Genetics Chapters 13 & 14
Genes are segments on the chromosomes that are responsible for inherited traits such as eye color, hair color, skin color, height, etc. • Now we’re going to learn how these traits are in- herited by the study of genetics.
I. Genetics: The branch of biology that studies heredity. • Heredity: The passing of characteristics from parents to offspring. • Mendel, an Austrian monk, carried out the 1st important studies of heredity. • The “father” of genetics. • He is credited with stating the principle that traits among off- spring result from combinations of dominant and recessive genes. • Traits :Characteristics that are inherited.
Monohybrid Cross: A cross involving 1 pair of contrasting traits. • Mendel’s 1st experiments are called monohybrid crosses because the 2 parent plants differed by a single trait—height. • Example: crossing a tall pea plant with a short pea plant.
Mendel carried out his experiments in three steps: • 1. Mendel allowed each variety of garden pea to self-pollinate for several generations. This method ensured that each variety was true-breeding (purebred) for a particular trait; that is all the offspring would show only one form of a particular trait. (Homo. Dom./Homo. Rec.) • Self-pollination-occurs when pollen is transferred from the anthers of a flower to the stigma of either the same flower or another flower of the same plant. • For example: All the tall pea plants had to produce many generations of nothing but tall plants. All the short pea plants had to produce many generations of nothing but short plants. • These true-breeding plants served as parental generation in Mendel’s experiments. • Parental generation = P1 generation (or P generation)
2. Mendel then cross-pollinated two P1 generation plants that had contrasting forms of a trait, such as a tall and a short pea plant. Mendel called the offspring of the P1 generation the 1st filial generation, or F1 generation. He then examined each F1 plant and recorded the number of F1 plants expressing each trait. • Offspring of the P1 generation = 1st Filial or F1 generation • Cross-pollination- involves flower of two separate plants.
3. Finally, Mendel allowed the F1 generation to self-pollinate. He called the offspring the F1 generation plants and the 2nd filial generation, of F2 generation. Again, each F2 plant was characterized and counted.
**The “x” means crossed. Mendel transferred pollen from one plant to another plant. **All of the f generation is tall. It’s as if the shorter parent had never existed. ** ¾ (75%) of the F2 generation is Tall & ¼ (25%) of the F2 generation is short. A ratio of 3:1. *It was as if the short (recessive) trait had reappeared from nowhere! *A dominant trait appears in every generation of the offspring. A recessive trait does not. P1 = 1st generationTall x short F1=2nd generation= Tall Tall Tall Tall F2=3rd generation=Tall Tall Tall short
III. Mendel’s Theories of Heredity—The foundationof genetics. • 1. For each inherited trait, an individual has two copies of the gene—one from each parent. • 2. Alleles are different gene forms (different versions). An individual receives one allele from each parent. Each allele can be passed on when the individual reproduces. • 3. When two different alleles occur together, one of them may be completely expressed, while the other may have no observable effect on the organism’s appearance.
Mendel’s 3rd Theory Cont. • Mendel described the expressed form of the trait as dominant. And the trait not expressed when the dominant form of the trait was present was described as recessive • Dominant: The expressed form of the trait. (The trait is visible). • Observable trait of an organism that masks a recessive form of the train. • Dominant genes (alleles) are represented by capital letters and the same letter in small case represents recessive genes. The dominant allele is always written first. • Ex. If tallness is dominant over short pea plants. Tt would be used if the pea plant is tall.
Blond Hair VS. Brown Hair
Mendel’s 3rd Theory Cont. • Recessive: The trait that is not expressed when the dominant form of the trait is present. • The hidden trait of an organism that is masked by a dominant trait. • For every pair of contrasting forms of a trait that Mendel studied, the allele for one form of the trait was always dominant and the allele for the other form of the trait was always recessive. • Example: Each of Mendel’s pea plants had 2 alleles that determined its height. • A plant could have: 2 alleles for tallness, TT 2 alleles for shortness, tt 1 for tall and 1 for short, Tt
+ = Sperm (1n) Egg (1n) Zygote (2n) 4. When gametes are formed, the alleles for each gene in an individual separate independently of one another. Thus, gametes carry only one allele for each inherited trait. When gametes unite during fertilization, each gamete contributes one allele. Each parent can contribute only one of the alleles because of the way gametes are produced during the process of meiosis.
Phenotype: The way an organism looks. It is the physical appearance of a trait. Example: The phenotype of a tall plant is tall regardless of the genes it contains. Genotype: The gene combination on organism contains (set of alleles). Genotype describes the genetic make up of a trait. The genotype of a tall plant that has 2 alleles for tallness is TT. **You can’t always know an organism’s genotype simply by looking at its phenotype.
Homozygous: Two alleles for a trait are the same. (The presence of two identical alleles for a trait.) Example—The true breeding tall plant that has two alleles for tallness (TT) would be homozygous for the trait height. Because tallness is dominant, a TT individual is homozygous dominant for that trait. A short plant would have 2 alleles for shortness (tt); it would always be homozygous recessive for the trait height.
Heterozygous: Two alleles for a trait are different. (The presence of two different alleles for a trait.) Example: The tall plant that has one allele for tallness and one allele for shortness (Tt) is heterozygous for trait height.
Mendel’s hypotheses brilliantly predicted the results of his crosses and also accounted for the ratios he observed. Because of their importance, Mendel’s ideas are often referred to as the Laws of Heredity: • Law of Dominance: • The dominant gene hides the recessive gene; the recessive gene only shows when the dominant gene is NOT present. • Law of Segregation: • The first law of heredity describes the behavior of chromosomes during meiosis when homologous chromosomes, and then chromatids are separated. • States that the two alleles for a trait segregate (separate) when gametes are formed.
Mendel believed that parent organisms could transmit any random combination of characteristics to their offspring. This became the foundation for the Law of Independent Assortment. • Law of Independent Assortment: • States that the alleles of different genes separate independently of one another during gamete formation. • Example: The alleles for plant height separate independently of the alleles for flower color. • Example: There could be a short plant with green pods. There could also be a tall plant with green pods. • We now know that this law applies only to genes that are located on different chromosomes or that are far apart on the same chromosomes. • Today, however, scientists know that some of the parents’ characteristics are inherited together as a group because many genes are located together on the same chromosome. • These are called linked genes, which were founded by Thomas Morgan & his graduate students. They studied the common fruit fly, Drosophila. • Example: The Drosophila fruit fly could be gray or black & wings could be long or vestigial.
Punnett Squares: Diagrams that predict the possible offspring of crosses. • We solve genetic problems and predict possible offspring by using the Punnett Squares. • The simplest is four boxes inside a square. • The possible gametes that one parent can produce is written along the top—usually the female. • The possible gametes that the other parent can produce is written along the left side—usually the male. • Each box inside the square is filled with two letters obtained by combining the allele along the top of the box with the allele along the side of the box. • The letters in the boxes are the possible genotypes of the offspring.
t t Tt Tt T • Monohybrid Cross: A cross involving one pair of contrasting traits. Example #1: Cross a heterozygous tall plant with a homozygous short plant. T=dominant for tall and t=recessive for short. tt tt t Possible offspring: Genotype: 2 Tt; 2 tt Phenotype: 2 Tall; 2 short Genotype ratio _2:2______________ Phenotype ratio _2:2_____________ _50__ % tall _50__ % short Tt tt X
Dihybrid Crosses:A cross involving 2 pairs of contrasting traits. • Dark fur color is dominant (D) and light fur (d) is recessive. • Rough coat texture (R) is dominant, while smooth coat (r) is recessive. • Ex: A female guinea pig is heterozygous for both fur color and coat texture is crossed with a male that has light fur color and is heterozygous for coat texture. What possible offspring can they produce?
Heterozygous fur color & coat texture XLight fur color & heterozygous coat texture ddRr DdRr ________ X ________ Dr dR dr DR Genotype: 2 DdRR 4 DdRr 2 Ddrr 2 ddRR 4 ddRr 2 ddrr dR DdRR DdRr ddRR ddRr DdRr Ddrr ddRr ddrr dr dR DdRR DdRr ddRR ddRr Phenotype: 6 dark & rough 2 dark & smooth 6 light & rough 2 light & smooth DdRr Ddrr ddRr ddrr dr
Probabilities Can Also Predict The Expected Results of Crosses: Probability—is the likelihood that a specific event will occur. Consider the possibility that a coin tossed into air will land on heads (1 possible outcome.) The total number of all possible outcome is two heads or tails. Thus, the probability that a coin will land on heads is ½ or 50%. Probability = number of 1 kind of possible outcome total number of all possible outcomes
X X XX XX X Example : Assume that in humans there is a 50/50 chance that a child will be a boy. If a certain mother and father have four sons, what are the chances that their fifth child will be a daughter. XY XY Y number of 1 kind of possible outcome OR 1 total number of all possible outcomes 2 XY XX X
Family Pedigrees can be used to study how traits are inherited • Pedigree: • A drawing that shows how a trait is inherited over several generations. It is a graphic representation of an individual’s family tree, where several generations may be illustrated. • A pedigree is helpful if the trait is a genetic disorder and the family members want to know if they are carriers or if their children might get the disorder. • A genetic traitthat appears in every generation of offspring is called dominant. • Scientists can determine several pieces of genetic information from a pedigree. • Autosomal trait—if a trait is autosomal, it will appear in both sexes equally. Autosomal does not involve the sex chromosomes.
Each chromosome carries genes for certain traits. Recall that in humans, the diploid number of chromosomes is 46 or 23 pairs. • There are 22 pairs of matching homologous chromosomes called autosomes. • An autosome is a chromosome other than X or Y sex chromosomes. The 2 chromosomes in a homologous pair of autosomes look exactly alike. • In a pair of homologous chromosomes, 1 comes from the mother, and one comes from the father. In other words, out of 46 chromosomes, 23 comes from mom and 23 from dad. • Homologous chromosomes have genes for the same traits arranged in the same order. • However, because there are different possible alleles (versions) for the same gene, the two chromosomes in a homologous pair are not identical to each other.
The 23rd pair (sex chromosomes) of chromosomes differs in males and females. These two chromosomes determine the sex of an individual and are called sex chromosomes. • Sex-linked trait— Traits controlled by genes located on sex chromosomes. • In humans, the chromosomes that control the inheritance of sex characteristics are indicated by the letters X and Y. • If you are a female, XX, your 23rd pair of chromosomes look alike. • If you are a male, XY, your 23rd pair of chromosomes look different
If a trait is sex linked, it is usually seen in males. Most sex-linked traits are recessive. • Because males have only one X chromosome, a male who carries a recessive allele on the X chromosome will exhibit the sex-linked condition. • A female who carries a recessive allele on one X chromosome will not exhibit the condition if there is dominant allele on her other X chromosome. • She will express the recessive condition only if she inherits two recessive alleles.
Polygenetic Trait: • A trait controlled by two or more genes. • Polygenic inheritance occurs when many genes interact to produce a single trait. • The genes may be on the same chromosome or on different chromosomes, and each gene may have two or more alleles. • Example: Familiar examples of polygenic traits in humans include eye color, height, weight, hair, and skin color. • All of these characteristics have degrees of intermediate conditions between one extreme and the other. When light-skinned people marry dark-skinned people, their offspring have intermediate skin colors. (F1 Generation) • When these children marry and produce the F2 generation, the resulting skin colors range from the light skin color to the dark skin color of the grandparent (the P1 generation), with most children having an intermediate skin color.
Incomplete Dominance • The phenotype of a heterozygote is intermediate between those of the two homozygotes (appearing halfway between the two parents). • A condition in which a trait in an individual is intermediate between the phenotype of the two parents. • Neither allele of the pair is completely dominant but combine and display a new trait. • We use all capital letters on these problems to show that it is different from the traditional genetic cross.
+ = Example #1: In Caucasians, the child of a straight-haired parent and a curly-haired parent will have wavy hair. Straight and curly hair are homozygous dominant traits. Wavy hair is heterozygous and is intermediate between straight and curly hair.
W W RW RW R Example #2: A cross between a homozygous red-flowered snapdragon is crossed with a homozygous white-flowered snapdragon. • RR=red • RW=pink • WW=white RW RW R Possible offspring: Genotype: 4RW Phenotype: 4 pink Genotype ratio _4:0______________ Phenotype ratio _4:0_____________ RR WW X
Codominance The equal expression of both alleles—both traits are displayed. In codominance, both alleles are expressed equally. Codomant alleles cause the phenotype of both homozygotes to be produced in heterozygote individuals.
Example #1: A cross between a homozygous red horse and a homozygous white horse results in heterozygous offspring with both red and white hairs in approximately equal numbers, producing the mixed color called roan. • Example #2: Crossing a black chicken with a white chicken results in a chicken that has black and white feathers, which appears checkered.
Multiple Alleles • A gene has three or more alleles for a trait. • Blood type problems show this inheritance pattern. • A capital “I” is used to show multiple alleles. • Human blood types are determined by the presence or absence of certain molecules on the surfaces of red blood cells. • Example: In the human population, the ABO blood groups (blood types) are determined by three alleles: IA, IB, and i on the surface of red blood cells. • The IAallele produces surface molecule A. • The IB allele produces surface molecule B. • The i allele produces no surface molecules.
The IA and IB alleles are both dominant over i, which is recessive. But neither IA and IB is dominant over the other. When IA and IB are both present in the genotype, they are codominant (equal expression).
When traits are controlled by genes with multiple alleles, an individual can have only two of the possible alleles for that gene. • Different combinations of the 3 alleles IA, IB, and i result in four different blood phenotypes, A, AB, B, and O. Blood Types and Genotypes in the ABO Blood Group System Blood Type Genotype A IA IA; IA i B IB IB; IB,i IAIB AB O ii • Example: A child with type O has a mother with blood type A and a father with blood type B. • The parental genotype for blood types must be __IAi; IBi__.
Traits Influenced by the Environment: • Environmental Influences: • Making inheritance more difficult to understand are interactions between genes and the environment. • The expression of some traits is affected by internal environments that are governed by age or sex. • Expression of other traits is affected by external factors in the environment such as temperature, chemicals, or light. • Example: Environmental influences fur color in the arctic fox—in the winter it has white fur; it has dark fur in the summer.
Human Genetic Disorders Caused by Mutations: • Changes in genetic material are called mutations. • The harmful effects produced by inherited mutations are called genetic disorders. • Many mutations are carried by recessive alleles in heterozygous individuals. This means two phenotypically normal people, who are heterozygous carriers of a recessive mutation, can produce children who are homozygous for the recessive allele.