Introduction to Genetics Genetics = the scientific study of heredity
The Work of Gregor Mendel • Gregor Mendel • Austrian monk • studied heredity • heredity—the passing on of characteristics from parents to offspring • Characteristics that are inherited are called traits. • First person to successfully predict how traits are transferred from generation to generation • Used garden peas in his experiments The Father of Genetics
Why pea plants? • Reproduce sexually, which means that they produce male and female sex cells, called gametes. • In a process called fertilization, the male gamete unites with the female gamete. • The resulting fertilized cell, called a zygote, then develops into a seed.
Pollen grains Transfer pollen Female part Male parts Cross-pollination • Mendel’s Experiment • He took pollen from a male plant and dusted it onto a female plant. • Parental generation (p) = the original pair of plants • Offspring • (f1) = first filial generation • (f2) = second filial generation
His first experiments are called monohybrid crosses because they only deal with ONE single trait (height, color) • mono means “one”
Concluded: • Each organism has two factors that control each of its traits. • These factors are genes and that they are located on chromosomes. • Genes exist in different forms called alleles. P1 Short pea plant Tall pea plant F1 All tall pea plants F2 3 tall: 1 short
Short plant Tall plant t t T T t T F1 All tall plants t T • Mendel called the observed trait dominant and the trait that disappearedrecessive. • Mendel concluded that the allele for tall plants is dominant to the allele for short plants.
F1 Tall plant Tall plant T t t T F2 Tall Tall Tall Short t t t t T T T T 3 1 Law of segregation • The way an organism looks and behaves is called its phenotype. Ex. Tall, yellow • The allele combination an organism contains is known as its genotype. Ex. TT, Tt • An organism’s genotype can’t always be determined by its phenotype. TtxTt cross
An organism is homozygous for a trait if its two alleles for the trait are the same. (True-breeding) • Exp. TT or tt • An organism is heterozygous for a trait if its two alleles for the trait differ from each other. (Hybrid) • Exp. Tt
Genetics & Probability • probability = the likelihood that a particular event will occur • Ex. Coin flipping: 1/2 probability that coin will flip head/tail • If you flip the coin 3 times what's the probability of flipping 3 heads? • 1/2 x 1/2 x 1/2 = 1/8 • *Past outcomes do not affect future ones!!* • The principles of probability can be used to predict the outcomes of genetic crosses. • What is the probability of parents having two male offspring in a row? (1/2*1/2=1/4)
Punnett Squares: Are used to predict and compare the genetic variations that will result from a cross. • The types of gametes go on the top and left sides of the square • The possible gene combinations appear in the four boxes
T t T T T T t T t t t t • If you know the genotypes of the parents, you can use a Punnett square to predict the possible genotypes of their offspring.
A A A a • You try this one:
Section 1 Check Question 1 The passing on of characteristics from parents to offspring is __________. A. genetics B. heredity C. pollination D. allelic frequency
The answer is B. Genetics is the branch of biology that studies heredity.
Section 1 Check Question 2 What are traits? Answer Traits are characteristics that are inherited. Height, hair color and eye color are examples of traits in humans.
Section 1 Check Question 3 Gametes are __________. A. male sex cells B. female sex cells C. both male and female sex cells D. fertilized cells that develop into adult organisms
Section 1 Check The answer is C. Organisms that reproduce sexually produce male and female sex cells, called gametes.
Section 1 Check Question 4 Which of the following genotypes represents a animal that is homozygous dominant for a trait? a. KK b. Kk c. kk
Section 1 Check Question 5 Which of the following genotypes represents a plant that is homozygous recessive for height? A. TT B. Tt C. tt
Dihybrid Cross round yellow x wrinkled green P1 Wrinkled green Round yellow All round yellow F1 F2 9 3 3 1 Round green Wrinkled yellow Round yellow Wrinkled green • Mendel’s Dihybrid Crosses
He found they appeared in a definite ratio of phenotypes 9:3:3:1— 9 round yellow: 3 round green: 3 wrinkled yellow: 1 wrinkled green. • He concluded that the seed shape and seed color—are inherited independently of each other. • This conclusion is known as the law of independent assortment.
Gametes from RrYy parent Ry RY rY ry RRYy RRYY RrYY RrYy RY RRYy RRYy RrYy Rryy Ry Gametes from RrYy parent rrYy RrYY RrYy rrYY rY RrYy rrYy Rryy rryy ry Dihybrid Punnett square Dihybrid crosses • A Punnett square for a dihybrid cross will need to be four boxes on each side for a total of 16 boxes.
Exceptions to Mendel • To be continued on the next exciting adventure of BIOLOGY…….
Exceptions to Mendel • Not all genes show simple patterns of dominant and recessive alleles. • Because the majority of traits are controlled by more then one gene/alleles.
Cases in which one allele is not completely dominant over another. Neither allele is dominant Ex. Red flower x White flower = Pink flower Incomplete dominance
Cases in which both alleles are expressed. Ex. White chicken x Black chicken = Speckled (white and black) chicken Codominance
Multiple Alleles • when a gene has more than two alleles • Ex. Mice coat color is determined by one gene with four different alleles. Three different colors result from the combinations of these alleles.
Blood Types - Multiple Alleles and Codominance • In humans, there are four blood types (phenotypes): A, B, AB, and O • Blood type is controlled by three alleles. A, B, O • O is recessive, two O alleles must be present for the person to have type O blood • A and B are codominant. If a person receives an A allele and a B allele, their blood type is type AB • Crosses involving blood type often use an I to denote the alleles - see chart.
Type O's are automatically OO and type AB is automatically AB. Type A and B can be homozygous or heterozygous.
Polygenic traits • traits controlled by two or more genes • Ex. At least three genes are involved in making the reddish-brown pigment in the eyes of fruit flies. Different combinations of alleles for these genes yield different eye colors.
Sex Linked • Some genes are located on the X chromosome. Females receive two alleles (XX) for these genes, but males only receive one (y). • When doing a punnet square, use large X's and Y's to denote male and female, use superscript letters to designate the alleles. • Ex. hemophilia (bleeding) and color blindness are recessive sex-linked traits (XcXc or XcY) • If the parent is a male, the genotype is automatically known. A colorblind male has to be b (recessive), since he only has one allele and colorblindness is recessive. A normal male must then be B (dominate) • Females can be heterozygous for the colorblindness trait - they are called carriers. A female can be XBXB - normal, XBXb - carrier, or XbXb - colorblind
The following shows a cross between a normal man and a woman who is a carrier.
1 codominance • In a certain cactus, prickly spines can be two pronged or one pronged. If a true breeding (homozygous) one-pronged cactus is crossed with a true breeding two-pronged cactus, the F1 generation has a mixture of spines, some are two-pronged, some are one-pronged. • Is this an example of codominance or incomplete dominance?
2 • A nurse at a hospital removed the wrist tags of three babies in the maternity ward. She needs to figure out which baby belongs to which parents, so she checks their blood types. Using the chart below, match the baby to its correct parents.