Intro to Genetics and Mendel Honors Biology Ms. Kim

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 http://wps.aw.com/bc_campbell_biology_7/29/7523/1925929.cw/nav_and_content/index.html The Novelty Gene Video

Gregor Johann Mendel (1843) Austrian Monk- “Father of Modern Genetics” Documented a mechanism of inheritance through his experiments with garden peas The scientific study of heredity is called GENETICS! Worked with pea plants in his monastery Correctly believed that heritable factors (genes) retain their individuality from generation to generation i.e. – marbles (no blending of colors!) Figure 14.1

Gregor Johann Mendel Mendel used the scientific method to identify two laws of inheritance Mendel discovered the basic principles of heredity By breeding garden peas in carefully planned (CONTROLLED) experiments

Mendel’s Experimental Method Why did Mendel choose pea plants? available in many varieties of traits They have seven distinct & observable traits easy to get he could strictly control which plants mated with which Grow quickly They reproduce quickly & have a short life cycle They produce many offspring in one cross

Mendel’s Experimental Cross Purebred white and purple flowers All Purple All White Both White/Purple Offspring were allowed to self pollinate White flowers reappear in some offspring What did Mendel notice? Did the trait for white flowers disappear in F1 generation?

Some genetic vocabulary Character: a heritable feature, such as flower color Trait: a variant of a character, such as purple or white flowers

Mendel observed the same pattern In many other pea plant characters

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 http://wps.aw.com/bc_campbell_biology_7/29/7523/1925929.cw/nav_and_content/index.html Colored Cotton Video

Mendel’s Experiment 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

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

What did Mendel Discover? P  F1  F2 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?

Genetics Vocab Mendel worked with his pea plants until he was sure that all were true- breeding varieties (pure bred) P generation  parental generation have offspring called the F1 generation (hybrids) Hybrid (F1) the offspring of two true breeding varieties If F1 generations self-fertilize/cross, their offspring are called the F2 generation

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 Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers F f

Mendel’s Experiments After studying pea plants, Mendel concluded that: Traits are passed from one generation to the next through genes. Each trait is controlled by a different form of a gene called an allele Some alleles are dominant to others called recessive traits New question: Have the recessive alleles disappeared or are they still present in the parents? Genes are the different parts of the DNA that decide the genetic traits a person is going to have. Alleles are the different sequences on the DNA-they determine a single characteristic in an individual.

What did Mendel 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

Recessive is… Represented by a lowercase letter (it is NOT the letter itself, though) an allele that does NOT produce a characteristic effect when present with a dominant allele only expressed when present with another (identical) recessive allele This is known as the homozygous condition aa or hh

Dominant is… Represented by a uppercase letter an allele that produces the same trait whether inherited with a another dominant allele (homozygous) or with a recessive allele (heterozygous) Aa or AA The allele that is expressed if present

Frequency of Dominant Alleles Not necessarily better, stronger, etc. than recessive alleles Ex: Polydactyl

Mendel’s Experiments Mendel crossed the first generation and saw that the recessive trait showed up in about 1 of 4 plants. Conclusion: Law of Segregation! What is the Law of Segregation?! Organisms inherit two copies of each gene (one from each parent) Organisms donate only one copy of each gene in their gametes (sex cells) Therefore, the two copies of each gene segregate (separate) during gamete formation (meiosis) Segregation (means separation): Alleles for the SAME trait (like height) separate (segregate )into different gametes; i.e., one sperm gets a "t" and the other gets a "T" (that's it !) Segregation of the alleles happens during the formation of gametes Each gamete will carry one form of the allele

The Law of Segregation: Mendel’s 1st Law Each gamete ONLY gets 1 allele

More about Alleles… located on homologous chromosomes Each individual has 2 alleles for the same gene located on homologous chromosomes Each parent passes 1 allele for each gene to his/her offspring In sperm or egg What stage of meiosis are alleles segregated into gametes? Meiosis Anaphase I

a A

Mendel’s Observations Used pea plants to see patterns in the way various traits were inherited Using his data, he saw that different traits are inherited separately Example: Green pea color isn’t always inherited with wrinkled pea shape Green peas can be smooth and round too! This is called the Law of Independent Assortment!

What is the Law of Independent Assortment? Allele pairs (traits) separate independently of each other during gamete formation (meiosis) Different traits are inherited separately Example – peas can be green and wrinkled OR green and round This explains genetic diversity among organisms Presence of one trait does not affect the presence of another trait! Law of Independent Assortment – each pair of alleles for different traits segregate independently of other pairs of alleles during gamete formation

Law of Independent Assortment: Mendel’s 2nd Law Says genes are inherited independently of other genes Genes are not linked unless on the same chromosome! Mendel assumed traits occur on different chromosomes! Occurs during Metaphase I

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

More Genetic Vocabulary An organism’s genotype Is its genetic (DNA) makeup A.k.a.-the allele combination (includes 2 alleles) An organism’s phenotype Is its physical outcome of the genotype Ex: blue eyes or AB blood type

Mendelian Genetics… aka- COMPLETE DOMINANCE If an organism is heterozygous (Hh), The effect of the recessive allele is HIDDEN Heterozygous and homozygous dominant have SAME phenotype The 1st allele is “completely dominant” over the 2nd allele

Phenotype Genotype 3 Purple White Ratio 3:1 Ratio 1:2:1 1 Purple 2 Figure 14.6 3 Phenotype Purple White Genotype PP (homozygous) Pp (heterozygous) pp Ratio 3:1 Ratio 1:2:1 1 Purple 2 Pp (heterozygous) Purple 1 1

Why Did Mendel Keep Getting the SAME results? We can answer this question using a Punnett square a diagram (box) used to predict probabilities of possible outcomes for offspring that will result from a cross between 2 parents SHOWS EXPECTED RESULTS (not necessarily actual)

Practice Which flower color is recessive? White Purple Neither Both PURPLE X WHITE PURPLE PURPLE & WHITE Which flower color is recessive? White Purple Neither Both

Practice Which flower color is recessive? PURPLE X WHITE PURPLE PURPLE & WHITE Which flower color is recessive? WHITE What would the genotype be for the recessive flower? A. PP homozygous dominant B. pp homozygous recessive C. Pp Heterozygous

Practice Which flower color is recessive? PURPLE X WHITE PURPLE PURPLE & WHITE Which flower color is recessive? WHITE What would the genotype be for the recessive flower? pp homozygous recessive Which flower color is dominant? White Purple Neither Both

Practice Which flower color is recessive? WHITE What would the genotype be for the recessive flower? pp homozygous recessive Which flower color is dominant? PURPLE What would the genotype be for the dominant flower color? A. PP homozygous dominant B. Pp heterozygous C. pp homozygous recessive D. Both A & B PURPLE X WHITE PURPLE PURPLE & WHITE

Genetics Vocab (pt 2) Monohybrid cross  cross where parents differ in only one trait (Rr x rr) Dihybrid cross  cross where parents differ in two traits (RrHh x rrHH) Punnett square – a diagram that shows the gene combinations that might result from a genetic cross of two parents