Chapter 14: Mendel and the Gene Idea

Problems with the blending hypothesis

-Predicts that all traits will converge on an average (red + white = pink) -Therefore trait variation within a population should decrease over generations -Doesn't explain how traits can skip a generation

Punnett square

-Punnett square: shows all possible genotypes/phenotypes for a given monohybrid cross -The rows and columns are the types of gamete each parent can produce

Understand Mendel's laws of segregation and independent assortment and how they relate to meiosis

-Segregation: alleles segregate (separate) into gametes -Independent assortment: It is random which of the 2 alleles a gamete will get

Monohybrid cross

-True-breeding parental generation is homozygous (one individual has the same alleles for a trait), written PP or pp -F1 generation is heterozygous (one individual has different alleles for a trait), written Pp -F2 generation is a mix of homozygous dominant, homozygous recessive, and heterozygous

Dihybrid cross

-two genes/two traits considered simultaneously

Degrees of Dominance

Alleles can show different degrees of dominance and recessiveness in relation to each other. In Mendel's classic pea crosses, the F1 offspring always looked like one of the two parental varieties because one allele in a pair showed COMPLETE DOMINANCE over the other. Ins such situations, the phenotypes of the heterozygote and the dominant homozygote are indistinguishable. For some genes, however, neither allele is completely dominant, and the F1 hybrids have a phenotype somewhere between those of the two parental varieties. This phenomenon, called INCOMPLETE DOMINANCE, is seen when red snapdragons are crossed with white snapdragons: all the F1 hybrids have pink flowers. This third, intermediate phenotype results from flowers of the heterozygotes having less red pigment than the red homozygotes. At first glance, incomplete dominance of either allele seems to provide evidence for the blending hypothesis of inheritance, which would predict that the red or white trait could never reappear among offspring from the pink hybrid. Is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in which the expressed physical trait is a combination of the phenotypes of both alleles. Another variation on dominance relationships between alleles is called CODOMINANCE; in this variation, the two alleles each affect the phenotype in separate, distinguishable ways. A form of dominance in which the alleles of a gene pair in a heterozygote are fully expressed thereby resulting in offspring with a phenotype that is neither dominant nor recessive.

Useful Genetic Vocabulary

Homozygous: an organism that has a pair of identical alleles for a character is said to be homozygous for the gene controlling that character. In the parental generation the purple pea plant is homozygous for the dominant allele (PP) while the white plant is homozygous for the recessive allele (pp). Homozygous plants "breed true" because all of their gametes contain the same allele-either P or p. Heterozygous: if we cross dominant homozygous with recessive homozygous, every offspring will have two different alleles- Pp in the case of the F1 hybrids of the flower color experiment. An organism that has two different alleles for a gene is said to be heterozygous. Heterozygous produce gametes with different alleles, so they are not true breeding. For example, P and p containing gametes are both produced by our F1 hybrids. Self pollination of the F1 hybrids thus produce both purple flowered and white flowered offspring. Phenotype: organisms appearance or observable traits Genotype: its genetic makeup In the case of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes.

True Breeding

Mendel chose to track only those characters that occurred in two distinct, alternative forms, such as purple or white flower color. He also made sure that he started his experiments with varieties that, over many generations of self pollination, had produced only the same variety as the parent plant. For example, a plant with purple flowers is true breeding if the seeds produced by self pollination is successive generations all give rise to plants that also have purple flowers.

Hybridization

Mendel cross pollinated two contrasting, true breeding pea varieties- for example, purple flowered plants and white flowered plants. This mating, or crossing, of two true breeding varieties is called a hybridization. The true breeding parents are referred to as the P GENERATION (parental generation) and their hybrid offspring are the F1 GENERATION. Allowing these F1 hybrids to self pollinate produces an F2 GENERATION.

Basics of the gene model of inheritance

Mendel developed a model to explain the 3:1 inheritance pattern that he consistently observed among the F2 offspring in his pea experiments. We describe four related concepts making up this model, the fourth of which is the law of segregation. 1. There are alternative versions of genes accounting for variations in inherited characters. Called "alleles" that code for distinct phenotypes. 2. For each character, an organism inherits two copies (alleles) of a gene, one from each parent (diploid) 3. If the two alleles at a locus differ, then one, the dominate allele, determines the organisms appearance; the other, the recessive allele, has no noticeable effect on the organisms appearance. 4. the law of segregation, states that the two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes. Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of the organism making the gamete. NOTE: if an organism has identical alleles for a particular character-that is, the organism is true breeding for that character- then that allele is present in all gametes. But if different alleles are present, as in the F1 hybrids, then 50% of the gametes receive the dominant allele and 50% receive the recessive allele. In a diploid organism (e.g., most animals), there are two alternative versions of chromosomes (one from mom, one from dad) On the chromosomes, we can find two alternative versions of genes called alleles Mendelian inheritance is all about: 1) how these alleles are transmitted in meiosis 2) how they interact with each other to determinate the phenotype

Mendel's Experimental Approach

One reason Mendel probably chose to work with peas is that there are many varieties. For example, one variety has purple flowers, while another variety has white flowers. A heritable feature that varies among individuals, such as flower color, is called a CHARACTER. Each variant for a character, such as purple or white color for flowers, is called a TRAIT. To achieve cross-pollination of two plants, Mendel removed the immature stamens of a plant before they produced pollen and then dusted pollen from another plant onto the altered flowers. Each resulting zygote then developed into a plant embryo encased in a seed (pea). Mendel could thus always be sure of the parentage of new seeds.

The Law of Independent Assortment

States that two or more genes assort independently-that is, each pair of alleles segregates independently of each other pair of alleles-during gamete formation. Mendel derived the law of segregation from experiments in which he followed only a single character, such as flower color. All the F1 progeny produced in his crosses of true breeding parents were MONOHYBRIDS, meaning that they were heterozygous for the one particular character being followed in the cross. We refer to a cross between such heterozygotes as a MONOHYBRID CROSS. DIHYBRIDS: Imagine crossing two true breeding pea varieties that differ both of these characters- a cross between a plant with yellow round seeds (YYRR) and a plant with green wrinkled seeds (yyrr). The F1 plants will be dihybrids, individuals heterozygous for the two characters being followed in the cross (YyRr).

Describe the main feature of the blending vs particulate hypotheses of inheritance

Two competing hypothesis: -Blending inheritance (common during Darwin's time) is the idea that the traits of offspring are a mix of parents' traits. -In the 19th century, many biologists believed that inherited characteristics were blends of parental traits -Darwin recognized this as a huge challenge to his theory: ultimately variation will be lost as all traits blend together, but didn't have a solution -Particulate inheritance (Mendel) is the idea that the traits of offspring come through discrete units, or genes. Mendel showed with his studies on pea plants that traits follow the particulate model of inheritance.

Relationship between Dominance and Phenotypes

We've now seen that the relative effects of two alleles range from complete dominance of one allele, through incomplete dominance of either allele, to codominance of both alleles. It is important to understand that an allele is called dominant because it is seen in the phenotype, not because it somehow subdues a recessive allele. Alleles are simply variations in a gene's nucleotide sequence. When a dominant allele coexists with a recessive allele in a heterozygote, they do not actually interact at all. It is in the pathway from genotype to phenotype that dominance and recessiveness come into play. Whether alleles appear to be completely dominant, incompletely dominant, or codominant depends on the level at which the phenotype is analyzed.

Heterozygous

a genotype with two different alleles

Homozygous

a genotype with two identical alleles

Dominant

an allele that "masks" another allele when heterozygous

Recessive

an allele that can be "masked" by a dominant allele when heterozygous

Testcross

breeding an organism of unknown genotype with a recessive homozygote is called a testcross because it can reveal the genotype of that organism.

Genotype

the combination of alleles that cause a particular phenotype

Phenotype

traits visible in an organism

Allele

version of a gene

Explain the relationship between genotype and phenotype in a genetic trait with typical dominant/recessive relationship

-A heterozygous genotype (dominant and recessive alleles in the same individual) appears like a homozygous dominant genotype (2 dominant alleles in the same individual)

Gamete formation in meiosis

-Consider a single gene on a pair of homologous chromosomes -Four haploid gametes formed: 2 get mom's allele, 2 get dad's -This is Mendel's law of segregation: alleles segregate into gametes

Mendel's approach

-Mendel crossed "true-breeding" pea plants with different traits -Flower color, seed color, seed shape, fruit color, fruit shape, plant height, etc -Also called phenotypes (traits or appearances)

Explain the difference between a monohybrid and a dihybrid cross

-Monohybrid: parents differ in alleles for 1 gene (usual 2x2 Punnett square) -Dihybrid: parents differ in alleles for 2 genes (big 4x4 Punnett square) NOTE: Won't be tested on Dihybrid

Explain the patterns of inheritance Mendel observed in terms of the gene model

-Particulate inheritance, not blending -Genotype responsible for phenotype -A Punnett square shows the possible combinations of the two alleles from each parent and the chances that a particular genotype will be present in offspring