CHAPTER 11

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 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     Gregor Mendol – father of genetics 1 st studies of heredity – the passing of characteristics to offspring Genetics – study of heredity The characteristics passed on called traits

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

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 only one tall or short) cross is one that looks at trait (let’s look at plant height –

THE 1

ST

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 1 st generation ( F 1 generation ) filial All the offspring were tall (the short plants were totally excluded)

THE 2

ND

GENERATION

   Next, Mendel crossed two plants from the F 1 generation The offspring from this cross are called the 2 nd filial generation ( F 2 GENERATION ) Mendel found that ¾ of the offspring were tall & ¼ were short (the short plants reappeared!!!!!!)

11.3 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 p signifies purple signifies white

Mendel Proposes a Theory

 The results from a cross between a true-breeding, white flowered plant ( plant ( PP pp ) and a true breeding, purple-flowered ) 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

Figure 11.7 A Punnett square analysis

Figure 11.8 How Mendel analyzed flower color

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 expressed if at least one dominant allele is present trait is the trait that will always be The dominant allele is always a capital letter A recessive trait will alleles are recessive only represented by be expressed if both 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

F1 GENERATION FATHER T t MOTHER T t T T T t F2 GENERATION t t - 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

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

ANSWER ON YOUR SHEET

TRAITS = BLUE SKIN & YELLOW SKIN BB – IS THIS HOMOZYGOUS OR HETEROZYGOUS?

HOMOZYGOUS IS BLUE SKIN OR YELLOW SKIN DOMINANT? BLUE

MENDEL’S DIHYBRID CROSSES    MONOHYBRID – MENDEL LOOKED AT ONE TRAIT IN HIS DIHYBRID CROSSES – HE LOOKED AT 2 TRAITS WANTED TO SEE IF TRAITS ARE INHERITED TOGETHER OR INDEPENDENTLY

DIHYBRID CROSS

  TOOK TWO TRUE BREEDING PLANTS FOR 2 DIFFERENT TRAITS (ROUND/WRINKLED SEEDS ------- YELLOW/GREEN SEEDS) 1 ST  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 – 1 ST 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 – 2 ND GENERATION   TOOK THE F1 PLANTS AND BRED THEM TOGETHER (PHENOTYPE WAS ROUND/YELLOW X ROUND/YELLOW) 2 ND GENERATION     FOUND ROUND/YELLOW - 9 FOUND ROUND/GREEN - 3 FOUND WRINKLED/YELLOW - 3 FOUND WRINKLED/GREEN - 1 ( 9 : 3 : 3 : 1 RATIO)

EXPLANATION OF 2 ND GENERATION  MENDEL CAME UP W/ 2 ND 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

PUNNETT SQUARES

  A QUICK WAY TO FIND THE GENOTYPES IN UPCOMING GENERATIONS 1 ST 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

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 1. HG 2. Hg

H h G g X H h G g

BOTH PARENTS ARE THE SAME 3. hG 4. hg

NOW LET’S DO A DIHYBRID CROSS H h G g X H h G g HG Hg HG HHGG HHGg hG HhGG hg HhGg Hg HHGg HHgg HhGg Hhgg hG hg HhGG HhGg HhGg Hhgg hhGG hhGg hhGg hhgg

WHAT ARE THE PHENOTYPIC RATIO’S?

H h G g X H h G g HG Hg HG HHGG HHGg hG HhGG hg HhGg Hg HHGg HHgg HhGg Hhgg hG hg HhGG HhGg HhGg Hhgg hhGG hhGg hhGg hhgg

Figure 11.10 Analysis of a dihybrid cross

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 PAIRED CHROMOSOMES THAT CONTAIN THE SAME TYPE OF GENTIC INFORMATION, SAME BANDING PATTERNS, SAME CENTROMERE LOCATION, ETC.

ARE THE THEY MAY HAVE DIFFERENT ALLELES, SO NOT PERFECTLY IDENTICAL WHY DO THEY HAVE DIFFERENT ALLELES?

CAME FROM DIFFERENT PARENTS

IMPORTANT THINGS TO KNOW     CROSSING OVER PROPHASE I  – OCCURS DURING 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

Figure 11.11 The journey from DNA to phenotype

11.6 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

Figure 11.12 Height is a continuously varying character

11.6 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

Figure 11.13 Pleiotropic effects of the cystic fibrosis gene,

cf

11.6 Why Some Traits Don’t Show Mendelian Inheritance 

Incomplete dominance

 not all alternative alleles are either fully dominant or fully recessive in heterozygotes  in such cases, the alleles exhibit incomplete dominance and produce a heterozygous phenotype that is intermediate between those of the parents

Figure 11.14 Incomplete dominance

11.6 Why Some Traits Don’t Show Mendelian Inheritance 

Environmental effects

  the degree to which many alleles are expressed depends on the environment for example, some alleles are heat-sensitive   arctic foxes only produce fur pigment when temperatures are warm the ch allele in Himalayan rabbits and Siamese cats encodes a heat-sensitive enzyme, called tyrosinase, that controls pigment production  tyrosinase is inactive at high temperatures

Figure 11.15 Environmental effects on an allele

11.6 Why Some Traits Don’t Show Mendelian Inheritance 

Epistasis

 in some situations, two or more genes interact with each other, such that one gene contributes to or masks the expression of the other gene   in epistasis, one gene modifies the phenotypic expression produced by the other for example, in corn, to produce and deposit pigment, a plant must possess at least one functional copy of each of two genes  one gene controls pigment deposition  the other gene controls pigment production

Figure 11.16 How epistasis affects kernel color

     Why is coat color in Labrador retrievers an example of epistasis?

E gene determines if dark pigment will be deposited in fur or not genotype ee , no pigment will be deposited in the fur, and it will be yellow genotype E_ , pigment will be deposited in the fur A second gene, the pigment will be B gene, determines how dark the Yellow dogs with the genotype eebb will have brown pigment on their nose, lips, and eye rims, while yellow dogs with the genotype these areas.

eeB_ will have black pigment in

Figure 11.17 The effect of epistatic interactions on coat color in dogs

11.6 Why Some Traits Don’t Show Mendelian Inheritance 

Codominance

 a gene may have more than two alleles in a population   often, in heterozygotes, there is not a dominant allele but, instead, both alleles are expressed these alleles are said to be codominant

11.6 Why Some Traits Don’t Show Mendelian Inheritance  The gene that determines ABO blood type in humans exhibits more than one dominant allele  the gene encodes an enzyme that adds sugars to lipids on the membranes of red blood cells   these sugars act as recognition markers for cells in the immune system the gene that encodes the enzyme, designated three alleles: I A ,I B , and i I, has   different combinations of the three alleles produce four different phenotypes, or bloodtypes (A, B, AB, and O) both I A and I B are dominant over i and also codominant

Figure 11.19 Multiple alleles controlling the ABO blood groups

Inheritance of Blood Type

67

11.8 Human Chromosomes

 Nondisjunction may also affect the sex chromosomes  nondisjunction of the X chromosome creates three possible viable conditions   XXX female  usually taller than average but other symptoms vary XXY male (Klinefelter syndrome)  sterile male with many female characteristics and diminished mental capacity  XO female (Turner syndrome)  sterile female with webbed neck and diminished stature

Figure 11.26 Nondisjunction of the X chromosome

11.9 The Role of Mutations in Human Heredity  Accidental changes in genes are called

mutations

 mutations occur only rarely and almost always result in recessive alleles  not eliminated from the population because they are not usually expressed in most individuals (heterozygotes)  in some cases, particular mutant alleles have become more common in human populations and produce harmful effects called genetic disorders

Table 11.3 Some Important Genetic Disorders

11.9 The Role of Mutations in Human Heredity  To study human heredity, scientists examine crosses that have already been made   they identify which relatives exhibit a trait by looking at family trees or pedigrees often one can determine whether a trait is sex-linked or autosomal and whether the trait’s phenotype is dominant or recessive

Figure 11.27 A general pedigree

11.9 The Role of Mutations in Human Heredity  Sickle-cell anemia is a recessive hereditary disorder  affected individuals are homozygous recessive and carry a mutated gene that produces a defective version of hemoglobin  the hemoglobin sticks together inappropriately and produces a stiff red blood cell with a sickle-shape  the cells cannot move through the blood vessels easily and tend to form clots  this causes sufferers to have intermittent illness and shortened life spans

Figure 11.29 Inheritance of sickle-cell anemia

11.9 The Role of Mutations in Human Heredity   The sickle-cell mutation to hemoglobin affects the stickiness of the hemoglobin protein surface but not its oxygen-binding ability In heterozygous individuals, only some of their red blood cells become sickled when oxygen levels become low  this may explain why the sickle-cell allele is so frequent among people of African descent  the presence of the allele increases resistance to malaria infection

Figure 11.30 The sickle-cell allele confers resistance to malaria