Chapter 12: Patterns of Inheritance

Why Gregor Mendel chose to study pea plants

1. other research showed that pea hybrids could be produced 2. many pea varieties were available 3. peas are small plants and easy to grow 4. peas can SELF-FERTILIZE (male and female sexual organs are enclosed within each pea flower, and gametes produced by the male and female parts of the same flower can fuse to form viable offspring) or be CROSS-FERTILIZED (preventing self-fertilization by removing a flower's male parts before fertilization occurs, then introduce pollen from a different strain, thus performing cross-pollination that results in cross-fertilization)

Mendel's Experimental Method

1. produce TRUE-BREEDING strains fro each trait he was studying 2. cross-fertilize true-breeding strains having alternative forms of a trait - perform RECIPROCAL CROSSES as well: using pollen from a white-flowered plant to fertilize a purple-flowered plant, then using pollen from a purple-flowered plant to fertilize a white-flowered plant 3. allow the hybrid offspring to self-fertilize and count the number of offspring showing each form of the trait

Monohybrid Crosses

A MONOHYBRID CROSS is a cross that follows only two variations on a single trait Mendel produced true-breeding pea strains for 7 different traits (flower color: purple or white, seed color: yellow or green, seed texture: round or wrinkled, pod color: green or yellow, pod shape: inflated or constricted, flower position: axial or terminal, plant height: tall or short) - each trait had 2 alternative forms (variations) - Mendel cross-fertilized the 2 true-breeding strains for each trait

Gregor Mendel

Born in 1822 Educated in a monastery Failed exams to be a teacher Became an abbot In the garden of the monastery he initiated his own experiments on plant hybridization

The products of some genes interact with each other and influence the phenotype of the individual

Epistasis: one gene can interfere with the expression of another gene

Before the 20th century, two concepts provided the basis for most thinking about heredity

Heredity occurs within species Traits are transmitted directly from parents to offspring This led to the belief that inheritance is a matter of blending traits from the parents

Principle of Independent Assortment

In a dihybird cross, the alleles of each gene assort independently

Learning Outcomes

Mendel's monohybrid crosses refute the idea of blending. One trait disappears in the first generation (F1), then reappears in a predictable ration on the next (F2). The trait observable in the F1 is called dominant, an the other recessive. In the F2, the ratio of observed dominant offspring to recessive is 3:1, and this represents a ratio of 1 homozygous dominant to 2 heterozygous to 1 homozygous recessive. The Principle of Segregation states that alleles segregate into different gametes, which randomly combine at fertilization. The physical basis for segregation is the separation of homologues during anaphase 1 of meiosis.

Learning Outcomes

Prior to Mendel, concepts of inheritance did not from a consistent model. The dominant view was of blending inheritance, in which traits of parents were carried by fluid and "blended" in offspring. Plant hybridizers before Mendel, however, has already cast doubt on this model by observing characteristics in hybrids that seemed to change in second-generation offspring. Mendel's experiments with plants involved quantifying types of offspring and mathematically analyzing his observations.

Botanists in the early 18th and 19th century produced hybrid plants

When the hybrids were crossed with each other, some of the offspring resembled the original strains, rather than the hybrid strains This evidence contraindicated the idea that traits are directly passed from parents to offspring

Testcross

a cross used to determine the genotype of an individual with dominant phenotype - cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp) - the phenotypic among offspring are different, depending on the genotype of the unknown parent

Alleles

alternative forms of a gene get an allele from mom and one from dad purple or white, tall or short

What property distinguishes Mendel's investigation from previous studies? a. Mendel used true-breeding pea plants. b. Mendel quantified his results. c. Mendel examined many different traits. d. Mendel examined the segregation of traits.

b

All organism's ___________ is/are determined by its __________. a. genotype; phenotype b. phenotype; genotype c. alleles; phenotype d. genes; alleles

c

The F1 generation of the monohybrid cross purple (PP) x white (pp) flower pea plants should a. all have white flowers. b. all have a light purple or blended appearance. c. all have purple flowers. d. have 3/4 purple flowers, and 1/4 white flowers.

c

The F1 plants from the previous question are allowed to self-fertilize. The phenotype ratio for the F2 should be a. all purple. b. 1 purple:1 white. c. 3 purple:1 white. d. 3 white:1 purple.

c

Which of the following is NOT a part of Mendel's five-element model? a. Traits have alternative forms (what we now call alleles). b. Parents transmit discrete traits to their offspring. c. If an allele is present it will be expressed. d. Traits do not blend.

c

Phenotypes like height in humans, which show a continuous distribution, are usually the result of a. an alteration of dominance fro multiple alleles of a single gene. b. the presence of multiple alleles for a single gene. c. the action of one gene on multiple phenotypes. d. the action of multiple genes on a single phenotype.

d

Dihybrid Cross

examination of 2 separate traits in a single cross for example: RR YY x rryy The F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait The F2 generation is produced by crossing member of the F1 generation with each other or allowing self-fertilization of the F1 - for example: RrYy x RrYy The F2 generation shows all four possible phenotypes in a set ratio: 9:3:3:1

The expression of some genes can be influenced by the environment

for example: coat color in Himalayan rabbits and Siamese cats - an allele produces an enzyme that allows pigment production only at temperature below 30c

Heterozygous

having 2 different alleles

Homozygous

having 2 of the same allele

Gene

information for a trait passed from parent to offspring Carried on chromosomes

F1 Generation (1st filial generation)

offspring produced by crossing 2 true-breeding strains For every trait Mendel studied, all F1 plants resembled only 1 parent - no plants with characteristics intermediate between the 2 parents were produced DOMINANT: the form of each trait expressed in the F1 plants RECESSIVE: the form of the trait not seen in the F1 plants

F2 Generation (second filial generation)

offspring resulting from the self-fertilization of F1 plants F2 plants exhibited both form of the trait in a very specific pattern: - 3/4 plant with the dominant form - 1/4 plant with the recessive form The dominant to recessive ratio was 3:1 Mendel discovered that this ratio is actually: - 1 true-breeding dominant plant - 2 not true-breeding dominant plants - 1 true-breeding recessive plant

Phenotype

outward appearance of an individual

Some human traits exhibit dominant/recessive inheritance

pedigree analysis is used to track inheritance patterns in families

Pleiotropy, a single gene can affect more than one trait

refers to an allele which has more than one effect on the phenotype This can be seen in human disease such as Cystic Fibrosis or Sickle Cell Anemia In these diseases, multiple symptoms can be traced back to one defective allele

Incomplete Dominance

the heterozygote is intermediate in phenotype between the 2 homozygotes

Codominance

the heterozygote shows some aspect of the phenotypes of both homozygotes The human ABO blood group system demonstrates: - multiple alleles: there are 3 alleles of the I gene (IA, IB, and i) - codominance: IA and IB are dominant to i but codominant to each other

Principle of Segregation

the two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization

Genotype

total set of alleles of an individual PP= homozygous dominant Pp= heterozygous pp= homozygous recessive