Genetic Inheritance MENDEL & MUTATIONS
Father of Genetics • Monk and teacher. • Experimented with purebred tall and short peas. • Discovered some of the basic laws of heredity. • Studied seven purebred traits in peas. • Called the stronger hereditary factor dominant. • Called the weaker hereditary factor recessive. • Presentation to the Science Society in1866 went unnoticed. • He died in 1884 with his work still unnoticed. • His work rediscovered in 1900. • Known as the “Father of Genetics”.
Mendel’s Observations • He noticed that peas are easy to breed for pure traits and he called the pure strains purebreds. • He developed pure strains of peas for seven different traits (i.e. tall or short, round or wrinkled, yellow or green, etc.) • He crossed these pure strains to produce hybrids. • He crossed thousands of plants and kept careful records for eight years.
Mendel’s Peas • In peas many traits appear in two forms (i.e. tall or short, round or wrinkled, yellow or green.) • The flower is the reproductive organ and the male and female are both in the same flower. • He crossed pure strains by putting the pollen (male gamete) from one purebred pea plant on the pistil (female sex organ) of another purebred pea plant to form a hybrid or crossbred.
Analyzing Mendel’s Results • Analyses using Punnett squares demonstrate that Mendel’s results reflect independent segregation of gametes. • The Testcross: • Can be used to determine the genotype of an individual when two genes are involved.
MENDEL’S LAWS OF HEREDITY WHY MENDEL SUCCEEDED • GregorMendol – father of genetics • 1st studies of heredity – the passing of characteristics to offspring • Genetics – study of heredity • The characteristics passed on called traits
PHENOTYPES & GENOTYPES • PHENOTYPE – THE WAY AN ORGANISM LOOKS AND BEHAVES – ITS PHYSICAL CHARACTERISTICS (i.e. – TALL, GREEN, BROWN HAIR, BLUE EYES, ETC.) • GENOTYPE – THE GENE COMBONATION (ALLELIC COMBINATION) OF AN ORGANISM – (i.e. – TT, Tt, tt, ETC.) • HOMOZYGOUS – 2 ALLELES ARE THE SAME • HETEROZYGOUS – 2 ALLELES DIFFERENT
From Genotype to Phenotype • Multiple Alleles: • Sometimes more than two alleles (multiple alleles) exist for a given trait in a population. • EX. ABO blood designation. • A and B are codominant. • Rh Blood group: • Rh is a cell surface marker on red blood cells • About 85% of the population is Rh+ (have the marker) • Problems: Mother is Rh negative has an Rh+ fetus.
MENDEL CHOSE HIS SUBJECT CAREFULLY • Used garden peas to study • Have male & female gametes (sex cells) • Male & female same flower • Know what pollination & fertilization mean • He could control the fertilization process • Not many traits to keep track of
PUNNETT SQUARES • A QUICK WAY TO FIND THE GENOTYPES IN UPCOMING GENERATIONS • 1ST DRAW A BIG SQUARE AND DIVIDE IT IN 4’S
PUNNETT SQUARE CROSS T T X Tt
CONT’D T T X T t T T T T T T T t T t T t
MENDEL WAS A CAREFUL RESEARCHER • USED CAREFULLY CONTROLLED EXPERIMENTS • STUDIED ONE TRAIT AT A TIME • KEPT DETAILED DATA
MENDEL’S MONOHYBRID CROSSES • MENDEL STUDIED 7 TRAITS CAREFULLY • 11.1 • Mendel crossed plants w/ diff. traits to see what traits the offspring would have • These offspring are called hybrids – offspring of parents w/ different traits • A monohybrid cross is one that looks at only one trait (let’s look at plant height – tall or short)
THE 1ST GENERATION • Mendel crossed two plants – 1 tall & 1 short (they came from tall & short populations) • These plants are called the parental generation (P generation) • The offspring were all called the 1st filial generation (F1 generation) • All the offspring were tall (the short plants were totally excluded)
THE 2ND GENERATION • Next, Mendel crossed two plants from the F1 generation • The offspring from this cross are called the 2nd filial generation (F2 GENERATION) • Mendel found that ¾ of the offspring were tall & ¼ were short (the short plants reappeared!!!!!!)
Mendel Proposes a Theory • By convention, genetic traits are assigned a letter symbol referring to their more common form • dominant traits are represented by uppercase letters, and lower-case letters are used for recessive traits • for example, flower color in peas is represented as follows • P signifies purple • p signifies white
Mendel Proposes a Theory • The results from a cross between a true-breeding, white-flowered plant (pp) and a true breeding, purple-flowered plant (PP) can be visualized with a Punnett square • A Punnett square lists the possible gametes from one individual on one side of the square and the possible gametes from the other individual on the opposite side • The genotypes of potential offspring are represented within the square
TO GO ANY FURTHER, WE MUST UNDERSTAND ALLELES, DOMINANCE, & SEGREGATION • Genes – a section of DNA that codes for one protein • These genes are what control & produce traits • The genes Mendel studied came in two forms (tall/short - round/wrinkled - yellow/green…….etc.) • Alternate forms of a gene are called alleles • Alleles are represented by a one or two letter symbol (e.g. T for tall, t for short)
ALLELES CONT’D • THESE 2 ALLELS ARE NOW KNOWN TO BE FOUND ON COPIES OF CHROMOSOMES – ONE FROM EACH PARENT
THE RULE OF DOMINANCE • A dominant trait is the trait that will always be expressed if at least one dominant allele is present • The dominant allele is always represented by a capital letter • A recessive trait will only be expressed if both alleles are recessive • Recessive traits are represented by a lower case letter
DOMINANCE CONT’D • LET’S USE TALL & SHORT PEA PLANTS FOR AN EXAMPLE • WHICH OF THESE WILL SHOW THE DOMINANT & RECESSIVE TRAIT? TT Tt tt DOMINANT TRAIT RECESSIVE TRAIT
THE LAW OF SEGREGATION • MENDEL ASKED HIMSELF……..”HOW DID THE RECESSIVE SHORT PLANTS REAPPEAR IN THE F2 GENERATION?” • HE CONCLUDED THAT EACH TALL PLANT FROM THE F1 GENERATION CARRIED TWO ALLELES, 1 DOMINANT TALL ALLELE & ONE RECESSIVE SHORT ALLELE • SO ALL WERE Tt
SEGREGATION CONT’D • HE ALSO CONCLUDED THAT ONLY ONE ALLELE FROM EACH PARENT WENT TO EACH OFFSPRING • HIS CORRECT HYPOTHESIS WAS THAT SOMEHOW DURING FERTILIZATION, THE ALLELES SEPARATED (SEGREGATED) & COMBINED WITH ANOTHER ALLELE FROM THE OTHER PARENT • The law of segregation states that during gamete formation, the alleles separate to different gametes
FATHER MOTHER F1 GENERATION T t T t tt TT Tt F2 GENERATION - the law of dominance explained the heredity of the offspring of the f1 generation - the law of segregation explained the heredity of the f2 generation
DIHYBRID CROSS • TOOK TWO TRUE BREEDING PLANTS FOR 2 DIFFERENT TRAITS (ROUND/WRINKLED SEEDS ------- YELLOW/GREEN SEEDS) • 1ST GENERATION • WHAT WOULD HAPPEN IF HE CROSSED JUST TRUE BREEDING ROUND W/ TRUE BREEDING WRINKLED (ROUND IS DOMINANT) ALL THE OFFSPRING ARE ROUND
DIHYBRID CROSS – 1ST GENERATION CONT’D • SO WHAT DO YOU THINK HAPPENED WHEN HE CROSSED TRUE BREEDING ROUND/YELLOW SEEDS WITH TRUE BREEDING WRINKLED/GREEN SEEDS ALL THE F1 WERE ROUND AND YELLOW
DIHYBRID CROSS – 2ND GENERATION • TOOK THE F1 PLANTS AND BRED THEM TOGETHER (PHENOTYPE WAS ROUND/YELLOW X ROUND/YELLOW) • 2ND GENERATION • FOUND ROUND/YELLOW - 9 • FOUND ROUND/GREEN - 3 • FOUND WRINKLED/YELLOW - 3 • FOUND WRINKLED/GREEN - 1 ( 9 : 3 : 3 : 1 RATIO)
EXPLANATION OF 2ND GENERATION • MENDEL CAME UP W/ 2ND LAW – THE LAW OF INDEPENDENT ASSORTMENT • GENES FOR DIFFERENT TRAITS ARE INHERITED INDEPENDENTLY FROM EACH OTHER • THIS IS WHY MENDEL FOUND ALL THE DIFFERNENT COMBONATIONS OF TRAITS
DIHYBRID CROSSES • A LITTLE DIFFERENT • H h G g X H h G g • MUST FIND OUT ALL THE POSSIBLE ALLELIC COMBONATIONS • USE THE FOIL METHOD LIKE IN MATH
FOIL – FIRST, OUTSIDE, INSIDE, LAST H h G g X H h G g 1. HG BOTH PARENTS ARE THE SAME 2. Hg 3. hG 4. hg
NOW LET’S DO A DIHYBRID CROSS H h G g X H h G g HG Hg hG hg HG HHGG HHGg HhGG HhGg Hg HHGg HHgg HhGg Hhgg hG HhGG HhGg hhGG hhGg hg HhGg Hhgg hhGg hhgg
WHAT ARE THE PHENOTYPIC RATIO’S? H h G g X H h G g HG Hg hG hg HG HHGG HHGg HhGG HhGg Hg HHGg HHgg HhGg Hhgg hG HhGG HhGg hhGG hhGg hg HhGg Hhgg hhGg hhgg
PROBABILITY • WILL REAL LIFE FOLLOW THE RESULTS FROM A PUNNETT SQUARE? • NO!!!!!! – A PUNNETT SQUARE ONLY SHOWS WHAT WILL PROBABLY OCCUR • IT’S A LOT LIKE FLIPPING A COIN – YOU CAN ESTIMATE YOUR CHANCES OF GETTING HEADS, BUT REALITY DOESN’T ALWAYS FOLLOW PROBABILITY
MEIOSIS • GENES, CHROMOSOMES, AND NUMBERS • CHROMOSOMES HAVE 100’S OR 1000’S OF GENES • GENES FOUND ON CHROMOSOMES
DIPLOID & HAPLOID CELLS • ALL BODY CELLS (SOMATIC CELLS) HAVE CHROMOSOMES IN PAIRS • BODY CELLS ARE CALLED DIPLOID CELLS (2n) • HUMANS HAVE THE 2n # OF CHROMOSOMES
DIPLOID AND HAPLOID CELLS CONT’D • HAPLOID CELLS • ONLY HAVE 1 OF EACH TYPE OF CHROMOSOME (DIPLOID CELLS HAVE 2 OF EACH TYPE) • SYMBOL IS (n) • SEX CELLS HAVE THE n # OF CHROMOSOMES
HOMOLOGOUS CHROMOSOMES • HOMOLOGOUS CHROMOSOMES ARE THE PAIRED CHROMOSOMES THAT CONTAIN THE SAME TYPE OF GENTIC INFORMATION, SAME BANDING PATTERNS, SAME CENTROMERE LOCATION, ETC. • THEY MAY HAVE DIFFERENT ALLELES, SO NOT PERFECTLY IDENTICAL • WHY DO THEY HAVE DIFFERENT ALLELES? CAME FROM DIFFERENTPARENTS
IMPORTANT THINGS TO KNOW • CROSSING OVER– OCCURS DURING PROPHASE I • CREATES GENETIC VARIABILITY (RECOMBINATION OF GENES) • IN MEIOSIS I, HOMOLOGOUS CHROMOSOMES SEPARATE (ANAPHASE I) • IN MEIOSIS II, SISTER CHROMATIDS SEPARATE • TETRAD – WHAT THE HOMOLOGOUS CHROMOSOMES ARE CALLED WHEN THEY PAIR UP DURING PROPHASE I
Why Some Traits Don’t Show Mendelian Inheritance • Often the expression of phenotype is not straightforward • Continuous variation • characters can show a range of small differences when multiple genes act jointly to influence a character • this type of inheritance is called polygenic
Why Some Traits Don’t Show Mendelian Inheritance • Pleiotropic effects • an allele that has more than one effect on the phenotype is considered pleiotropic: one gene affects many characters • these effects are characteristic of many inherited disorders, such as cystic fibrosis and sickle-cell anemia