Mendelian Genetics Chapter 11 Gregor Mendel Father of

Mendelian Genetics Chapter 11

Gregor Mendel – “Father of Genetics”, first Geneticist, one of the most important scientists in history n n n Austrian Monk (18221884 Worked with ordinary garden peas Used science and math skills learned from University.

Genetics – the scientific study of heredity

Mendel’s Peas – background information • • The anther (♂, male) produces sperm Gametes The ovary (♀, female) produces eggs Sperm + Eggs: FERTILIZATION The peas Mendel started with were TRUEBREEDING.

More Peas n n True-breeding: parents always have offspring identical to themselves when allowed to selfpollinate. (Is selfpollination sexual or asexual reproduction? ) For example: Plants with white flowers produce only offspring with white flowers.

Important terms n Trait = specific observable characteristic n Mendel produced hybrids n Hybrid q n The offspring produced by parents with differing forms of a trait. Contrast with true-breeding Some genetics notation q q Parent generation = P F 1 = first generation of offspring from a cross, (F 2 = second generation, etc. )

Mendel’s crosses…results Seed Shape Seed Color Round Yellow Seed Coat Color Gray Pod Shape Pod Color Flower Position Smooth Green Axial Tall Short Wrinkled Green White Constricted Yellow Terminal Round Yellow Gray Smooth Green Axial Plant Height Tall

Mendel’s crosses…results Seed Shape Seed Color Round Yellow Seed Coat Color Gray Pod Shape Pod Color Flower Position Smooth Green Axial Tall Short Wrinkled Green White Constricted Yellow Terminal Round Yellow Gray Smooth Green Axial Plant Height Tall

Genes n Mendel eventually concluded that traits are passed from one generation to the next. n Today, we know that… Traits are determined by GENES q Chemical factors that determine a trait. q Consist of DNA sequences n n Mendel did not know anything about genes, DNA, chromosomes, meiosis, etc. Alleles - Different forms of a gene n n Ex. Eye color is a trait, you could have 2 blue eye color alleles, you could have 2 brown eye color alleles, you could have one of each. You get one allele from each parent for each trait.

Dominant and Recessive Genes n Principle of Dominance: Some alleles are dominant and others are recessive. n Dominant: An allele that is expressed (visible) whether it is the only version of the gene present or when in combination with a recessive allele. n Recessive: An allele ONLY expressed when dominant allele is NOT present

Symbols to represent Dominant Genes n n If the symbol “T” represents the dominant allele (or form of the gene), then… it doesn’t matter whether both of the alleles an individual has are dominant (TT) or… there is just one copy of the dominant allele (Tt). Either way, the individual looks the same (it has the dominant trait).

Symbols to represent Recessive Genes n n n If the symbol “t” represents the recessive allele (or form of the gene), then… both of the alleles an individual has must be recessive (tt) to have the recessive trait. The presence of one dominant allele (T) hides the recessive allele (t) when there is one of each (Tt).

Important vocabulary n n Homozygous q two identical alleles for a trait. (ex. SS or ss) q TRUE-BREEDING for a trait Heterozygous q two DIFFERENT alleles for a trait (ex. Ss) q HYBRID for a trait Phenotype q physical characteristics, appearance Genotype q genetic make up, the information in DNA

End part one n If a teacher was to describe a plant as tall with green seeds (outward appearance) they would be describing the plant’s __________. n If the teacher said that this plant is a cross between Tt and TT they would be describing the ________. n ►PLANTS CAN HAVE THE SAME PHENOTYPE BUT DIFFERENT GENOTYPES!!!

(T = Tall Term that describes genotype t = short) Genotype Phenotype TT Tall Tt Tall tt Short

Mendelian Genetics Chapter 11

Segregation n SEGREGATION q n Separation of alleles during gamete formation (meiosis) Gamete q q Sex cells carry a single copy of each gene (ex. The allele for short OR the allele for tall, not both)

Genetics and Probability n Probability q q n n likelihood that a particular event will occur. For example: What is the probability of rain tomorrow? How likely is it that a flipped penny will land heads-up? Tails-up? Probability is used to predict the outcomes of genetic crosses.

Steps to a setting up a Punnett Square 1. 2. 3. 4. 5. Identify parental genotypes for the cross (ex. TT, tt, Tt) Draw the Punnett Square--a grid with each set of parental alleles on its own side. Separate the alleles (why? ) Match alleles from rows and columns to Gametes (sperm make offspring genotypes. or eggs) are haploid (one set Identify the kinds (and their numbers) of of chromosomes) offspring you have.

Steps to a setting up a Punnett Square: An Example 1. 2. Both parents are heterozygous for plant height, with tall being dominant to short. Describe their offspring.

Probability n n Look at the cross we just did. How many offspring have two alleles for tallness (TT)? Two for shortness (tt)? Both (Tt)?

Probabilities n n n ¼ or 25% have two alleles for tallness. ¼ or 25% have two alleles for shortness. ½ or 50% have one allele for tallness and one for shortness. We write this in terms of a ratio 1 TT: 2 Tt: 1 tt This ratio of allele combination types is called the genotypic ratio.

Probabilities n n n How many plants will be tall and how many will be short? We also write this in terms of a ratio (3 tall: 1 short). This ratio of physical types is called the phenotypic ratio.

3 tall (75%), 1 short (25%) expressed as a ratio, 3: 1 n Why? q q 1 homozygous for tallness (TT) and the 2 heterozygous plants (Tt) You lump these together because you cannot tell any of these offspring apart for this trait—TT offspring look exactly like Tt offspring. 1 homozygous recessive (tt)

n This is exactly what Gregor Mendel did in his pea experiments.

A brief explanation… n n All of the reproduction we will be discussing regarding Mendel’s garden peas will be sexual reproduction (a new organism formed by the joining together of a male and a female gamete). However, pay close attention to who the parents are — sometimes there is a cross between two different parents (usually with contrasting forms of some trait, like flower color or height). Other times, the peas are allowed to do what comes naturally to peas—self-pollination (the same plant fertilizes itself).

Sexual Reproduction in Plants Male (♂) sexual organs Female (♀) sexual organs

Cross-pollination – either by wind, Pollen insect or geneticist (containing male gametes)

Self-pollination Convenient, but boring

Mendel’s first crosses n n n Mendel’s Monohybrid cross Tall allele/Tall allele The parent TT generation (P) is a combination of two different purebreeding types The first generation (F 1) produced only TALL plants The second generation (F 2 ) produced tall and short plants short allele/short allele tt

Predictions n n n Mendel found that the approximately three dominant to one recessive ratios showed up consistently. Probabilities predict average outcomes for a LARGE number of events, not exact outcomes. Flip a coin twice q q q n Heads once, tails once Heads twice Tails twice Flip a coin many times, likely to get very close to a 50: 50 ratio

Predicting Genetics n n n Genetics is similar Larger numbers result in closer to expected values. This explains why humans don’t always have the same number of male children and female children despite the fact that the odds of any one child’s gender is 50% female, 50% male.

Mendel’s Principle n Independent Assortment q q Genes for different traits segregate independently during gamete formation Accounts for much of the genetic variation in living things n n Usually, all combinations of traits are possible, ex. Tall round seeds, purple flowers Short Tall Etc. round seeds, white flowers wrinkled seeds purple flowers wrinkled seeds white flowers

Summary of Mendel’s Principles n n Genes determine inheritance. Genes are passed from parent to offspring. Some genes may be dominant and others recessive Adults (sexually reproducing) have two copies of each gene—one from each parent. They segregate during gamete formation. Alleles for different traits assort independently (Independent Assortment)