregation The two alleles for a characteristic separate (segregate) during gamete formation (Anaphase I of meiosis) end up in different gametes Each gamete gets 1 allele ONLY

Mendel’s Model Mendel developed a model to predict inheritance Mendelian Genetics states That heterozygotes will have the SAME phenotype as homozygous dominant individuals Aka-Complete Dominance

Mendelian Genetics COMPLETE DOMINANCE If the 2 alleles at a locus differ (heterozygous) Then the dominant allele determines organism’s appearance The recessive allele has no noticeable effect on organism’s appearance (it is hidden) The 1st allele is “completely dominant” over the 2nd allele

More Genetic Vocabulary An organism’s phenotype Is its physical appearance Ex: Blue eyes, brown hair, etc An organism’s genotype Is its genetic makeup A.k.a.-the allele combination Ex: aa, Aa, AA http://www.hippocampus.org/Biology;jsessionid=C83A30FA6EDF5E6BAEEDC6F71B0363E3