Ch. 11 Mendelian Genetics

Important Vocab Genetics: the study of heredity Fertilization: the joining of sperm and egg Genes: chemical factors that determine traits (a specific characteristic) Alleles: different forms of a gene Ex: tall vs short, yellow vs green Trait: a specific characteristic (color, size, shape, etc.)

More Important Vocab True-breeding: individuals that will produce offspring identical to themselves if allowed to self-pollinate TT, tt, RR, rr Hybrid: individuals that will not produce offspring identical to themselves if allowed to self-pollinate Tt, Rr, Ss

Principle of Dominance States that some alleles are dominant and others are recessive An organism that has a dominant allele for a particular trait will always show that trait An organism with a recessive allele for a trait will exhibit that form only when the dominant allele is absent

Who is the Father of Genetics? Gregor Mendel What organism did he study? Pea plants Started with true-breeding plants He crossed a true-breeding tall plant (TT) with a true-breeding short plant (tt) Found that all offspring were tall TT x tt = all tall Why? Principle of dominance

Probability and Genetics Probability: the likelihood that a particular event will occur Why do we look at probability when studying genetics? All the traits you receive from your parents depends on chance! What do we use to predict the outcome of a genetic cross between two individuals? Punnett Squares

Probabilities Predict Averages Probabilities predict the average outcome of a large number of individuals You are more likely to get the predicted outcome with 500 individuals rather than 5 individuals.

(One factor crosses) Monohybrid Crosses

Vocab for Punnett Squares Homozygous: organisms that have two identical alleles for a particular trait Aka – true-breeding Heterozygous: organisms that have two different alleles for a particular trait Aka – hybrid Phenotype: physical characteristics Yellow, green, tall, short Genotype: genetic makeup YY, Yy, yy, TT, Tt, tt

Let’s Practice the Vocab! T= tall, t = short Homozygous tall = TT Heterozygous = Tt Homozygous short = tt Y = yellow, y = green Homozygous yellow = YY Heterozygous = Yy Homozygous green = yy Let’s try some monohybrid (single factor) crosses!

Practice with Punnetts! Heterozygous tall x heterozygous tall What are the genotypes? Tt x Tt Punnett Square Phenotypic ratio? Tall:short 3:1 Genotypic ratio? TT:Tt:tt 1:2:1 T t T TT Tt t Tt tt

More Practice with Punnetts! Homozygous tall x homozygous short What are the genotypes? TT x tt Punnett Square Phenotypic ratio? Tall:short 4:0 Genotypic ratio? TT:Tt:tt 0:4:0 T T t Tt Tt t Tt Tt

More Practice with Punnetts! Heterozygous tall x short What are the genotypes? Tt x tt Punnett Square Phenotypic ratio? Tall:short 2:2 Genotypic ratio? TT:Tt:tt 0:2:2 T t t Tt tt t Tt tt

Your turn to try! T T T TT TT t Tt Tt Homozygous tall x heterozygous tall What are the genotypes? TT x Tt Punnett Square Phenotypic ratio? Tall:short 4:0 Genotypic ratio? TT:Tt:tt 2:2:0 T T T TT TT t Tt Tt

(Two Factor crosses) Dihybrid Crosses

Back to Mendel! When looking at a single trait (i.e. height), the two alleles will separate from each other The 2 T’s separate from each other Now let’s look at 2 traits at the same time. Mendel asked: Does the segregation of one pair of alleles affect the segregation of another pair of alleles? Tall vs short and yellow vs green No, they will separate independently. What the alleles for height do will not affect what the color alleles do.

Principle of Independent Assortment States that genes for different traits can segregate independently during the formation of gametes. Gametes: sex cells What are the two types of gametes? Sperm Egg

Dihybrid Crosses Let’s look at two factors: seed shape and color Round (R) vs wrinkled (r) seed shape Yellow (Y) vs green (y) seed color Homozygous round and yellow = RRYY Wrinkled and green = rryy Heterozygous for both traits = RrYy

Dihybrid Crosses RRYY rryy RY ry RY ry RY ry ry RY Let’s cross 2 truebreeding plants: Round yellow x wrinkled green Step 1: What are the genotypes? RRYY rryy Step 2: What are the possible gamete combinations? RRYY rryy RY ry RY ry RY ry ry RY

Dyhybrid Crosses Step 3: Draw out the Punnett Square RY ry RrYy RrYy

Dyhybrid Crosses Step 4: Figure out the Phenotypic ratio Round yellow: round green: wrinkled yellow: wrinkled green 16:0:0:0

Now you try! Cross two individuals that are heterozygous for both traits. Round vs wrinkled Yellow vs green What are the genotypes? RrYy What are the gametes? RrYy = RY, Ry, rY, ry

RrYy x RrYy 9: 3: 3: 1 RY Ry rY ry RRYY RRYy RrYY RrYy RRYy RRyy RrYy Phenotypic ratio: Round/yellow : round/green : wrinkled/yellow : wrinkled green RrYY RrYy rrYY rrYy RrYy Rryy rrYy rryy 9: 3: 3: 1

Other types of Dominance Incomplete Dominance Codominance

Incomplete Dominance Case in which one allele is not completely dominant over another Heterozygote phenotype is somewhere between the two homozygote phenotypes Example: Snapdragons Red (R) and white (r) RR = red and rr = white What about Rr? These are pink!

Let’s Practice! R R r Rr Rr r Rr Rr Red snapdragon x white snapdragon Genotypes? RR x rr Punnett Square Ratios: Phenotypic Red: pink: white 0:4:0 Genotypic RR: Rr: rr R R r Rr Rr r Rr Rr

Now It’s Your Turn: R r R RR Rr r Rr rr Pink x Pink Genotypes? Rr x Rr Punnett Square Ratios: Phenotypic Red: pink: white 1:2:1 Genotypic RR: Rr: rr R r R RR Rr r Rr rr

Codominance Situation in which both alleles contribute to the phenotype Heterozygote is usually spotted or speckled compared to the solid colored homozygotes Example: Chickens B = black, W = White BB = black chicken WW = white chicken BW = black and white speckled chicken

Let’s Practice! B B W BW BW W BW BW Black chicken x White Chicken Genotypes? BB x WW Punnett Square Ratios: Phenotypic Black: speckled: white 0:4:0 Genotypic BB: BW: WW B B W BW BW W BW BW

Your Turn! B W B BB BW W BW WW Speckled chicken x speckled chicken Genotypes? BW x BW Punnett Square Ratios: Phenotypic Black: speckled: white 1:2:1 Genotypic BB: BW: WW B W B BB BW W BW WW

Bloodtyping is Codominant! Types A and B are both dominant to O Bloodtype A B AB O Genotype Antigens Antibodies Will take blood from: Can’t take blood from: IAIA or IAi IBIB or IBi IAIB ii A B A and B None B A None A and B A or O B or O AB, A, B, O O --- B or AB A or AB A, B, AB Antigen: substance that triggers an immune response Antibody: protein that helps destroy pathogens

Meiosis

Chromosomes Genes are located on chromosomes which are found in the nucleus. Mendel’s principles of genetics requires 2 things: Each organism must inherit a single copy of every gene from each parent. When an organism produces its own gametes, those two sets (copies) of genes must be separated from each other so that each gamete contains just one set of genes.

Chromosomes Homologous chromosomes: term used to refer to chromosomes that each have a corresponding chromosome from the parent of the opposite sex A cell that contains both sets of homologous chromosomes is said to be diploid (meaning 2 sets) Having 2 complete sets of chromosomes agrees with Mendel’s idea that the adult organism contains 2 copies of each gene A cell that contains only 1 set of chromosomes is called haploid – i.e. gametes

Chromosomes Humans: How many chromosomes does each of your typical body cells have? 46 (diploid) How many chromosomes does each of your gametes have? 23 (haploid – one set of chromosomes)

Meiosis Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. Divided into 2 major steps: Meiosis I Meiosis II

Meiosis I Prophase I Each chromosome pairs with its corresponding homologous chromosome to form a tetrad (made of 4 chromatids) Homologous chromosomes then exchange portions of there chromatids during crossing-over Results in the exchange of alleles between homologous chromosomes and produces new combinations of alleles

Meiosis I Metaphase I Anaphase I Telophase I and Cytokinesis Spindle fibers attach to the chromosomes Chromosomes line up across middle of cell at metaphase plate Anaphase I Fibers pull homologous chromosomes towards opposite ends of cell Telophase I and Cytokinesis Nuclear membranes reform Cell separates into 2 haploid daughter cells

Meiosis II Prophase II Metaphase II The 2 cells created by Meiosis I now enter into a second meiotic division At this point, each cell’s chromosomes contains 2 sister chromatids (same genes, but different alleles  one may have “A” while the other may have “a”) Metaphase II Chromosomes line up at metaphase plate Spindle attaches to centromeres

Meiosis II Anaphase II Telophase II and Cytokinesis Sister chromatids separate and move towards opposite ends of cell Telophase II and Cytokinesis Nuclear membrane reforms Cytoplasm divides Result: 4 genetically different haploid gametes

Mitosis vs Meiosis Mitosis produces 2 identical diploid daughter cells Meiosis produces 4 genetically different haploid gametes

Sex-linked Traits Human Heredity

Human Heredity Karyotype: a picture of chromosomes arranged in homologous pairs

Karyotypes Human karyotypes show 46 chromosomes 23 chromosomes came from the sperm (father) 23 came from the egg (mother) Chromosome pairs #1-22 are called autosomes. These determine all sorts of traits – eye color, hair color, height, shapes of features

Sex Chromosomes Chromosome pair #23 are called sex chromosomes because they determine a person’s sex Females have 2 X’s = XX Males have 1 X and 1 Y = XY All human egg cells carry 1 X. Only ½ of human sperm cells carry an X, whereas the other ½ carry a Y Results in a 50/50 chance of having either a girl or boy. Who determines the sex of the baby? Why?

Sex-linked Genes Sex-linked genes are genes located on the X chromosome. Most sex-linked traits are recessive. Males are more likely to show sex-linked traits. This is because males have only 1 X – if this X is “bad”, the sex-linked trait will be expressed. Females have 2 Xs – both Xs have to be “bad” in order for the trait to be expressed. Examples: Colorblindness, Hemophilia, muscular dystrophy

Sex-linked Traits In order to figure out the chance of getting a sex-linked trait, we follow the same procedure as we did for single-factor traits. However, we have to add sex chromosomes (X and Y) into the equation. Let’s use colorblindness as an example.

Colorblindness - Genotypes Males have 2 possible genotypes: Normal Male = XY Colorblind male = XcY Females have 3 possible genotypes: Normal female (not a carrier) = XX Normal female (carrier) = XcX This means she doesn’t show the trait, but can pass it on to her offspring Colorblind female = XcXc

Colorblindness with Punnetts Normal male x colorblind female What are the genotypes? XY x XcXc Punnett Square What is the chance they will have a colorblind son? 50% What is the chance they will have a colorblind daughter? 0% X Y Xc XcX XcY Xc XcX XcY