Heredity Genetics Chapter 12 Gregor Mendel Worked in

Heredity / Genetics Chapter 12

Gregor Mendel Ø Worked in the mid-1800 s Ø Studied pea plants Ø Developed rules that accurately predict the patterns of heredity Ø Known as father of genetics for his contributions to the study of heredity.

Heredity Ø Genetics - The study of heredity. Ø Heredity - The inheritance of a set of characteristics from one’s parents. Ø The physical features that are inherited are referred to as characteristics.

Heredity Ø The units that determine biological characteristics are called genes. Ø Characteristics can show up as one of several possible forms called traits.

Traits Ø Mendel studied 7 traits of the pea plants.

Mendel’s First Experiments Ø Used a pair of contrasting traits Breeding a short plant with a tall plant, purple & white flowers, etc. l Kept all other traits the same in each plant l The resulting offspring with different characteristics are referred to as hybrids. l • These are called monohybrid crosses.

Mendel’s First Experiments Ø Followed these plants through three generations l Group of offspring from a given group of parents. • P 1 = original parents • F 1 = offspring of parents l first filial generation • F 2 = offspring of F 1 l second filial generation (filial = son/daughter)

Mendel’s First Experiments Ø P 1 = purple and white Counted the number of traits in the F 1 generation l Found F 1= all purple l

Mendel’s First Experiments Ø Crossed F 1 x F 1 Counted the number of traits in the F 2 generation l F 2 = purple and white l 3: 1 ratio l

Results Ø Tested all of the contrasting traits and compared the ratio of traits that resulted from each cross They all ended up being 3: 1

Mendel’s Conclusions Ø At the time, people thought the traits of offspring were a blend of the traits from parents. Ø Mendel’s results showed that only one of two traits (purple or white) were expressed for each characteristic.

Ø Today, we know that different traits result from different versions of genes. l Each version of a gene is called an allele.

More Conclusions Ø An offspring’s traits do not match one- to-one with the parents’ traits. Ø Offspring do not show a trait for every allele that they receive. The trait that results from a set of alleles is the phenotype. l The set of alleles that an individual has for a characteristic is called the genotype. l

Simplified…. . Ø Phenotype – an organism’s physical appearance (trait) l Ex. Purple, white Ø Genotype – an organism’s genetic composition l Ex. PP, Pp Ø Genotype determines phenotype.

Even More Conclusions Ø Mendel did not know about meiosis, but concluded that each trait (purple or white) was controlled by a pair of alleles, one from each parent. l Homozygous – same alleles • Ex. TT or tt l Heterozygous – different alleles • Ex. Tt

Ø For each pair of traits, one always seemed to “win” over the other whenever both alleles were present. Ø The other allele had no effect on the organism’s physical form. The expressed allele is called dominant. l The allele that is not expressed (hidden) when the dominant allele is present is called recessive. l

Law of Segregation Ø The two alleles separate from each other during formation of sex cells. Ø Either of these traits could end up in any gamete and chance decides which alleles will be passed on.

Mendel’s Second Experiments Ø Used the lack of pattern in this round. l Round seed did not always show up in the yellow seed trait. Ø Dihybrid cross involves two separate characteristics

Dihybrid Cross Ø A cross between individuals that involves two pairs of contrasting traits. l Homozygous round yellow x Homozygous green wrinkled • RRYY x rryy

Mendel’s Conclusions Ø The inheritance of one characteristic did not affect the inheritance of the second. Ø Law of Independent Assortment: Most genes are inherited independently and do not influence each other’s inheritance.

Probability Ø Probability: the likelihood an event will occur. Ø Probability: Number of times an event is expected to occur Number of opportunities for an event to occur Example: Seed Color l. Dominant (Yellow) occurred 6, 022 times l. Recessive (Green) occurred 2, 001 times

And the answers are…… Dominant Trait Recessive Trait 6, 022 + 2, 001 = 0. 75 or 75% Yellow = 0. 25 or 25% Green

Probability Examples In begonias, a red flower (R) is dominant to a white (r). Ø What is the probability of producing a white flower if two heterozygous plants are crossed? A red flower? Ø What is the probability of producing three white flowers in a row?

Punnett Squares Ø Biologists use a Punnett Square to predict the probability of traits which will be inherited. Ø Visual diagram that shows probability of different genotypes and phenotypes in a cross without a calculation.

Monohybrid Cross Ø A cross between two individuals involving one pair of contrasting traits. Ø What is the probability of producing a white flower if two heterozygous plants are crossed? Ø A red flower?

Practicing Punnett Squares Monohybrid Cross Tall (T) is dominant in pea plants, Short (t) is recessive. Genotype: Tt Phenotype : Tall Genotype: Phenotype: Genotype: Phenotype:

Practicing Punnett Squares Monohybrid Cross Tall (T) is dominant in pea plants, Short (t) is recessive. Genotype: TT Phenotype : Tall Genotype: Tt Phenotype : Tall Genotype: Phenotype: Genotype: Phenotype:

Practicing Punnett Squares Monohybrid Cross Tall (T) is dominant in pea plants, Short (t) is recessive. Genotype: TT Phenotype : Tall Genotype: tt Phenotype : Short Genotype: Phenotype: Genotype: Phenotype:

In guinea pigs, black coat color (B) is dominant over brown coat color (b) 1. A homozygous dominant (BB) guinea pig is crossed with a homozygous recessive(bb) guinea pig. a. What is the phenotype of the F 1 offspring? b. What is the genotype of the F 1 offspring?

B = black b = brown 2. A homozygous dominant (BB) guinea pig is crossed with a heterozygous (Bb) guinea pig. a. What is the phenotype of the F 1 offspring? b. What is the genotype of the F 1 offspring?

B = black b = brown 3. A homozygous recessive guinea pig is crossed with a heterozygous guinea pig. a. What is the phenotype of the F 1 offspring? b. What is the genotype of the F 1 offspring?

B = black b = brown 4. A heterozygous guinea pig is crossed with a heterozygous guinea pig. a. What is the phenotype of the F 1 offspring? b. What is the genotype of the F 1 offspring?

Ratios of offspring Ø Genotypic ratio: The ratio of genotypes which occur in the offspring. l HW #4 = 1 BB : 2 Bb : 1 bb Ø Phenotypic ratio: The ratio of phenotypes which occur in the offspring. l HW #4 = 3 black : 1 brown

Genotypic Ratio Phenotypic Ratio 1 TT : 2 Tt : 1 tt 3 tall : 1 short Heterozygous tall Genotype: Genotype: Phenotype:

Genotypic Ratio Phenotypic Ratio 2 TT : 2 Tt 4 tall : 0 short Homozygous tall Genotype: TT Heterozygous tall Genotype: Tt Genotype: Phenotype: Genotype: Phenotype:

Homework Tonight 1/18 Complete the genotypic and phenotypic ratios (c – d) for the guinea pig problems.

Dihybrid Cross Ø Dihybrid crosses show the probabilities when you cross two traits. l Option 1: Use Punnett Squares and Rule of Multiplication to solve OR l Option 2: Use Larger Punnett Square to solve.

Dihybrid Crosses Option 1 Ø Make a Punnett Square for each gene, then use rule of multiplication. Ø In plants, round seeds (A) are dominant to wrinkled (a) and yellow seeds (B) are dominant to green (b). Parent 1 is heterozygous round, yellow l Parent 2 is heterozygous round, yellow l What is the probability that their offspring will be Aa. Bb?

Dihybrid Cross Option 1 Continued A A AA a Aa aa B B BB b Bb Probability of Aa = ½ b Bb bb Probability of Bb = ½ Rule of Multiplication ½x½=¼ Probability of offspring Aa. Bb = ¼

Dihybrid Cross Option 2 Ø Make one big punnett square to show all possible combinations. Ø An Example: l Parent 1 is Aa. Bb • Possible gametes = AB, Ab, a. B, or ab l Parent 2 is Aa. Bb • Possible gametes = AB, Ab, a. B, or ab

Dihybrid Cross Parents AB Ab a. B ab AB AABb Aa. BB Aa. Bb Ab AABb AAbb Aa. Bb Aabb a. B Aa. Bb aa. BB aa. Bb ab Aa. Bb Aabb aa. Bb aabb Probability of offspring Aa. Bb = 4/16 or ¼

Dihybrid Cross Ratios Ø Genotypic: l 4 Aa. Bb : 2 AABb : 2 Aabb : 2 Aa. BB : 2 aa. Bb : 2 aabb : 1 AABB Ø Phenotypic: l 9: 3: 3: 1 • 9 round, yellow : 3 round, green : 3 wrinkled, yellow : 1 wrinkled, green

Human Somatic Cells Ø Human somatic cells have a total of 46 chromosomes— 23 from each parent l These cells contains 44 autosomes (22 pair). • Autosomes are chromosomes that do not determine gender. l Each cell has 2 sex chromosomes. • Females are XX • Males are XY.

Human Reproductive Cells Ø Sperm and Egg are haploid and contain 23 chromosomes. l Form a zygote during fertilization with 46 chromosomes. Ø Gender is determined by sex chromosomes. Egg always carries X chromosome. l Sperm can carry X or Y l • If X, offspring will be XX = female. • If Y, offspring will be XY = male.

Pedigrees Ø A pedigree is a chart that follows the inheritance of a single trait through several generations of a family. l Simple way to model inheritance Ø Can be used to determine inheritance pattern and predict future inheritance. l Answer questions about sex linkage, dominance, and heterozygosity. • Carrier: has allele for genetic disorder but does not show symptoms.

Example Pedigree I 1 2 II 1 2 3 4 III 1 2

Pedigrees Ø Squares represent males; circles represent females. Ø Shaded shapes indicate presence of trait. Ø Horizontal lines connect parent to each other. Ø Vertical lines connect parents to their children (arranged L R birth order).

Pedigrees Ø Brackets across top connect siblings. Ø Roman numerals are often used to show generations. Ø Sometimes lines must be drawn to show unusual relationships.

Sex-Linked Inheritance Ø Sex-linked genes l Genes located on the sex chromosomes X and Y. Ø Most sex-linked genes are on the X chromosome. Ø Recessive disorders on the X chromosome affect males more than females because females can be carriers without having the disorder.

Sex-Linked Disorders Colorblindess Ø Gene for color vision is located on the X chromosome. Dominant allele gives normal vision. l Recessive alleles produce colorblindness l Ø 10% of males in US suffer from at least one form of colorblindness

Colorblindness XBXB – Normal female XBXb – Normal (carrier) female Xb. Xb – Colorblind female XBY – Normal male Xb. Y – Colorblind male

Red-Green Colorblindness

Red-Green Colorblindness

Red-Green Colorblindness

The individual with normal color vision will see a 5 revealed in the dot pattern. An individual with Red/Green (the most common) color blindness will see a 2 revealed in the dots.

Alternative Inheritance

Incomplete Dominance Ø Genetic condition in which neither allele is completely dominant or recessive l Snapdragons • RR = red. • rr = white. • Rr = pink.

Incomplete Dominance Ø Parent 1 is red and Parent 2 is white. Ø Write genotypic and phenotypic ratios for the offspring. R R r r Rr Rr Phenotypic 0 Red : 4 Pink : 0 White Genotypic 0 RR : 4 Rr : 0 rr

Codominance Ø Condition in which both alleles of a gene are expressed equally. l Neither is dominant, nor do they blend. Ø Example: Chickens l FB - Black Feathers • A chicken that is FB FB will have all black feathers l FW - White Feathers • A chicken that is FW FW have all white. l But, a chicken that is FB FW will have black and white feathers.

Codominance Example Ø Parent 1 has black feathers; Parent 2 has black and white feathers. Ø What is the probability that these parents will have offspring with all white feathers? FB FB 0% will have all white feathers. FB FB FB FW

Polygenic Traits Ø Traits that are controlled by more than one gene Ø Inheritance is complicated and the traits show a very wide range of phenotypes l Ex. Skin color in humans

Multiple Alleles Ø A gene with more than 2 possible alleles. Ø Multiple alleles on Chromosome #9 control the ABO blood groups. l Three alleles IA, IB, and i • IA & IB are codominant, • Both dominant over i. l Result in 4 blood phenotypes • A, AB, B, and O

Blood Type Example Ø Parent 1 is heterozygous Type A. Parent 2 is homozygous Type B. Ø What is the probability that they will have a child with blood type B? IA i IB IA IB IB i 50% for Type B child

Blood Types Ø Another blood group factor is Rh, which is determined by dominant / recessive alleles • R = positive allele • RR = Positive • Rr = Positive • r = negative allele • rr = Negative

Blood Types in Humans Ø Based on antigens on the surface of RBC’s Ø If the body does not recognize the antigen, antibodies attack it and try to destroy it.

Donors and Receivers Type A blood has the antibody for type B blood, so it can’t receive blood from type B or AB donors. Ø Type O is the universal donor because it contains no antigens. Ø Type AB is the universal receiver because it contains no antibodies. Ø

Blood Group Alleles Can Donate To Can Receive From A IA IA or IA i A and AB A and O B IB IB or IB i B and AB B and O AB IA IB AB A, B, AB, O O ii A, B, AB, O O