Heredity / Genetics Chapter 12

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

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

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

Traits Mendel studied 7 traits of the pea plants.

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

Mendel’s First Experiments Followed these plants through three generations Group of offspring from a given group of parents. P1 = original parents F1 = offspring of parents first filial generation F2 = offspring of F1 second filial generation (filial = son/daughter)

Mendel’s First Experiments P1 = purple and white Counted the number of traits in the F1 generation Found F1= all purple

Mendel’s First Experiments Crossed F1 x F1 Counted the number of traits in the F2 generation F2 = purple and white 3:1 ratio

They all ended up being 3:1 Results 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 At the time, people thought the traits of offspring were a blend of the traits from parents. Mendel’s results showed that only one of two traits (purple or white) were expressed for each characteristic.

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

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

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

Even More Conclusions 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. Homozygous – same alleles Ex. TT or tt Heterozygous – different alleles Ex. Tt

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

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

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

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

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

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

And the answers are…… Dominant Trait Recessive Trait 6,022 6,022 + 2,001 6,022 6,022 + 2,001 = 0.75 or 75% = 0.25 or 25% Yellow Green

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

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

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

Practicing Punnett Squares Monohybrid Cross Tall (T) is dominant in pea plants, Short (t) is recessive. Parents (P1) Genotype: Tt Phenotype : Tall T t TT Genotype: Tt Phenotype : Tall Tt T Genotype: Phenotype: Genotype: Phenotype: tall tall tt Tt Genotype: Phenotype: Genotype: Phenotype: t tall short

Practicing Punnett Squares Monohybrid Cross Tall (T) is dominant in pea plants, Short (t) is recessive. Parents (P1) Genotype: TT Phenotype : Tall T T TT Genotype: Tt Phenotype : Tall TT T Genotype: Phenotype: Genotype: Phenotype: tall tall Tt Tt t Genotype: Phenotype: Genotype: Phenotype: tall tall

Practicing Punnett Squares Monohybrid Cross Tall (T) is dominant in pea plants, Short (t) is recessive. Parents (P1) Genotype: TT Phenotype : Tall T T Tt Genotype: tt Phenotype : Short t Tt Genotype: Phenotype: Genotype: Phenotype: tall tall Tt Tt Genotype: Phenotype: Genotype: Phenotype: t tall tall

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 F1 offspring? b. What is the genotype of the F1 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 F1 offspring? b. What is the genotype of the F1 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 F1 offspring? b. What is the genotype of the F1 offspring?

B = black b = brown 4. A heterozygous guinea pig is crossed with a heterozygous guinea pig. a. What is the phenotype of the F1 offspring? b. What is the genotype of the F1 offspring?

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

Parents (P1) 1TT : 2 Tt : 1tt 3 tall : 1 short Phenotypic Ratio Genotypic Ratio Phenotypic Ratio 1TT : 2 Tt : 1tt 3 tall : 1 short Heterozygous tall Genotype: Parents (P1) Heterozygous tall Genotype: Genotype: Phenotype:

Parents (P1) T T TT TT T tall tall Tt Tt t tall tall 2TT : 2 Tt Genotypic Ratio Phenotypic Ratio 2TT : 2 Tt 4 tall : 0 short Homozygous tall Genotype: Parents (P1) TT T T Heterozygous tall Genotype: TT TT Tt T Genotype: Phenotype: Genotype: Phenotype: tall tall Tt Tt Genotype: Phenotype: Genotype: Phenotype: t tall tall

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

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

Dihybrid Crosses Option 1 Make a Punnett Square for each gene, then use rule of multiplication. 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 Parent 2 is heterozygous round, yellow What is the probability that their offspring will be AaBb?

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

Dihybrid Cross Option 2 Make one big punnett square to show all possible combinations. An Example: Parent 1 is AaBb Possible gametes = AB, Ab, aB, or ab Parent 2 is AaBb

Dihybrid Cross Parents AB Ab aB ab AABB AABb AaBB AaBb AAbb Aabb aaBB Probability of offspring AaBb = 4/16 or ¼

Dihybrid Cross Ratios Genotypic: Phenotypic: 4 AaBb : 2 AABb : 2 Aabb : 2 AaBB : 2 aaBb : 2 aabb : 1 AAbb : 1 AABB Phenotypic: 9:3:3:1 9 round, yellow : 3 round, green : 3 wrinkled, yellow : 1 wrinkled, green

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

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

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

Example Pedigree I 1 2 3 II 4 III 1 2

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

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

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

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

Colorblindness XBXB – Normal female XBXb – Normal (carrier) female XbXb – Colorblind female XBY – Normal male XbY – 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 Genetic condition in which neither allele is completely dominant or recessive Snapdragons RR = red. rr = white. Rr = pink.

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

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

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

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

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

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

Blood Types 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 Based on antigens on the surface of RBC’s 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. Type O is the universal donor because it contains no antigens. Type AB is the universal receiver because it contains no antibodies.

A and AB A and O B and AB B and O A, B, AB, O 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 A, B, AB, O O ii