Introduction to Genetics

Introduction to Genetics Biology

Gregor Mendel • Genetics – the study of patterns of inheritance and how traits or characteristics are passed on from parent to offspring • Gregor Mendel – the father of genetics. He used pea plants to study patterns of inheritance.

Self-Pollination vs. Cross Pollination • Mendel controlled the pollination of pea plants. • Pea plants normally reproduce by self pollination (male/female parts from same plant unite) • Mendel cross-pollinated plants by removing the anthers (male part of plant) and pollinating the stigma (female) part of another plant

Pea Plants • Mendel studied seven different traits in pea plants • Trait – a specific characteristic that varies from one individual to the next. • Each trait had two contrasting characteristics (alleles) • For example: • Seed Shape – smooth/wrinkled • Seed Color – yellow/green • Plant Height – tall/short

Mendel’s Vocabulary • True-breeding – plants that produce only like offspring when self-pollinated. • Hybrid – cross between two organisms of different traits • P1 Generation – The original pair of organisms in a series of crosses • F1 Generation – First generation of offspring as a result of P1 cross.

Mendel’s Work • Mendel crossed true-breeding short plants with true-breeding tall pea plants • 100% of the offspring were tall • Mendel crossed F1 plants (tall plants) to see if recessive trait reappeared • 25% of offspring were short; 75% were tall in F2 generation

Section 11-1 P Generation F1 Generation F2 Generation Tall Short Tall Tall Tall Tall Tall Short

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

Mendel’s Conclusions • Every individual receives two factors (alleles) for each trait, one from mom and one from dad. • Factors can sometimes be dominant or recessive • Dominant = one factor masks the presence of another • Recessive = factor that is masked by the presence of a dominant allele • Law of Segregation – pair of factors (alleles) are segregated/separated during the formation of gametes

Mendel’s Conclusions (con.) • Law of Independent Assortment – Alleles for different characteristics are distributed to gametes independently • Alleles/Traits are not linked to one another

Probability • Principle of Probability– the likelihood that a particular event will occur • Probability is identified as a ratio, fraction, or percent • 50% • ½ • 1:2

Probability • Example – Coin Flip • 50% chance of getting heads • Could get heads 10 consecutive flips • Over many trials results will be near 50% • Past coin flips do not affect future coin flips

Punnett Squares • Diagrams that apply the principles of probability to predict the outcome of genetic cross • Uppercase letters represent dominant alleles Ex. R • Lowercase letters represent recessive alleles Ex. r

Punnett Squares (con.) • Homozygous – organisms with two of the same alleles for a trait (RR or rr) • Heterozygous – organisms with two different alleles for a trait (Rr) • Genotype – The genetic makeup of an organism (RR, Rr, rr) • Phenotype – The physical characteristics of an organism (Round)

How to use a Punnett Square • Monohybrid Cross- “one-trait” cross • Identify the genotype for each parent • Write the alleles from one parent on the top and the alleles for the other parent on the left side of the square • Fill in the table like a multiplication table, capital letter always goes first

Ex. #1

Ex. #2 RR x rr

Analyzing results Genotypic ratio-compares possible genotypes # homozygous dominant: # heterozygous: #homozygous recessive In Ex. #1 it is 1:2:1 In Ex. #2 it is 0:4:0

Analyzing results (Con’t) Phenotypic ratio-compares possible phenotypes # with Dominant trait: # with recessive trait In Ex. #1 it is 3:1 In Ex. #2 it is 4:0

Analyzing results (Con’t) Percentages-% chance of something occuring In Ex. #1 there is a 75% chance the plant will be tall and a 25% chance the plant will be short.

Patterns of Inheritance Complete Dominance: In the heterozygous individual, only the dominant allele is expressed, the recessive allele is present but unexpressed.

Ex. In pea plant green pods are dominant over yellow pods. Crosstwo pea plant that are heterozygous for pod color. What is the phenotypic ratio? What is the genotypic ratio?

ANSWER Genotypic ratio: 1:2:1 Phenotypic ration is 3:1

Exceptions to Mendel’s Rules: Not all traits are clearly dominant or recessive • Incomplete Dominance – One allele is not completely dominant over another • Example: Japanese Four O’Clock Plant • RR = Red Flowers • RW = Pink Flowers • WW= White Flowers

Incomplete Dominance in Four O’Clock Flowers

Ratios Find genotypic ratio the same way we did for complete dominance Now there are 3 phenotypes so the ratio is # Dom: # Heterozygous: # Rec Find Genotypic ratio and phenotypic ratio for the previous problem Genotypic ratio: 0:4:0 Phenotypic ratio: 0:4:0

Codominance – In a heterozygote both alleles are dominant and are expressed Example: coat color in cattle • RR = Red Coat • RW = Roan Coat • WW = White Coat

RR = Red WW = White RW = Roan

Ex. Cross 2 roan coat cows. Genotypic Ratio- #RR: #RW: #WW 1:2:1 Phenotypic Ratio- #red: #roan: #white 1:2:1

Dihybrid Cross • Cross involving two traits (hair color & eye color or pod color & pod shape) • Traits assort independently of each other • Mendel used to discover the principle of independent assortment

Dihybrid Cross Go to Section:

How to use a Dihybrid Cross • Identify the 2 traits • Assign a capital letter to the dominant form of the first trait and the same lower case letter for the recessive form. • Choose a different letter and do the same for the second trait. • Identify the genotype for each parent

5. Identify the gametes for each parent Combine: 1st letter with 3rd letter 1st letter with 4th letter 2nd letter with 3rd letter 2nd letter with 4th letter 6. Fill in the Punnett Square -ABC order -keep like letters together -capital always goes first

Ex. RrTt x RrTt

Analyze Using Phenotypic Ratio 1st trait dominant, 2nd trait dominant: 1st trait dominant, 2nd trait recessive: 1st trait recessive, 2nd trait dominant: 1st trait recessive, 2nd trait recessive In previous example, the phenotypic ratio is 9:3:3:1

Sex-Linked Traits Sex-linked traits are determined by genes found only on the sex “X” chromosome REMEMBER: XX-Females Xy-Males EX. Red-Green Colorblindess

GENOTYPES Possible female Genotypes XBXB-normal XBXb-normal (carrier) XbXb-colorblind Possible male genotypes Xby-normal Xby-colorblind

Ex. Cross a female carrier with a normal male What is the % chance they will have a colorblind son? 50%

Multiple Alleles Multiple Alleles: Having more than 2 alleles for a trait. Ex. Human blood type

Human Blood Type 3 alleles: A, B, and O A and B are codominant O is recessive

Possible Blood Type Combinations GENOTYPES PHENOTYPES AA-homozygous dominant Type A BB-homozygous dominant Type B AO-heterozygous Type A BO-heterozygous Type B AB-codominant Type AB OO-homozygous recessive Type O

EX. Cross a mother who has type O blood with a father who has typeAB blood. What are the possiblephenotypes? Answer: Type A and Type B (both are heterozygous)

Polygenic Traits • Polygenic Traits – traits can also be controlled by more than one gene • Example – Skin color is controlled by 3 to 6 genes that control melanin production

Pedigrees Pedigree: diagram that shows how a trait is inherited over several generations. Symbols Circle: female Square: male Diamond: unknown sex

Pedigrees (con’t) • Shaded in shape =person exhibits the trait • Half shaded=the person is a carrier • Not shaded=the person is unaffected • Marriage line-horizontal line from male to female • Child line-vertical line which extends from the marriage line

Introduction to Genetics