Mendelian Genetics Chapter 11

Genetics What is Genetics? The scientific study of heredity What is Heredity? – the passing of traits from parents to offspring

Inheritance You get your genes from your parents In meiosis, half of the chromosomes in a pair come from the Dad, half come from the Mom Chromosome: Thread-like structure within the nucleus that contains the genetic information passed from one generation of cells to the next What we know today is based on the work of Gregor Mendel – Austrian monk

“The Father of Modern Genetics” Gregor Mendel 1856-1865 – Austrian monk who laid groundwork for understanding biological inheritance – studied 7 inherited traits in pea plants (true-breeding) of his monastery garden – COUNTED the plants and compiled data (QUANTITATIVE APPROACH to science) Paper was published in 1866, but not enough was understood to truly value this work.

Key terms to know Allele – each form of a gene for a certain trait (R or r) Gene – sequence of DNA that codes for a protein and thus determines a trait Genotype – combination of alleles for a given trait (RR or Rr or rr) Phenotype – Appearance of trait (round seeds or wrinkled seeds) Homozygous - when you have 2 of the same alleles for a given trait (RR or rr) Heterozygous – when you have 2 different alleles for a trait (Rr)

Key terms to know Dominant – Allele that is expressed, Represented by a Capitol letter. Recessive – Allele that is masked or hidden, Represented by a lowercase letter. F1 generation: Filial or First Generation Test cross (Punnett square): determine the genotype of an individual with a dominant phenotype

Characters and Traits Character – heritable feature that varies among individuals (ex. Flower color) Trait – each variant for a character (ex. Purple vs. white flowers) Originally believed that traits blended together to give offspring results

Figure 14.1 A genetic cross

Mendel chose to use plants that were true-breeding… P generation – parentals; true-breeding (On their own create identical offsprings) parents that were cross-pollinated F1 generation – hybrid offspring of parentals that were allowed to self-pollinate F2 generation – offspring of F1’s

Figure 14.2 Mendel tracked heritable characters for three generations

Figure 11-3 Mendel’s Seven F1 Crosses on Pea Plants Section 11-1 Seed Shape Seed Color Seed Coat Color Pod Shape Pod Color Flower Position Plant Height Round Yellow Gray Smooth Green Axial Tall Wrinkled Green White Constricted Yellow Terminal Short Round Yellow Gray Smooth Green Axial Tall *Flower color – purple (P) vs. white (p) Seed coat color and flower color are often put in for one another – thus, the EIGHT traits!!! Go to Section:

Mendel’s 4 ideas… Alternative versions (alleles) of genes account for variations in inherited characters. For each character, an organism inherits two alleles, one from each parent. If the two alleles differ, the dominant allele is expressed in the organisms appearance, and the other, a recessive allele is masked. The two alleles for each character segregate during gamete production. (Law of Segregation)

Figure 14.3 Alleles, alternative versions of a gene

Figure 14.4 Mendel’s law of segregation (Layer 2)

From the law of segregation…… Came the Law of Independent Assortment Genes for different traits can segregate independently during the formation of gametes. In other words….. Just because a seed is round does not mean that it has to be green.

Figure 14.5 Genotype versus phenotype

Probability & Genetics

Punnett Square Device for predicting the results of a genetic cross between individuals of a known phenotype. Developed by R.C. Punnett Rules: 1. must predict possible gametes first 2. male gametes are written across top, female gametes on left side 3. when read Punnett, start in upper left corner and read as if a book 4. WRITE OUT GENOTYPES IN ORDER

Example…… Character – flower color Alleles – Purple (P) and white (p) Note: Purple is dominant with a capital letter and white is recessive shown with a lowercase of dominant trait Genotypic combos possible – two dominants: PP (homozygous dominant) two recessives: pp (homozygous recessive) One of each: Pp (heterozygous)

Phenotypic possibilities – physical appearance PP – purple pp – white Pp – purple (white is masked, but still part of genotype)

Monohybrid crosses – only one character considered Steps to do: Write out genotypes of parents Write out possible gametes produced Draw 4 box Punnett square Put male gametes on top, female on left side Fill in boxes Determine genotypes by reading Punnett starting from top left Determine phenotypes by reading from genotype list Ex. White flowered plant X Purple flowered plant Yellow peas X Green peas Tall plant X short plant

Dihybrid (Two-Factor)Cross Because genes separate independently we can determine the possible outcomes of a two-factor cross. Example: Mendel’s Peas F1 Hybrids for Shape and Color: RrYy Foil – First, Last, Inner, Outer Possible gametes passed on to offspring: RY, ry, rY, and Ry – Place in order (dominant to recessive) RY, Ry, rY, ry then place on cross

Beyond Dominant and Recessive Incomplete Dominance One allele is not completely dominant over the other – something in the middle is expressed Ex. Red and White Snapdragons Result can be heterozygous (Rr) or two separate dominant alleles (RW) each resulting in a mixture of both alleles Incomplete dominance & co-dominance. Go over on the day you assign worksheet (01/08/08?)

Another way that incomplete dominance can be expressed Red= RR White= WW RW= pink- each allele is equally expressed to result in a blended product

One way to express incomplete dominance RR (Red) X rr (White)= (Rr)Pink Rr- results in a blended result of PINK

Beyond Dominant and Recessive Codominance Both alleles are expressed in the phenotype Ex. Cow Hair Color RR – Red WW – White RW – Roan (Red & White) Practice Codominance/Incomplete Dominance #1-4

Beyond Dominant and Recessive Multiple Alleles Genes have more then two alleles Ex. Blood Type Type A blood- AA or AO alleles A is dominant to O Type B blood- BB or BO alleles B is dominant to O Type AB- codominant- A and B alleles A nor B is dominant so both are expressed on organisms RBC Type O- recessive- OO alleles Both alleles must be recessive in order to have type O.

More on blood types….. The blood type determines what antibodies are located within the blood. Type A blood has type B antibodies. If type B blood is put into their bodies, their immune system reacts as if it were a foreign invader, the antibodies clump the blood - can cause death. Type AB blood has no antibodies, any blood can be donated to them - they are called the "universal acceptors" Type O blood has no surface markers on it, antibodies in the blood do not react to type O blood, they are called the "universal donors"

Polygenic Traits Traits that are controlled by the interaction of several genes. Example: Reddish brown eyes in varying degrees found in fruit flies is controlled by 3 genes Human skin color is controlled by 4 different genes which result in a variety of skin color.

Sex-linked Genetics Ex. Colorblindness

Sex Chromosomes- last pair (23rd) in a karyotype MALE KARYOTYPE FEMALE KARYOTYPE

Sex Chromosomes- last pair (23rd) in a karyotype Male – XY and Females – XX The 23rd pair of chromosomes will determine the gender of an individual Very few genes are located on the Y chromosome……Most are located on the X Sex linked alleles will ALWAYS be tracked on the X chromosome ONLY when we conduct practice genetic problems

Sex-Linked Genes Ex. Colorblindness is carried on the sex-chromosomes It is a recessive trait – Xc How many genes do females need to express the trait (colorblindness)? 2 Xc Xc How many genes do males need to express the trait (colorblindness)? 1 XcY

Sex-Linked Punnett Square Let C = Normal Vision and c = Colorblind Cross: Normal Male ( ) x Carrier Female ( )

Sex-Linked Punnett Square C Let C = Normal Vision and c = Colorblind X Y x X X = Normal Male x Carrier Female X Y X C C c 1st put male genotype on the top of the table & female genotype on the left side C C c

Sex-Linked Punnett Square C C – Normal Vision and c - Colorblind X Y x X X - Normal Male x Carrier Female X Y X C C c C 2nd, cross them X X X Y C C C C C c c c

Sex-Linked Punnett Square C C – Normal Vision and c - Colorblind X Y x X X -Normal Male x Carrier Female X Y X 3rd, list the sex and appearance of each possible offspring C C c C Offsprings: 1 Normal Female 1 Normal (Carrier) Female 1 Normal Male 1 Colorblind Male X X X Y C C C C C c c c

PRACTICE and HW Complete problems 1-3 on the sex linked genetic practice problems sheet NOW! Complete the remaining 3 Co-dominant and Incomplete dominant practice problems and Sex Linked practice problems # 4-8 from today’s class for HW