Chapter 14 Mendel and the Gene Idea

Genetics u The scientific study of inheritance. u Genetics is a relatively “new” science (about 150 years).

Genetic Theories 1. Blending Theory - traits were like paints and mixed evenly from both parents. 2. Incubation Theory - only one parent controlled the traits of the children. Ex: Spermists and Ovists

3. Particulate Model - parents pass on traits as discrete units that retain their identities in the offspring.

Gregor Mendel u Father of Modern Genetics.

u Mendel was a pea picker. u He used peas as his study organism.

Why Use Peas? u Short life span. u Bisexual. u Many traits known. u Cross- and self-pollinating. u (You can eat the failures).

Cross-pollination u Two parents. u Results in hybrid offspring where the offspring may be different than the parents.

Self-pollination u One flower as both parents. u Natural event in peas. u Results in pure-bred offspring where the offspring are identical to the parents.

Mendel's Work u Used seven characters, each with two expressions or traits. u Example: u Character - height u Traits - tall or short.

Monohybrid or Mendelian Crosses u Crosses that work with a single character at a time. Example - Tall X short

P Generation u The Parental generation or the first two individuals used in a cross. Example - Tall X short u Mendel used reciprocal crosses, where the parents alternated for the trait.

Offspring u F1 - first filial generation. u F2 - second filial generation, bred by crossing two F1 plants together or allowing a F1 to self-pollinate.

Results - Summary  In all crosses, the F1 generation showed only one of the traits regardless of which was male or female. u The other trait reappeared in the F2 at ~25% (3:1 ratio).

Mendel's Hypothesis 1. Genes can have alternate versions called alleles. 2. Each offspring inherits two alleles, one from each parent.

Mendel's Hypothesis 3. If the two alleles differ, the dominant allele is expressed. The recessive allele remains hidden unless the dominant allele is absent. Comment - do not use the terms “strongest” to describe the dominant allele.

Mendel's Hypothesis 4. The two alleles for each trait separate during gamete formation. This now called: Mendel's Law of Segregation

Law of Segregation

Mendel’s Experiments u Showed that the Particulate Model best fit the results.

Vocabulary u Phenotype - the physical appearance of the organism. u Genotype - the genetic makeup of the organism, usually shown in a code. u T = tall u t = short

Helpful Vocabulary u Homozygous - When the two alleles are the same (TT/tt). u Heterozygous- When the two alleles are different (Tt).

Test Cross u Cross of a suspected heterozygote with a homozygous recessive. u Ex: T_ X tt If TT - all dominant If Tt - 1 Dominant: 1 Recessive

Dihybrid Cross u Cross with two genetic traits. u Need 4 letters to code for the cross. u Ex: TtRr u Each Gamete - Must get 1 letter for each trait. u Ex. TR, Tr, etc.

Number of Kinds of Gametes u Critical to calculating the results of higher level crosses. u Look for the number of heterozygous traits.

Equation The formula 2 n can be used, where “n” = the number of heterozygous traits. Ex: TtRr, n=2 2 2 or 4 different kinds of gametes are possible. TR, tR, Tr, tr

Dihybrid Cross TtRr X TtRr Each parent can produce 4 types of gametes. TR, Tr, tR, tr Cross is a 4 X 4 with 16 possible offspring.

Results u 9 Tall, Red flowered u 3 Tall, white flowered u 3 short, Red flowered u 1 short, white flowered Or: 9:3:3:1

Law of Independent Assortment u The inheritance of 1st genetic trait is NOT dependent on the inheritance of the 2 nd trait. u Inheritance of height is independent of the inheritance of flower color.

Comment #1 u Ratio of Tall to short is 3:1 u Ratio of Red to white is 3:1 u The cross is really a product of the ratio of each trait multiplied together. (3:1) X (3:1)

Probability u Genetics is a specific application of the rules of probability. u Probability - the chance that an event will occur out of the total number of possible events.

Genetic Ratios u The monohybrid “ratios” are actually the “probabilities” of the results of random fertilization. Ex: 3:1 75% chance of the dominant 25% chance of the recessive

Rule of Multiplication u The probability that two alleles will come together at fertilization, is equal to the product of their separate probabilities.

Example: TtRr X TtRr u The probability of getting a tall offspring is ¾. u The probability of getting a red offspring is ¾. u The probability of getting a tall red offspring is ¾ x ¾ = 9/16

Comment u Use the Product Rule to calculate the results of complex crosses rather than work out the Punnett Squares. u Ex: TtrrGG X TtRrgg

Solution “T’s” = Tt X Tt = 3:1 (Tall:Short) “R’s” = rr X Rr = 1:1 (Red:White) “G’s” = GG x gg = 1:0 (Yellow:green) Product is: (3:1) X (1:1) X (1:0 ) = 3:3:1:1 Tall, Red, Green peas (3x1x0)

Variations on Mendel 1. Incomplete Dominance 2. Codominance 3. Multiple Alleles 4. Sex-Linked 5. Polygenic Inheritance

Incomplete Dominance u When the F1 hybrids show a phenotype somewhere between the phenotypes of the two parents. Ex. Red X White snapdragons F1 = all pink F2 = 1 red: 2 pink: 1 white

Result u No hidden Recessive. u 3 phenotypes and 3 genotypes u Red = C R C R u Pink = C R C W u White = C W C W

Codominance u Both alleles are expressed equally in the phenotype. u Ex. Sickle Cell Anemia u AA=Normal blood cells u AA’=Some normal some sickle u A’A’= All Sickle shaped

Result u No hidden Recessive. u 3 phenotypes and 3 genotypes

Multiple Alleles u When there are more than 2 alleles for a trait. u Ex. ABO blood group u I A - A type antigen u I B - B type antigen u i - no antigen

Result u Multiple genotypes and phenotypes. u Very common event in many traits.

Alleles and Blood Types Type Genotypes A I A I A or I A i B I B I B or I B i AB I A I B O ii

Comment u Rh blood factor is a separate factor from the ABO blood group. u Rh+ = dominant u Rh- = recessive u A+ blood = dihybrid trait

Linked genes u There are many genes, but only a few chromosomes. u Therefore, each chromosome must carry a number of genes together as a “package”.

Linked Genes u Traits that are located on the same chromosome. u Result: u Failure of Mendel's Law of Independent Assortment. u Ratios mimic monohybrid crosses.

Crossing-Over u Breaks up linkages and creates new ones. u Recombinant offspring formed that doesn't match the parental types.

If Genes are Linked: u Independent Assortment of traits fails. u Linkage may be “strong” or “weak”.

Linkage Strength u Degree of strength related to how close the traits are on the chromosome. u Weak - farther apart u Strong - closer together

u End of part 1

Chromosomal Basis of Sex in Humans u X chromosome - medium sized chromosome with a large number of traits. u Y chromosome - much smaller chromosome with only a few traits.

Human Chromosome Sex u Males - XY Females - XX u Comment - The X and Y chromosomes are a homologous pair, but only for a small region at one tip.

Sex Linkage u Inheritance of traits on the sex chromosomes. u X- Linkage (common) u Y- Linkage (very rare if exists at all)

Males u Hemizygous - 1 copy of X chromosome. u Show ALL X traits (dominant or recessive). u More likely to show X recessive gene problems than females.

X-linked Disorders u Color blindness u Duchenne's Muscular Dystrophy u Hemophilia (types a and b) u Immune system defects

X-linked Patterns u Trait is usually passed from a carrier mother to 1/2 of sons. u Affected father has no affected children, but passes the trait on to all daughters who will be carriers for the trait.

Can Females be color-blind? u Yes, if their mother was a carrier and their father is affected.

Sex Limited Traits u Traits that are only expressed in one sex. u Ex – prostate

Sex Influenced Traits u Traits whose expression differs because of the hormones of the sex. u These are NOT on the sex chromosomes. u Ex. – beards, mammary gland development, baldness

Polygenic Inheritance u Factors that are expressed as continuous variation. u Lack clear boundaries between the phenotype classes. u Ex: skin color, height

Genetic Basis u Several genes govern the inheritance of the trait. u Ex: Skin color is likely controlled by at least 4 genes. Each dominant gives a darker skin.

Result u Mendelian ratios fail. u Traits tend to "run" in families. u Offspring often intermediate between the parental types. u Trait shows a “bell-curve” or continuous variation.

Genetic Studies in Humans u Often done by Pedigree charts. u Why? u Can’t do controlled breeding studies in humans. u Small number of offspring. u Long life span.

Pedigree Chart Symbols Male Female Person with trait

Sample Pedigree

Dominant Trait Recessive Trait

Human Recessive Disorders u Several thousand known: u Albinism u Sickle Cell Anemia u Tay-Sachs Disease u Cystic Fibrosis u PKU u Galactosemia

Sickle-cell Disease u Most common inherited disease among African-Americans. u Single amino acid substitution results in malformed hemoglobin. u Reduced O 2 carrying capacity. u Codominant inheritance.

Recessive Pattern u Usually rare. u Skips generations. u Occurrence increases with inbred matings.

Human Dominant Disorders u Less common then recessives. u Ex: u Huntington’s disease u Achondroplasia u Familial Hypercholsterolemia

Inheritance Pattern u Each affected individual had one affected parent. u Doesn’t skip generations. u Homozygous cases show worse phenotype symptoms. u May have post-maturity onset of symptoms.

General Formal R = F X M X D R = risk F = probability that the female carries the gene. M = probability that the male carries the gene. D = Disease risk under best conditions.

Example u Wife has an albino parent. u Husband has no albinism in his pedigree. u Risk for an albino child?

Risk Calculation u Wife = probability is 1.0 that she has the allele. u Husband = with no family record, probability is near 0. u Disease = this is a recessive trait, so risk is Aa X Aa =.25 u R = 1 X 0 X.25 u R = 0

Risk Calculation u Assume husband is a carrier, then the risk is: R = 1 X 1 X.25 R =.25 There is a.25 chance that any child will be albino.